摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2043-2046 Water Distillation through Poly(tetrafluoroethy1ene) Hydrophobic Membranes in a Stirred Cell M. 1. Vazquez-Gonzalez and L. Martinez Departamento de Fisica Aplicada , Facultad de Ciencias, Universidad de Malaga, 29011-Malaga , Spain The aim of this paper is the study of the transport of pure water through microporous hydrophobic membranes in a stirred cell containing two phases at different temperatures. The dependence of the phenomena on stirring rate and on the average temperature has been investigated. The influence of these operating conditions on mass-transfer rate is discussed, taking into account mass and heat transfer within the membrane and adjoining liquids. The concept of temperature polarization is introduced into the transport equations and shown to be important in the interpretation of experimental results.If a membrane separates two liquid phases having the same concentration, but at different temperatures, mass transfer through the membrane is observed. In analysing these ther- mally induced processes it is convenient to consider two dif- ferent processes : (1) thermo-osmosis, occurring across dense membranes, by a dissolution-diffusion mechanism ; and (2) distillation, occurring across porous membranes, by an evaporation-diffusion-ondensation mechanism. In the latter case the observed fluxes are usually much higher than in the thermo-osmotic case. We have studied thermo-osmosis pre- viously.' In the present work we focus our attention on the distillation process.In this process the liquids or solutions must not wet the membrane, otherwise the pores will be filled immediately as a result of capillary forces. This implies that non-wettable porous hydrophobic membranes must be used in the case of aqueous solution^.^.^ When the liquid phases contain pure water and there is no temperature difference, the system is in equilibrium and no transport occurs. If the temperature of one of the two liquid phases is higher than that of the other, a temperature differ- ence exists across the membrane, resulting in a vapour pres- sure difference. Thus, water will evaporate on the hot side; the vapour flows from the warm to the cold side where it condenses. In this way water transport takes place across the membrane from the hot to the cold side.The advantages of membrane distillation are that the distil- lation process takes place at moderate temperatures and that a relatively low temperature difference between the two liquids contacting the microporous hydrophobic membrane gives relatively high fluxes. The need to supply heat to the evaporation surface of the membrane means that the temperature gradients must be in the liquid phase adjacent to the membrane. The same occurs in the condensation surface side. Although the bulk phases on the two sides of the membrane are stirred, the effective temperature difference between the two sides of the mem- brane is not the same as the temperature difference between the bulk solutions (Fig.1).This loss of driving force brought about by thermal gradients in the fluids bounding the mem- brane is known as temperature polarization and has been applied to studies of thermo-osmosis, 'v4*' before being used in membrane distillation.6-8 In the present work we have measured the water distilla- tion through three different membranes for different oper- ation conditions and the theories of mass and heat transfer within the membrane and adjoining fluids have been devel- oped with a view to discussing the results obtained and to characterizing the temperature polarization. Experimental Three commercial PTFE membranes were used. These hydrophobic membranes are marketed by Gelman instru- ments Co.as TF-200, TF-450 and TF-1000, with nominal pore sizes of 0.2, 0.45 and 1.0 pm and porosities of 0.80. They have a limited mechanical strength, and in practice must be supported by different nets made generally of polymer fibres. In this way, they are composite membranes formed by an actual porous PTFE layer with a thickness of 60 pm on a polypropylene screen support. Scanning electron micro-graphs show differences between these screen supports for the three membranes. Pure water (doubly distilled and deionized) was used in the experiments. The water was filtered through a Millipore filter of 0.45 pm nominal pore size before being introduced into the measuring apparatus. All the measurements were made with an experimental device similar to that employed in earlier thermo-osmotic studies.' It is a cell formed by two symmetric cylindrical semicells 0.1 m long, separated by the membrane which is placed in a methacrylate holder with two stainless-steel grids between which the membrane was fixed. The membrane surface area exposed to the flow was 4 = 32.2 x m2.The water in the chamber was stirred by a chain-drive magnetic-cell stirrer assembly, both semicells being sur-rounded by concentric thermostatic chambers, through which flows of fluids at different temperatures were maintained. Two thermocouples were sealed in the membrane holder so that the temperature of each half-cell could be recorded and controlled. The cold chamber was connected to a glass tube placed horizontally and inserted in the top of the half-cell. In the present work a series of experiments has been per- formed with distilled water and three microporous hydropho- bic membranes.A temperature difference was maintained between both sides of the membrane and the volume flux was Fig. 1 Schematic representation of the membrane distillation J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 measured by collecting and weighing the water flowing through the glass tube placed horizontally in the cell. The experiments were carried out for different average tem-peratures in the system and different stirring rates of the bulk phases. The purpose of this paper is to study the experimen- tal situation and to evaluate some characteristic parameters of the membrane system from the flux measurements. Theory Simultaneous heat and mass transport characterize the mem- brane distillation process.The mass transport through the membrane is driven by a vapour pressure difference, resulting from the imposed temperature difference. This mass transport may be explained by the following mechanisms.6 (a)When non-condensable gases are contained in the pores of the membrane (e.g. air) as a stagnant film, the molecular diffusion model applies :9 J, = (l/Y,nXD&/XsXn/r/RT)(P,-'2) (1) where D is the water diffusion coefficient, A4 the water molec- ular weight, E the membrane porosity, x the tortuosity factor, 6 the membrane thickness, Y,,the mole fraction of air (log- mean), T the temperature, and P, and P, the pressures of water vapour corresponding to the T,, and Tm2 tem-peratures, respectively, and R the gas constant. (b) In most cases, when the pore sizes and the mean free molecular paths in the membrane distillation process are of the same order of magnitude, the Knudsen diffusion model applies : O -P,).I, = (~/~H~&/X~X~M/?~RT)''~(P,(2) where r is the membrane pore radius.The two models mentioned suggest the following equation by which the transfer may be described: J = C(P1 -P2) (3) where C can be considered a combined mass-transfer coeffi- cient through the membrane, accounting for the mass-transfer resistances of both molecular and Knudsen diffusion. Inspection of both models suggests that C will be slightly temperature dependent, decreasing < 3% with a 10"C increase in mean temperature.As vapour pressures within the membrane are not directly measurable, it is convenient to express eqn. (3) in terms of temperatures : J = C I dP/dT IT,,,(Tml -(4) This equation is a good approximation for values of T,, -Tm2< 10°C. In eqn. (4) dP/dT can be evaluated from the Clausius-Clapeyron equation at the average membrane temperature T,. Since, as opposed to the temperatures Tbl and G2,temperatures T,, and Tm2are difficult to measure, Tbl -Tb2 is, as a rule, inserted in the above equation. In order to do this, we must introduce the heat-transfer coeffi- cients (hl, h2) in the liquid films near the membrane, the latent heat transfer (A) accompanying vapour flux and the heat transfer by conduction (k,) across the membrane.In this way, for the stationary thermal flux across the membrane system in Fig. 1 we can write: hl(Tbl -'ml) = (km/SXTml -Tm2) + JA = h2(T,2 -q2) (5) From eqn. (4) and (5): -T,,)/JA = [CA(dP/dT)]-'[1 + (k,,,/6h)]+ (l/h) (6) where h = l/(l/h, + 1/h2) is the overall film heat-transfer coeficien t . On the other hand, from eqn. (4) and (5) Tml -Tm2 = + (H/hl) + (H/h2)1-'(Tb1 -Tb2) (7) where H = CA(dP/dT) + k,/6 (8) and z = [l + (H/h,) + (H/h2)]-l = [l + (H/h)]-l (9) is the temperature polarization coefficient. Eqn. (6) may be used for the analysis of experimental results for which Kl, &, and J are reported, as dP/dT is a function of T, = + Tb,)/2 assuming the temperature polarization is similar on either side of the membrane. Spe- cifically, a fit to a linear function of the values of (Tbl -T,,)/JA opposite to those of l/(dP/dT)A should yield an intercept of l/h and a slope of (l/C)[l + (kddh)],from which C may be obtained.Results and Discussion Experiments were carried out for a fixed temperature differ- ence Tbl -Tbz = 10°C between the bulk phases 1 and 2 (Fig. 1). The stirring rate used varied from 150 to 350 rpm in steps of 50 rpm, and the average temperature from 25 to 55"C, in steps of 5 "C. The flux results so obtained are shown in Table 1. The distillate flux increases monotonically with stirring rate, corresponding"*" to a decrease of the heat resistance in the boundary layers of the membrane.On the other hand, the distillate flux increases when the absolute temperature level in the membrane increases, corresponding to an increase of dP/dT with temperature. Eqn. (6) of the transport model shows how J increases when h and dP/dT augment each other. Plots of eqn. (6) for the experimental water flux corre- sponding to the same stirring rate and different average tem- peratures have been carried out in order to evaluate h and C, resulting in plots having a correlation coefficient > 0.98. Three representative plots are shown in Fig. 2. The h values obtained from the intercept of plots of eqn. (6) are shown in Table 2 for the different stirring rates and mem- branes, the h error being ca. 10%. The differences between the h values for the same stirring rate and different membranes Table 1 Mass fluxes per unit area (/lo-' kg m-' s-l ) for the three membranes at various stirring rates and average temperatures, T, membrane stirringrate T,/"C type (rpm) 25 30 35 40 45 50 55 TF 200 150 1.23 1.51 1.63 2.00 1.89 2.27 2.41 200 1.35 1.68 1.82 2.23 2.20 2.84 2.78 250 1.43 1.81 1.96 2.39 2.45 2.84 3.05 300 1.49 1.90 2.07 2.52 2.64 3.03 3.27 350 1.53 1.98 2.15 2.61 2.80 3.19 4.44 TF 450 150 0.91 1.21 1.36 1.59 1.74 2.08 2.08 200 1.00 1.35 1.52 1.79 1.96 2.34 2.36 250 1.06 1.46 1.64 1.94 2.12 2.53 2.57 300 1.10 1.55 1.73 2.06 2.25 2.67 2.72 350 1.14 1.61 1.80 2.15 2.35 2.78 2.85 TF lo00 150 1.27 1.58 1.86 2.27 2.45 2.51 3.12 200 1.41 1.82 2.14 2.27 2.78 2.85 3.46 250 1.52 1.99 2.35 2.79 3.03 3.11 3.70 300 1.59 2.13 2.52 2.96 3.22 3.31 3.88 350 1.66 2.24 2.65 3.09 3.37 3.46 4.01 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.5 4 1 I 0 1 2 [l/,?($)]/10-9 kg K J-' Pa-' Fig. 2 AT/JA us. l/rZ(dP/dT) corresponding to the results obtained for the TF 200 (+), TF 450 (A) and TF lo00 (+) membrane when the stirring rate was 250 rpm are attributed to the differences in the membrane screen sup- ports. On the other hand, for the same membrane the h values increase with the stirring rate as suggested by the heat transfer theory for stirred cells.' '9'' In order to evaluate the C coefficients from the slope of plots of eqn.(6),the thermal conductivity of the porous mem- branes, k, ,was calculated as k, = Ek, + (1 -&)k, where k, and k, are the thermal conductivities of the gas phase and the PTFE phase, re~pectively:'~ k, = 0.027 W m-' IC',k, = 0.22 W m-' K-'. In this way, the following Table 2 Overall film heat-transfer coefficient (/W rnW2K-') for the different membranes at different stirring rates stirring rate (rpm) TF 200 TF 450 TF loo0 150 952 938 1311 200 1102 1127 1500 250 1216 1282 1643 300 1307 1410 1754 350 1570 1518 1843 0.5 I \ 1 0.4 0.3 0.5 0.4 T 0.3 0.2 I 11 I 200 300 4 0 stirring rate (rpm) Fig.4 Temperature polarization coefficient for the different mem- branes us. stirring rate. Average temperature 318 K. +, TF 200;A, TF 450;+,TF 1OOO. C values are obtained: C(TF 200) = (22 & 2) x kg m-'s-' Pa-' C(TF 450) = (14 f2) x lo-' kg m-' s-' Pa-' C(TF 1000)= (18 & 2) x lop7kg m-' s-' Pa-' results which are independent of the stirring rate in the system. On the other hand, values of the mass-transfer coefficients predicted from the molecular diffusion theory, C,, and the Knudsen diffusion theory, CK , have been evaluated for 40 "C, which is intermediate in the experimental range studied. The calculations have been performed by using the numerical values for the relevant geometric parameters reported here- after. The tortuosity factor is typi~ally'~ = 2, the diffusivity x of water vapour in air at 40 "C and at normal pressure is13 2.88 x m2 s-' and Xn in eqn.(1) is taken to be 0.9. The results obtained are: CK(TF 200) = 18 x lop7kg m-' s-' Pa-' CK(TF 450) = 41 x kg m-' s-' Pa-' CK(TF 1OOO)= 90 x kg m-'s-' Pa-' CD = 14 x lop7kg m-* s-l Pa-' Considering the differences in their pore sizes, the three membranes perform in a similar way, as evidenced by the experimental mass-transfer coefficients. This suggests a mechanism based largely on molecular diffusion, which is independent of pore size. In fact, the C, value predicted by the molecular diffusion theory is very similar to those obtained for the TF 450 and TF lo00 membranes. Only for the TF 200 membrane does the C value obtained indicate that the combined molecular/Knudsen diffusion coefficient best describes the mass fluxes observed.This interpretation is coherent with the fact that for this membrane the pore size is more similar to the mean-free molecular path of water vapour than that for the other membranes. The experimental C and h values allow us to quantify the z temperature polarization coefficient. Fig. 3 shows the results obtained for the three membranes (at a representative stirring rate) as a function of the average temperature in the system, 0.2 I I 1 1 294 304 314 324 '+,j calculated according to eqn. (8) and (9). This behaviour is T/K different from that obtained in thermo-osmosis, where the zFig. 3 Temperature polarization coefficient for the different mem- branes us.average temperature in the system. Stirring rate 300 rpm. polarization coefficient is independent of the temperature. ' +, TF 200;A, TF 450;+,TF 1OOO. This is because in membrane distillation the H coefficient for 2046 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the membrane, as shown in the expression (8), accounts for the energy fluxes due to the convective transport of the evaporating vapour occurring simultaneously with the heat conduction across the membrane. It is the first contribution, increasing with the temperature, that is not present in the 2 3 4 5 A. C. M. Franken, J. A. M. Nolten, M. H. V. Mulder, D. Barge- man and C. A. Smolders, J. Membr. Sci., 1987,33, 315. G.C. Sarti and C. Gostoli, Membranes and Membrane Processes, ed. E. Drioli and M. Nakagaki, Plenum Press, New York, 1986. H. Vink and S. A. A. Chisthi, J. Membr. Sci., 1976, 1, 149. F. Bellucci, J. Membr. Sci., 1981,9, 285. thermo-osmosis process. 6 R. W. Schofield, A. G. Fane and C. J. D. Fell, J. Membr. Sci., Finally, Fig. 4 shows the z values obtained for the three membranes as a function of the stirring rate, also calculated according to eqn. (8) and (9). In this case the z variations are due to the h variation with the stirring rate, this behaviour being similar to that obtained in thermo-osmosis. All these results highlight the fact that any attempt to compare a membrane distillation transport model with mea- sured data must take into account the significant influence of temperature polarization, which depends on the heat-transfer coefficient of the membrane and of the liquid layers. 7 8 9 10 11 12 13 1987,33, 299. J. M. Ortiz de Zarate, F. Garcia-Lopez and J. I. Mengual, J. Chem. SOC., Faraday Trans., 1990,86,2891. S. Bandini, C. Gostoli and G. C. Sarti, Desalination, 1991,81,91. R. B. Bird, W. E. Stewart and E. N. Lightfoot, Transport Pheno- menu, John Wiley, New York, 1960. M. Mulder, Basic Principles of Membrane Technology, Kluwer, Dordrecht, 1992. T. G. Kaufmann and E. F. Leonard, AZChE J., 1968,14,421. H. Hikita and Y. Konishi, AZChE J., 1984,30,945. J. H. Perry, Chemical Engineers Handbook, McGraw-Hill, New York,4th edn., 1963. 14 M. Imai, S. Furusaki and T. Miyauchi, I. E. C. Proc. Des. Den, References 1982,21, 421. 1 C. Fernandez-Pineda and M. I. Vhzquez-Gonzalez, J. Chem. SOC., Faraday Trans. I, 1988,84,647. Paper 4/00520A; Received 27th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002043

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2047-2056 Mechanistic Aspects of Biological Redox Reactions involving NADH Part 5.-AMl Transition-state Studies for the Pyruvate-L-Lactate interconversion in L-Lactate Dehydrogenase Shoba Ranganathan and Jill E. Gready" Department of Biochemistry, University of Sydney, NSW 2006,Australia The catalytic mechanism for the interconversion of pyruvate to L-lactate by the enzyme L-lactate dehydrogenase (LDH), in the presence of the cofactor nicotinamide adenine dinucleotide (NAD), has been studied using semi- empirical AM1 quantum mechanical calculations. We have characterized the structure of the LDH transition state (TS), in isolation and in the presence of key active-site groups, using a supermolecule model. An initial investi- gation with isolated substrate and cofactor analogues resulted in TS structures for hydride-ion transfer from the cofactor analogue, planar trans-I-methyldihydronicotinamide to eight conformers of the substrate analogue, protonated pyruvic acid.Fragments of essential active-site residues were then introduced in stages. With trun- cated Arg-171 and His-195 residues, the TS for hydride transfer from the cofactor analogue to the substrate pyruvate resembled the active-site configuration in the X-ray crystallographic structure of the abortive LDH-NADH-oxamate ternary complex. The substrate species is carbonyl-protonated and thus the rate-limiting chemi- cal step is hydride transfer. These results contrast with earlier work indicating that carbonyl-protonated pyruvate is unstable in the free state (K.E. Norris, G. B. Bacskay and J. E. Gready, J. Comput. Chem., 1993, 14, 699). Introduction of the Val-138 fragment gave closer agreement with experiment for the orientation of the cofactor analogue's carboxamide side chain in the TS and for the reversibility criteria for the reaction. For each TS located, stable reactant and product complexes have been isolated by following the reaction coordinate, and the optimized structures, energies and charge distributions of the TS, stable reactant and product complexes and the isolated reactants and products are reported. There is significant charge transfer in the TS, with a charge of ca. +0.4 on the nicotinamide species. The catalytic conversion of pyruvate to L-lactate in L-lactate dehydrogenase (LDH), in the presence of the cofactor nicotin- amide adenine dinucleotide (NAD), involves the transfer of both a proton and a hydride ion:' CH,(CO)CO,-+ NADH + H+ e CH,CH(OH)CO,-+ NAD+ The overall reaction kinetics have been the subject of exhaus- tive experimental investigation^'-^ while site-directed muta- genesis and protein engineering ~tudies~-~ have identified the key residues in the active site and their roles.In the enzy- matic reaction, the proton donor has been identified as His-195 (residue numbering as defined by Eventoff et d7),by pH titration and chemical modification work while the hydride ion migrates from the reduced form of the cofactor, NADH.' From the X-ray crystallographic structure of the ternary LDH-NADH-oxamate complex [Protein Data Bank (PDB) entry 1LDM8], several key residues have been identi- fied in the active site, with roles ascribed to binding and reac- tion involving the active complexes and/or the transition state (TS).While Arg-171 (substrate binding) and His-195 (substrate binding and proton donor) are essential, other resi- dues having a possible direct bearing on the reaction mecha- nism and their suggested roles are: Arg-109 (part of a flexible loop) in the activation of His-195 and stabilization of the TS, Asp-168 in the pK, modulation of His-195, and Val-138 in the orientation of the cofactor's carboxamide side chain. A view of the active site, showing the enzyme-bound substrate, the nicotinamide ring of the cofactor and key binding and catalytic residues and the peptide backbone atoms of the mobile loop, is presented in Fig.1. The enzymatic reaction involves the formation of a chiral centre on the active carbon of the substrate and is stereospecific both for the transfer of the pro-R-hydrogen of NADH and for the formation of only one stereoisomer of lactate, i.e. L-lactate. The accepted mechanism involves a concerted proton and hydride transfer' taking place in a hydrophobic and highly charged active site from which solvent has been excluded by the closure of a flexible loop, triggered by cofactor and sub- strate binding. The transition state has been suggested to be more pyruvate-like than lactate-like.g The enzyme itself is proposed to have evolved to bind preferentially the higher- energy pair, pyruvate and NADH, over the lactate-NAD+ pair,'*6 in accordance with the theory of matched internal states," which postulates that the kinetically significant tran- sition state will lie between internal intermediates of approx- imately equal Gibbs energy.Also, the orientation of the carboxamide side chain of the nicotinamide may have a direct bearing on cofactor activation. In the enzyme-bound cofactor, this side chain adopts trans geometry although the cis form is more stable in the free cofactor." It has been proposed that out-of-plane orientations of the side chain could facilitate the catalytic reaction.', Whether the stereo- chemical directionality and control is provided by a non-planar distortion of the nicotinamide ring in the transition state' is another interesting question.Recent theoretical studies on LDH include an AM1 study on the substrate model formaldehyde," a PM3 potential-energy surface calculation'6 and computation of electrostatic interaction energies.' However, several questions noted above still need to be addressed; in particular, the enzyme reaction mechanism at a molecular level, the nature of the transition-state species, the relative energies of the stable intermediates, the molecular components of the charge dis- tribution for the species involved in the transition state and the role of key active-site residues. In the AM1 study of the LDH reaction by Wilkie and Wil- liams,' the substrate pyruvate was substituted by formalde- hyde, the cofactor by dihydropyridine and the proton donor His-195 residue by protonated imidazole.Since LDH is spe- cific for the reduction of 2-0x0 acids, containing the group Val-138 Fig. 1 LDH active site, based on the PDB structure of the LDH- NADH-oxamate ternary complex (1LDM) : (a)stereo representation and (b) two-dimensional view, with key residues and distances (in A) labelled. Dashed lines represent hydrogen bonds. -CO-CO2 -, this choice for the substrate is not a good rep- resentation of the substrate pyruvate, particularly given indi- cations from our earlier work of the unusual properties of protonated pyruvate.' '*I9 The cofactor analogue used, dihy- dropyridine, is unsubstituted and cannot account for the sug- gested activation and orientation effects (cisltrans and in-plane/ out-of-plane) of the carboxamide side chain.Andres et ~1.'~have conducted an extensive exploration of the PM3 energy hypersurface calculation for the pyruvate reduction mechanism in LDH, using pyruvate (see Fig. 2) as substrate, 1-methyldihydronicotinamide as the cofactor ana- logue, l-methylguanidinium ion representing the Arg-171 residue and protonated 4-methylimidazole ion as the proton donor, His-195 (illustrated in Fig. 3). This study reports the potential-energy surface for the two processes : proton trans- fer from the protonated imidazole to the carbonyl oxygen of the pyruvate and hydride-ion transfer from nicotinamide to the carbonyl carbon of pyruvate.The TS for the catalytic reaction has been located and the overall process has been interpreted as a stepwise mechanism in which the initial step is proton transfer, followed by hydride-ion migration. Struc- tures corresponding to important points on the energy hyper- surface have been provided. However, in all the structures shown, the carboxamide side chain of the cofactor analogue adopts a novel orientation, pointing towards the guanidinium moiety in contrast to its orientation towards the imidazole J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 d Fig. 2 Structures of substrate species: (a) protonated pyruvate, (b) protonated pyruvic acid, (c) pyruvate and (d) pyruvic acid.r3 is the 06-H bond length in (a). ring in the lLDM structure. Whether reorientation of the carboxamide to reflect the X-ray crystallographic structure would have an impact on the energy hypersurface is uncer- tain and requires study. Also, the molecular charge distribu- tion for the TS structure has not been provided. In the systematic investigation of the LDH enzyme mecha- nism, the structures and energetics of possible substrates and intermediates involved in the catalytic step have been report- ed in our earlier quantum chemical In the present work, we have carried out quantum mechanical cal- culations on the structure of the LDH transition state, in iso- lation and in the presence of key active-site groups, using a supermolecule model.Following an initial computational investigation of isolated substrate and cofactor analogues, the active-site residues have been introduced in stages. At each face b "b)-l-c2 '. N4*l @04 Fig. 3 Structures of (a) trans-1-methyldihydronicotinamide (cofactor analogue), (b) 1-methylguanidinium ion (Arg fragment), (c) Cmethylimidazolium ion (His fragment), (6)acetamide (Val fragment) and (e)acetate ion (Asp fragment). r4 is the N1-H bond length in (c). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 stage, the possibility of TSs involving both cis and trans con-formers of the carboxamide side chain on the nicotinamide moiety were investigated. The optimized structures, energies and charge distributions of the TS species obtained and the corresponding reactant and product complexes are reported.Computational Models In the enzyme environment, the substrate pyruvate forms an ion pair with Arg-171 which effectively neutralizes its charge. Thus, in addition to pyruvate, the corresponding neutral form, pyruvic acid, was considered as a second model sub- strate. In ref. 18, four possible non-concerted reaction mecha- nisms were set out corresponding to protonation of the carbonyl oxygen of pyruvate and pyruvic acid followed by hydride transfer to the carbonium ion so formed [(i) and (ii)], as well as hydride transfer to the carbonyl carbon of pyruvate and pyruvic acid, followed by protonation of the carbonyl oxygen [(iii) and (iv)]. These postulated mechanisms are now investigated by studying the hydride-transfer reaction from NADH to pyruvate or pyruvic acid or their protonated forms, protonated pyruvate and protonated pyruvic acid.The cofactor NADH has been modelled by the cis and trans con-formers of 1-methyldihydronicotinamide, as in an earlier study on dihydrofolate reductase (DHFR) substrate ana-logues,2l l-methylnicotinamide cation representing the oxi- dized form of the cofactor. At the outset, the hydride transfer from l-methyldihydro- nicotinamide to all the stable of the four substrate species, (a) protonated pyruvate, (b) protonated pyruvic acid, (c) pyruvate and (d) pyruvic acid, has been studied, with the carboxamide side chain in both cis and trans orientations.The chemical structures of the lowest- energy conformers of these substrate species are given in Fig. 2, while that of 1-methyldihydronicotinamideis shown in Fig. 3. The C4 hydrogen labelled H, is transferred to the substrate as a hydride ion. The existence of TS structures for any of these proposed reactions would indicate at least a possibility of such a pathway in the reaction. Different relative orienta- tions of the substrate analogues and the nicotinamide moiety were first sampled in order to locate the lowest-energy TS. Starting from each transition-state structure obtained, con- strained reaction paths22 were followed to locate possible minima, representing the corresponding stable reactant and product complexes. While the gas-phase study of isolated substrate and cofac- tor species permits several favourable relative orientations of the reactants, the active-site residues in the enzyme would impose directional constraints on the approach of the reac- tants.Also, the gas-phase model cannot mimic the true enzyme environment or test for the proposed concerted reac- tion. Therefore, in the second stage, we have introduced frag- ments of two essential residues in their X-ray crystallographic positions:' Arg-171, to bind the carboxylate group of the substrate pyruvate and His-195 as proton donor. The cofac- tor analogue, trans-1-methyldihydronicotinamide,was also initially positioned in its X-ray crystallographic location. With the inclusion of the Arg-171 fragment and protonated His-195 moiety as proton donor, the choice of the substrate species is limited to pyruvate. Arg-171 has been modelled by protonated 1-methylguanidine while His-195 has been rep- resented by protonated 4-methylimidazole, as illustrated in Fig. 3.These fragments have also been used in the theoretical studies of Krechl and co-~orkers.~~*~~ In the TS structure determination, no a priori assumptions have been made as to the mechanism of the reaction: the reactants and products alone have been defined, so that the nature of the transition state would reveal the underlying mechanism. The reactants comprise 1-methylguanidinium cation, 4-methylimidazolium cation, pyruvate and 1-methyldihydronicotinamide,while the product species are 1-methylguanidinium cation, 4-methyl- imidazole, lactate and the 1-methylnicotinamide cation.The 1-methylguanidinium cation is chemically unchanged by the redox reaction. In order to test any dependence of the carboxamide side- chain orientation on the TS, the next stage was to introduce the fragment, CH3CONH2 , representing Val-138, into the supermolecule model. Acetamide was chosen as a suitable model for this residue, to represent the backbone carbonyl of Val-138 in the X-ray structure' and the neighbouring atoms of the peptide backbone. With the introduction of this frag- ment, the rotation of the carboxamide side chain is restricted in a manner similar to that apparent in the enzyme. Finally, Arg- 109 (represented by the 1-methylguanidinium cation, as in the case of Arg-171) and Asp-168 (modelled by acetate ion) were added to the system, to reproduce the charged functional groups of other key residues in the enzyme active site.The structures of the fragments depicting Val-138 and Asp-168 are shown in Fig. 3. Computational Methods The geometries of all equilibrium and TS structures were optimized at the semi-empirical level of theory. The AM1 methodology of Dewar et was adopted, since it is con- sidered the best general-purpose semi-empirical treatment available and, in particular, it provides a better description of intermolecular interactions than MNDO or MIND0/3.2s The calculations were performed on an IBM RS 6000 work-station using the SYDPAC program,26 which incorporates efficient geometry ~ptimization~~ and transition-state search algorithms based on the algorithm of Dewar et aL2' and the GDIIS method of Csaszar and P~lay.~~ Transition states were initially located using the method of Dewar et d2'and refined by the GDIIS procedure.*' The relaxed potential- energy surface in the vicinity of the TS was then calculated as a two-dimensional grid in terms of two internal coordinates describing the path of the migrating species.Structures corre- sponding to low-energy regions of the grid, on either side of the saddle point, were then minimized to obtain stable inter- mediates, representing reactant and product complexes. Results and Discussion Transition-state Structures Hydride Transfer from 1-Methyldihydronicotinamideto Isolated Neutral and Protonated Substrate Species The nicotinamide ring of the cofactor is represented as planar in the X-ray crystallographic structure for the LDH-NADH- oxamate ternary complex' with the carboxamide side chain in a trans coplanar orientation.Taking account of these observations as well as previous AM1 and ab initio results on geometry optimizations of oxidized and reduced nicotina- mide species," the nicotinamide ring was introduced as a planar species. The carboxamide side chain was constrained to be coplanar with the nicotinamide ring, in either the trans or cis orientation. The coplanarity constraint was relaxed only if no TS structure could be obtained with the fixed car- boxamide orientation.Protonated Pyruuate. A TS search was carried out for the reaction of protonated pyruvate (bearing formal charge 0) and 1-methyldihydronicotinamide,yielding L-lactate and 1-methylnicotinamide cation as products, using AM 1 opti- mized geometries'8*20*21 as starting structures. No transition state was located for this substrate species, with either cis-or trans-1-methyldihydronicotinamide. The unusually long Cl-C2 bond (1.6-1.7 A18) in all protonated pyruvate con- formers is further elongated during the TS search, so that the reactant supermolecule tends towards decarboxylation. This observation is in agreement with the behaviour of the proto- nated pyruvate species during geometry optimization,' resulting in a methylhydroxy carbene-carbon dioxide complex.Thus, hydride transfer from the cofactor analogue to protonated pyruvate is unsuitable for modelling the cata- lytic reaction. Protonated Pyruuic Acid. With protonated pyruvic acid (formal charge +1) as the substrate analogue and trans-l-methyldihydronicotinamide, eight transition states (designated TSla-TSlh, in the order of increasing heat of formation) were located. The products of this reaction are L-lactic acid and the 1-methylnicotinamide ion. These struc- tures have been verified as saddle points, by the existence of a single negative frequency of vibration. The TS structures cor- respond to the eight conformers of carbonyl-protonated pyruvic acid.18 For each TS, stable reactant and product complexes have been isolated, by following the reaction coor- dinate backwards or forwards.The relative orientation of the substrate and cofactor species in the TSs is dependent on intermolecular hydrogen-bonding interactions between the OH groups of the protonated pyruvic acid and the carbonyl oxygen of the carboxamide side chain. Therefore, the orienta- tion of the nicotinamide moiety with respect to the proto- nated pyruvate species shows little correspondence to that of the cofactor and the inhibitor, oxamate, in the ternary complex 1LDM.8 Similar intermolecular hydrogen-bonding patterns involving the carboxamide carbonyl oxygen have been observed in the study of the hydride-transfer reaction between 1-methyldihydronicotinamide and folate and dihy- drofolate analogue substrates of DHFR.21 The structure of the lowest-energy transition state (TSla) is shown in Fig.4. The heats of formation of the free reactant and product species, the TS and the reactant and product complexes and the relevant intermolecular geometric parameters (defined by rl, r2 and 6), for the structures TSla-TSlh are listed in Table 1. No transition states were identified with the cis cofactor analogue, in contrast with the model DHFR reaction21 where both cis and trans cofactor analogues lead to TS struc- tures. Solely in the case of TSlh, the TS could not be located when the trans carboxamide side chain was constrained to the plane of the nicotinamide ring. This side-chain torsion was then relaxed, in order to locate the TS and the corre- sponding reactant and product complexes. For these struc- tures, the torsion of the carboxamide side-chain (4)shows significant deviations from the normal value of 180" (Table l), with the carbonyl group directed towards the si face3' of the cofactor analogue. The existence of TS species establishes this reaction as a possible pathway for the LDH enzymic reaction.The sub- strate species in the TS is carbonyl protonated. On the basis of these results, a fast protonation of the substrate carbonyl group followed by hydride-ion transfer from the cofactor appears tenable as a mechanism for the LDH catalytic reac- tion. Pyruuate and Pyruuic Acid. Pyruvate (formal charge -1) and pyruvic acid were then studied for possible hydride transfer from cis-and trans-1-methyldihydronicotinamide, resulting in the formation of deprotonated L-lactate and de- protonated L-lactic acid, respectively, as products, along with the oxidized form of the cofactor, 1-methylnicotinamide cation.For both of these substrate analogues, no transition- state structures were found. Thus, the possibility of hydride J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 N H U NIC Fig. 4 Transition state (TSla) for hydride (HR)transfer from trans-1-methyldihydronicotinamide(NIC) to substrate protonated pyruvic acid (PPA): (a) stereo representation and (b) two-dimensional view. rI, r2, 8 and 4 are the intermolecular parameters. The dashed line represents the intermolecular hydrogen bond (distance given in A).transfer prior to protonation of the substrate moiety seems unlikely. Hydride Transfer from 1-Methyldihydronicotinamideto Pyruuate in the Presence of Arg-171 and His-195 Fragments With the introduction of the Arg-171 fragment (l-methyl- guanidinium ion), the substrate pyruvate forms an ion pair, bearing no net charge. The overall formal charge on the reac- tants (1-methylguanidinium ion, protonated 4-methyl-imidazole, pyruvate and 1-methyldihydronicotinamide) remains +1, the proton and the charge residing with the 4- methylimidazole species. On the other hand, for the product species, the unit positive charge is on the l-methylnicotin- amide ion, the 4-methylimidazole is neutral, while the proton and the hydride ion are now associated with L-lactate.The starting structure for each reactant and product species was its fully optimized AM 1 geometry. The initial intermolecular arrangement of the reactant and product species was based on the X-ray crystallographic locations of the heavy atoms from the Protein Data Bank entry lLDM,' i.e. the cofactor analogue was trans planar and the cis orientation (4= 00) was generated from this geometry by rotation of the carbo- xamide side chain. However, no planarity constraints were imposed on the nicotinamide ring and all coordinates were optimized during the TS search. In Fig. 5, a stereo view of the TS structure (TS2) obtained is given. The different molecular fragments are held together by electrostatic and hydogen-bonding interactions.A com- parison of TS2 with the corresponding atomic coordinates from the PDB entry lLDM, illustrated in Fig. 1, shows that the relative orientations of the reacting species are remark- ably similar, considering the complete optimization of all atomic positions and the absence of the protein backbone to restrict the movement of the Arg-171 and His-195 fragments J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 AM1 results for the hydride-ion transfer reaction from trans-1-methyldihydronicotinamideto eight conformers of proto- nated pyruvic acid ~~ 4H/ species kcal mol-' rl/A r2/A $/degrees 4/degrees TSla R 57.58 co 1.126 -180.0 RC 27.66 2.885 1.128 121.4 180.0 TS 36.90 1.596 1.235 142.7 180.0 PC -7.22 1.129 8.332 302.4 180.0 P 2.97 1.130 03 -180.0 TSlb R 60.66 00 1.126 -180.0 RC 29.43 3.957 1.128 244.4 180.0 TS 37.90 1.601 1.228 206.3 180.0 PC -8.39 1.129 6.316 129.4 180.0 P 1.58 1.129 03 -180.0 TS lc R 58.18 00 1.126 -180.0 RC 27.89 3.221 1.128 108.8 180.0 TS 38.07 1.61 1 1.229 143.0 180.0 PC -7.97 1.131 4.804 50.2 180.0 P 1.74 1.128 00 -180.0 TSId R 67.84 co 1.126 -180.0 RC 33.99 3.861 1.128 248.1 180.0 TS 42.50 1.635 1.218 207.9 180.0 PC -9.17 1.131 7.914 276.0 180.0 P 5.60 1.130 00 -180.0 TSle R 67.84 co 1.126 -180.0 RC 34.08 3.479 1.128 255.3 180.0 TS 42.50 1.636 1.218 207.9 180.0 PC -11.49 1.132 8.137 71.7 180.0 P 5.60 1.130 co -180.0 TSlf R 67.47 co 1.126 -180.0 RC 32.44 4.117 1.126 100.5 180.0 TS 46.16 1.621 1.230 147.0 180.0 PC -9.21 1.132 8.136 78.8 180.0 P 5.60 1.130 00 -180.0 TSlg R 67.31 co 1.126 -180.0 RC 29.44 3.950 1.127 115.5 180.0 TS 48.10 1.699 1.204 146.7 180.0 PC -12.02 1.131 7.136 284.5 180.0 P 1.74 1.130 co -180.0 TSlh R 67.85 00 1.126 -180.0 RC 34.06 3.156 1.129 123.2 165.5 TS 56.5 1 1.602 1.242 161.6 149.2 PC -6.40 1.131 5.707 300.14 126.8 P 5.60 1.130 co -180.0 For each transition state (TSla-TSlh) obtained, heats of formation (AfH/kcal mol- ') and structural parameters, r1, r2, 6 and 4 (Fig.4) are given, for the free reactant (R) and product (P) species, the tran- sition state (TS) structure and the stable reactant and product com- plexes (RC and PC). Structures TSla-TSlh correspond to protonated pyruvic acid conformers PPA1, PPA9, PPA3, PPA17, PPA11, PPA13, PPA7 and PPA15, respectively, of ref.18. Boltzmann-weighted mean AfH values: R, 57.80; RC, 27.87; TS, 37.17; PC, -11.82 and P, 1.73. and to restrain the cofactor to a catalytically favourable orientation. With the introduction of two residue fragments, the amino group of the carboxamide side chain is hydrogen bonded to N1 of the imidazole moiety. On the other hand, the side chain carbonyl group is not involved in the hydrogen-bonding network, in contrast to its major role in the intermolecular arrangement observed for TS 1. This change in the carbonyl group hydrogen-bonding arrange- ment from TS1 to TS2, produces a molecular complex with similar hydrogen-bonding interactions to that observed in the enzyme ternary complex structure (1LDM).In the crystal structure,* the carbonyl oxygen of the carboxamide side chain of the cofactor appears to be hydrogen bonded to a 205 1 0 €30 ARG ON OH C Fig. 5 Transition state (TS2) for hydride (HR)transfer from trans-l-methyldihydronicotinamide(NIC) to substrate pyruvate (PYR), in the presence of 1-methylguanidinium ion (ARG) and protonated 4-methylimidazole (HIS): (a) stereo representation and (b) two-dimensional view. Dashed lines represent intermolecular hydrogen bonds. Key intermolecular distances (in A) are given. water molecule, while the amino group on the side chain par- ticipates in hydrogen bonding to the enzyme. The TS structure represents the hydride-ion transfer step, from the cofactor analogue to the carbonyl-protonated pyru- vate.By following this reaction coordinate, stable reactant and product complexes were located. Interatomic distances r3 and r4 indicate the position of the transferred proton with respect to 06 of the substrate [Fig. 2(a)] and N1 of the 4- methylimidazolium ion [Fig. 3(c)], respectively. The heats of formation, the intermolecular orientation coordinates, rl, r2, 8 and 4 (defined as for TSl), and the proton-transfer coordi- nates, r3 and r4, for the free reactants and products, TS2 and the reactant and product complexes, are presented in Table 2. In both reactant and product complexes, the substrate species carries the imidazole proton, while its carboxylate group forms an ion pair with the 1-methylguanidinium cation.Attempts to locate a TS corresponding to the proton transfer from the 4-methylimidazolium ion to the substrate pyruvate were unsuccessful, in agreement with the PM3 results.I6 The TS search also failed with the cis-cofactor analogue. The values of rl, r2 and 8 for TS2 are, respectively, 1.422 A, 1.302 A and 173.0", which are similar to the corresponding PM3 results, 1.417 A,1.308 8,and 172.9", of Andres et ~1.'~ However, there is considerable difference in the orientation of the cofactor moiety in TS2 compared with that shown in ref. 16. To quantify this difference, the orientation of the nicotin- amide (n) moiety with reference to the substrate pyruvate (p) can be defined by z, the dihedral angle Nl(n)-C4(n)-C2(p)-Cl(p) (numbering as in Fig.1 and 2). In TS2, z has a value of 174.2" compared with the value of 195.3" for the PDB entry, ILDM. The value of z computed 2052 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 AM1 results for the hydride-ion transfer reaction from trans-1-methyldihydronicotinamideto pyruvate, in the presence of (a) Arg-171 and His-195 and (b) Arg-171, His-195 and Val-138 residue fragments, characterized by transition states (a)TS2 and (b) TS3 species Af H/kcal mol -' r1/A 4 eldegrees +/degrees r3/A rJA TS2 R 178.1 1 M 1.126 -180.0 co 0.997 RC 54.43 3.049 1.128 150.4 149.4 1.002 2.455 TS 73.85 1.422 1.302 173.0 148.5 0.985 2.683 PC 37.33 1.126 3.276 101.7 153.5 0.967 4.089 P 173.49 1.124 ca -180.0 0.967 a3 TS3 R 127.43 03 1.126 -180.0 co 0.997 RC -24.68 4.704 1.127 117.3 174.0 1.961 1.021 TS 14.80 1.415 1.303 170.9 201.7 0.983 2.622 PC -29.11 1.123 3.247 111.6 172.2 0.970 4.21 1 P 122.8 1 1.124 ca -180.0 0.967 co The heats of formation (A,H/kcal mol-'), structural parameters for the reaction coordinate, rI, rz, 8 and 4 (Fig.4) and proton-transfer coordinates, r3 (Fig. 2) and rg (Fig. 3), are given for the free reactant (R) and product (P) species, the transition-state (TS) structure and the stable reactant and product complexes (RC and PC). for the PM3 transition state16 from the published Cartesian coordinates is only 79.5", with a nicotinamide ring rotation of 115.8' from its PDB orientation.Also, there is considerable difference in the heat of formation values for TS2 (73.85 kcal mol-') and the corresponding TS (44.73 kcal mol- ') report-ed by Andres et The non-planar deformation of the amino group of the car- boxamide side chain can be attributed partly to the inherent limitation of the AM 1 model in representing n-resonance effects in the amide group. The amino group of the carbox- amide side chain is hydrogen-bonded to N1 of 4-methylimidazole. This favourable interaction is lacking for the cis orientation of the carboxamide group and is probably the reason why a TS structure corresponding to cis-l-methyl- dihydronicotinamide has not been located. The nicotinamide ring retains its planar geometry in the transition state, although the carboxamide side chain is 32" out of plane (4 = 148.5'), with the carbonyl group directed towards the si face of the nicotinamide ring.The transition state obtained for this reaction clearly con- tains the protonated pyruvate species, bound to the gua- nidinium ion by the formation of an ion pair. This observation leads us to two important conclusions. First, iso- lated protonated pyruvate, which exhibited an abnormal Cl-C2 bond elongation and was prone to decarboxyl-ation," is stabilized by the formation of an ion pair with the guanidinium ion. The ion pair formed by protonated pyru- vate and 1-methylguanidinium ion in the TS is stable under AM1 geometry optimization conditions and the Cl-C2 bond length (1.546 A) of the protonated pyruvate species is similar to the value 1.508 A, calculated for the lowest-energy conformer of protonated pyruvic acid.' Similar results were obtained by an independent AM1 study on the pyruvate- protonated 1-methylguanidine ion pair, for which the com- puted Cl-C2 bond length (1.513 A) of the pyruvate species is almost identical to that determined for pyruvic acid (1.514 A).2o Secondly, as the carbonyl oxygen of the pyruvate moiety is protonated in the transition state (the reactant species protonated 4-methylimidazole being deprotonated in the TS), the catalytic mechanism is most probably stepwise, with protonation preceding hydride transfer.Hydride Transfer from 1-Methyldihydronicotinamideto Pyruvate in the Presence of Arg-171, His-195 and Val-138 Fragments The addition of an acetamide moiety (shown in Fig.3), rep- resenting the Val-138 residue, to the reactant and product models described above, introduces a hydrogen-bonding interaction of the backbone carbonyl of Val-138 with the amino group of the trans carboxamide side chain of 1-methyldihydronicotinamide. At the outset, the acetamide moiety occupied the position of the corresponding heavy atoms in the PDB entry 1LDM. The transition-state structure obtained (TS3 in Fig. 6) with fragments of three residues (Arg-171, His-195 and Val-138) is quite similar to TS2. The orientation coordinates, rl, r2 and 8, are almost unchanged: 1.416 A, 1.303 A and 170.9", respec- tively. The orientation of the nicotinamide ring with reference ARG 'R AL Fig.6 Transition state (TS3) for hydride (H,)transfer from trans-l- methylidihydronicotinamide (NIC) to substrate pyruvate (PY R), in the presence of 1 -methylguanidinium ion (ARG), protonated 4-methylimidazole (HIS) and acetamide (VAL): (a)stereo representation and (6) two-dimensional view. Dashed lines represent intermolecular hydrogen bonds. Key intermolecular distances (in A) are given. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 to pyruvate (z = 172.8") remains almost unchanged compared with TS2 (z = 174.2"). The carboxamide side chain is now turned only 19" out of the plane of the nicotinamide ring, but towards the re face3' (4 = 201.7"). Although the carboxamide side chain is not coplanar with the nicotinamide ring, as in the crystal structure for the ternary complex (lLDM), the Val-1 38 fragment mimics reasonably well the carboxamide hydrogen-bonding interaction encountered in the enzyme environment.A comparison of TS3 with the PDB structure (Fig. 1) shows that the acetamide fragment is slightly dis- placed compared with the location of the Val-138 residue. This movement is a consequence of the residue fragments not being tethered to the peptide backbone of the enzyme. The Val-1 38 fragment is able to counteract the hydrogen-bonding interaction of the amide hydrogen atoms with N1 of the imidazole ring found in TS2 and partially rectify the out-of- plane torsion of the carboxamide side chain. The cis orienta-tion of the carboxamide side chain is repulsive towards the carbonyl group of the Val-138 fragment and no TS resulted from this conformer. Table 2 lists the heats of formation, the orientation coordinates (rl, r2, 0 and 4), and the proton- transfer coordinates (r3 and r4) for the isolated reactant and product species, the transition state and the stable reactant and product complexes for the substrate-cofactor system in the presence of three residue fragments.TS3 contains protonated pyruvate as the substrate species, forming an ion pair with the 1-methylguanidinium ion, as in the case of TS2. However, the substrate species in the corre- sponding reactant complex is not protonated, in contrast to the reactant complex in TS2. This is reflected in the Lowdin bond orders31 calculated for r3 and r4 of 0.028, 0.785 and 0.841,0.004 for the reactant complexes corresponding to TS3 and TS2, respectively, compared with values of 0.850 for r3 in protonated pyruvic acid '' (lowest-energy conformer) and 0.857 for r4 in the 4-methylimidazolium ion.As for the TS2 calculations, no TS could be isolated for the proton-transfer step. Thus, while the proposed mechanism of substrate pro- tonation, followed by hydride transfer from the cofactor ana- logue, is not perturbed by the introduction of the Val-138 fragment, it appears that the stage of the protonation step may be very sensitive to the details of the active-site environ- ment. Comparison of the values of rl and r2 for the TSs obtained so far (Tables 1 and 2) indicates that in all TS1 structures the migrating hydride ion is closer to the nicotinamide species than to the substrate species, i.e.it is reactant-like. This is also the case for TS2 and TS3, except that the TS is less reactant-like than TS1. 8 values for TS2 and TS3 indicate a roughly linear reaction coordinate, while those for TSla- TSlh are quite variable but non-linear. Hydride Transfer from 1-Methyldihydronicotinamideto Pyruvate in the Presence of Arg-171, His-195 and Val-138 Fragments :Inclusion of Arg- 109 and Asp- 168 Fragments Two other key residues, Arg-109 (implicated in polarizing the carbonyl group of the substrate pyruvate and in stabilization of the transition state'v4) and Asp-168 (forming a charge couple with His-195 and implicated in the stabilization of protonated Hi~-195~),have been modelled by the 1-methylguanidinium ion (as in the case of Arg-171) and acetate ion (Fig.2), respectively. The fragments representing these residues were initially placed in their respective X-ray crystallographic locations. Their inclusion in the super-molecule at the reactant or product level, both individually and together, did not result in any further TS structures. In fact, energy-minimized structures for the reactant and product ensembles could not be obtained. While the Arg-109 fragment repelled the protonated His-195 fragment, the 2053 Asp-168 fragment abstracted the H atom attached to N3 of the 4-methylimidazole ion.The inability to locate a TS struc- ture suggests that, within the limitations of the present model, these residue fragments do not play a major role in the stabil- ization of the transition state. Since the residue fragments are not tethered to a protein backbone, the individual reactant and product species are able freely to adopt orientations which would be impossible in the enzymic environment. This stage represents the limit of our simple supermolecule approach to the highly complex LDH catalytic system. Activation Energies for Reactions represented by TS1,TS2 and TS3 The activation energies (E,) for the hydride-transfer reactions characterized by TS1, TS2 and TS3 have been computed from the heats of formation, given in Tables 1 and 2 and illustrated in Fig.7, and are presented in Table 3. The E, values have been calculated relative to the free reactants and the reactant complex, for the forward reaction, and relative to the free products and the product complex, for the reverse reaction. In the case of TS1, the heats of formation have been averaged over the eight TS structures located (TSla-TSlh) and the Boltzmann-weighted mean values given in Table 1 were used to compute E, . Considering E, (in kcal mol- ') for the free reactants and products [reaction (1) of Table 31, the ,150 , , , 100-50-0--50 ' I I I I I R RC TS PC P species Fig. 7 Heats of formation (Af H/kcal mol- ') for the free reactants (R) and products (P), the reactant and product complexes (RC and PC) and the transition state (TS) for TS1 (---) (mean values from Table l), TS2 (-.-.) and TS3 (-). Substrate species for TSl contain the carboxylic acid group (C0,H) while those for TS2 and TS3 contain the carboxylate function (C02-). Table 3 Activation energies (EJ for the forward and reverse reac- tions characterized by TS1, TS2 and TS3 and the corresponding dif- ference in E, (AE) reaction EJrelative to TS1 TS2 TS3 (1) forward reactants -20.6 -104.3 -112.6 reverse products 35.4 -99.6 -108.0 -56.0" -4.6" -4.6" (2) forward reactant complex 9.3 19.4 39.5 reverse product complex 49.0 36.5 43.9 -39.7b -17.1b -4.4b " AE,. AE2. 2054 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Total net AM1 charges for substrate (SUB), hydride ion (HR),cofactor analogue (NIC), fragments of Arg-171 (ARG), His-195 (HIS) and Val-138 (VAL) species at the transition states (TS)and the corresponding reactant and product complexes (RC and PC) for TS1, TS2 and TS3; partial charges for atoms C8 and 09 of the cofactor analogue and the carbonyl bond dipole (p/D)are also given AM1 charges species SUB HR NIC ARG HIS VAL C8 09 PP TS 1 RC 0.96" -0.04b ---0.42 -0.59 3.02 TS 0.67" -0.01 0.34' ---0.37 -0.46 2.50 PC 0.09d -0.9W ---0.35 -0.33 2.02 TS2 RC 0.01" -0.01 0.98 0.01f -0.38 -0.49 2.59 TS -0.40' 0.00 0.44' 0.96 0.W -0.34 -0.36 2.11 PC -0.94' -0.99' 0.95 0.W -0.37 -0.40 2.30 TS3 RC -0.93h -O.OOb 0.96 0.96' 0.01 0.38 -0.50 2.64 TS -0.40" 0.01 0.43' 0.96 0.W 0.01 0.36 -0.39 2.26 PC -0.95' -0.99' 0.96 -0.01J 0.00 0.37 -0.42 2.40 Molecular species for charge calculation are as follows :" protonated pyruvic acid, 1-methyldihydronicotinamide,'1-methylnicotinamide ion, lactic acid, protonated pyruvate, Cmethylimidazole, lactate, pyruvate, protonated Cmethylimidazole.activation barrier for TS formation from the free reactants decreases in the order TS1 (-20.6) % TS2 (-104.3) > TS3 (-112.6), while the barrier for TS formation from the free products also decreases in the same order, TS1 (35.4) % TS2 (-99.6) >TS3 (-108.0). It is observed that the TS structure is higher in energy than the free-product structure in the case of TS1 alone. The difference in E, for the forward and reverse reactions (AE1), which is, of course, the reaction enthalpy, can serve as a measure of the reversibility of reaction (1).AEl and, consequently, the reversibility of reaction (l),increases in the order: TS1 6TS2 = TS3. The E, values for the reactant and product complexes [reaction (2) of Table 31 indicate an increase in the activation barrier for TS formation from the reactant complex in the order TS1 (9.3) <TS2 (19.4) <TS3 (393, while the corresponding barriers for TS formation from the product complex are higher, TS1 (49.0) > TS2 (36.5)c TS3 (43.9), with no regular trend observed. However, the reversibility of reaction (2), estimated in terms of the reac- tion enthalpy as the difference AE,, between the pertinent E, values, shows an increase: TS1 <TS2 < TS3.For TS3 alone, the two reaction enthalpies AE, and AE2 show small similar absolute values, reflecting energetically similar free reactants and products as well as reactant and product com- plexes. Thus, for the reaction characterized by TS3, the inter- nal states are apparently matched, in accordance with the theory of Albery and Knowles," while a substantial improvement in the reversibility of the reaction has been achieved by the inclusion of important residue fragments in the initial model comprising isolated substrate and cofactor analogues, with increased rates of reaction. Ionic Charge Distribution in the Transition States The charges for the substrate, hydride ion, l-methylnicotin- amide, 1-methylguanidinium cation, 4-methylimidazole and acetamide species at the TS structures TS1, TS2 and TS3 can be estimated as the sum of the component atomic charges. The total AM1 charges for these molecular entities, com- puted from Lowdin population analysis,32 are given in Table 4.The charge distributions for each species in the reactant and product complexes, derived from each TS, have also been provided in Table 4, for comparison.For TS1 (and its reac- tant and product complexes), the mean charges have been computed for the conformers TSla-TSlh. The difference in the sign of the charge on the substrate between TS1 and the other two TSs is a direct consequence of the change in the substrate species itself from protonated pyruvic acid to proto- nated pyruvate (or for the reactant complex corresponding to TS3, pyruvate).The positive charge on the cofactor analogue reflects the significant charge transfer associated with the TS structure, for each model. The migrating hydride ion, however, remains almost uncharged, as was observed by Cummins and Gready" for hydride-transfer reactions between the same cofactor analogue and folate and dihydro- folate substrate analogues of DHFR. A comparison of the species partial charges of each TS with those of the corresponding reactant and product com- plexes clearly shows the positive charge development on the 1-methylnicotinamide species. These values (ca. 0.4) suggest that the TSs are more reactant-like than product-like. Analysis of the atomic charges of the substrate and cofac- tor moieties in the free reactants and products (given in Fig.8) shows that the charges on C2 of the substrate species and on N1 of the nicotinamide species (see Fig. 3 for numbering) undergo considerable change. C3 and C5 of the nicotinamide ring are affected to a smaller extent while the charge on the active carbon C4 is less variant. Considering the charges on C2 (substrate species) and N1, C3 and C5 (nicotinamide species) for TS1, TS2 and TS3, the TSs appear more reactant- like than product-like. In all of the charge distributions shown in Fig. 8, the car- boxamide side chain of the nicotinamide species exhibits a high degree of carbonyl-group polarization, including those for the free reactant and free product species. Experimental and molecular modelling studies by La Reau and Ander- aimed at elucidating possible molecular mechanisms for the high degree of stereospecificity of the reaction, con- cluded that the high stereospecificity could not be accounted for by steric exclusion effects from obligatory binding of the carboxamide side chain of the cofactor in the anti conformer.They suggested instead that the stereospecificity was due to an electrostatic effect of polarization of the carbonyl bond by active-site positively charged groups (His-195, Arg- 109 and Arg-171) and the dipole of the 'x2F helix which leads to a reduction in electron density on C4 and stabilization of the TS. With respect to the C4 charge, the present results do not support this contention as there is no evidence for significant electron depletion on C4 in the TSs.The charges on C8 and 09 of the carbonyl group in the TSs have been compared with those for the respective reactant and product complexes in Table 4. The carbonyl bond dipole moments34 (p)derived from these charges have also been computed and given in Table 4. Considering that p is 1.97 Dt for the l-methyl- nicotinamide ion and 2.26 D for l-methyldihydronicotin-amide, it is apparent that the p values for the reactant and product complexes (excluding the product complexes of the t 1 D (Debye) x 3.33564 x C m. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 -0.41 Q -0.32 0 -0.43 -0.25 -0.21 (b1 0.31 -0.1 4 -0.05 0.01 -0:06 -0.13 -0.46 0 -0.26 (c) 0.01 0.10 -0.36 -0.39 0 0 -0.25 o k 0*010.05\K/0-0.23 -0.24 I 0-17 -0.10 0110 Fig.8 AM1 partial charges for C, N and 0 atoms of the substrate and cofactor moieties in (a) the free reactants (pyruvate and 1- methyldihydronicotinamide), (b)the free products (lactate and 1-methylnicotinamide ion), (c) TS1 (substrate analogue is protonated pyruvic acid here), (d) TS2 and (e)TS3. For TS1, mean charges for the structures TSla-TSlh are given. TS1) exceed those observed for the free reactant and product species as well as those of the TSs. Of the three TS models, TS2 and TS3 have p values which are similar and compara- ble with those calculated for the isolated cofactor species. The p values for the reactant and product complexes of TS2 and TS3 are marginally greater than those for the TSs.The anomalous p values for TS1 are a consequence of the net charges on the polarizing substrate species, as compared with the charges on the substrate species in TS2 and TS3. In summary, while there is no significant increase in carbonyl polarization upon TS formation, it is possible that enhanced carbonyl polarization of the reactant and product complexes may be involved in activation of the cofactor and may thus influence the stereospecificity of the enzyme reaction. Conclusions The present study has provided an insight into the catalytic mechanism of the LDH enzyme reaction. Different plausible reaction mechanisms have been explored, at the semi-empirical AM1 level, in a systematic manner, starting with isolated substrate analogues and gradually introducing molecular species indicative of the function of key active-site residues.The results of these computations can be sum-marized as follows : (1) In all of the transition states obtained, the substrate species is protonated. This finding lends support to the pro- posal that the enzyme reaction is a two-step process, with proton transfer to the substrate preceding hydride-ion trans- fer from the cofactor. The fact that the transition states located were for hydride-ion transfer indicates that the overall kinetics of the reaction would be controlled by the hydride-transfer process. This conclusion is in agreement with Wilkie and Williams' hypothesis that proton transfer and hydride transfer are kinetically coupled but dynamically uncoupled.' (2) All the transition states obtained in our study are for the trans orientation of the carboxamide side chain of 1-methyldihydronicotinamide. No transition state has been iso- lated for the corresponding cis conformer.The nicotinamide ring retained its planar conformation in all the TS structures reported. This result differs from that obtained using ab initio methods by Wu and Ho~k~~but needs to be treated with some caution, as semiempirical methods are inadequate for predicting small ring distortions. However, the present results indicate that nicotinamide-ring distortion is not essential for the formation of a viable TS.The question of whether non- planar nicotinamide conformations in the TS could account for the stereospecificity of the LDH reaction12 is thus not addressed in the present study. The trans carboxamide side chain tends to adopt a conformation almost coplanar with the nicotinamide ring in TS3, in which hydrogen bonding of the carboxamide amino group with the Val-138 residue frag- ment has been explicitly introduced. However, without mod- elling the active-site enzyme environment in a more comprehensive manner, the relevance of suggested out-of- plane orientations of the carboxamide side chain in the tran- sition ~tate'~*l~ cannot be assessed. (3) The transition state TS3, containing fragments of key active-site residues, Arg-171, His-195 and Val-138, is judged to be representative of the LDH transition state in that it represents a reaction having 'matched internal states'.'' (4) There is considerable charge transfer accompanying the formation of the transition state, with the nicotinamide moiety carrying a positive charge of ca.0.4. Interestingly, the migrating hydride ion and the active carbon (C4) of the nico- tinamide species remain virtually uncharged. Based on the pattern of atomic charge distribution, the TS can be classified as more reactant-like than product-like. The enhanced polar- ization of the carboxamide carbonyl group observed in the reactant and product complexes may be involved in directing the stereospecificity of the enzyme reaction, but we found no support for the proposal of La Reau and Anderson33 for increased polarization of the carbonyl group in the TS or for electron depletion at C4 of the nicotinamide species in the TS.The present study has located TS structures for simple models of the LDH active site, using semi-empirical AM1 methodology, and the nature of the substrate species in the TS has been characterized. The planar conformation of the 2056 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 nicotinamide ring and the trans orientation of the carbo- xamide side chain have been confirmed, although the slightly out-of-plane orientation of the carboxamide side chain could be due to the limitations of the present models, in represent- ing the complete enzyme environment in the vicinity of the 11 12 13 P.L. Cummins and J. E. Gready, J. Mol. Struct. (Theochem), 1989,183,161. H. Eklund and C-I. Branden, in Pyridine Nucleotide Coenzymes, Part A, ed. D. Dolphin, 0. Avramovic and R. Poulson, Wiley, New York, 1987, p. 55. S. A. Benner, Experimentia, 1982,38,633. active site. The intrinsic nature of the carboxamide carbonyl polarization is evident from the charge distribution. However, possible mechanistic roles of some of the key active-site residues, such as Arg-109 and Asp-168, have not been established. Computations at a higher level of theory (ab initio 3-21G) are under way to test the stability of the results obtained at the semi-empirical AM1 level. Also, a more com- plete investigation incorporating the entire enzyme environ- ment and solvent is being undertaken, using a combined quantum and molecular mechanical approach.14 15 16 17 18 19 F. H. Westheimer, in Pyridine Nucleotide Coenzymes, Part A, ed. D. Dolphin, 0. Avramovic and R. Poulson, Wiley, New York, 1987, p. 255. J. Wilkie and I. H. Williams, J. Am. Chem. SOC., 1992, 114, 5423. J. Andres, V. Moliner, J. Krechl and E. Silla, Bioorg. Chem., 1993,21, 260. J. L. Gelpi, R. M. Jackson and J. J. Holbrook, J. Chem. SOC., Faraday Trans., 1993,89, 2707. K. E. Norris and J. E. Gready, J. Mol. Struct. (Theochem), 1993, 279,99. K. E. Norris, G. B. Bacskay and J. E. Gready, J. Comput. Chem., 1993, 14, 699. The award of an H.B. and F.M. Gritton Research Fellowship 20 K. E. Norris and J. E. Gready, J. Mol. Struct.(Theochem), 1992, 258,109. is gratefully acknowledged by S.R. 21 P. L. Cummins and J. E. Gready, J. Comput. Chem., 1990, 11, 791. References 22 J. J. P. Stewart, L. P. Davis and L. W. Burggraf, J. Comput. Chem., 1987,8,1117. 1 2 3 4 5 6 7 8 A. R. Clarke, T. Atkinson and J. J. Holbrook, Trends Biochem. Sci., 1989,14, 101. C. R. Dunn, H. M. Wilks, D. J. Halsall, T. Atkinson, A. R. Clarke, H. Muirhead and J. J. Holbrook, Philos. Trans. R. SOC. London B, 1989,332, 177. J. J. Holbrook, A. Liljas, S. J. Steindel and M. G. Rossmann, in The Enzymes, ed. P. D. Boyer, Academic Press, New York, 2nd edn., 1975, vol. 11, p. 191. A. R. Clarke, D. B. Wrigley, W. N. Chia, D. A. Barstow, T. Atkinson and J. J. Holbrook, Nature (London), 1986,324,699. K. W. Hart, A. R. Clarke, D. B. Wrigley, W. N. Chia, D. A. Barstow, T. Atkinson and J. J. Holbrook, Biochem. Biophys. Res. Commun., 1987,146,346. A. R. Clarke, H. M. Wilks, D. A. Barstow, T. Atkinson, W. N. Chia and J. J. Holbrook, Biochemistry, 1988,27, 1617. W. Eventoff, M. G. Rossmann, S. S. Taylor, H-J. Torff, H. Meyer, W. Keil and H-H. Kiltz, Proc. Natl. Acad. Sci. USA, 1977,74,2677. J. L. White, M. L. Hackert, M. Buehner, M. J. Adams, G. C. Ford, P. J. J. Lentz, I. E. Smiley, S. J. Steindel and M. G. Rossmann, J. Mol. Biol., 1976, 102, 759. 23 24 25 26 27 28 29 30 31 32 33 34 35 J. Krechl and J. Kuthan, J. Mol. Struct. (Theochem), 1988, 170, 239. M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. J. P. Stewart, J. Am. Chem. SOC., 1985,107,3902. J. J. P. Stewart, J. Comput. Aided Mol. Des., 1990,4, 1. P. L. Cummins, SYDPAC, unpublished work. P. L. Cummins and J. E. Gready, J. Comput. Chem., 1989, 10, 939. M. J. S. Dewar, E. F. Healy and J. J. P. Stewart, J. Chem. SOC., Faraday Trans. 2, 1984,80,227. P. Csaszar and P. Pulay, J. Mol. Struct. (Theochem), 1984, 114, 31. D. Voet and J. G. Voet, in Biochemistry, Wiley, New York, 1990, p. 347. L. C. Cusachs and P. Politzer, Chem. Phys. Lett., 1968,1, 529. M. A. Natiello and J. A. Medrano, Chem. Phys. Lett., 1984, 105, 180. R. D. La Reau and V. E. Anderson, Biochemistry, 1992,31,4174. U. Burkert and N. L. Allinger, in Molecular Mechanics, Amer-ican Chemical Society, Washington, DC 1982, p. 196. Y-D. Wu and K. N. Houk,J. Am. Chem. SOC., 1991,113,2353. 9 A. R. Clarke, T. Atkinson and J. J. Holbrook, Trends Biochem. Sci., 1989, 14, 145. 10 W. J. Albery and J. R. Knowles, Biochemistry, 1976, 15, 5631. Paper 3/07631H; Received 30th December, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949002047

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2057-2060 Direct Observation of Native and Unfolded Glucose Oxidase Structures by Scanning Tunnelling Microscopy Qijin Chi, Jingdong Zhang, Shaojun Dong* and Erkang Wang* Laboratory of Electroana Iytical Chemistry , Changchun Institute of Applied Chemistry , Chinese Academy of Sciences, Changchun, Jilin 130022,P.R. China Native and unfolded glucose oxidase (GOD) structures have been directly observed with scanning tunnelling microscopy (STM) for the first time. STM images show an opening butterfly-shaped pattern for the native GOD. When GOD molecules are extended on anodized, highly ordered pyrolytic graphite (HOPG), a helical structure composed of double-stranded chains was obtained under STM. These results are in good agreement with pre- vious description of the GOD molecular structure. A simple model of the unfolding process for GOD molecules was proposed to explain these observations.Electrochemical evidence was provided to support the results obtained with STM and the proposed model. In contrast to other microscopic techniques such as SEM, transmission electron microscopy (TEM) and scanning TEM (STEM), STM is capable of working in a wide variety of environments either at room temperature in air or even in aqueous solution.''2 This advantage as well as the superior resolution means that STM has been extensively used not only in imaging various solid surfaces but also in revealing the structural details of biological materials3 In the latter, a series of exciting achievements have been made and devoted to deoxyribonucleic acid and recA-DNA com-plexes.' '*' High-resolution STM images of DNA have been obtained in vacuum: in and in liquid environ-ment~,~,'~and structural features of the double helix have been discerned,6 even down to atomic res~lution.~ Some reports discuss STM imaging of enzyme^,'^-^^ a typical example being phosphorylase kinase which was shown to have a bilobate structure.' 8719 In addition, virus particles, polypeptides, molecular bilayers, the purple membrane, amino acids, polysaccharides and many other proteins have been imaged with varying degrees of res~lution.~ Although the tunnelling mechanism through biological macromolecules is not yet fully understood, the major obs- tacle to imaging biological specimens is not tunnelling through the sample but rather fixation of the sample to the substrate.21 Several methods have been developed to over- come the fixation problem, which include covalently binding the sample to the substrate6 using biological materials to form aggregates22 and electrochemical deposition.' 0,20i23 However, unlike the special properties of the biological speci- mens exploited in those studies, it is demonstrated here that GOD can be successfully imaged with STM when the enzyme is deposited on freshly cleaved HOPG or irreversibly adsorbed onto an anodized HOPG surface.GOD is a flavin enzyme with flavin adenine dinucleotide (FAD) as the redox prosthetic group, its biological function being to catalyse glucose to form gluconolactone.From bio- chemical st~dies~~*~~ GOD is known to be a structurally rigid glycoprotein of ca. 160000 Dat and consists of two identical polypeptide chains, each containing a FAD redox centre. However, direct observation of its structural features has never been performed. Experimental GOD (from Aspergillus, EC 1.1.3.4) was obtained from Sigma (Type 11, 35 300 U g- ',G-6125). Other reagents used were of t 1 Da = 1 u. analytical-reagent grade. All solutions were prepared with doubly distilled water. GOD was dissolved in doubly distilled water and the exact concentration of the enzyme solution was determined by spectrophotometric assays at 452 nm using a molar absorptivity of 21.6 dm3 mo1-' cm-1.26 Electrochemical experiments were carried out on an FDH 3204 potentiostat (Shanghai) with a Gould Series-60000 X-Y recorder (Shenyang, China).A three-electrode system was employed with an Ag/AgCl (saturated KC1) reference elec- trode, a platinum plate auxiliary electrode and an HOPG working electrode. All buffer solutions were purged with nitrogen gas before electrochemical experiments. A Varian DMS-90 UV-VIS spectrophotometer was used for spectro- photometric assays. STM imaging was performed under a normal atmosphere at room temperature (20 & 2°C) with a TO~OMETRIXTMX 2000; this STM instrument was com- bined with a monitor which was used in observing the dis- tance between the sample and the tip.The air humidity was kept at 40-60%. The Pt/Ir (80/20) tips were mechanically formed by cutting. Scanning was in the constant-current mode except for the images of bare anodized HOPG, at a bias voltage of 0.2-0.4 V and a currrent of 1.0-1.5 nA. Post- image processing of all images involved low-pass filtering to remove high-frequency noise without two-dimensional Fourier transformation. GOD was deposited in a 15 mm3 droplet of 4.3 x mol dm3 enzyme solution on the freshly cleaved HOPG surface. The sample was then dried at room temperature (drying time 40-60 min) and finally washed carefully with doubly distilled water. Another sample was prepared by the same process except that the freshly cleaved HOPG was firstly anodized at +2.0 V for 2 min.Results and Discussion Fig. 1 shows the absorption spectrum of 4.3 x mol dm3 GOD solution. Absorption is significant in the visible region with maximum values at 380 and 452 nm, but above 452 nm the absorption decreases sharply. This is consistent with a previous report.26 When p-hydroquinone was used as a medi- ator, electrochemical experiments demonstrated that GOD can rapidly catalyse glucose to gluconolactone. Therefore, the present results confirm that the GOD used retains its native structure with biological activity. GOD can be adsorbed on the freshly cleaved HOPG to form a monolayer ; usually molecular images are easily obtained at the edge of the HOPG surface, and possibly the fixation of GODmolecules is more stable in these regions.A typical image of GOD molecules is shown in Plate l(a). Four 2058 0.09 0.08 0.07 0.06 a2 0.05 524 0.04 0.031 0.02' 0.01 ' 0.00' 300 400 500 600 A/nm Fig. 1 Absorption spectrum of 4.3 x lop6 mol dm-3 GOD solu-tion individual enzyme molecules are clearly discerned and are distributed uniformly on the substrate surface. Many scans were performed in other regions of the substrate and this pattern was found to be reproducible. The scanning range was then limited to 31 x 31 nm2 or smaller, which allows detailed observation of an enzyme molecule [Plate l(b)]. Plate l(c) was obtained from Plate l(b) by rotating the sample by 90" and reducing the scan area; the corresponding three-dimensional image is shown in Plate l(d).As visualized from Plate l(c) and (4, the individual enzyme molecule exhibits a butterfly-shaped structure containing two sym-metric wings and a depressed bridge or centre. This pattern is similar to the STM image of phosphorylase kinase,I8 but the former is smaller. Quantitative size determinations were made with the TOPOMETRIX Instrument software. Fig. 2 shows typical line profiles obtained through the various direction noted in Plate l(c). The largest length across the wing tips [see Fig. 2(4, (b)]is 13.8 f0.5 nm (n = 12) and the largest width across the bridge [Fig. 2(c)] is 18.1 f 0.6 nm (n = 12). The average length of a wing (parallel to the edge) is 12.2 0.4nm (n = 20) and the average width of each wing [perpendicular to the edge; Fig.2(4] is 8.9f 0.4(n = 20). The width of the bridge was measured (parallel to the edge through the bridge) as 4.7f 0.3 nm (n = 16). The average height of the molecule was measured as 1.27 & 0.06 nm (n = 15) on the basis of the calculated average displacement of the probe tip from the substrate while the molecule was scanned. As different electronic work functions exist between the substrate (HOPG) and the adsorbed enzyme, the mea- sured vertical distance will not be consistent with the physical height. In other words, the absolute thickness of the molecule cannot be accurately obtained from these STM measure-ments. However, the relative height shown here is expected to be proportional to the absolute thickness.Thus, the size of a GOD molecule in this study can be approximately described as 18 x 12 x 1.3 nm3, but the exact dimensions of an individ- ual molecule are still unknown, although it is considered in general to be ca. 9.0 nm in diameter.24 In addition, the line J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 L rn 2.95 w III Y0.93' 0.00 11.92 23.84 0.64' I I 0.00 11.50 22.00 distance/nm Fig. 2 (a), (b), (c) and (dj sectional cuts obtained from Plate l(c) along the direction of ab, cd,ef and gh, respectively. profile shown in Fig. 2(c) indicates clearly that the bridge portion of the molecule between two wings appears to be depressed; this result further supports the above description of a butterfly-shaped pattern.In previous studies, HOPG was activated by various means, such as chemical ~xidation,~' laser activation,28 elec- trochemical ~xidation~~,~' and reaction with dioxygen at ele- vated temperat~re,~'.~~ to create active sites and to accelerate heterogeneous electron transfer between HOPG and the redox systems. Here, electrochemical pretreatment at a high positive voltage was used to produce active sites on the HOPG surface. Plate 2 shows the STM images of bare HOPG after anodizing it at +2.0 V for 2 min; clearly nanometre-sized pits were formed in the substrate plane [Plate 2(b)]. These etching pits provide effective active sites for the adsorption of GOD on the HOPG. The GOD mol- ecules were adsorbed irreversibly onto the anodized HOPG to yield a stable sample.A series of STM images with differ- ent scan ranges are shown in Plate 3. These images are obvi- ously different from those shown in Plate 1, and indicate that the tertiary structure of the enzyme has been lost on this surface. As shown in Plate 3(a)-(c), several structures, includ- ing chains, clusters and fragments, exist simultaneously on this surface. In addition, sometimes a tetramer of unfolded GOD molecules could also be observed, as shown in the left- hand side of Plate 3(d). In these various structures, a typical example is the isolated fibre shown in Plate 3(e) and (f), which can be clearly discerned to be composed of two poly- peptide chains twisted into a rope-like structure. A cross- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Plate 1 STM images of GOD adsorbed on the freshly cleaved HOPG obtained with different scan scopes. (a) and (b)topographic views; (c) obtained from (b) after rotating it by 90" and reducing the scan area; (d) three-dimensional image of (c). U = 0.3 V; I = 1.2 nA; scan area = 88.33 x 88.33 nm2 (a),31.42 x 31.42 nm2 (b), 22.82 x 22.82 nm2 (c), (d). Plate 2 Topographic views of bare HOPG electrochemically pretreated at +2.0 V for 2 min obtained in constant-height mode. U = 0.4 V; scan area = 120 x 120 nm2 (a), 10 x 10 nm2 (b). Q, Chi et al. (Facingp. 2058) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Plate 3 STM images of GOD adsorbed on the anodized HOPG at various scan scopes. (a)-(f) Topographic views.U = 0.2 V; I = 1.0 nA; scan area = lo00 x loo0 nm2 (a),400 x 400 nm2 (b),200.08 x 200.08nm2 (c), 100 x 100 nm2 (d),70.17 x 70.17 nm2 (e)and 45 x 45 nm2 (f). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 5*50*2.023.76 0.00 22.98 45.96 distance/nm Fig. 3 (a), (b)and (c)line profiles obtained from Plate 3 (d) and (f) along the directions of ab, cd and ef, respectively section of Plate 3(e) along the ab direction through the chain is shown in Fig. 3(a),and the helical pitch was measured as 15.1 f0.7 nm (n = 15). The chain width was determined by line scans perpendicular to the chain [e.g. Fig. 3(b)];the average value is 12.5 f1.4 nm (n = 22). Size determination of each strand chain was made by drawing a section through ef shown in Plate 3(f); the result is given in Fig.3(c).The diam- eters of two strand chains are 8.0 and 6.3 nm, respectively. After deducting the space influence, these two strand chains can be considered to be the same size. Therefore, the rope- like pattern observed is composed of two identical chains and represents the unfolded form of an individual enzyme mol- ecule. In order to understand these observations better, a simple model is proposed in Fig. 4. Fig. 4(a) shows the native struc- ture of the enzyme, which has tertiary and quaternary struc- tures with a folded form. When the enzyme is adsorbed onto G. GL G. J / I \ e HOPG e HOPG e HOPG e HOPG (b 1 (c) (d1 (e1 Fig. 4 Proposed model of the extending process of the GOD mol-ecules on the anodized HOPG. G, GL and large solid spots represent glucose, gluconolactone and FAD, respectively.the freshly cleaved HOPG, the enzyme molecules retain their native or approaching native structure as shown in Fig. 4(a) and (b)owing to the weak enzymesurface interaction. There- fore, under this situation a butterfly-shaped pattern com-posed of two wings and a depressed bridge could be observed from STM imaging. Each wing represents the folded form of a polypeptide chain, whereas the bridge denotes the non- covalent bonding part of two polypeptide chains of a GOD molecule. In contrast, a strong interaction between enzyme and the substrate surface occurs when GOD is adsorbed onto the anodized HOPG surface, which results in GOD mol- ecules gradually unfolding, as shown in Fig.4(b)-(e). The extent of unfolding of the enzyme molecules depends on the degree of enzyme-surface interaction. Since the anodized HOPG surface is not uniformly flat (see Plate 2), unlike the untreated HOPG surface, the strength of the enzyme-surface interaction must vary. Therefore, when the GOD molecules are adsorbed onto the anodized HOPG surface, at least four kinds of structures are expected to be present: (1) the poly- peptide chain of GOD fully extended [Fig. 4(e)], giving an individual chain structure ;(2) when the fully extended poly- peptides aggregate together, trimers or tetramers of the unfolded molecules would be formed; (3) the native GOD molecules extend only partly as shown in Fig.4(c) and (d) or the unfolded molecules coil randomly, giving an intermediate or shorter chain structure; (4) a strong interaction between the enzyme molecules and the substrate may also result in the enzyme molecules being broken to form fragments. Indeed, all these structures were observed with STM, as shown in Plate 3. Clearly the typical structure shown in Plate 3(e)and (f) originate from the fully unfolded form of an individual GOD molecule. On the other hand, it is well known that the redox centre (FAD) of the GOD molecule is embedded deep in the enzyme in the native structure. The distance between either of its two FAD centres and the electrode surface exceeds the distance across which electrons are transferred at measurable rates, hence direct electron transfer between the native enzyme and the electrode surface cannot occur.A large number of attempts have been made to achieve direct electron exchange between GOD and various elec-tr~des;~~-~'however, when the GOD molecules are adsorbed on the HOPG surface and gradually extend, the redox centre of the enzyme is also gradually exposed to the HOPG surface, the distance between the redox centre of enzyme and the substrate surface becoming shorter and shorter [see Fig. 4(b)-(e)]; thus direct electron transfer from adsorbed GOD to the HOPG surface is expected to be pos- sible. In this study, electrochemical experiments confirm this prediction. When GOD is deposited on the untreated HOPG, no electrochemical signals resulting from direct elec- tron transfer can be measured, indicating that the adsorbed GOD retains its native or approximately native structure and the distance between the redox centre and the HOPG surface remains larger than the critical distance under this situation. In contrast, the well defined cyclic voltammograms of GOD adsorbed on anodized HOPG were obtained and shown in Fig.5. A couple of redox peaks appear at -0.4 to -0.5 V us. Ag/AgCl, which result from the direct reduction and oxida- tion of the adsorbed GOD and can be described by the fol- lowing equation: GOD(FAD) + 2e + 2H+ eGOD(FADH,) (1) As expected, the GOD adsorbed on the anodized HOPG surface could not catalyse its substrate (glucose) oxidation, indicating that the adsorbed GOD had been denatured to lose biological activity owing to extending to an unfolded form.Consequently the electrochemical results shown here lend further support to the STM observations described 2060 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 5 T. P. Beebe Jr., T. E. Wilson, D. F. Ogletree, J. E. Katz, R. Balhorn, M. B. Salmeron and W. J. Siekhaus, Science, 1989,243, 370. 6 A. Cricenti, S. Selci, A. C. Felici, R. Generosi, E. Gori, W. Djac- zenko and G. Chirotti, Science, 1989,245, 1226. 7 D. D. Dunlap and C. Bustamante, Nature (London), 1989, 342, 204. 8 G. Lee, P. G. Arscott, V. A. Bloomfield and D. F. Evans, Science, 1989,244,475. 9 P. G. Arscott, G. Lee, V. A. Bloomfield and D.F. Evans, Nature 10 (London), 1989,339,484. S. M. Lindsay, T. Thundat, L. Nagahara, U. Knipping and R. L. Rill, Science, 1989, 244, 1063. 11 M. Amrein, A. Stasiak, H. Gross, E. Stoll and G. Travaglini, Science, 1988,240, 514. 12 M. Amrein, R. Durr, A. Stasiak, H. Gross and G. Travaglini, Science, 1989, 243, 1708. 13 R. Czajika, C. G. J. Koopal, M. C. Feiters, J. W. Gerritsen, R. J. M. Nolte and H. van Kempen, Bioelectrochem. Bioenerg., 1 " ' 1 ' 1 ' ' . 1 0.3 0.1 -0.1 -0.3 4.5 4.7 E/V vs. AgjAgCl 14 15 1992, 29,47. P. K. Hansma, V. B. Elings, 0.Marti and C. E. Bracke, Science, 1988,242,209. S. A. Darst, E. W. Kubalek and R. D. Kornberg, Nature Fig. 5 Cyclic voltammograms of bare anodized HOPG (dashed line) and GOD adsorbed on the anodized HOPG (solid lines) obtained in 0.1 mol dm-3 phosphate buffer (KH,PO, + Na,HPO,) (pH 6.7) with scan rates (mV s-l) of (a) and (b) 30, (c) 50, (d) 70, (e) 100 and (f) 150 16 17 18 (London), 1989,340,730.M. H. Jericho, B. L. Blackford, D. C. Dahn, C. Frame and D. Macleean, J. Vac. Sci. Technol. A, 1990,8,661. L. Haggerty, B. A. Waston, M. A. Barteau and A. M. Lenhoff, J. Vac. Sci. Technol. By 1990,9, 1219. R. D. Edstrom, M. H. Meinke, X. Yang, R. Yang and D. F. Evans, Biochemistry, 1989,28,4939. above and to the model shown in Fig. 4. Finally, it should be pointed out that the interaction between GOD and the anod- 19 20 V. B. Elings, R. D. Edstrom, M. H. Meinke, X. Yang, R. Yang and D. F. Evans, J. Vac. Sci. Technol. A, 1991,8,652. R. W. Keller, D.Bear and C. Bustamante, J. Vac. Sci. Technol. ized HOPG surface possibly includes physical and chemical interactions, but the real mechanism of interaction is not clear at present. 21 22 B, 1991,9, 1291. M. Salmeron, T. P. Beebe Jr., J. Odriozola, T. Wilson, D. F. Ogletree and W. Siekhaus, J. Vac. Sci. Technol. A, 1990,8, 635. M. J. Miles, T. McMaster, J. J. Carr, A. S. Tatham, P. R. Conclusions Shewry, J. M. Field, P. S. Belton, D. Jeenes, B. Hanley, M. Whittam, P. Cairns, V. I. Morris and N. Lambert, J. Vac. Sci. (1) Glucose oxidase can be adsorbed on freshly cleaved HOPG and retains its native structure, a butterfly-shaped pattern, consisting of two symmetrical wings and a depressed bridge, which has been clearly observed by STM. (2) In order to observe the double-stranded chains of the GOD molecule, it is necessary to make the enzyme extend fully to the unfolded form.A simple method, by adsorption of 23 24 25 26 27 28 Technol. A, 1990,8,698. S. M. Lindsay and B. Barris, J. Vac. Sci. Technol A, 1988,6,544. S. Nakamura, S. Hayashi and K. Koga, Biochim. Biophys. Acta, 1976,445,294. The Worthington Manual, Worthington BiochemicaI, Freehold, S. Nakamura and S. Fujiki, J. Biochem., 1968,63, 51. L. Porte, D. Richard and P. Gallezot, J. Microsc., 1988, 152, 515. R. S. Robinson, K. Sternitzke, M. T. McDermott and R. L. NJ, 1977, pp. 37-39. the enzyme onto the anodized HOPG surface, has been developed to prepare an unfolded GOD sample. A typical rope-like structure composed of two twisted polypeptide chains has been successfully visualized by STM.(3) Since the redox centre of the enzyme can be exposed sufficiently to the anodized HOPG surface, direct electron transfer between the adsorbed GOD and the treated sub- 29 30 31 32 33 34 McCreery, J. Electrochem. SOC., 1991,138,2412. A. A. Gewirth and A. J. Bard, J. Phys. Chem., 1988,92,5563. C. A. Goss, J. C. Brumfield, E. A. Irene and R. W. Murray, Anal. Chem., 1993,651378. H. Chang and A. J. Bard, J. Am. Chem. SOC.,1990,112,4598. H. Chang and A. J. Bard, J. Am. Chem. SOC., 1991,113,5588. Y. Degani and A. Heller, J. Phys. Chem., 1987,91,1285. P. N. Bartlett, R. G. Whitaker, M. J. Green and J. Frew, J. strate has been achieved. Chem. Soc., Chem. Commun., 1987, 1603. 35 Y. Degani and A. Heller, J. Am. Chem. SOC., 1988,110,2615. Thanks are due to Prof. Zemu Yu,Bailin Zhang and Jin Li for their technical help and useful discussions. The support of the National Natural Science Foundation of China is greatly 36 37 W. Schuhmann, T. J. Ohara, H-L. Schmidt and A. Heller, J. Am. Chem. SOC., 1991,113, 1394. Y. Kajiya and H. Yoneyama, J. Electroanal. Chem., 1992, 328, 259. appreciated. 38 Y. Kajiya and H. Yoneyama, J. Electroanal. Chem., 1992, 341, 85. References 39 C. G. J. Koopal, M. C. Feiters, R. J. M. Nolte, B. de Ruiter, R. B. M. Schasfoort, R. Czajka and H. Van Kempen, Synth. 1 G. Binnig and H. Rohrer, ZBM J. Res. Deo., 1986,30,355. 2 P. K. Hansma and J. Tersoff, J. Appl. Phys., 1987,61, R1. 3 S. R. Snyder and H. S. White, Anal. Chem., 1992,64,116R. 4 R. J. Driscoll, M. G. Youngquist and J. D. Baldeschwieler, 40 Met., 1992,51, 397. C. G. J. Koopal, M. C. Feiters, R. J. M. Nolte, B. de Ruiter and R. B. M. Schasfoort, Biosensors Bioelectronics, 1992,7,461. Nature (London), 1990,346,294. Paper 41006421; Received 1st February, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002057

出版商:RSC    

年代:1994

数据来源: RSC

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J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2061-2064 2061 Influence of lmmobilising Anions on the Redox Switching of PolyaniIine V. W. Jones, M. Kalaji" and G. Walker Department of Chemistry, University of Wales, Bangor, UK LL57 2UW C. Barbero and R. Kotz Paul Scherrer lnstitut , CH-5232 Villigen PSI, Switzerland The immobilisation of heteropolyacids into polyaniline films has been studied using the probe beam deflection technique, X-ray photoelectron spectroscopy and scanning electron microscopy. The results indicate that the heteropolyanions modify the redox behaviour and morphology of polyaniline. The concept of using conducting polymers to enhance cata- lytic activity continues to arouse much interest.' One strategy is to introduce highly dispersed metal particles such as plati- num, palladium, iridium and ruthenium into the polymer matrix to improve catalytic efficiency of reactions such as hydrogen evolution,' oxygen reduction3 and the reduction of di~xygen.~ Another strategy is to introduce metal-containing ions as the dopants.' Recent reports indicated that heteropolyanions of the Keggin type can be incorporated into conducting polymer films, such as polyaniline, prepared chemically or The idea of trapping such large anions should have a marked effect on the mechanism of redox switching of the conducting polymers.In the case of polyanil- ine, it has been that the first oxidation is accom- panied by proton expulsion and anion insertion in order to maintain charge neutrality. Therefore the trapping of the anions should influence this process in a way which can decrease the intrinsic switching time between the insulating and conducting forms.In this study, we present results on polyaniline (PANI) films containing Keggin-type anions, prepared electrochemi- cally and analysed using cyclic voltammetry, probe beam deflection technique (PBD), X-ray photoelectron spectros- copy (XPS) and scanning electron microscopy (SEM). The data indicate that PANI films prepared with Keggin-type acids exhibit cation exchange in aqueous HCl solutions. Fur- thermore, the doping levels of the PANI films vary according to the heteropolyacid used. Experimental Polyaniline films were grown potentiodynamically on plati- num or indium-doped tin oxide (ITO) electrodes by cycling the potential between 0.4 and 0.95 V us.the saturated calomel electrode (SCE) at a sweep rate of 50 mV s-'. The syntheses were carried out under an argon environment from solutions containing 0.01 mol dm-3 aniline in 0.1 mol dm-3 solution of the Keggin-type acid in acetonitrile. The acids used were phosphomolybdic acid (PM), phosphotungstic acid (PT) and tungstosilicic acid (TS). The electropolymerisation was carried out in acetonitrile because of the poor solubility of the anilinium salt of the heteropolyanions in water. After the polymerisation, the electrodes were rinsed with acetonitrile followed by distilled water before studying their redox behav- iour in aqueous acidic solutions.The films prepared from solutions of Keggin acids will be referred to as PANI/Keggin whereas the polyaniline films prepared in hydrochloric acid will be referred to as PANI/Cl-. Film thicknesses were calcu- lated from the charge-to-thickness ratio measured by * To whom correspondence should be addressed ellip~ornetry'~and compared to thicknesses calculated from SEM. The PBD arrangement has been described previously. l4 The PBD signal, along with the cyclic voltammogram, were recorded on an X-Y recorder (BBC SE780). The electro- chemical cell used was a conventional optical glass cuvette (2 cm x 2 cm x 4 cm). A planar glass (1 cm wide) covered with 200 nm of gold or platinum was used as a working electrode. The counter electrode was a platinum wire and an SCE was used as a reference electrode.The distance between the counter and working electrodes was large enough to prevent any interference due to reactions occurring at the counter electrode with the laser beam. Thin films, less than 300 nm, were used to assure fast establishment of chemical equi- librium inside the film during potential scans. Probe beam deflection is a technique that measures the concentration gradient in front of the electrode by monitor- ing the refractive index gradient with a light beam.14 The electrochemical oxidation-reduction process is accompanied by counterion exchange with the bathing solution to main- tain electroneutrality. The ion concentration in the solution changes, creating a gradient of the refractive index normal to the electrode surface.A beam travelling parallel to the surface undergoes a deviation proportional to the concentration gra- dient; therefore deviation of the beam is proportional to the extent and direction of the ion flux. Positive deflection corre- sponds to insertion of ions in the film, while negative deflec- tion implies release of ions to the solution. Scanning electron micrographs (Hitachi S-520 SEM) were taken of PANI/Cl- and PANI/Keggin films grown on ITO. The films were first cycled in 1 mol dm-3 HC1, washed with triply distilled water and then dried under vacuum at 60°C prior to any measure- ments. The films were sputter coated with gold (5 nm) (Polaron equipment) to prevent charging of the glass.XPS investigations were carried out on polymer films deposited on gold electrodes. The samples were investigated in the pristine state because even slight Ar' sputtering altered the composition significantly. No charging effects were observed. The polymer-coated electrodes were cycled in 1 mol dm-3 HCl prior to recording XPS data to ensure that the ionic composition of the films corresponds to the electro- chemically active material and not to the as-prepared films. The films were examined in the oxidised state. In addition to the overview spectrum, the emission peaks of N Is, C Is, P 2p, W 4f, Si 2p, Mo 3d and 0 1s were investigated. Scattering cross-sections for quantitative analysis were taken from ref. 15. Results and Discussion Fig.1 shows a cyclic voltammogram and the corresponding probe beam deflectogram for a PANI/TS film obtained in 0.1 1000 800 600 400 % 3 200 3 ou -200 -400 I -600I I I I -0.2 0.0 0.2 0.4 0.6 0.8 EfV vs. SCE (a +Fig. 1 Cyclic voltammogram (-) and deflectogram .) o PANI/tungstosilisic recorded in 0.1 mol dm-3 HCl at a sweep rate of 10 mV s-' mol dm-3 HCl. The redox behaviour of PANI/TS resembles that of PANI films grown under 'normal' conditions, i.e. in 0.1 mol dm- HCl.'o.'l However, the corresponding deflec- togram exhibits a negative deflection accompanying the oxi- dation of the polymer which is indicative of the expulsion of ions (protons) from the polymer matrix. A positive deflection on the reverse sweep corresponds to the insertion of cations.This behaviour is in contrast to the deflectograms obtained for PANI/Cl- which exhibit a positive deflection during the first oxidation peak in 0.1 mol dm-3 HC1,16 indicating ion (anion) insertion in the film. PANI/PT (Fig. 2) exhibits similar behaviour to PANIDS in that proton expulsion and insertion accompany the oxida- tion and reduction, respectively. However, the cyclic voltam- mogram exhibits an anodic shift in the first oxidation 2oo 1 400150-100-200 50-J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 101 ,-lo00 500 '4. -500 -1 000 -0.2 0.0 0.2 0.4 0.6 0.8 EfV vs. SCE (aFig. 3 Cyclic voltammogram (-) and deflectogram .) c PANI/phosphomolybdic recorded in 0.1 rnol dm-3 HC1 at a sweep rate of 100 mV s-' potential. The peak separation for the first redox couple is less than that for PANIDS and PANI/Cl-.PANI/PM (Fig. 3), on the other hand, exhibits a remarkable change in the redox behaviour. Whereas PANI/Cl- shows an asymmetry in the shape of the cyclic voltammogram, the peak separation for the first redox couple in PANI/PM is negligible. More- over, the colour of the film in the reduced state is dark blue (Fig. 4), whereas PANI/Cl- is yellow. ''The electrochemistry of heteropolyacids has been previously studied" and no similar observation was noted which excludes the conclusion that what is observed in PANI/PM is solely due to some film formation by the phosphomolybdates. However, it is not pos- sible at this stage to exclude the fact that the redox chemistry observed is due to a combination of PANI switching and electroactivity of trapped heteropolyanions.This is further 1 0.i 0-% --50--200?!5 0 -1 00-400 -1 50-V -600 -200 / I I I I -0.2 0.0 0.2 0.4 0.6 C 3 EfV vs. SCE Fig. 2 Cyclic voltammogram (-) and deflectogram (. . .) of Fig. 4 Absorbance spectra of PANI/phosphomolybdic film on IT0 PANI/phosphotungstic recorded in 0.1 mol dm-3 HCI at a sweep taken at different potentials. Note the absorbance around 650 nm in rate of 100 mV s-' the reduced state which is due to the phosphomolymbdate anion. Table 1 XPS results obtained for the different PANI/Keggin films ~~ ratio PANI/PT PANI/PM PANIPS ~~ ~ W:P 11 (12) 0:P 47 (40) 30 (40)C:N 6 (6) 2.6 (6) N:P 6 30 Mo:P 9.5 (12) Si : W w:o N:W formula (C6HSN)6(PW1 204d3 -doping level, n 0.5 The expected value according to the acid or PANI formula is given in brackets.Cannot be determined. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 (a1 Fig. 5 Scanning electron micrographs of (a) PANI/CI -,(b)PANI/phosphotungstic, (c) PANI/tungstosilicic and (d) PANI/phosphomolybdic.C. All films were grown on IT0 supported by the fact that the blue colour exhibited by the film in the reduced state is most likely to be due to the phos- phomolybdic anion trapped within the films matrix. The reduction of the Mo6+ ions produces a proportion of the Mo5+ ions. Transfer of electrons from Mo5+ to Mo6+ ions is responsible for the intense 'charge-transfer absorption'. The corresponding deflectogram indicates proton expulsion upon oxidation of the polymer followed by anion insertion occurring at potentials above 0.3 V.The interpretation of the PBD results for all the films studied is coherent with the irreversible immobilisation of the heteropolyanion inside the polymer layer. In the reduced state, the negative charge of the immobilised heteropolyanion is compensated by protons. Upon oxidation, the negative charge of the immobilised ion is used to compensate for the positive charge in the polymer backbone, and the protons are expelled from the film. This contrasts with PANI/Cl- films in which the oxidation is accompanied by anion insertion.The difference in behaviour of PANI films prepared in the different heteropolyacids can be partially explained using the XPS data which also provide evidence for the immobilisation of the Keggin-type anions within the polymer matrix. Table 1 shows a summary of the XPS data obtained for PANI films grown in different heteropolyacid solutions, and indicates the ratio of the detected elements, the.deduced formulae of the polymer salt and the doping level (degree of anion charges per polymer ring). The XPS data show that the hetero- polyanions are present after the polymerisation and are retained inside the films even after cycling in an aqueous solution containing 1 mol dm-3 HCl. In the case of phos- photungstic and tungstosilicic acid, the amount of heterpoly- anion present is enough to compensate for the charge in the emeraldine state.In the case of phosphomolybdic acid, the amount is lower than is necessary to compensate for the charge in the half-oxidised state which explains the need for anion insertion as shown in the corresponding deflectogram above 0.3 V. The XPS data for PANI/PM exhibit a marked deviation from the expected values. It has been previously shown, using scanning tunnelling microscopy, that phosphomolybdic acid can undergo an electrochemical decomposition reaction to yield particles of smaller diameter (probably molybdenum oxide).” However, the kinetics of the process and the ratio of products to reactants are still under investigation.2 There-fore, the deviation from the expected values in the XPS mea- surements may be due to the presence of a mixture of phosphomolybdic acid and molybdenum oxide. The origin of the second redox couple in the PANI/PM is still unclear. Our initial studies on phosphomolybdic acid indicate two quasi-reversible reactions in the same potential window used for PANI/PM.However, the peak positions are different to those observed with PANI/PM which may suggest that the redox reactions of the phosphomolybdate anion can be mediated by electron transfer through the polymer network. Furthermore, this is supported by the fact that as the polymer is oxidised, the initial colour change is from blue to green, but changes over a period of time back to blue.22 This suggests that the polymer is initially oxidised, but is then involved in a charge-transfer process with the anions.The possibility of this charge-transfer process is still under investigation. The morphology of the polymer film is also changed by the dopant anion. Scanning electron microscopy (Fig. 5) shows evidence of compact morphological structures consisting of globular microspheroids except for PANI/tungstosilicic, which was found to have a smooth, undulating surface. It is worth noting here that PANI/tungstosilicic exhibited the highest electrical conductivity amongst all the films studied, including PANI/Cl-.22 The results clearly indicate that some anions can be immo- bilked within PANI film and that they can alter the electrical and optical behaviour of the host matrix.The switching time of such polymers is currently being studied using ultra- microelectrodes. The spectroscopic properties and the influ- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ence of anion guests in heteropolyanions on the behaviour of PANI films are also under unvestigation. Support from the SERC (M.K.), Pilkington (V.J.), PSI (M.K.) and the University of Wales/Bangor (M.K.) is gratefully acknowledged. The authors would like to thank B. Schnyder for carrying out the XPS measurements and the referees for their helpful comments. References 1 J. Simonet and J. Rault-Bertholet, Prog. Solid State Chem., 1991, 21, 1. 2 K. Kost, D. Bartak, B. Kazec and T. Kuwana, Anal.Chem., 1990,162,151. 3 C. Bose, S. Basak and K. Rajeshwar, 204th ACS Meeting, Wash-ington DC, 1992, Abstract No. 69. 4 T. Vork and E. Barendrecht, Electrochim. Acta, 1990,35, 135. 5 G. Bidan, M. Lapkowski and J. Travers, Synth. Met., 1989, 28, C113. 6 A. Pron, Synth. Met., 1992,46,277. 7 G. Bidan, E. Genies and M. Lapkowski, J. Chem. SOC., Chem. Commun., 1988,533. 8 M. Hasik, A. Pron, I. Kulszewicz-Bajer, J. Pozniczek, A. Biel-anski, Z. Piwowarska and R. Dziemaj, Synth. Met., 1993, 55-57, 972. 9 M. Kalaji, L. M. Peter, L. Abrantes and J. Mesquita, J. Electro-anal. Chem., 1989,274,289. 10 M. Kalaji, L. Nyholm and L. M. Peter, J. Electroanal. Chem., 1991,313,271. 11 M. Kalaji, L. Nyholm and L. M. Peter, J. Electroanal. Chem., 1992,325,269.12 M. Vuki, M. Kalaji, L. Nyholm and L. M. Peter, J. Electroanal. Chem., 1992,332,315. 13 R. Greef, M. Kalaji and L. M. Peter, Faraday Discuss. Chem. SOC.,1989,88, 277. 14 R. Kotz, C. Barbero and 0.Haas, J. Electroanal. Chem., 1990, 2%, 37, and references therein. 15 J. Yeh and I. Lindau, At. Data Nucl. Data Tables, 1985,32, 1. 16 C. Barbero, M. C. Miras, 0.Haas and R. Kotz, J. Electrochem. SOC., 1991, 138, 669. 17 M. Kalaji, V. W. Jones, C. Barbero and R. Kotz, Abstracts of the 44th Meeting of the International Society of Electrochemistry, Germany, 1993, Abst. PI. 7.15. 18 B. Keita and L. Nadjo, J. Electroanal. Chem., 1987,227, 77. 19 N. Greenwood and A. Earnshaw, Chemistry of the Elements, Pergamon Press, Oxford, 1986, p. 1185. 20 A. Kowal, Abstracts of the International Conference on Scanning Tunnelling Microscopy, Interlaken, Switzerland, 1991, Abst. PH/77. 21 A. Kowal, personal communication. 22 M. Kalaji and G. M. Walker, in preparation. Paper 4/00682H; Received 4th February, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949002061

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2065-2070 Conductivity and Dielectric Relaxation in Hydrated Fused Salts G. P. Johari, D. A. Wasylyshyn and S. K. Jain Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7 The dielectric properties of Ca(NO,), . 2.9H20,Cd(NO,), -2.37H20and (Ca + Cd)(NO,), -2.92H20have been measured in the supercooled liquid state immediately above their respective calorimetric glass-transition tem- peratures. Over the frequency range 12-105 Hz, their permittivity and loss spectra are featureless. By subtrac- ting the contributions from electrode impedance.'and dc conductivity, the remaining E*, which is due to a polarization process, is described well by a Davidson-Cole relaxation function with /? in the range 0.26-0.40 (f0.05).The activation energies for conduction are 200-220 kJ mol-'.p decreases with decreasing tem- perature, as does the contribution to permittivity from orientation polarization. The latter, which differs from the behaviour of most liquids, indicates a decrease in the number densities of the molecular and ion-pair dipoles and/or their dipole moments with decreasing temperature. Analyses of the data in E* and M* ( =E*-') formal-isms give the same results. The former is direct, although it requires knowledge of the exponent for the elec- trode impedance. The data also fit an asymmetric distribution of conductivity relaxation times at least as well as in the literature, but this fit appears to be misleading. In the basic macroscopic theory of dielectrics, Cole' has important in determining their behaviour.The data have also reminded us that electromagnetic measurements alone do not been described by a distribution of conductivity relaxation distinguish between conduction and polarization currents, as time^,^.^'^ but this significantly underestimates M* at high only the total current Jtotal= J, + aP/at, appears in frequencies. Part of the data on Ca(NO3),.2.9H2O and Maxwell's equation. Any separation of conduction current J, Cd(NO,), * 2.37H20 has been briefly reported before in a dif- and polarization current aP/dt should be made in other ways, ferent context," where an analysis in terms of a combination by, for example, justifiably describing the free electric charges of conductivity and polarization processes was deferred.' (ions) as mutually exclusive to bound ones (dipoles) on a Most of the dielectric data are new.certain timescale. (The timescale distinguishes a permanent ion pair from a transient ion-pair dipole.) The procedure thus introduces a dc conductivity, go, with J, = a,E and a polar- Experimental ization P = xoE where xo is the electric susceptibility and E The details of the equipment and measurement technique the electric field. Thus E/Jtota,= (l/a,)/(l + icoxo/ao),with a have been described before.".' Briefly, the permittivity, E', Maxwell's relaxation time formally equal to xo/o,, and co as and loss, E", were measured by means of a GenRad 1689 the angular frequency of the electric field. Variation in the digibridge, over the frequency range 12-10' Hz.The glass- form of the denominator of this equation leads to various transition temperature, %, was measured by differential scan- shapes of the spectra. Virtually all analyses of the dielectric ning calorimetry at a heating rate of 20 K min-'. The data of ionic solids2 (inorganic, organic or polymeric) are cur- uncertainties in the measurement of E' and E" are 0.1% and rently done without dividing the Jtotalinto its components. 0.5% respectively, of temperature is kO.1 K and of Tp is f1 This has led to a general description of the dielectric data, K. even for materials containing molecular dipoles, in terms of a All chemicals were AnalaR (BDH) grade. The electrolytes distribution of conductivity relaxation times arising from a were heated to a temperature 20-40°C above their melting stretched exponential decay of the electric field under the points and kept for 2 h.This caused a loss of water of crys- constraint of a constant displacement tallization. Their water content was determined gravimet- In recognition of the presence of molecular dipoles in ionic rically and by Karl-Fischer titration. The samples used in substances, and of the requirement from the law of chemical this study were transparent and remained in their super- equilibrium that ions and dipolar ion pairs coexist in fused cooled state at room temperature for several days without salts,' the analysis of M* is more appropriately done in terms crystallization.of two processes.6 It has been demonstrated6 that the dielec- tric properties of ionic solids are attributed to: (i) a Maxwel- Results and Data Analysis lian conductivity, i.e. with a single relaxation time and (ii) a dipolar relaxation with a Davidson-Cole type asymmetric Representative data on conductivity, 0, and permittivity, E', distribution of relaxation times.' Nevertheless, complex per- spectra of Ca(NO,), -2.9H20 at different temperatures are mittivity, &*, can be directly analysed by computing the inter- shown in Fig. 1, of Cd(NO,), 2.37H20 in Fig. 2 and of (Ca facial or electrode polarization effects and the dc conductivity + CdNNO,), -2.92H20 in Fig. 3. None of the spectra show a from the measured data.8.9 For several network oxide low-frequency plateau or shoulder in E' that could be associ- this analysis gave the same results as the M* ated with the limiting, low-frequency value, E~ The dc con- gla~ses,~ .analysis.6 ductivity plateau is discernible as an intermediate-frequency Here we report the dielectric properties of three electrolytes shoulder for some temperatures (as for example at 251.0 K in containing water (of crystallization) as molecular dipoles and Fig. 2; it is more clearly discernible on a linear scale plot of interpret these properties in terms of both E* and M* formal-a). Contributions from electrode polarization effects domi- isms. These substances are weak electrolytes, i.e. they contain nate at frequencies below this shoulder, and contributions dipolar ion pairs in thermal equilibrium with ions.The from conductivity and/or dipolar relaxation effects dominate results show that both conduction and polarization are at frequencies above it. , +\ +, +\ (c) -9 -5 -6 h-I E q -7 b Y Q,-0 -8 , v I I/(111 I 1 1 III'I' ' '')Ill" ' ''1IIII I 1"1111 i r88111a i iilllll I isr,sA-910 100 1000 10000 100000 frequency/Hz Fig. 2 Variation of ac conductivity and permittivity against fre- quency for Cd(NO,), .2.378,0 at different temperatures/K: (a) 239.1, (b)243.7, (c) 247.1,(d)251.0 -5 -6 rh I E q -7 b v Q, --8 -9 I 10 100 1000 10000 100000 frequency,/Hz Fig. 3 Variation of ac conductivity and permittivity against fre- quency for (Ca + CdXNO,), 2.92H ,O at different temperatures/K: e (a)234.5,(bl 238.3,(cl 242.1 Following Cole and co-w~rkers,'*~,~ we represent the elec- trode impedance, Zel,as a complex function, Z,*,= Zo(io)-", where 2, is the characteristic of the electrode/dielectric inter- face, and the exponent n is equal to 0.5 in the present case.This is equivalent to a 'constant-phase element' in series with the bulk dielectric properties of the material. For an electrode impedance, Zel, in series with a dielectric sample, the measured conductance, G,,,, in S, and permittivity, &beasare given byY8 Gmeas (GIC,) + im' (1)CO + IWE;,,, = 1 + Z,*,[(G/C,) + ico~']C, (this follows from two admittances, YT = G + ios'C, and Y; = l/Z,*,, being taken in series).In eqn. (l), C, is the capacitance of the empty cell (its value, which differed for dif- ferent measurements, was between 12 and 15 pF), o the angular frequency, E' the true or bulk permittivity (from dipolar reorientation) of the sample and G its true or bulk conductance, which is the sum of the dc and ac (or dipolar) contributions. For frequencies when G % OE'C,and lZ,*,I= 2, Go-" 4 1, eqn. (1) is expanded as a Taylor series, and after truncating at the first order it may be written as, co co and (3)L \ 1 L0-l J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 These are transformed to the equations, used here, EL,,, = &ic+ Eiip where the conductivity, a = a, + adip = e,G/C,. Here, e, is the permittivity of vacuum (=8.8514 pF m-I), a, and adip are the dc and dipolar contributions to a and &iC and &Gip are those to E".The last terms on the right-hand side of eqn. (2)-(6) represent the contribution from interfacial impedance (the units of Z, are formally i2 Hz" or S-' s-"). At fre- quencies low enough for ddip to be negligible and &Lip to be equal to the equilibrium value, E,, a,,,, varies as w-", and EL,,, as m-("+'), and for n = 0.5, as found here and in earlier eqn. (4) becomes : Before proceeding further, the validity of three conditions, namely, (i) G >> we'C,, (ii) 2, Gw-" + 1 and (iii) the impor- tance of terms higher than first order [these terms have been neglected in order to obtain eqn. (2) and (3) by Taylor expan- sion of eqn.(l)] was examined for the sets of data used for the calculations. For the lowest-temperature data sets, the conditions were satisfied by only one or two lowest-frequency data. Eqn. (1) was also Taylor-expanded to fourth-order terms to see if the inclusion of such terms would increase the number of data points needed to determine a. and 2,.Cal-culations with simulated (not measured) data showed that this did not increase the number of low-frequency data points needed, and that the effect of inclusion of such terms remained insignificant up to the frequency range of 1 MHz, which is higher than the frequency used here. The calcu- lations were done as follows. For a pair of a,,,, values at frequencies miand wj (j > i), ameas(mi) -ameas(mj) lr co= { 2, a;[ cos( -) -]}(w; 1/2 -0;'j2) (8)4 eo For four sets of wi or oj,there are six pairs of mi, wj.From these, an average value of 2,'; was calculated by using eqn. (8). For the same values of mi, a. was then calculated by substituting the Zoo; value into eqn. (7). 2, was then deter- mined by substituting go into Zoo;. The calculations were refined by independently adjusting a. and 2, by infinitesimal amounts until a. in eqn. (4) and &bip in eqn. (6)became constant with decreasing frequency at the low-frequency end of the spectrum. It should be pointed out that according to eqn. (6), as w -0, E' from electrode impedance, or (EL,,, -&hip), approaches co for all positive values of n. Thus, relatively small errors in the estimated a, and 2, will cause -&iip)to become positive (instead of remaining zero) as a+ 0.It did become positive in some cases, but only after a plateau value of &kip or E, had been reached. Few analytical procedures could entirely avoid this occurrence. We also realize that the measured values as such can be fitted to eqn. (1) by using computational fitting algo- rithms, a step that needs to be considered and justified in a manner that removes the subjectivity of parameter fitting. We chose to separate the contributions so that the physical meaning of each parameter could be brought to the fore in an old-fashioned manner. The data obtained from ca. 30 sets of isothermal spectra, particularly those which gave the needed information, are summarized in Table 1.Representative plots for the analysis of a,,,, into adc, dintand adipcontributions, and of E' into &Lt and &iipcontributions are shown in Fig. 4. Similarly, repre- sentative plots for the analysis of EL,,, and EL,,, into dc, inter- facial and dipolar contributions are shown in Fig. 5. The &, ~~and E& values obtained from the analysis are plotted in a complex plane in Fig. 6. These show a skewed arc or a depen-dence according to the empirical Davidson-Cole relaxation function,' Eo -E, (9)E* = E, + (1 + ioz,)B with values of e0, E,, p and fo (&"-peak frequency) sum- marized in Table 1. Other empirical stretched exponential relaxation functions such as the KWW Havriliak-Negami's empirical eq~ation'~with its extra parameter 1 -a, and Jonscher's equation" also seemed to fit the data.In view of the large errors associated with the resolved dipolar contributions (e.g. needing to subtract 100.09 from 101.00 to obtain the dipolar contribution to E" at 20 Hz), it seemed inappropriate to use the data for a comparison of the relative merits of the various equations. For simplicity we use eqn. (9). Fig. 7 shows an Arrhenius plot of the dc conductivity and the dielectric relaxation rate of eqn. (9) for the various glasses. The plots of a. are linear over the entire temperature range, but thefo data deviate from a straight line considerably at temperatures wheref, <lo0 Hz. The seven sets of E' and E" data at frequencies < 100 Hz have consider- able measurement errors and were not as reliable for deter- mining &iipand &iipafter subtraction of the very large contributions from electrode impedance and dc conduction particularly at the low-frequency end of the complex-plane plots where fo lies.The values of fo < 100 Hz are therefore meaningless. The values of AE are also approximate, but /3 values at such temperatures are precise. Practical corrections Table 1 Dielectric relaxation and other parameters and T, of several fused electrolytes" 252.9 Ca(NO,), * 2.9H,O 5410 7.8 (T, = 237 K) 32.0 5.0 0.28 249.2 1490 7.7 29.0 0.9 0.28 246.0 373 7.6 27.4 0.25 0.30 242.6 85.0 7.5 23.3 0.10 0.36 239.1 20.0 7.4 15.8 0.07 0.30 236.5 8.45 7.1 12.7 0.05 0.26 Cd(NO,), .2.37H20 (T, = 233 K) 25 1.o 1240 5.0 16.0 0.8 0.40 247.1 189 4.7 14.8 0.2 0.38 243.7 37.0 4.7 11.6 0.10 0.42 239.1 6.35 4.5 7.5 0.05 0.30 (Ca + Cd)(N0,)2* 2.928,O (7''= 235 K) 246.7 3910 7.4 26.0 5.0 0.30 242.1 663 7.3 24.0 0.7 0.28 238.3 115 7.2 21.6 0.15 0.28 234.5 25.0 7.0 16.0 0.07 0.28 ;* Errors are: *5% in 0,; kO.1 in E~ k0.5 in c0; k0.05 in fl and 20% info (= 1/2nz,). 2068 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3020 1 1 -lo{ I -121 I 1 I111111100 1 I (""" ' ' 'I'"" ' '")IIJ10 lo00 10000 100000 f req uency/H z Fig. 4 Analysis of measured ac conductivity (0)into dc (-), interfacial (+) and dipolar ( x ) contributions and measured permit- tivity (0)into interfacial (+) and dipolar (x) contributions for Ca(NO,), .2.9H20 at 239.1 K for an electrode impedance by an arbitrary (unmeasurable) quantity 2, become difficult whenf, approaches a range of frequency where the E' and E" data are less accurate. Discussion Dielectric properties of ionic materials and polymer electro- lytes are generally attributed to charge transport a10ne,~-~ and are analysed in terms of M*. ColeI6 has summarized the difficulties one encounters in such an analysis. Nevertheless, the analysis offers the advantage of conveniently reducing the overwhelmingly large contribution from interfacial polariza- tion and dc conductivity as it suppresses the features at the low-frequency side of the M* spectrum and accentuates those at the high-frequency side.For ionic materials and polymer electrolytes the M* analysis has led to two conclu-sions:2-4,6,10 (a) the dc conductivity is non-Maxwellian, i.e. instead of a single relaxation time it involves an asymmetric distribution of conductivity relaxation times according to a stretched exponential relaxation function,'2*'3 and (b)a faster relaxation process contributes to the dielectric properties, for the stretched exponential relaxation function underestimates the high-frequency data.3y4'6 Our analysis of the data is strictly in terms of a dc conduc- tion and dielectric or polarization process. It contrasts the commonly used approach, based on the KWW parameterPKWWfor dc conduction, in which a polarization process is assumed to be absent from the dielectric behaviour observed at 10-3-108 Hz, and/or Odc is not distinguished from ddip .2-4 It also differs from an earlier procedure6 in which the inter- facial polarization contributions to CT and E' were reduced to I" 9-\LU8-7-$ 6-yI3-\ ,+..' a h ooo frequency/Hz Fig.5 Analysis of measured E' and E" (0)into dc (A),interfacial ( +) and dipolar ( x ) contributions for (Ca + CdXNO,), * 2.928,O at 238.3 K virtually zero by converting E* into M*, although the results obtained there6 are identical to those obtained by the present method. The data can be analysed also in (total) complex imped- -7-(4 6-5-4-3-2-A-1-##--'+ + 0 A-I I I I 1 I I I I I I I I I -0451'") "i'"3 ++yt4--+-+.+2-1-/N+ \-'t 0 I I 1'1 I I I I I 1 I 'I I I I 1 I I 4 5 6 7 8 9 101112131415161718192021 2223 & Fig.6 Cole-Cole plots for (c) Ca(NO3),.2.9H,O (239.1 K), (b) Cd(N03),.2.37H,0 (243.7 K) and (a) (Ca + CdXN0,),.2.92H20 (238.3 K) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 4, A + 3.95 4.00 4.05 4.10 4.15 4.20 4.25 4.30 103 KIT Fig. 7 Arrhenius plots of the dc conductivity and relaxation rate for (a) Ca(NO,), .2.9HZ0, (b) Cd(NO,), .2.37HZ0 and (c) (Ca+ CdXNO,), * 2.92HZ0.Data for relaxation rates below 100 Hz are meaningless as discussed in the text. ance, Z* (= 2'-iZ), formalism. A complex plane plot of Z* for substances shows several depressed circular areas.All three representations are derivable from a single set of measurements, but the identification and interpretation of the frequency of maxima or other features with characteristic times led to quite different answers. This raises questions on the relevance of an intercomparison of the derived quantities against other characteristic times (e.9. NMR, mechanical, calorimetric) in the same material, and whether a self-consistent interpretation of these quantities is meaningful in terms of molecular processes. An example of this is the dielec- tric behaviour of 14.2mol.o/o LiC1-propylene glycol mixtures, to which both conduction and dipolar processes contribute, and yet the data in M* formalism can be mistaken for con- duction process alone with a temperature-independent PKWW parameter." In this study also we anticipate a dipolar contri- bution to E* from water dipoles, yet the data can be fitted to dc conduction alone with a distribution of conductivity relax- ation times" implicit in the formalism for the stretched expo- nential decay of electric field due to ionic diffu~ion.~ The data in Table 1 and Fig.7 give an activation energy of 210 f10 kJ mol-' for dc conduction. The Arrhenius equa- tion should describe the dielectric relaxation rate as well, but deviations from this equation may occur, as evident in Fig. 7, owing to at least two effects: (1) analytical errors associated with the use of limited data, particularly at the low-frequency end, cause large errors in the calculations offo at low tem- peratures, and (2) the possibility that the effective size of ion-pair dipoles becomes smaller as the density increases on decreasing the temperature.This reduction in size would increase the relaxation rate. In the absence of independent information on the size of ion pairs, it is difficult to resolve the importance of the second effect. In our experience, the dc conductivity can be estimated reasonably accurately from the data over the limited frequency range. Subtraction of very large contributions to E' and E" from electrode polarization and dc conductivity, as mentioned earlier, prevents an accu- rate determination of the dielectric relaxation time. The dipolar relaxation may be seen as an a process, in con- trast with the modes of localized diffusion observed in silicateg.'' and other glasses6 (i.e.far below their TJ,whose E* has been similarly analysed.E~ decreases with temperature, which is opposite to that required by the Curie law, AE (or E~ -E,) -C/(T-To). E, also decreases with temperature and is attributed to the decrease in both the strength of the high-frequency, or low-temperature, relaxation already observed" and the infrared contribution to E' for ionic solids.' The distribution parameter P decreases with decreasing temperature, or equivalently the half-width of the spectrum increases. Most of the changes in the dielectric parameters seen in Table 1 are not unusual, but the decrease in the ALEwith tem- perature is remarkable.One framework for this analysis is based on the Kirkwood-Frohlich equation,20.2 where Ndis the number density of molecular dipoles, k, T the thermal energy, po is the dipole moment of an isolated mol- ecule and g is the dipolar orientational correlation factor. A decrease of AEwith temperature is caused by either an over- whelmingly large decrease in N, (which usually increases on cooling) or a decrease in g or both. There is also a further effect here which is related to the ion-pair dipole formation in weak electrolytes. In such cases po decreases with the distance of separation between the ions in an ion pair, which depends upon the density of the material. Furthermore, concentration of such permanent ion pairs may decrease or increase depending on the magnitude of the parameters for the ion s ion-pair association constant,22 KA=-4x~a~exp( Z,Z, 2) 3000 a&g k, T where Z1 and 2, are the ionic charges, e the electronic charge, N the Avogadro number, and a the ion-size param- eter.KA , or equivalently, the concentration of ion-pair dipoles, is sensitively dependent upon the product go T, since a is expected to remain constant with changing temperature. Without further information on the po of ion-pair dipoles, we tentatively conclude that the decrease in AEwith temperature is due to a combination of these effects. To calculate KA , it is necessary to know E~ of different melts, which can be deter- mined more accurately when the concentration of water is high. In that case the strength of the dipolar relaxation associated with ion pairs would become an independent measure of their concentration.Our aim here is to suggest that the effects observed should occur in most ionic melts. Finally, it is instructive to compare the three types of analyses of the dielectric data represented only in the M* for-malism, namely: (i) with a distribution of conductivity relax- ation times using the KWW parameter,2-4s'0 (ii) with a Maxwellian conductivity and dipolar rela~ation~**.~ and (iii) with a distribution of conductivity relaxation times,23 but using the Davidson-Cole parameter.' Fig. 8 shows represen- tative spectra of M" for the fused electrolytes fitted to: (i) KWW asymmetric distribution of ionic conductivity relax- ation timeslo and (ii) dc conductivity and dipolar relaxation J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 analysed, the electrode impedance distorts the shape of the low-frequency part of the M’ and M” spectra. M” O.O4I0.03 n 0.06 0.05 0.04 M” 0.03 0.02 0.01 C 0.04 1 I log (frequency/Hz) Fig. 8 M” spectrum showing ine Maxwellian conductivity relax- ation and dipolar relaxation in (a)Ca(NO,), * 2.9H20 (246.0 K), (b) Cd(NO,), .2.37H20 (243.7 K), and (c) (Ca + Cd)(NO,),.2.928,0 (238.3 K). Rectangles are the measured values. The low-frequency peak is due to Maxwellian conductivity (A)and the high-frequency peak due to a polarization process ( x ). The calculated values for a distribution of conductivity relaxation times are shown by a contin- uous line.(a) M, = 0.125 and PKWW= 0.50, (b) M, = 0.203 and BKWW= 0.52 and (c) M, = 0.132 and PKWW= 0.50. Note that the last calculations underestimate the values at high frequencies. process with parameters given in Table 1 (fit of the Davidson-Cole equation to the M” data23 is not given, for although it fits the data better than the KWW parameter, it too implies a distribution of ionic conductivity relaxation times and ignores polarization effects from dipolar reorientation). As is generally ob~erved,~-~.~ the former fit is unsatisfactory at high frequencies. The latter seems to provide a satisfactory description of the dielectric data. We recommend that for separating ionic from polarization cur- rents in dielectric materials, analyses can be done either in the E* (Fig.4-6) or M* (Fig. 8) formalism, bearing in mind that each has its weaknesses: the analysis of E* data requires an assumed value of n, and the M* data conceal the effects of electrode impedance. When simulated (not real) data are Conclusions The dielectric properties of several fused salts containing water of hydration have contributions from both the ionic and dipolar diffusion, or polarization of water and ion-pair dipoles. Although the data can be described also in terms of a distribution of conductivity relaxation times without regard to dipolar diffusion, such a description seems inadequate and misleading. Direct analysis of the dielectric properties by separating the conduction and polarization effects is less concealing of the various processes than the analysis done after conversion of the E* data into an electric modulus formalism.For inter-preting the dielectric properties of ion-containing materials, it seems preferable to use E* directly so that the effects of elec- trode polarization can be separated and meaningfully inter- preted. References 1 R. H. Cole, J. Noncryst. Solids, 1991, 131-133, 1125. 2 For reviews of the experimental data, see C. A. Angell, Chem. Rev., 1990, 40,523; Annu. Rev. Phys. Chem., 1992, 43, 693; J. Wong and C. A. Angell, Glass, Structure by Spectroscopy, Marcel Dekker, New York, 1976, ch. 11. 3 P. B. Macedo, C.T. Moynihan and R. Bose, Phys. Chem. Glasses, 1972,13, 171. 4 C. T. Moynihan, L. P. Boesch and N. L. Laberge, Phys. Chem. Glasses, 1973, 14, 122. 5 R. A. Robinson and R. H. Stokes, Electrolyte Solutions, Butter-worths, London, 1959. 6 G. P. Johari and K. Pathmanathan, Phys. Chem. Glasses, 1988, 29, 219. 7 D. W. Davidson and R. H. Cole, J. Chem. Phys., 1951,19, 1484. 8 J. F. Johnson and R. H. Cole, J. Am. Chem. Soc., 1951,73,4536. 9 R. H. Cole and E. Tombari, J. Noncryst. Solids, 1991, 131-133, 969. 10 S. K. Jain and G. P. Johari, Phys. Chem. Glasses, 1989,30, 135. 11 G.P. Johari, Polymer, 1986, 27, 866. 12 R. Kohlrausch, Ann. Phys., 1887,12, 393. 13 G.Williams and D. C. Watts, Trans. Faraday SOC.,1970,66,80. 14 S. Havriliak and S. Negami, Polymer, 1967,8, 161. 15 A. K. Jonscher, Nature (London), 1977, 267, 673; J. Muter. Sci., 198 1, 16, 2037; Dielectric Relaxation in Solids, Chelsea Dielec- trics Press, London, 1983. 16 R. H. Cole, Annu. Rev. Phys. Chem., 1989, 40,1; J. Noncryst. Solids, 1991, 131-133, 11 19. 17 K. Pathmanathan and G. P. Johari, J. Chem. Phys., 1991, 95, 5990. 18 P. N. Huang and G. P. Johari, J. Mol. Liquids, 1993,56,225. 19 R. P. Lowndes and D. H. Martin, Proc. R. SOC.London, Ser. A, 1970,316,351. 20 J. G.Kirkwood, J. Chem. Phys., 1939,7,911. 21 H. Frohlich, Theory of Dielectrics, Oxford University Press, Oxford, 2nd edn., 1958. 22 C. W. Davies, Zon Association, Butterworths, London, 1962. 23 K. Pathmanathan and J. R. Stevens, J. Appl. Phys., 1990, 68, 5128. Paper 4/00182F; Received 12th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002065

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2071-2076 207 1 Structure and Transport in Concentrated Micellar Solutions with a Lower Consolute Boundary J. M. Keller and H-D. Ludemann Universitat Regensburg, lnstitut fur Siophysik und Physikalische Biochemie ,8400 Regensburg, Germany Gregory G. Warr* Department of Physical and Theoretical Chemistry, The University of Sydney, NSW 2006,Australia Self-diffusion coefficients and viscosities of aqueous solutions of dodecyltributylammonium bromide (C,,NBu,Br) have been measured as a function of concentration and temperature by pulsed-field-gradient 'H NMR and couette viscometry. These solutions form no liquid-crystalline phases, remaining isotropic at all com-positions, but demix above a lower critical temperature of 58°C between 8 and 80 wt.% solution.Diffusion results are first interpreted using the conventional approach in which the partition of both surfactant and water between bulk and micelle bound states is examined. An alternative approach to self-diffusion in concentrated surfactant solutions incorporating the viscosity results has proven more fruitful, revealing a change in solution structure near the high-concentration side of the two-phase body. Based on this a mechanism for the phase separation involving hydration of the surfactant is proposed and compared with previous results on this and related systems. The usual sequence of self-assembly phases encountered with increasing concentration in cationic surfactant systems such as dodecyltrimethylammonium bromide is micellar (spheres and/or rods), hexagonal, (cubic) lamellar.This is understood to be due to changes in the surfactant packing parameter, u/uol,, where u is surfactant tail volume, a, is the area per molecule at the aggregate surface and-I, is the fully extended chain length of the surfactant tail. As concentration is increased, so is the ionic strength, leading to more effective screening of the electrostatic interactions between head groups which largely determine uo. However, when the tri- methylammonium head group is replaced with a bulky, hydrophobic tributylammonium group, this sequence is inter- rupted.' Steric interactions between tributyl head groups within the same micelle constrain the surfactants to form spherical micelles at all concentrations.The result is a solu- tion that contains no liquid-crystalline mesophases, but only a single, isotropic solution from the c.m.c. up to at least 90 wt. % surfact ant. Small-angle neutron scattering (SANS) has confirmed that only small, spherical micelles are present in dodecyltributyl- ammonium bromide (C,,NBu,Br) solution between 1 and 80 wt.% over a wide temperature range.2 At moderate concen- trations and elevated temperatures (>58 "C in H,O or 48 "C in D20) the solution displays an unanticipated demixing into two conjugate solutions ca. 10 and 80 wt.% in surfactant. SANS spectra display the characteristics of long-range attrac- tions between micelles near the two-phase region. Recent modelling of the scattering spectra3v4 using a dispersion force interaction has shown an increase in the magnitude of the attractions with increasing temperature.This is hardly sur- prising, as lower consolute behaviour can scarcely arise from anything but an increasing attraction of some kind. The diffi- culty lies in the interpretation of this as an effective potential between micelles, without really understanding the mecha- nism. The same question has dogged non-ionic systems, which undergo a similar demixing on warming. Although they also have the added complication of asphericity, the molecular origin of the effective intermicellar attraction is still a subject of C,,NBu,Br solutions are thus of interest both for their existence as concentrated dispersions of spherical micelles and for the interactions which lead to lower consolute behav- iour.Both SANS and preliminary diffusion coefficient measure- ments by NMR have shown that the monomer concentration in C, ,NBu,Br systems increases with increasing total sur-factant concentration above the c.m.c.,p9 The resultant screening of electrostatic interactions between micelles by the equilibrium monomer solution nullifies electrostatic stabiliza- tion of the dispersion at concentrations above CU. 30 wt.%. Above this concentration the Debye length is only a few A. The result is that a short-range attraction is sufficient to cause the phase separation. (Note that steric interactions within micelles provided by the butyl arms about the quat- ernary nitrogens still maintains the sphericity of the micelles.) The monomer behaviour explains why such an attractive interaction can play such an important role in the phase behaviour of C,,NBu,Br solutions, but not its origin.In this study we have used pulsed-field-gradient NMR to measure the self-diffusion coefficients of both surfactant and water in C,,NBu,Br solutions over a wide range of tem- peratures and compositions in order to examine structural changes in solution which may be masked in SANS or QELS studies" by long-range concentration correlations near the phase boundary. Solution viscosities have also been deter- mined in order to provide further information on solution structure, and a basis for temperature scaling of the diffusion.Experimental Dodecyltributylammonium bromide (C ,NBu,Br) was pre-pared by reaction of bromododecane (Aldrich) with tri-n- butylamine (Aldrich) in acetonitrile. The crude product, which is extremely hygroscopic, was recovered by rotary evaporation followed by freeze drying. This was then purified by recrystallization from acetone-ether, and stored in a desic- cator out of the light. Viscosities were measured in a Deer constant stress rheo- meter using Couette geometry. The shear stresses were in the range 0.01-2.0 Pa, corresponding to shear rates of between 0.5 and 200 s-'. The maximum shear rate provides a guide to the shortest timescale probed, which in this study was 0.01 s. Samples in the rheometer were jacketed and thermostatted to within 0.2"C.All solutions studied were found to be Newto- nian over the entire range investigated. Self-diffusion coefficients of the solutions were studied in glass capillaries with inner diameter 1.5 mm and outer diam- eter of 7 mm. Details of the apparatus" and the filling procedure' have been published previously. The self-diffusion coefficients were obtained in a Bruker MSL-300 spectrometer operating at a proton frequency of 300.1 MHz in a home-built probe head in a Hahn spinwxho pulse sequence by the pulsed-field-gradient method introduced by Stejskal and Tanner.13 In the presence of the field pulses, the decay of the echo amplitude A is given by ~(2z)= A(o)~xP(- WG)~XPC-((Y~~)~D(A (1)-8/31] where z is the time between the 90" and 180" pulses, D the self-diffusion coefficient, 6 the duration of the gradient pulse, A the time between the two gradient pulses and g the gra- dient strength, given by g = kl.Here I is the current intensity and k the coil constant, which is obtained from the known diffusion coefficient of water at ambient pressure and 298 K14 and from benzene data, known from tracer measurements.' In all experiments z was between 100 and 300 ms, this corre- sponding to the time over which diffusion is probed. D was determined by recording 10-15 spin+choes with increasing g values while holding all other parameters con- stant. Subsequent Fourier-transformation of the echoes facili- tates the analysis of the data and permits detection of impurities.The measured diffusion coefficients are regarded as reliable to f5%. Results and Discussion Fig. 1 shows the self-diffusion coefficients of water and C12NBu3+ ion as a function of composition at various tem- peratures. Results at all temperatures and compositions studied are listed in Table 1. These results agree with prelimi- lo-* (a) .-J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 nary ~tudies,~ with both solute and solvent diffusion coeffi- cients decreasing as the surfactant content increases. The water self-diffusion coefficient decreases gradually across the composition range examined, whereas that of the surfactant decreases rapidly up to ca. 10 wt.%, then levels off and decreases only slowly with further increase in surfactant con- centration. All C12NBu3Br solutions demix on warming to above 60°C between 10 and 80 wt.% surfactant.Diffusion coefficients of both surfactant and water increase smoothly with increasing temperature as the phase boundary is approached (Table 1). It is usual to treat self-diffusion in surfactant solutions by considering the distribution of material between micellar and monomer states, together with diffusion in each of these states. In the analysis which follows we first take this approach, but finding the information obtained to be limited we also examine the effect of intermolecular interactions on the concentration and temperature dependence of the self- diffusion coefficients using a more general approach which incorporates solution viscosities.Based on these results, we propose a mechanism for the observed phase separation which is consistent also with previous data. Self-diffusionin Micellar Solutions :Two-state Model The measured self-diffusion coefficient of a surfactant in micellar solution is an average of its diffusion coefficients in the micellar and monomer states : D(Cl,NBu,) = f'mic 'mic(C12NB~J + (1 -f'mic)Dmonorner(C12NB~3) (2) where Pmicis the (mole) fraction of solute present in micelles and 1-Prnic is the fraction present as monomer.16 The diffusion of water in aqueous solutions has also been analysed using this two-state approach.' 7*18 Some fraction of the water is regarded as being bound to or associated with the micelle, f'hund, and is constrained to diffuse with the micelle.The rest, l-f'bund, is all viewed as free or bulk water. Thus we also have D(H20) = Pbound Dhund(H20) + (l -'bo~nd)~free(~2~)(3) Dfree(H20) and 'rnonorner(C12NBu3) in eqn. (2) and (3)describe the diffusion of small molecules in an environment containing various obstructions, which in this case are mono- meric surfactant molecules and micelles. Obstructions are a v) simple way of introducing interactions into diffusion behav- .^g3 10-12Iiour within a liquid which has been widely applied to micellar wt.% surfactant solutions. Jonsson et al. have made an extensive analysis of the self- diffusion of small molecules in colloidal systems' incorpor-ating the effect of large, essentially stationary obstructions like micelles.Large, spherical obstructions were found to reduce the diffusion coefficient of small molecules as Di = D,J(l + 4/2), where 4 is the volume fraction of obstructing particles. This agrees well with Monte Carlo simulations of diffusion in hard-sphere systems, but experimental results for model colloidal dispersions have shown that Difalls away more steeply than predicted above 4 = 0.2.'77'9 This expres- sion is independent of the size of the obstructions, although other forms apply for non-spherical species. 100 In these surfactant solutions, a two-state model including wt.% surtactant obstructions may provide a measure of changes in water Fig. 1 Self-diffusion coefficients of (a) C,,NBu,+ and (b)water as a structure with temperature, which has been postulated as the source of lower consolute behaviour in surfactants systems.function of concentration determined at various temperatures : .,20°C; 0,30°C; +, 40°C; 0,50°C; A, 60°C; A, 70°C; 0,80°C; Because the diffusion of CI2NBu3 is much slower than that 0,90"C; x ,100"C.Lines are included as guides to the eye. of water at all concentrations measured we neglect the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Diffusion coeficients of dodecyltributylammonium bromide-water mixtures‘ ~ ~~ surfactant : concentration (wt.%) T/”C 0.5 1.9 5 20.1 30.4 46.3 64.4 80.5 20 2.33( -10) 9.94( -11) 7.95( -11) 1.98(-11) 21.5 1.29(-11) 9.9q -12)25 2.w-10) 1.1q- 10) 30 2.84(- 10) 1.28( -10) 1.q- 10) 4.17( -11) 2.83( -11) 1.88(-11) 1.q- 11) 7.77( -12) 34 2.49( -11)40 3.43( -10) 1.63(-10) 1.26(-10) 5.46( -11) 3.91(-11) 3.35( -11) 2.59( -11) 1.25(-11) 50 4.07(-10) 2.w-10) 1.58(-10) 6.9q -11) 5.q-11) 4.q -11) 3.77( -11) 1.94(-1I) 60 4.94(- 10) 2.58( -10) 1.9q- 10) 8.91(-11) 6.98(-11) 5.94( -11) 5.06( -11) 2.92( -11) 61 7.21(-11) 62 7.45(-11) 6.06(-11)63 7.52(-11) 5.45(-11) 64 9.77( -11) 7.61( -11) 5.51( -11) 65 1.00(-10) 7.72(-11) 66 1.03(-10) 8.35(-11) 5.9q -11) 70 6.09- 10) 3.2q-10) 2.361-10) 4.38(-11) 80 7.95( -10) 4.2q-10) 2.82( -10) 5.89( -11) 90 1.08( -09) 5.77(-10) 3.55( -10) 8.26(-11) 100.5 1.58( -09) 8.41(-10) 4.48( -10) 1.14(-10) water: concentration (wt.%) T/“C 0.5 1.9 5 20.1 30.4 46.3 64.4 80.5 10 1.5q-09) 20 2.02( -09) 1.9q-09) 1.53( -09) 1.1q -09) 1.08(-10) 21.5 7.6q -10) 3.92(-10) 25 2.3q -09) 30 2.59( -09) 2.42( -09) 1.87( -09) 1.47(-09) 9.63(-10) 5.01( -10) 1.73(-10) 34 1.13(-09) 40 3.22(-09) 2.35( -09) 2.35( -09) 1.86(-09) 1.29(-09) 6.73( -10) 2.38( -10) 45 3.56( -09) 50 3.91( -09) 3.69( -09) 2.92( -09) 2.32( -09) 1.w-09) 8.52( -10) 3.24( -10) 53.5 1.73( -09) 55 2.52( -09) 1.78(-09) 9.55(-10) 56 1.83( -09) 9.9q -10) 57 2.62( -09) 1.87(-09) 1.02(-09) 58 2.73( -09) 1.87( -09) 1.04(-09) 59 1.94(-09) 1.w-09) 60 4.68(-09) 4.47( -09) 3.53( -09) 2.8q -09) 1.97(-09) 1.09(-09) 4.9q -10) 61 2.86( -09) 1.12(-09) 62 2.89( -09) 2.02( -09) 63 2.94( -09) 1.13( -09) 64 3.74( -09) 2.99( -09) 9.41( -10) 65 3.87( -09) 3.18( -09) 1.04(-09) 66 3.q -09) 67 3.47( -09) 70 5.45(-09) 5.5q -09) 6.3q -10) 80 6.29( -09) 6.13( -09) 8.15( -10) 90 7.42( -09) 1.06(-09) 100.5 8.10(-09) 8.41( -09) 1.49( -09) Numbers in parentheses are exponents.motion of these obstructions as both monomer and micelle In our previous investigation of this system, a hydrophobic and use the following form of eqn. (3) molecule was solubilised into the micelles, providing a tracer for their diff~sion.~ Whilst helpful at low concentrations, the probe was found to diffuse faster than the surfactant at high concentrations. This was attributed to solute transport by a jump mechanism between colliding micelles, due to the where D0(H20) is the diffusion coefficient of bulk water” strong attractions between them. It is therefore not possible and Dbound(H20) is the experimental diffusion coefficient of to measure a reliable value for Dmic in these solutions.In the surfactant at the composition and temperature of interest. order to obtain an estimate for Pmicwe have assumed that $ is the volume fraction of surfactant (in either monomer or micelles are stationary, or at least move so slowly as to act micelles) and is approximated by the weight fraction. Values only as obstructions to the diffusion of the monomer, i.e. of Pbound calculated from eqn. (3’) are shown in Fig. 2. Over is almost con- D(C12NBU3) = (l -Pmic)D0(C12NBu3)/(1 + $/2) (2’)the entire temperature range studied Pbound stant, decreasing only slightly with increasing temperature.where Do(C,,NBu3) is the diffusion coefficient of monomeric There is no major change in the amount of bound water as surfactant at infinite dilution, based on measurements made the consolute boundary is approached. below the c.m.c. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1.00, i AA AAAAI I -0.80 z)c 0 0 0 3 'D 0.60 C 30 * * * * * I)* 0.40- 0.-Y i,2 w- I0.20 C c z T O .a. .a .. ,a, TrC Fig. 2 Fraction of bound water in C,,NBu,+ solution, as a func- tion of temperature for various concentrations of surfactant (wt.%): a,5.0;0,20.1; +,30.4; 0'46.3; A, 64.4;A, 80.5 This assumption is probably reasonable in concentrated solution, but less so in more dilute systemsg Surfactant diffu- sion in concentrated solutions is addressed in more detail below.Like Phund,we find that Pmicis temperature indepen- dent within experimental uncertainty. Pmicand Phundas a function of solution composition at 30 "C are plotted together in Fig. 3. These results are representative of those obtained at all temperatures examined (cf. Fig. 2). Note the qualitatively different behaviour of the two components : the surfactant is partitioned mainly into micelles even at very low concentra- tions, whereas with water the binding of water to the micelles is more gradual. Nevertheless, even as low as 20 wt.% sur- factant over 40% of the available water is apparently bound to surfactant. At 80 wt.% solution there is essentially no unbound water according to this interpretation.An 80 wt.% surfactant solution corresponds to a surfactant : water mole ratio of 1 :6, which is less than enough water to hydrate a tributylammonium group fully.2 At such high surfactant concentrations there is no 'bulk' water, so conclusions based on binding and obstruction of water molecules are rendered meaningless. Similarly, the divi- sion of diffusion into free and bound components becomes less appropriate as surfactant concentration increases. When there is no bulk water in the solution, through what do we imagine surfactant monomers and water molecules now diffuse? Whatever the medium is, it is not the one we chose as our reference point for Do(C12NBu,), so little can be con- cluded about the structure of concentrated solutions from this approach.We therefore take an alternative approach to 1.oo A t IA A A A0.75I A A U A -23 0.50 C .-A4-2 0.25-A I0.00 0 25 50 75 100 surfactant (wt.%) Fig. 3 Fraction of bound water (A) and fraction of micellised sur- factant (A) in C,,NBu3+ solution, as a function of concentration of surfactant (wt.%) at 30 "C diffusion process, examining the interactions affecting D(C,,NBu,) and D(H20). Diffusion in Concentrated Interacting Systems In hydrodynamics it is usual to deal with gradient diffusion processes, which for a solution of components i,j, k, . . . has a unique mutual diffusion ~oefficient~~-~~ (4) where q is the viscosity, Rh the hydrodynamic radius, 4 the volume fraction, and pi the chemical potential of species i, providing the chemical potential gradient or 'thermodynamic force' for diffusion.K(i$, t) is a mobility coefficient which describes the hydrodynamic interactions in the system.2s Self- diffusion then is the motion of a tracer molecule (i$tracerz 0) identical with other molecules. It thus generates neither chemical potential gradient nor backflow or pressure gra- dient by its motion, and hence self-diffusion coefficients are largely independent of thermodynamic interactions in the system, except through K(i$,t). In general the self-diffusion coefficient of a component of a solution at a particular concentration i$ may therefore be written as Di = DoiKi(i$, t), where Doi = k, T/67rqRh is an infinite-dilution or Stokes' law diffusion coeficient.Each tracer particle i has its own unique mobility function Ki. The general problem of the calculation of Di or Ki remains unsolved, although for model hard-sphere dispersions good agreement has been found between theory and experiment for short time diffusion coefficient^.^^^^^ However, theory and experiment for long-time diffusion coefficients differ substan- tially even for these model system^.^' As it is the long-time diffusion result measured in this study, we must therefore confine ourselves to a less quantitative interpretation. The ratio of water to surfactant self-diffusion coefficients is W2O) -RlI(C12NBU3) K(H2O) D(C12NBu3) -Rh(H20) K(C12NBU3) (5) At infinite dilution this measures the radius ratio of the entities present.From previous work2 the radius of C,,NBu,Br micelles is known to be between 18 and 20 1$ throughout the single-phase region, giving a micelle :water radius ratio of 15-18, depending on the extent of hydration of the micelles. For monomeric C12NBu3 this ratio should be around 5-8; however, this would only be evident for experi- ments below the c.m.c. of the surfactant. Fig. 4 shows D(H20)/D(C12NBu,) over the full range of solution composi- tions and temperatures examined. As the concentration tends 20 40 60 80 '1 10 surfactant (wt.%) Fig. 4 Ratio of self-diffusion coefficients, water : C12NBu,+, as a function of concentration determined at various temperatures : .,20°C; 0,30°C; +, 40°C; 0,50°C; A, 60°C; A, 70°C; 0,80°C; 0,90"C; x ,100"C.Lines are included as guides to the eye. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 to zero, D(H,O)/D(C,,NBu,) decreases steeply towards ca. 15, then dipping suddenly to between 5 and 10 at 0.5 wt.%, consistent with expectations. The results at different tem- peratures all converge at low concentration, which is also expected since molecular sizes are independent of tem-perature. As surfactant concentration is increased two effects are observed. First, the D(H20)/D(C12NBu3) ratio increases and goes through a maximum near the critical composition of 46 wt.%.2 Secondly, the magnitude of this maximum decreases by almost 50% as temperature increases from 20 to 60°C.At even higher concentrations D(H20)/D(C, ,NBu,) again decreases at all temperatures and ventures into the upper range of the anticipated molecular size ratio. Compare Fig. 4 with the expected behaviour for a colloidal disper~ion.'~*'~?~~As the volume fraction of dispersed phase increases, diffusion coefficients of both the particles and solvent decrease until the particles become immobile at a volume fraction of 0.55. At this point the solvent diffusion coefficient is still at least half of its infinite-dilution value and the ratio, if plotted as in Fig. 4, would increase with volume fraction and diverge at 4 = 0.55. The present system differs from a colloidal dispersion in two ways: at low concentra- tions there is a changing partition of surfactant between monomer and micelle, causing the initial rapid rise in D(H20)/D(C1,NBu3).At higher concentrations the dynamic nature of the micelles permits enhanced diffusion of sur-factant by an exchange mechanism not available to the water.' This leads to the observed decrease in D(H20)/D(Cl 2NBU3). It is known from previous studies that there is an effective attractive potential between micelles which increases as the lower consolute boundary is approached.2 Without yet addressing the mechanism of attraction, it can be concluded that the frequency of sticky collisions between micelles must increase with temperature, and we would expect monomer transfer between micelles to increase, increasing D(C12NBu,) relative to that of water. This reduces D(H2O)/D(Cl2NBu,) with increasing temperature, as is observed (Fig.4). A more direct measure of interactions in solution may be obtained by combining diffusion coefficients of each com- ponent i of the solution with temperature and viscosity by comparing values of Diq/T. This quantity should reflect only changes in the (mean) size of the units, and their interaction potential through Ki.The results of Fig. 4 and a previous SANS investigation2 both suggest that micelle sizes are nearly constant up to at least 65 wt.% and are independent of temperature. Fig. 5 shows the reduced viscosity, qlq,,,,, ,of C,,NBu,Br solutions as a func- tion of concentration and temperature. Viscosity increases with concentration, as would be expected for any dispersion.For moderately concentrated solutions, the reduced viscosity decreases as temperature is increased, again suggesting the influence of micellar dynamics. Dq/T for both surfactant and water at various tem-peratures as a function of composition is shown in Fig. 6. Data for pure water are interpolated from ref. 20. Curves at all temperatures have a characteristic shape for each of the two species. For the surfactant the data at all temperatures lie on a master curve within experimental error, whereas for water trends with temperature remain evident, especially at high surfactant concentrations. For the surfactant [Fig. qa)] in the low-concentration region, Dq/T decreases with increas- ing concentration as the surfactant redistributes itself weight fraction surfactant Fig.5 Viscosity of C,,NBu,Br solutions as a function of concen- tration and temperature: W, 20°C; 0,30"C; +,40°C; 0,50"C; A, 60"C;A, 70 "C;a,80 "C. Lines are included as guides to the eye. between monomer and micelles. Above around 5 wt.% sur- factant, Dq/T increases monotonically with concentration at all temperatures studied, and any temperature trends are obscured by experimental scatter. This indicates a common structural progression with concentration for the surfactant aggregates, and that water plays the major role in determin- ing the temperature behaviour. In contrast with the surfactant behaviour, Dq/T for water [Fig. qb)] increases with surfactant concentration to a maximum at around 65 wt.%, before decreasing again.This decrease signals a change in R,(H,O), or more probably K(H,O), and hence in the solution structure below the two- phase body which has not previously been recognised. The temperature dependence of Dq/T of water, which can be seen from the spread of the data at a particular surfactant concentration in Fig. qb),is also revealing of solution struc- ture. Dq/T of water is independent of temperature over the range investigated up to 30 wt.% surfactant. This is typical of micellar solutions : the micelle/monomer partition of sur-factant is unchanged with temperature, hence the average Y 7 160 :120 Y I-7 80 02 40--.r-s 01 surfactant (wt.%) NI I I 60 E Y0 : 40 Iz I 20 40 60 80 100 surfactant (wt.%) Fig.6 Self-diffusion coefficients scaled by temperature and viscosity as Dq/T for (a) C,,NBu,+ and (b)water as a function of concentra- tion determined at various temperatures: W, 20°C; 0, 30°C; +, 40°C; 0,5OoC;A, 60°C; A, 70°C; a,80°C hydrodynamic size of the solute molecules remains the same, and intermolecular interactions are dominated by long-range electrostatic effects. The temperature dependence of Dg/T increases upon nearing the two-phase body, and is greatest at 65 wt.% where it decreases by a factor of two between 20 and 60°C. This also reflects the progressive failure of the conven- tional micellar solution description employed in the two-state model, eqn.(2’). In contrast with the qualitative differences in Dg/T, note that Pbunddisplays no temperature dependence (Fig. 2). There is also some evidence for a substantial change in solution structure in this concentration range from earlier SANS experiments on this system. In 80 wt.% solutions of C12NBu3Br in D20, the scattered intensity is far lower than would be expected compared with more dilute solutions of the same substance.2 In fact it is 20 times less than in an 8 wt.% solution, which is impossible to reconcile with a simple dispersion of micelles in water. An increasing monomer con- centration with increasing total surfactant adequately suc-cessfully rationalises the low scattering intensity, but at 80 wt.% surfactant this means a dispersion of micelles in a ‘solvent’ which is itself ca.50% by volume surfactant., At 65 wt.% total surfactant, the ‘solvent’ is only ca. 25% surfactant. The behaviour of Dq/T demonstrates that intermolecular interactions between surfactant molecules are only very little affected by temperature, whereas K(H20) is strongly tem- perature dependent. This provides direct evidence for the central role of water structure effects in the phase equilibria of this system, as we have previously suggested.’ In 80 wt.% solution there is insufficient water to solvate fully all of the surfactant present; the overall water :surfactant mole ratio is 6 : 1 and in the ‘solvent’ it is 24 : 1. Neither of these is suffi- cient to solvate the hydrophobic chains of the dodecyltri- butylammonium ion fully, so we envisage the H-bond network of water to be significantly disrupted.At 65 wt.%, the mole ratios are 13 : 1 overall, and 72 : 1 in the ‘solvent’. 72 is just the number of water molecules required to hydrate each of the monomers in the solvent fully.,* While the level of agreement is fortuitous, it does correlate with the high level of bound water found at this composition using the two-state approach. The water network at these intermediate composi- tions must be highly strained and ordered, as almost all water molecules are fully occupied solvating surfactant alkyl groups. We propose that phase separation occurs in order to relieve the strain in this fragile network, and that increasing temperature disrupts the order so that the network can no longer satisfy the solvation requirements of all the solute present.Evidence from previous work demonstrates that ‘v29’ solutions too dilute to phase separate are electrostatically sta- bilked, and contain a large amount of ‘bulk’ water. This work has shown that C,,NBu,Br solutions undergo a struc-tural change between 65 and 80 wt.%, and that this change affects water molecules more significantly. At 80 wt.% an extended H-bond network cannot be supported throughout the liquid phase. Mixtures below the two-phase body are thus trapped: they remain stable as long as the hydration struc- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ture of the surfactant molecules can be maintained. Once dis- rupted, as in the concentrated solutions, separation into conjugate phases becomes favourable; one containing 90% or more bulk water, and the other isolated water molecules in a solution of amphiphilic solvent molecules. We thank Mr.E. Treml and Mr. P. Ashburner for the prep- aration and purification of C,,NBu,Br. We also thank Mr. I. Luck, who performed the measurements of solution vis-cosities. Financial support from the Fonds der Chemie and the Australian Research Council is gratefully acknowledged. References 1 S. A. Buckingham, C. J. Garvey and G. G. Warr, J. Phys. Chem., 1993,97,10236. 2 G. G. Warr, T. N. Zemb and M. Drifford, J. Phys. Chem., 1990, 94, 3086. 3 C. Manohar and V. K. Kelkar, Langmuir, 1992,8,18.4 V. K. Kelkar, J. Narayanan and C. Manohar, Langmuir, 1992,8, 22 10. 5 J. A. Lum Wan, G. G. Warr, L. R. White and F. Grieser, Colloid Polym. Sci., 1987,265, 528. 6 B. Lindman, A. Carlsson, G. Karlstrom and M. Malmsten, Adv. Colloid Interface Sci., 1990,32, 183. 7 R. Kjellander and E. Florin, J. Chem. SOC., Faraday Trans. 1, 1981,77,2053. 8 R. Kjellander, J. Chem. SOC.,Faraday Trans. 2, 1982,78,2025. 9 M. Jansson and G. G. Warr, J. Colloid Interface Sci., 1990, 140, 541. 10 M. Drifford, L. Belloni and M. Dubois, J. Colloid Znterface Sci., 1987, 118, 50. 11 F. X. Preilmeier, E. W. Lang, R. J. Speedy and H-D. Ludemann, Ber. Bunsenges. Phys. Chem., 1988,92, 11 11 , 12 T. Vardag, F. Bachl, S. Wappmann and H-D. Ludemann, Ber.Bunsenges. Phys. Chem., 1990,94, 336. 13 0.E. Stejskal and J. E. Tanner, J. Phys. Chem., 1965,42,288. 14 R. Mills, J. Phys. Chem., 1973,94, 685. 15 A. F. Collings and R. Mills, Trans. Faraday SOC.,1970,66,2761. 16 B. Feaucompre and B. Lindman, J. Phys. Chem., 1987,91,383. 17 B. Jonsson, H. Wennerstrom, P. G. Nilsson and P. Linse, Colloid Polym. Sci., 1986,264, 77. 18 P. 0. Eriksson, G. Lindblom, E. E. Burnell and G. J. T. Tiddy, J. Chem. Soc., Faraday Trans. 1,1988,84,3129. 19 P. Venema, R. P. W. J. Struis, J. C. Leyte and D. Bedeaux, J. Colloid Interface Sci., 1991, 141, 360. 20 H. Weingartner, Z. Phys. Chem. NF, 1982,132, 129. 21 D. W. Davidson, in : Water-A Comprehensive Treatise, ed. F. Franks, Plenum Press, New York, 1973, vol. 2, ch. 3. 22 G. K. Batchelor, J. Fluid Mech., 1976, 75, 1. 23 G. K. Batchelor, J. Fluid Mech., 1983, 131, 155. 24 R. Bearman, J. Phys. Chem., 1961,65, 1961. 25 W. B. Russel, D. A. Saville and W. R. Showalter, Colloidal Dis- persions, Cambridge University Press, Cambridge, 1991. 26 W. van Megen, S. M. Underwood, R. H. Ottewill, N. St. J. Williams and P. N. Pusey, Faraday Discuss. Chem. SOC., 1987, 83,47. 27 A. van Blaaderen, J. Peetermans, G. Maret and J. K. G. Dhont, J. Chem. Phys., 1992,%, 4591. 28 P. H. Ashburner, B.Sc.(Hons) Thesis, University of Sydney, 1989. Paper 3/07591E; Received 30th December, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949002071

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2077-2083 Formation of Two-dimensional Structures from Colloidal Particles on Fluorinated Oil Substrate Genady S. Lazarov, Nikolai D. Denkov, Orlin D. Velev and Peter A. Kralchevsky* Laboratory of Thermodynamics and Physico-chemical Hydrodynamics, Faculty of Chemistry, Sofia University, 1 126 Sofia, Bulgaria Kuniaki Nagayama Protein Array Project, ERA TO, JRDC, 5-9-1 Tokodai, Tsukuba 300-26,Japan We propose a new type of liquid substrate, perfluorinated oil (F-oil), for the formation of two-dimensional arrays from colloidal particles. The appropriate conditions for particle ordering (experimental cell, type and concentra- tion of surfactants, etc.) are reported. Large and well ordered structures from pm-sized latex particles are obtained.Ordered clusters of globular protein (ferritin) macromolecules are also observed. The structures formed are directly transferred (after the F-oil evaporation) onto a solid substrate for subsequent study by means of optical and electron microscopy. The mechanism of the ordering process is studied and the advan- tages and disadvantages of the liquid substrates (in comparison with the solid ones) are discussed. Some possible ways for control of the ordering process and for improvement of the quality of the arrays are pointed out. The interest in ordered two-dimensional (2D) colloid struc- tures fixed on solid substrates'-" is stimulated by their poss- ible applications in optical device~~-~q~ some otherand techniques like data storage, microelectronics and synthetic membrane production.2D arrays from pm5-89' ' and sub- pm5999'0 latex particles, from a number of proteins and protein complexes' 2-25 and even from viruses,' 8,20 were obtained and analysed with respect to their structure and orientation. Since 3D crystals are obtained from a very limited number of integral membrane proteins,26 2D crys-tallization provides a unique possibility for investigating their structure.' 8,27-30 The experience gained during the experi- ments on formation of ordered two-dimensional structures '931932from colloid particles'-' and proteins' 2-25,33,34 shows that the choice of an appropriate substrate is of crucial importance for the ordering process. As a rule, the 2D arrays obtained at a single fluid interfa~e~.'~.~are built up by small domains comprising 1-34 no more than several hundred particles.Most probably the reasons are related to the specific mechanism of the ordering process controlled by diffusion, and to the small magnitude of the forces acting between the adsorbed particles. Some recent experimental studies3 report the formation of 2D structures of high quality from pm-sized latex particles in a Langmuir trough. It is not yet clear to what extent this method will be applicable for obtaining 2D arrays of high quality from sub-pm latex particles and biocolloids. Solid substrates'-20 (mica, glass, graphite, metals) can also be used for colloid particle array formation. In the case of latex particles it is proven7v8 that the ordering process starts when the thickness of the water layer containing the particles becomes approximately equal to the particle diameter.It is ~hown~.~that the mechanism of the ordering process consists of two stages: (i) nucleus formation and (ii) array growth. The first stage is governed mainly by lateral capillaryforce^^'-^^ between the particles that appear when the liquid layer thickness becomes equal to the particle diameter. As shown the~retically~'-~~ the corresponding attractive energy can be many orders of magnitude larger than the thermal energy, kT, even for nm-sized particles. The second stage (array growth) is connected with the evaporation of water from the already ordered region^.^*^ Water evaporation causes the appearance of particle flux directed from the disor- dered toward the already ordered regions, see ref.7 for details. Thus new particles are continuously reaching and sticking to the boundary of the ordered array. The control of the water evaporation rate turned out to be a convenient tool for improvement of the array quality and for overall control of the ordering process. It is possible that similar factors play an important role in the experiments with proteins.*' The investigations of the mechanism of 2D array formation on solid substrate^^'^ also revealed the main shortcomings of these substrates. The two most important ones are the rough- ness of the solid surface and the irreversible sticking of the particles against the substrate before their incorporation inside the array.The first problem becomes very important for small particles (in particular, biomolecules). In the works of Harris et a1.12-'4 it was overcome by using a cleaved mica for substrate (the so-called 'mica spreading technique'). The sticking of the particles causes the appearance of defects in the 2D structure and makes impossible the application of any 'annealing' procedure for improvement of the quality of the obtained array. The method developed by Nagayama and co-fo r array formation in a thin aqueous layer on a mercury substrate combines the advantages of both aforementioned types of substrate. On the one hand the mercury surface is molecularly smooth and tangentially mobile.On the other hand, the immersion capillary forces and the hydrodynamic flux are most probably the important factors for the process of ordering. The 2D structure obtained is then successfully transferred to a solid support for fixation and investigation by electron microscopy and image recon- struction. Ordered arrays from a large number of proteins, latex particles and viruses were obtained by this rneth~d.~' The application of mercury as a substrate needs a special experimental pr~cedure.~~.~' The high surface tension of mercury demands ultra-clean experimental conditions because the mercury surface is easily contaminated. The necessity of a low-humidity oxygen atmosphere and the cleaning procedure require complicated equipment.This sug- gests that one might search for some alternative liquid as a substrate which can combine the advantages of mercury with simpler experimental conditions. The aim of this study is to investigate the possibility of the application of perfluorinated oil (F-oil) as a liquid substrate for 2D array formation. Experiments with pm-sized latex par- ticles, which allow direct observation of the ordering process, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 and with the globular protein ferritin are performed. We use perfluoromethyldecalin (PFMD) which possesses some of the appropriate features of the mercury substrate (molecularly smooth and tangentially mobile surface) and some additional advantages : (i) it is chemically inert and ha~ardless,"~~' (ii) it allows the merging and rearranging of already ordered domains into larger ones, (iii) the 2D structure formed can be gently deposited onto another surface after evaporation of all F-oil, and (iv) it is difficult to contaminate the fluorocarbon surface (the common surfactants adsorb poorly at the fluorocarbon/water interface4').As a final result, large and well ordered domains from latex particles and clusters from ferritin molecules are obtained under appropriate conditions and after that are transferred onto the solid substrate. Experimental Materials We use perfluoromethyldecalin, PFMD (commercial name Flutec PP-7, Rhone Poulenc S.A.) as a substrate. PFMD has a higher density than water.It has high vapour pressure at room temperature and can be easily evaporated after the for- mation of a 2D array. Thus the obtained array can be trans- ferred to a solid substrate for fixation and investigation, see Fig. 1. Some of the relevant physico-chemical properties of PFMD are: density, p250c= 1.94 g ~m-~; boiling tem-perature, Tb= 141"C; melting temperature, T, = -10"C; insoluble in water or liquid hydrocarbons; molecular mass, M, = 512.1; refractive index, n:' = 1.315; relative permit- tivity, E, = 1.99; surface tension, Q = 19.2 mN m-'; inter-facial tension against pure water, y = 53.4 mN m- '. To ensure the spreading of drops from the latex suspension or ferritin solution on the fluorinated oil one should use appropriate surfactants (see, e.g., ref.42 and references therein). We chose the surfactant perfluorononyl-pol yoxyet hylene, C,Fl,CH2CH(OH)CH2(OCH2CH,),0CH3, abbreviated hereafter as PFPE (M, = 995). This substance was labor- atory synthesised and used without additional purification. As a cosurfactant we used the fluorinated alcohol C,F,,(CH,),OH with M, = 464 (Daikin Kougyou Ltd., Japan). Since saturated perfluorocarbons and some fluorinated sur- factants have found biomedical application^,^^*^' one can conclude that these substances do not denature the proteins in the organism. Nevertheless, we cannot be sure that the fluorinated surfactant and cosurfactant used by us do not affect the structure of the ferritin molecules. A more detailed investigation on the protein structure after 2D array forma- tion requires special techniques like electron diffraction and capillary.water \ forces evaporation / F-oil solid substrate F-oil evaporation8 Fig. 1 Experimental system: (a) 2D ordering of particles in an aqueous layer over F-oil substrate; (b) the formed 2D array is de- posited on a solid substrate after the evaporation of water and F-oil image reconstruction and can be the subject of a separate study. The latex suspension used (JSR, Japan) contained particles of diameter 1.70 pm with concentration 1 wt.%. In some experiments glucose (Chameleon, Japan) was used without additional purification. The experiments on 2D protein array formation were per- formed with aqueous solutions containing g ml- ' ferri-tin (Sigma) and 0.15 mol 1-' NaCl. The ferritin was purified by gel chromatography (gel Sephacryl S300, Pharmacia) and filtered through a 0.22 pm filter unit (Millipore) just before use. The purification quality (in particular, the absence of aggregates) was controlled by measuring the size of the par- ticles by means of a dynamic light scattering method (Malvern 47OOC, Malvern Instruments Ltd.).The main frac- tion used in the experiments consisted of single molecules of diameter 12 f0.5 nm. The acidity of the solutions corre- sponded to pH 5.4 & 0.1 as measured after the purification procedure. Methods Wetting of PFMD by an Aqueous Phase To find appropriate conditions for the formation of a stable aqueous layer on the PFMD substrate we carried out pre- liminary experiments on the spreading of water drops on the surface of PFMD.Certain quantities of the surfactant and cosurfactant were dissolved in PFMD. The PFMD phase was placed in a cylindrical glass vessel of diameter 3.5cm and volume 35 ml. Then 20 p1 drops of pure water were gently deposited in the central part of the fluorocarbon surface. These drops spread over a certain area and the radii of the formed aqueous lenses were used as a measure of the wetta- bility of the PFMD phase by water. The average diameter of pure water drops spread over pure PFMD was about 3 mm. The drop diameter increased up to 17 mm with the increase of the surfactant (PFPE) concentration.At PFPE concentra- tions above mol 1-' the fluorocarbon phase became turbid and the drop diameter did not increase any more. This means that the solubility of PFPE in PFMD is about 10-3 moll-and it is meaningless to use higher surfactant concen- trations. We improved the wettability further by adding cosurfactant (the fluorinated alcohol) to the system. Fluori- nated alcohol alone did not change the wettability even at concentrations as high as lo-' mol 1-'. However, the mixture of PFPE and alcohol ensured better spreading of the water droplet, i.e. the fluorinated alcohol plays the role of a cosurfactant. For the experiments with latex and ferritin we used 5 x mol 1-' alcohol and lop3 mol 1-' PFPE which corresponded to a spread water drop of 22 mm diam- eter. As discussed below, this composition provided spread- ing of aqueous layers which were very stable and did not rupture during the experiments with latex and ferritin.Note that the system is very sensitive to the nature and concentration of the surfactants. A deviation from the optimal composition quoted above changes substantially (for the worse) the spreading and 2D array formation behaviour. Experimental Cell and Method for 20 Array Formation The experimental cell used for obtaining ordered structures is a modification of the one described previ~usly.~ It consists of a Teflon ring which is pressed against a glass plate. To design an appropriate cross-section of the Teflon ring for the present study we performed experiments on the wetting of Teflon by water and F-oil.These experiments showed that the pure water formed a three-phase contact angle air/water/Teflon close to 90°, which substantially decreased when the water phase is pre-equilibrated with F-oil containing fluorinated J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 surfactants. The independently measured oil/water/Teflon contact angle was very close to 90". These experiments suggest the use of a Teflon ring of par- ticular cross-section, as shown in Fig. 2. The inner wall of the Teflon ring is cut in a way which allows the three-phase contact line air/water/Teflon to be 'attached' to the concave corner of the wall. Thus, by loading the cell with appropriate volumes of F-oil and suspension one can ensure the forma- tion of a thin, almost flat, aqueous layer with slightly concave upper surface (Fig.2). As shown previously7 such a configu- ration ensures better ordering of the latex particles. For control of the water evaporation rate the cell was covered with a thermostatted glass container. We were able to vary the evaporation rate and the array formation rate by controlling the temperature of the water circulating through the glass ~ontainer.~ To form sufficiently thin aqueous films over the F-oil sub- strate we controlled the oil level uia ejection and injection of oil by means of a microsyringe. Taking into account that the three-phase contact line ferritin solution (latex suspension)/ Teflon/air is fixed at the concave corner of the wall, the varia- tion of the oil level results in different meniscus shapes which, in turn, is important for controlling the process of array for- mation.By sucking out some oil we succeeded in promoting the thinning of the aqueous film. In the experiments with latex particles the ordering process was observed from below by a metallographic type optical microscope in transmitted or reflected light. The final 2D array, deposited on the glass plate after the water and oil evaporation, was studied and photographed by an optical microscope (Optiphot 2, Nikon) and by a scanning electron microscope (SEM). The samples for SEM (JEOL, Superprobe 733) were prepared by vacuum-coating with thin layers of carbon and gold.In the experiments with ferritin the following procedure was used. We loaded the cell with the necessary amounts of F-oil and ferritin solution and waited for ca. 1 h to allow the ferritin molecules to be adsorbed at the water/air surface. Then we sucked out some F-oil which gave rise to a fast film thinning. At a given moment (when the water layer thickness became smaller than ca. 1 pm) Newton rings appeared, owing to interference of reflected monochromatic light from the lower and upper surfaces of the aqueous film. If we did not change the oil level in the initial stage, it would take a long time (>7 h) for water to evaporate before observation of the first Newton rings. The reason for this sluggish thinning of the aqueous layer is the decreased evaporation rate due to the adsorbed monolayer of ferritin molecules at the air/water interface.The obtained ferritin-containing aqueous films were stable and did not rupture during the thinning process. After film formation the system was left for a period of a few days to allow total evaporation of the water and oil. In order to prepare samples for electron microscopy we put a specimen grid at the bottom of the cell before loading it with F-oil and ferritin solution. The specimen grid was coated with Formvar plastic film. After drying, the ferritin structures formed in the aqueous film were deposited directly on the specimen grid and were studied by transmission electron microscopy (Hitachi H-500). Results and Discussion Description of the Main Stages of the 2D Array Formation Process After loading the cell with a latex suspension one observes the formation of a slightly concave aqueous layer (Fig.2) similar to the one in the earlier experiments on glass sub- strate~.~~"The water evaporates and the layer thins with time. At the moment when the layer thickness becomes equal to the particle diameter many small 'islands' of ordered par- ticles form in the centre of the substrate. If two such islands approach each other closely, so that the aqueous menisci formed around them overlap, they attract each other due to lateral capillary force^,^^-^^ merge and form one large island. The latter often represents a single, well ordered domain. Later the water in the central zone of the substrate is totally evaporated and the islands become dry.After that moment they no longer coalesce upon collision. This can be explained with the irreversible coagulation of the neighbouring particles within an island when the water is evaporated. It is known that the polystyrene latex particles immersed in water are strongly charged owing to dissociation of ionic surface groups. This surface electrical charge gives rise to a strong electrostatic repulsion between the particles as long as they are immersed in the aqueous phase. The electrostatic repul- sion disappears after drying, and the particles irreversibly adhere to each other owing to the van der Waals attraction. At the same time a circular meniscus is formed around the central zone of the substrate where the layer thickness is still larger than the particle diameter.At the boundary of this meniscus small groups of particles are formed (Fig. 3) which slowly move towards the centre of the substrate. During this motion the water is evaporated from between and around the particles and they are pressed against each other by the lateral capillary forces acting in the thin aqueous layer. The shape of the liquid meniscus is visualized in reflected mono- chromatic light by the Newton interference fringes around each group of particles (Fig. 3). When two such groups come Fig. 2 Experimental cell for 2D array formation on F-oil substrate: 1, glass plate; 2, Teflon ring; 3, glass container for controlling the water evaporation rate; 4, micro-syringe for varying the oil level and the meniscus shape; 5, jacket through which thermostatting liquid flows Fig.3 Groups of ordered particles (formed when the layer thickness becomes equal to the particle diameter) are seen as dark spots. When two such groups approach each other closely they coalesce owing to the lateral capillary forces. The meniscus shape is visualised in reflec- ted monochromatic light by the interference fringes. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 closer together, they merge into a larger group (this process is also driven by lateral capillary forces), which can rearrange in a single domain (but only if there is water around the particles). With time the central zone is filled with such more or less well ordered groups of particles.After a certain period of time one observes the formation of a ‘2D foam’ of particles which encircles the region of the already formed islands (Fig. 4). The observations with a high magnification objective show that this foam consists of circu- lar fields free of particles surrounded by narrow ordered domains. In reflected light, the presence of a concave aqueous surface is observed in the regions free of particles. A similar 2D foam was observed previously’ with latex particles on a glass substrate in the presence of water-soluble surfactant (sodium dodecyl sulfate). Note that similar structures are observed also with proteins on mercury, see Fig. 4 in ref.25. This type of structure can be formed due to the action of capillary forces. During the later stages of the ordering process one can observe the formation of a ring-shaped homogeneous particle monolayer. This process resembles very much the corre-sponding stage of array formation on solid substrates. The particles carried by the convective water flux are moving in a radial direction towards the centre of the substrate and upon reaching the ordered zone are incorporated in it. Large and well ordered monolayers are formed during this process, see Fig. 5. Transitions from a monolayer to a bilayer and vice versa are observed. During the final stage of water evaporation, multilayers are formed in the close vicinity of the cell wall.After the evapo- ration of the water from the F-oil surface, the oil itself starts to evaporate. Thus, the latex structure formed gradually approaches the upper surface of the glass plate and after complete evaporation of the oil, the latex array is transferred to the solid substrate. In summary, the experiments described show that a 2D latex array can be formed over the F-oil substrate. They also prove that if the groups of particles are not completely dry, they can merge upon collision to form larger domains. The results imply that one must strive for precise control of the process to obtain very large (mm size) ordered domains of high quality. Control of the Process Control of the Evaporation Rate The experiments described in the preceding section show that 2D array formation is a complex dynamic process.We should be able to control the experimental conditions during every stage of the process. Our experiments showed that it is preferable to decrease gradually the temperature at the top of the cell during the ordering process. Thus by varying in a vertical direction the temperature gradient inside the cell we were able to regulate the particle flux from the periphery towards the ordered regions in the central zone (see ref. 7 for details). One of the main reasons for obtaining the 2D foam described above was the low intensity of the convective par- ticle influx. By gradually decreasing the temperature of the water circulating in the glass container above the cell (see Fig.2), condensation of water on the glass cover is initiated. Thus the water vapour pressure in the cell is decreased, the evaporation from the aqueous film is increased and the influx of water from the thicker periphery towards the ordered par- ticle structures is accelerated. In this manner we were able to ‘compress’ the 2D foam into the central zone, and to enlarge the well ordered regions. Fig. 5 shows an ordered monolayer of particles obtained in this way. Photographs are taken by using optical microscopy and SEM. A more rapid decrease of the temperature at the top of the cell results in the formation of multilayers. On the other hand, an increase of the temperature (i.e. suppression of the evaporation) leads to a transition from dense monolayer for- mation back to 2D foam formation and even to a cessation of the ordering process.In conclusion, control of the evapo- ration rate can be a useful tool for improvement of array quality. Impact of Glucose upon the 20 Array Quality The addition of a small amount of glucose (0.1 wt.%) does not change significantly the process and the final result. A high concentration (10 wt.%) completely stops the evapo- ration and no ordering is observed even for a period of up to 24 h. A medium concentration (0.25 wt.%), however, turns out to improve the array formation process. As expected, the evaporation from very thin aqueous layers containing glucose was slower. Because of the decreased drying of the water Fig. 5 Large, well ordered domains of 1.7 pm latex particles obtained in the presence of glucose (0.25 wt.%) by regulation of the Fig.4 Formation of a ‘2D foam’ in the region where the thickness evaporation rate: (a) optical microscope view; (b) SEM micrograph.of the water layer is equal to or smaller than the particle diameter. A slight deformation or shrinking of the latex particles may take The bright spots are areas free of particles. place in the vacuum chamber of the electron microscope. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Fig. 6 View of the transition zones monolayer-bilayer-triple layer, 1A-2A-3A, obtained on the F-oil substrate. No tetragonal lattices are seen at the transition boundaries. layer between the smaller groups of particles, their coalescence into larger domains was more pronounced. As a result, very large, well ordered domains, comprising more than 15 O00 particles, were obtained.Effect of the ‘Softness’ of the Substrate Surface The ordering process on F-oil shows many similarities with that on a glass s~bstrate.~ However, there are several differ- 1A 20 2A 4 m4 *4 m 1 --1-Fig. 7 Schematic presentation of the transition zone between mono- layer and bilayer: (a) on a solid substrate and (b) on an F-oil sub- strate Fig. 8 Subduction of two approaching ordered domains takes place upon collision (instead of merging) when the evaporation rate is rather high; scanning electron micrograph 208 1 ences between the two cases. They are connected mainly with the ‘softness’ (vertical flexibility) of the surface of the F-oil substrate and are briefly described and discussed below.Almost Complete Absence of the Square Lattice It has been demonstrated7.’ that when using glass plate as a substrate, multilayers can be obtained with the following sequence of layers: lA-20-2A-30-3A--.-. Here the ciphers correspond to the number of layers and the symbols mean hexagonal (A) or tetragonal (0)packing of the par- ticles. This order exactly coincides with the phase diagram calculated and observed by Pieranski et al.43 for an equi- librium system of charged particles confined inside a thin liquid layer between two solid plates. In the present experiments we have not observed tetrago- nal lattices between the hexagonally packed domains, see Fig.6. Inspection of the transition regions between monolayers and bilayers of hexagonally packed particles by SEM con-firms the absence of a developed tetragonal lattice in between them. The lack of a tetragonal lattice is most probably a result of the mobile and ‘soft’ oil/water interface which can bend at the transition zone between the multilayers [cf: Fig. 7(a)and (b)]. Subduction of Ordered Domains One under the Other: ‘Superlattices’ As described above, two ordered groups of particles can merge when they approach each other closely (unless they have already dried). In such a way, larger and well ordered regions were formed. However, if the evaporation rate is rather high, the approaching domains (which are almost dry) can tuck one underneath another upon collision instead of merging and rearranging (see Fig.8). If two hexagonally packed monolayers (placed one over the other) have lateral axes of symmetry, rotated with respect to each other at an angle not divisible by 60°, they form a peculiar structure which we term the ‘superlattice’. Superlattices exhibit an interesting optical property, known in the literature as the ‘moire effect’.44 For instance, in Fig. 9(a) one can see regions of hexagonal structure with different unit-cell constants (but which are always larger than the constant of the particle monolayer). The presence of superlattices with different unit- cell constants (Fig. 10) is due to the different relative orienta- tions of the two layers in the different domains.44 To our knowledge, this is the first observation of such a phenomenon in experiments on 2D array formation from latex particles.‘Black Holes ’ We sometimes observe the spontaneous formation of large 3D agglomerates of latex particles which we term ‘black holes’. They are seen as dark spots in transmitted light and, like their cosmic namesakes, draw in all particles nearby and bring about intensive hydrodynamic fluxes. We hypothesise that the ‘black holes’ form due to the pres- ence of some larger particle (a result of the polymerisation procedure during latex production) or particle aggregate in the water layer, which then bends the two film surfaces. Since the oil/water interfacial tension is rather low, the oil/water interface gives way and becomes concave around the larger particle (or aggregate).The neighbouring latex particles (which are slightly heavier than water) tend to fill this con- cavity, which, in turn, spontaneously grows and sinks into the oil phase. The larger the number of particles gathered, the greater the sagging of the oil/water interface. We tried to avoid the formation of such features because they disturbed the formation of good 2D arrays. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Fig. 9 Photograph of ‘superlattices’ with different unit-cell con- stants (due to the different relative orientation of the two overlapping layers) in transmitted light: (a) overview; (b) superlattice at higher magnification Fig.10 Illustration of the superlattice constant dependence on the relative orientation of two overlapping layers Fig. 11 Transmission electron micrograph of ferritin structures con- taining ordered regions 2DFerritin Structures We studied by transmission electron microscopy (TEM) specimen grids with deposited ferritin structures. In some cases we observed groups of hexagonally ordered ferritin molecules (see Fig. 11). The lattice constant measured from the photographs was ca. 13 nm, which is close to the diam- eter of the ferritin molecules. Note that a partial destruction of the Formvar film took place during the observation of the sample by TEM. Deposition of the ordered structures Onto the microscope ‘pecimen grid (after F-oil evaporation) was also employed as a method of TEM sample preparation.The deposition process could be additionally promoted by gentle ejection of the oil after 2D array forma- tion. We hope that further experiments, with more precise control of the conditions, will allow us to obtain a 2D protein array of higher quality. Conclusions The experiments performed demonstrate that F-oil substrates can be applied to the formation of 2D ordered structures from latex particles or ferritin molecules. Under appropriate conditions, larger and well ordered 2D arrays from latex par- ticles can be obtained, see Fig. 5. The arrays can be trans- ferred onto solid substrates without damage after complete evaporation of the F-oil, Fig.1 and 5. The results confirm that the capillary forces and the convective particle flux are the main factors governing the 2D array formation process. The quality of the arrays was increased by controlling the evaporation rate and the meniscus shape. Unforeseen pheno- mena like domain subduction, the moire effect and ‘black holes’, which are connected with the specific properties of the oil/water interface, were observed by optical microscopy or SEM. Ordered 2D clusters from ferritin molecules were obtained in a similar way. They were then deposited directly onto the specimen grids and studied by TEM. A further development of this experimental technique can lead to obtaining larger ordered domains from colloidal particles, i.e.latex microspheres, globular or membrane proteins. This study was supported by the Research and Development Corporation of Japan (J.R.D.C.) under the Nagayama Protein Array Project of the Program ‘Exploratory Research for Advanced Technology’ (ERATO). The surfactant, PFPE, and the cosurfactant were kindly supplied by Dr. M. Matsu- mot0 from Kyoto University- We gratefully acknowledge the preparation of the electron microscope photographs by Dr. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2083 L. Surchev, Dr. C. D. Dushkin and Mr. Tz. Iliev and useful discussions with Dr. H. Yoshimura. References 21 22 23 H. Yoshimura, S. Endo, M. Matsumoto, K. Nagayama and Y. Kagawa, J. Biochem., 1989,106,958. H. Yoshimura, M. Matsumoto, S. Endo and K.Nagayama, Ultramicroscopy, 1990,32, 265. T. Akiba, H. Yoshimura and K. Namba, Science, 1991, 252, 1 2 3 4 5 T. Alfrey Jr., E. B. Bradford and J. W. Vanderhoff, J. Opt. SOC. Am., 1954,44,603. I. M. Krieger and F. M. ONeill, J. Am. Chem. SOC., 1968, 90, 3114. J. W. Goodwin, R. H. Ottewill and A. Parentich, J. Phys. Chem., 1980,84,1580. H. W. Deckman and J. H. Dunsmuir, Appl. Phys. Lett., 1982,41, 337. H. W. Deckman, J. H. Dunsmuir, S. Garoff, J. A. McHenry and D. G. Peiffer, J. Vac. Sci. Technol. B, 1988,6, 333. 24 25 26 27 28 29 1544. N. Ishii, H. Taguchi, M. Yoshida, H. 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ISSN:0956-5000    

DOI:10.1039/FT9949002077

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2085-2093 Determination of Aggregate Structures by Combined Light-scattering and Rheological Studies Stephen D. T. Axford" and Thelma M. Herrington Department of Chemistry, University of Reading, Whiteknights, Reading, Berkshire , UK RG62AD The structures of aggregates of a clay, sodium bentonite, formed during the rapid (diffusion-limited) aggregation of a destabilised colloidal suspension have been studied using a combination of three experimental techniques. The first two of these were static and dynamic (quasi-elastic) light-scattering measurements, while the third involved rheological studies. The system was studied as a function of the pH of the aggregating suspension, between pH 2.3 and 10.2. Analysis of the light-scattering results leads directly to a determination of the fractal dimension, d,, of the aggregates formed.The value of d, for the aggregates showed a rapid transition from 3.0 below pH 4.3, to 1.8 above pH 4.3. This implied a close-packed structure for aggregates formed under highly acidic conditions, but a more open one in less acidic and in slightly alkaline suspensions. Rheological measure- ments showed minima in the Bingham yield stresses for both aggregated and unaggregated bentonite suspen- sions, at the cross-over point, pH 4.3, between the two structures. In addition, the ratio of the storage modulus to the loss modulus took only two values: a higher one below pH 4.3, and a lower one above pH 4.3. It was concluded that hetero-flocculation is induced in the aggregation of sodium bentonite below a certain acidity, resulting in aggregates which have a card-house, as opposed to band-like, structure.Furthermore, these open aggregates showed more viscous, and less elastic, behaviour than those with a higher fractal dimension. A recent study by Herrington and Midmorel of the rapid aggregation of dilute kaolinite suspensions concluded that the flocs showed a transition from loosely packed, volu- minous, card-house structures in slightly acidic conditions, to compact, flake-like, structures in moderately acidic condi- tions. Their work relied on the determination of fractal dimensions by dynamic scaling (e.g. Weitz et aL2) measure-ments, made during a study of the growth of the aggregates by photon correlation spectroscopy (PCS).In the current work, the aggregates of another clay, sodium bentonite (predominantly a montmorillonite clay), are studied. The technique of dynamic scaling is again used, but in addition, static light-scattering experiments are performed, which are also able to give information directly about the fractal struc- ture of the aggregates. The results from both sets of experi- ments are compared with the information implied in a series of rheological studies. The basis behind dynamic scaling measurements is the theory of aggregation kinetics originally propo~ed~,~ by Smoluchowski, who also dealt with the coagulation and coalescence of larger aggregates, and precipitation. Sub-sequent ~tudies~-~ of colloidal aggregation tested this theory and found that it was generally a good description of the initial stages of aggregation.Only recently, with the advent of computer-based simulations of aggregation, has aggregate behaviour which deviates from the simple Smoluchowskian scheme been more accurately modelled. Typically, such advances have taken place in the simulation of both perikinetic' and orthokineticg aggregation. The application of the concept of fractal behaviour for col- loidal aggregates leads to the relationship between the radius, r, of an aggregated cluster of mass, m,and the radius, a, and mass, rno ,of a single particle, uiz. :".(ym0 (1) This expression serves as a formal definition for the fractal dimension, d,, of any object which shows fractal-like behav- iour.It is now generally recognised that random aggregates are fractal objects,"." a result based on the observation of a power-law dependence of scattered light intensity, Z(q), on the scattering vector, q, where q is related to the scattering angle, 8, by q = (4nn/A)sin(8/2); here n is the refractive index of the medium, and Iz. the wavelength of the incident light. There- fore, when the cluster size is suficiently large to show true fractal behaviour (i.e. containing a sufficient number of single particles to enable the cluster to show the same, repeating, internal structure over a length much greater than the char- acteristic size of the component particles themselves), this dependence may be written where df is the fractal dimension of the aggregate, defined by eqn.(1) above. This relationship is the basis for the sub- sequent determinations of fractal dimension of large clay aggregates, by measurements of static light intensity. One such recent studyI2 by Lin et ai. has shown the differences in colloidal gold and silica aggregate structure, when the aggre- gation mechanism is either diffusion- or reaction-limited. The time dependence of the growing characteristic cluster mass, rn, can be determinedI3 from the Smoluchowski equa- tions. For diffusion-limited cluster aggregation, it can be shown that a linear relationship between mass and time ensues. Because the rate of increase in mass, rn, of an aggre- gate growing under diffusion-limi ted conditions, remains con- stant as a function of time, the mass term in eqn. (1) may be replaced by time.Thus, we may immediately write an expres- sion for the radius, I, at some time, t, after the start of the aggregation cc (t)'ldf (3)a If it proves possible to measure r experimentally, as a func- tion of time, then a value of d, may be derived using eqn. (3). Dynamic, or quasi-elastic, light-scattering measurements, which are the basis of PCS (see below), enable the average hydrodynamic radius of the clusters to be determined. There- fore, providing that the rate of aggregation remains constant throughout the timescale of the experiment, the fractal dimension of the aggregates may be deduced.Extensive rheological studies14-' of a similar clay, kaolin- ite, have been made previously, though most have used samples with a large volume fraction of solids. Similar studies have also been made'**'' on the flow properties of concen- trated bentonite suspensions. In the current work, despite the inherent difficulty in working with very dilute suspensions, measurements relating to aggregate and network structure have been made. There are some fundamental differences between the kaolinite used in previous' studies, and the ben- tonite clays described here. The main differences are struc- tural, with one particularly important physical result. The structure of kaolinite consists of repeated sheets of tetra- hedrally coordinated Si atoms, and octahedrally coordinated A1 atoms, stacked in a 1: 1 ratio.A typical basal spacing is ca. 7 A. On contact with water, there is no penetration of water between the aluminosilicate layers, and the clay does not expand. Bentonite comprises layers with two tetrahedral sheets sandwiching an octahedral sheet, and is thus referred to as a 2 :1 layer clay. Typical basal spacings are of the order of 9 A. In the presence of water, this is taken up by the clay between the layers, leading to interlayer swelling, and increas- ing the basal spacing to between 12 and 15 A. Thus the effec- tive volume fraction taken up by bentonite dispersed in water is greater than that calculated simply from the mass of ben- tonite added to the water; this behaviour is the main differ- ence compared with the non-expanding kaolinite.These effects are described in more detail2' by van Olphen. The use of two complementary light-scattering techniques should lend more support to the resulting postulated struc- tures, than would be the case using only one method. Cer- tainly, recent studies by Lin et ~1.~' .and Rarity et ~ 1have ~ shown the two methods to be capable of giving an under- standing of the fractal geometry, and a measurement of the fractal dimension, of the aggregates. Likewise, the inclusion of rheological measurements may well demonstrate that the suggested aggregate structures, as obtained from both kinds of light-scattering experiments, are those most likely to exist. Theory Static Light Scattering Of the two light-scattering techniques used, the first case to be considered is that of static light scattering.Here, the angular dependence of the intensity of light, scattered by aggregates in suspension, is recorded. It is necessary to know the dependence of scattered light intensity on both scattering vector, q,defined above, and cluster mass, m. This relation- ship may be written (4) where I,&) is the intensity of light scattered by a single fractal cluster with mass, rn, and concomitant radius of gyra- tion, R, ,at an angle corresponding to the scattering vector, q. The structure factor for a cluster of mass, m, is SJqR,). This latter variable is responsible for the widely varying behaviour of eqn.(4) seen with very differently sized aggregates. Eqn. (4) may be rewritten for a suspension consisting of a large number of aggregates of different masses, uiz. m= 1 where n, is the fraction of clusters with mass, m. This expres- sion is dependent on both the aggregate size distribution, through the n,m2 term, and the shape of the clusters, through the S,(qR,) term. The radius of gyration, R,, of the aggregates, is the characteristic length for each cluster, and can be related to the radius, a, of a single particle, and the mass of the aggregate, through the fractal scaling of the clus- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ters R, = aml/df (6) In addition, the limiting values for the structure factor may be written as a function of qR, as follows (7) It is possible to see that eqn.(5) leads to two limiting expres- sions for I(q), depending on the relevant S(qR,) term in eqn. (7). Clearly, for small aggregates, with qR, 41, the internal structure is not resolved, and the cluster behaves essentially as a point particle as far as the light probe is concerned. Such an aggregate scatters light coherently, and the resulting intensity scales solely as a function of m2, from eqn. (5). However, for large aggregates, satisfying qR, 9 1, the fractal nature of the cluster is revealed, since the light scattered from different parts of the aggregate adds incoherently. Substitut- ing eqn. (6) and (7) into (3,and eliminating R,, gives a dependence of scattered intensity for large aggregates which is linear both in m, and in q-df.It is also abundantly clear that there will be a cross-over regime, with qR, x 1, where neither asymptotic region of eqn. (7) is reached. This point has been studied in some detail by Rarity et ~1.~~and Lin et ~1.'~previously. A more complex expression for S(4R.J has been derived2' by Lin et al., which shows the desired behav- iour at both extremes of qR,, and which also describes the transition in the cross-over region. If it is desired to make ~ measurements of the fractal dimension of a suspension of aggregates by a static light-scattering technique, then the main prerequisite is that the average radius of the clusters is much greater than the length probed by the light source.This length is essentially l/q, the inverse of the magnitude of the scattering wave vector. Dynamic Light Scattering Dynamic, or quasi-elastic, light scattering is a most useful experimental tool for studying colloidal aggregates. By mea- suring the temporal fluctuations in scattered light intensity over very short timescales, the technique can give informa- tion about the motion of the scattering particles. In a normal colloidal suspension, this is Brownian motion, arising from collisions with the molecules of the suspending medium. The velocity of a particle is then determined solely by its ability to diffuse through the fluid. It thus becomes possible to derive an average diffusion coefficient, (D,,,), for the aggregates in suspension, and this in turn may be related to an average hydrodynamic radius, r, using Stokes' law.However, it should be remembered that dynamic scattering only yields informa- tion about diffusion coefficients, and that all other param- eters must be derived from these. In the application of dynamic light scattering to the tech- nique of PCS, the intensity correlation function is obtained by making successive measurements of scattered light inten- sity as a function of time elapsed from some reference point. The empirical normalised intensity function, g(2)(z), is deter- mined directly by where the intensity, I, is measured at the arbitrary reference time, t, and then later at a time, t + z. Eqn. (8) is then related to the theoretical normalised field autocorrelation function, g(')(z), by the Siegert relation g'2'(z) = 1 + cIg"'(z) 12 (9) J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 where C is a constant. The autocorrelation function, g(')(T), for a dispersion of monosized particles, may be related to the diffusion coefficients as follows g(')(z) = exp(-q2Dt) (10) However, for polydisperse systems, such as are formed by the aggregation of a monodisperse sample, the autocorrelation function becomes where n, is the number density (normalised to Zz= n, = 1) of aggregates with mass, rn, and diffusion coefficient, D,. In order to make comparison with real experimental data, the correlation function obtained experimentally needs to be described more simply than by eqn.(1 l), which includes diffu- sion coefficients for every size of aggregate. In the simplest analysis, known as a cumulants analysis, therefore, the right- hand side of eqn. (11) may be expanded about the mean (q2D,), which yields Here the term (q2D,) is called the first cumulant, pl, of the correlation function. The departure from a pure exponential, which would be the form of eqn. (ll), i.e. it would reduce to eqn. (10) if all the aggregates were of the same size, is now modelled as a polynomial in the logarithm of the correlation function. The initial slope of a plot of the left-hand side of eqn. (12) against z [obtained by differentiating eqn. (12) with respect to z,and setting z = 01, gives the mean diffusion coef- ficient, (D,), and p2/p1gives a measure of the polydispersity of the sample.Whilst the above cumulants analysis yields an expression for (Dm), a model-independent form for the mean diffusion coefficient may be given as follows, in terms of the diffusion coefficients for every cluster mass Here, the individual diffusion coefficients are weighted by the scattering intensity and the number of clusters of each aggre- gate mass. Note that the above analysis makes no allowance for the effects of a rotational contribution to the autocorrela- tion function, as is described21 by Lin et al., for fractal flocs with qR, > 1. Another study, by Axford et al., concluded23 that for certain systems, such as those described below, rota- tional diffusion effects do not contribute significantly to the value of the first cumulant, (q2D,).In a typical experiment, the average diffusion coefficient is calculated from eqn. (8), (9) and (12); only when a more com- plete understanding of the aggregate size distribution is required, is a rather more complex deconvolution of the light intensity data applied in terms of eqn. (8), (9), (11) and (13). Such an analysis then requires estimates of the cluster struc- ture factors, S(qR,),used in eqn. (13); these may be generated by Rayleigh-Gans-Debye theory. Their use in interpreting PCS results in terms of cluster aggregation and polymer theory has been successfully demon~trated~~ by Herrington and Midmore. Simple Rheology Theory In this section, some of the simpler aspects of a small part of an enormous subject are dealt with.Given the nature of the study being undertaken, our interest is inevitably in the flow behaviour of suspensions, which will tend to be non-Newtonian fluids, and in whatever information may generally 2087 be derived about the possible structures of particles dispersed in a liquid. The most typical non-Newtonian fluid is a shear-thinning liquid. Such fluids show a widely varying shear viscosity as a function of shear rate, in complete contrast to a Newtonian liquid which has a constant shear viscosity. In the Newtonian case the shear stress, 0, generated in simple shear flow, is directly proportional to the applied shear rate, p, i.e., Is = qj (14) where q is the viscosity, and is constant.However, for a shear-thinning fluid, the apparent viscosity decreases with increasing shear rate. One of the simplest models for non-Newtonian flow is that of 'Bingham' plastic behaviour. In this case a fluid does not start to flow until a critical, or yield, stress is reached. In other words, the apparent viscosity is extremely large until this critical stress is attained; thereafter, the viscosity falls sig- nificantly, and the fluid shear stress increases as a function of shear rate. In practice, this subsequent rise in stress is not usually linear with shear rate, though this assumption is made in the Bingham model. Thus, for this idealised model, we may write 0 = OY + qpj (15) where oYis the yield stress, and qp the plastic viscosity, both of which are constants.In reality, the concept of a yield stress is sometimes flawed, and fluids do flow at very low shear rates, often showing Newtonian behaviour. However, the derivation of a yield stress for a fluid may well have many important practical purposes, in the sense that for normal applications and typical shear rates, there is some point at which the fluid starts to flow with a considerable reduction in apparent viscosity. In the current work, the calculation of a yield stress for a suspension in which there are a number of inter-particulate forces may reveal information about the nature of the binding between particles, and hence indicate the presence of long-range structure within the suspension.Another rheological aspect we shall consider is that of the viscoelastic behaviour of a material. The term 'visco-elastic' means that the material shows properties which are a mixture of rigidity (i.e. elastic properties) and fluidity (i.e. viscous properties). It is often useful, in attempting to understand the viscoelastic behaviour of a material, to discuss the response of that material to an applied small-amplitude oscillatory shear. It should be recalled that the shear rate is the rate of change of strain with respect to time. In oscillatory shear we define a complex shear modulus, G*,(much like the modulus for an elastic, or Hookean, solid) relating stress, 0, to strain, y, uiz. = G*(W)Y(t) (16) where stress and strain are inevitably functions of time as a result of the oscillatory shear applied. The complex modulus is also a function of applied frequency, o,and can be resolved into two components, as follows G* = G' + iG" (17) where G'and G" are referred to as the storage modulus and loss modulus, respectively.Thus it is found that there are two components to the resulting stress in the sample. One part is always in phase with the applied strain, while the other is out of phase, by 42. In the simplest of interpretations, the rela- tive magnitudes of G' and G" will give an indication of the degree of rigidity, as opposed to fluid-like behaviour, of a material. Therefore, should very different values of G’/G’’ be found for suspensions containing clusters or networks formed under a variety of experimental conditions, it may be possible to interpret the results as indicating the presence of different structures within the fluid.Experimental The experiments to determine aggregate structure were carried out using sodium bentonite, predominantly a smec- tite, or montmorillonite, clay; this was supplied as a finely divided powder by Allied Colloids. All exchangeable cation sites within the clay are taken up by sodium ions; the clay was also free from the presence of other ions and salts. Ca. 5 g of bentonite were added to 250 ml water, and dispersed using an ultrasonic bath. The resulting suspension was left for 24 h to allow the sedimentation of any larger particles. The suspension was decanted and allowed to stand for another 24 h.Again, the suspension was decanted in order to discard those particles which had sedimented. Two further suspen- sions were prepared, each one a ten-fold dilution of the first. The volume fractions of solids, 4, for the three preparations were found to be ca. and respectively. The effective volume fractions of each dispersion may well be up to 50% greater than calculated, owing to the swelling nature of the bentonite clay; however, the order of magnitude of 6 is as given. The two most dilute suspensions were sized using PCS, and the average particle diameter was found to be 212 nm, with a polydispersity index (the ratio of the variance to the mean) of between 0.1 and 0.2. This diameter is, of course, only an effective hydrodynamic diameter, and the actual shape of the clay particles is almost certainly not spherical.In fact, given that the clay is a layered aluminosilicate structure, the smallest particles are likely to be flake-like fragments. This suggested structure for the single particles is supported25*26 by scanning electron microscopy of bentonite. In each experiment, 5 ml of a suitable strength of bentonite suspension was placed in a cylindrical Burchard cell, made from high optical quality quartz. To this was then added 4 ml of aqueous HCl, of between and mol I-’. Finally, to initiate aggregation, 1.0 ml of 0.1 mol 1-’ aqueous KC1 was added, making a total sample volume of 10 ml. There- fore, in all experiments, the concentration of KCl was con- stant, at 0.01 mol 1-’, whilst the pH was varied at will.All’ the solutions were kept at 25°C at all times. The Burchard cell was then placed in the PCS apparatus, and the increase in hydrodynamic radius monitored as a function of time. The choice of KCl solution strength was the minimum which removed the electrostatic repulsions between the clay par- ticles and enabled aggregation to take place. The final pH of the aggregating mixture was measured using a standard glass electrode after the run was completed. It was confirmed that the final pH was in fact reached immediately on the addition of the HCl and KCI to the bentonite, and took values between pH 2.3 and 10.2. The reaction was followed until the increase in radius had slowed somewhat, probably due to sedimentation of the largest aggregates.The conditions of each run were chosen so that the growth in radius took place over a suitable timescale of, say, between 30 min and 2 h. At the end of this time, intensity measure- ments as a function of scattering angle were made. Results were collected over a short period during which the size of the aggregates changed negligibly. These static light-scattering experiments were carried out on aggregates with an average diameter of ca. 2000 nm, which ensured that the condition qR, % 1 was met. At this point, the measurement of time in dynamic scaling measurements is briefly considered. Because the technique J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 deals with perikinetic aggregation, it is possible to define a dimensionless time parameter, E, in terms of the theoretical Smoluchowski rate coefficient, k,, the initial particle number concentration, No,and the time elapsed from the start of the aggregation, t. This ‘reduced’ time takes the form E = k, Not, and is the time parameter quoted in the results presented in the following section. Whilst not strictly necessary for dynamic scaling measurements, in which solely the logarithm of time is important, it enables aggregation experiments, taking place at different particle concentrations, to appear to occur over similar dimensionless timescales. Finally, suspensions of the bentonite with 4 = were treated with aqueous HC1, to give a pH range between 1.1 and 9.3.Two samples at each pH were made, and one was then treated with aqueous KCl in order to induce aggre- gation. It should be realised, however, that the aggregated clay is still likely to show some long-range order in the form of networking or gelling behaviour, with bonding existing throughout the dispersion, because of the somewhat higher (4> 0.01 in water) volume fraction, compared with the dis- persions studied by light scattering alone. Obviously, the majority of clay particles will reside within compact aggre- gates, but some will remain loosely bonded in the suspending fluid. Therefore, all rheological measurements on these aggre- gated systems are likely to derive from contributions from both the aggregates and the suspending liquid.Experiments using two types of Bohlin rheometer were then performed on these samples. Of the two different sets of measurements obtained, one used shear viscometry to deter- mine yield stresses, whilst for the other the suspension was subjected to oscillatory flow in order to derive the relative magnitudes of the storage and loss moduli of the aggregates. In these latter experiments the methods of Tadros and co- worker~~~-~’were followed. This involves initially fixing the oscillatory frequency and measuring the rheological para- meters as a function of strain amplitude. This allows us to find the linear viscoelastic region, where G*, G and G” are independent of strain. Once the linear region has been deduced, measurements may be made as a function of fre- quency, at fixed amplitude.Thus it is often desirable to con- centrate on either one frequency or a small range of frequencies, and make comparisons between the measured rheological parameters of different systems under the same experimental conditions. In this way, variations in visco- elastic properties may be seen for a range of colloidal clay systems. Results Light-scattering Results The results of dynamic scaling measurements using PCS made on the bentonite are shown in Fig. 1 and 2. It was found that plots of log(r/a) against dimensionless time, E, elapsed from the start of the aggregation, showed only two gradients, namely ca. 0.33 and ca. 0.56, corresponding to fractal dimensions of 3.0 and 1.8, respectively.No other slope was seen at any pH, or in any experiment. Fig. 1 shows the results obtained at low pH for three runs, all of which exhibit a high fractal dimension. Fig. 2, in contrast, shows four plots, all obtained at higher pH values where the fractal dimension is much lower. The results of static light-scattering experiments, on the same samples, are illustrated in Fig. 3 and 4. Here are plotted log[Z(q)] against -log q, such that the slope equals the fractal dimension, d,. Fig. 3 is for aggregates formed at relatively low pH, whilst Fig. 4 is for suspensions which are less acidic. Reliable plots were obtained in slightly fewer cases than with J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 0.7 0.6 1.8 2.0 2.2 2.4 2.6 2.8 log E Fig. 1 Logarithmic plots of aggregate size against time from the start of aggregation, for three suspensions of sodium bentonite, as determined by dynamic light scattering. All results were obtained below pH 4.3, as follows: A, pH 2.3; 0,pH 4.01 ;0,pH 4.21. All the plots have slopes of ca. 0.33, corresponding to d, x 3.0. quasi-elastic light scattering, probably because of dust con- tamination which causes large problems at the smaller scat- tering angles. Again, it may be seen in Fig. 3 and 4, that essentially only two slopes are found, corresponding to fractal dimensions of around 2.9 for aggregates in a more acidic medium, or of about 1.9, at higher pH. 0.9 0.8 0.7 0.6 k ,,& .O 0.4 / ,o / / / 0.3 o?' I IIIIII II1IIII 1.4 1.8 2.2 2.6 3.O 3.4 log E Fig.2 As for Fig. 1, but for aggregates formed above pH 4.3, as follows: A,pH 4.55; 0, pH 5.7; 0,pH 7.5; 0,pH 10.21. The four plots all have slopes of between 0.5 and 0.6, corresponding to d, z 1.8. n r I cn ,-I2.0 nllo lIIIII1I'I 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 -log (q/nm-') Fig. 3 Logarithmic plots of intensity against scattering vector, obtained from two static light-scattering experiments. The plots are for the following pH values: 0,pH 4.01; 0,pH 4.21. The slopes in each case equal ca. 2.9-3.0. 4.0 -3.6 r I v) m ,o-' 3.2 A, ' 11'""'~""'""'1''''''' 1.5 1.6 1.7 1.8 1.9 2.0 -log (q/nm -I) Fig. 4 As for Fig.3, but at higher pH values: A,pH 4.55; 0, pH 5.7. The two plots have slopes, equal to the fractal dimension, of between 1.9 and 2.2. If all the results of determinations of fractal dimension as a function of pH are combined, then we obtain the plot shown in Fig. 5. It is clear from this that there is a sudden transition at ca. pH 4.3, between an aggregate structure which possesses the maximum possible fractal dimension at less than pH 4.3, and a structure with a much lower fractal dimension above this point. Rheological Results Two different types of rheological study were performed on the bentonite. In the first, shear viscometry was used on sus-pensions of aggregated and unaggregated bentonite, over a wide pH range. Fig.6 shows typical plots of shear stress against applied shear rate, for one of each kind of sample. It can be seen in Fig. 6 that the suspensions behave as Bingham plastics with a clearly defined Bingham yield stress, obtained by extrapolating the linear portion of the plots back to a shear rate of zero. Fig. 7 shows all the yield stress results, for both aggregated and unaggregated bentonite, as a function of pH. It is seen that there is a minimum in the yield stress at ca. pH 4.3 in both cases, though the curve is more pronounced for the aggregated suspension. Also, in this case, the yield stresses all tend to lie at lower values than the corresponding values, where these exist, for unaggregated bentonite. It should be noted that Fig. 6 shows that the suspensions do actually flow at low shear rates, and therefore the yield stress is perhaps only a useful tool, rather than a physical reality.However, it 3.0 1--+----JI, 1a't 4'1 I I 1 2.0.-g n L+ 1.0 12 3 4 5 6 7 8 91011 PH Fig. 5 All the dynamic (0)and static (+)light-scattering results of the previous four figures, plotted to show fractal dimension as a func-tion of the pH of the aggregating bentonite suspension 2090 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 10.5 1.2 10.0 1.o 8 0.8 2 0.6 m Q-. c L v) * c3P 9.5 9.O 0.4 8.5 I I I I I I I I I I 0.2 0 50 100 150 200 2 3 4 PH 5 6 7 shear ratels-’ Fig. 6 Two plots of shear stress against shear rate.One is for a typical aggregated suspension of sodium bentonite, at pH 4.3 (+); the other is an unaggregated suspension, at pH 6.8 (A). does describe a point where there is a change in the flow properties of the liquid, and as such will help with an under- standing of the forces existing, either between or within the aggregates or particles in the fluid. The results of the second class of experiments, involving oscillatory shear, are shown in Fig. 8 and 9. Fig. 8 shows the results of calculations of G/G” as a function of applied strain frequency, a,for three typical aggregated suspensions, at various values of pH. The oscillatory frequency is seen in Fig. Fig. 9 The magnitude of G/G“ (the ratio of storage to loss modulus) for aggregated sodium bentonite, shown as a function of pH. Measurements were made over an oscillatory frequency range, GO,of 0.1-1.0 Hz, as in Fig.8. 8 to vary between 0.1 and 1.0 Hz, over which range the values of G’/G”may be taken, to a reasonable approximation, as being constant. Clearly, over a complete range of fre- quencies, the ratio G/G” will actually take all values from zero to infinity, since at very high frequencies the dispersion’ shows only elastic behaviour and G’/G”-+ a;at low fre- quencies only viscous behaviour will be observable and G/ G” -+0. However, by concentrating on the region, in this case one decade, where G/G remains independent of frequency, we may make comparisons between the viscoelastic behav- ~~~~ iour of different suspensions.The use of such measurements l~l~l~1.4 llll 1.2 1.o 0.8 0.6 0.4 123456789 9” Fig. 7 Plots of all the yield stresses, for both aggregated (+) and unaggregated (A) systems, as a function of pH in the bentonite sus- has been proved re~ently~’-~’ to be of considerable value in studying the stability and flocculation of colloidal disper- sions. These observations are necessarily only applicable over the frequencies covered in Fig. 8, but obviously any observed viscoelastic rheological parameter is ultimately frequency dependent. Measurements of G‘/G as a function of pH for all the aggregated bentonite samples are shown in Fig. 9. Here it becomes clear that G’/G”takes only two values, one of about 10.2 for bentonite aggregated at less than pH 4.3, and the other of ca.9.1, for aggregates formed above pH 4.8. There-fore, there appears to be a critical pH value, much as for the fractal dimension calculations shown in Fig. 5, either side of which, G‘/G is well defined and constant. Discussion pension. We may now consider the results of all three types of experi- ment in more detail. The results shown in Fig. 5 indicate a clear change in fractal dimension of the bentonite aggregates which are formed in diffusion-limited aggregation. If we con- sider the nature of the smallest clay fragments in the colloidal dispersion, then certain clays, e.g. kaolinite, are kn~wn~**~’ 10.0 to have negatively charged faces and positively charged edges. 5One consequence of this dual charge character is the possi- bility of heteroflocculation between the edges and faces of the clay particles, resulting in a three-dimensional, voluminous, ‘card-ho~se’~~.~~structure.If, however, the growth of the clusters proceeds via face-face aggregation, a far more dense, close-packed structure will result. Kaolinite was shown’ by Herrington and Midmore, using dynamic light scattering, to 9.5 c3 09.0 E 0 0 0 0 00 I I I I IIII form aggregates with two possible fractal dimensions over a 0.I 02 0.4 0.6 0.8 1.0 range of pH values, very much like the plot in Fig. 5. There-o/Hz fore, it would seem reasonable to assume that the bentonite Fig. 8 Plots of the ratio of the magnitude of the storage modulus to particles are behaving quite similarly to the kaolinite, and loss modulus, for aggregated sodium bentonite, as a function of the forming two rather different structures upon aggregation.frequency of the applied oscillatory shear. Two typical cases are Note that no value for the fractal dimension of the aggre- shown: 0,pH 6.8; 0,pH 4.3. gates was found to be greater than 3.0, which is, of course, the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 maximum possible, when the solids in the aggregate are com- pletely space-filling. Similarly, the minimum measured fractal dimension is ca. 1.8, a value which corresponds to that fo~nd~.~~for the rapid, i.e. diffusion-limited, aggregation of gold and other sols. It may be reasonable to suppose that bentonite particles aggregating via a face-edge mechanism, and giving a random, open structure, will lead to the forma- tion of clusters not unlike those generated by the rapid aggre- gation of monodisperse spheres, clusters which will also have a low fractal dimension3' in such a system.The transition from high to low fractal dimension takes place at pH 4.3, as can be seen in Fig. 5. At df = 3.0, the only realistic candidate for the structure of the aggregates is a band-like one, with clay particles packing face-to-face, build- ing a layer by layer, and leaving no space in the structure. For d, = 1.8, however, there are still two possibilities, namely, a less tightly formed band-like structure, or a card-house built up by edgeface aggregation. The evidence would seem to point to edge-face, heteroflocculated, structures at high pH, since it is quite difficult to envisage a band-like structure which will not immediately rearrange to a close-packed one with d, = 3.0.Also, given that the background ionic strength is fairly high (with 0.01 mol 1-' KCl present), so that there is a very thin electrical double layer around the aggregate, then the positively charged edges will not be swamped by the effects of the larger, negatively charged faces. At lower ionic strengths the negative charge on the faces then overlaps the edges, and envelops the whole of the clay fragment. If the aggregation was carried out in water alone, as was done with kaolinite clay by Herrington and Midmore,' it proved impos- sible to form aggregates with the lowest fractal dimension for this very reason, that the positive edges seemed unable to participate in interparticle electrostatic 'bond' formation.If the structure were band-like at low fractal dimension, it is difficult to conceive why the background ionic strength would be such an important influence. It is necessary to consider the various factors which lead to the formation of one structure in preference to the other. A number of different mechanisms may be envisaged which account for the observed transition of pH 4.3. First, it may be the case that, as the pH is lowered, the increasing ionic strength finally reduces the face-face repul- sions to such an extent that the particles favour aggregation as a band-like structure.Obviously at higher pH values, with long-range face-face repulsion still existing, positivenegative charge coupling (i.e. face-edge) is energetically favourable. The situation appears to change at low pH, since the ability to aggregate through van der Waals attractions over a larger surface area (i.e. face-face) is then apparently preferable. This latter scenario does, however, require that the face-face elec-trostatic repulsions are removed, and so does not appear to take place until the acidity is increased to pH 4.3. However, it should be borne in mind that the relative increase in ionic strength is quite small, e.g. an acidity of pH 4 implies that H+ and C1- ions from added HCl represent only ca.1% of the total number of ionic species, when the background solution is 0.01 mol 1-' KCl. Further evidence that the differences in ionic strength with changing pH are not responsible for the transition between face-face and face-edge bonding is given by the reasonable independence of transition pH with molar- ity of KC1. In other words, experiments at any molarity of KCl, sufficient to induce aggregation, always resulted in a structure transition at around pH 4.3. Secondly, once any electrical double layers have been SUE-ciently reduced in thickness for aggregation to occur, as is the case at every pH studied here with 0.01 mol 1-' KCl present, then the likely magnitude and extent of electrical charge on both the edges and the faces of the clay particles may be con- 2091 sidered.Accepting that for clay particles in water alone the faces are negatively charged, and the edges positively charged, then it would appear that the strongest interparticle bonds should result from face-edge interactions. However, even if all the surfaces were negatively charged, then aggre- gation would still occur, as with e.g. polystyrene spheres, pro- vided that the background electrolyte concentration is sufficient to reduce the range of electrostatic forces, and hence allow attractive van der Waals potentials to overcome the repulsive terms. This situation certainly exists here, but we must also consider the initial, i.e. electrostatic, particle- particle interaction, which is either repulsive (face-face) or attractive (face-edge), depending on the proposed structure.It would appear that the attractive electrostatic interaction leads to the favoured structure above pH 4.3, provided that the clay particles retain their dual charge character, and despite the fact that face-face bonding would take place over a much larger surface area. Thus we may conclude that the overall bond energy between two particles is strongest for an attractive electrostatic interaction, even if this bonding takes place over the relatively small area of a face-edge bond. If the pH of the dispersion is lowered, i.e. made more acidic, then electron donation from negatively charged oxygen atoms on the faces, to the increased number of H30+ ions in the water, will lead to a reduction in the total negative surface charge.Such interactions will result in a form of hydrogen bonding, since H30+ is heavily hydrated, giving rise to a network of bound H+ and H,O over the faces of the clay. This surface reaction will have two effects: weakening the face-edge electrostatic attractive forces ; and decreasing the face-face electrostatic repulsions. If, during the formation of aggregates, it becomes energetically favourable to over- come the much reduced repulsive terms, in order ultimately to form bonding interactions over the large surface area of the faces, then a compact structure will be produced. In this situation, the pH at which bonding between surface oxygen atoms and aqueous H30+ becomes significant may be quite well defined, and is unlikely to be dependent on clay or KCl concentrations to any great extent.Lastly, one further contribution to the dramatic reduction of face-face repulsions at lower pH, may be the partial decomposition of the clay, with a concomitant release of A13+ ions into the aqueous phase. The efficiency of these ions in removing any residual electrostatic repulsions, and thus encouragmg the formation of the most stable (i.e. with the largest possible fractal dimension, bonded by the largest areas available on the clay) aggregates or networks, will be in accordance with the Schultze-Hardy rule. The sharpness of the transition at pH 4.3 may well be indicative of the passage of such trivalent ions from the clay into the solution, at this particular level of acidity.This effect might reasonably be expected to be independent of the molarity of KCl, because the increase in ionic strength due to the formation of aqueous A13+ could exceed that caused by the KC1 alone. Further- more, the start of such clay decomposition may well be so sensitive to pH that the sharp transition at pH 4.3 is inevit- able for a wide range of clay volume fractions and back- ground electrolyte concentrations. If we conclude that the possible structures for bentonite aggregates are close-packed and band-like with a high fractal dimension below pH 4.3, or open and card-house-like above pH 4.3, then an examination of the rheological results should be made, in order to see whether these assumptions are justi- fied, and supported by the results.The results of the yield stress measurements were shown in Fig. 7. Here values of yield stress are presented for suspen- sions of bentonite with and without 0.01 mol 1-' KCl present. In the former, aggregates will have formed with the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 two possible fractal dimensions, depending on the pH of the suspension, while in the latter, aggregation only occurs at the lowest pH, i.e. pH 1.1, where the hydrogen ion concentration is sufficiently high to induce aggregation, and play the role of the added salt. Both sets of results in Fig. 7 show a minimum at between pH 4 and 5. If we consider the aggregated benton- ite below pH 4.3,then a rise in yield stress is seen as the pH is reduced.Since the aggregates form close-packed structures in this region, it is clear that an increase in ionic strength will lead to an increase in the face-face attraction between par- ticles, and so give rise to an increase in yield stress, since this is a measure of the binding energy between the particles in the fluid. This effect will be particularly noticeable if the repulsion terms have been dramatically reduced by the pres- ence of A13+, as discussed previously. Above pH 5, there is an increase again in yield stress, this time for aggregates which possess a card-house structure. The reduction in hydrogen ion concentration would appear to favour the strengthening of the faceeedge bonds between the particles.This effect manifests itself in an increase in yield stress from 0.46 Pa at pH 4.3,to 1.2 Pa at pH 9.3.It is not easy to see how the reduction in acidity strengthens the face-edge bonds, but one possibility is that reducing [H’] leads to the positively charged edges being more strongly attracted to the negative faces, since the conditions of greater [OH-] and smaller [H’] will tend to increase the thickness and range of influ- ence of the electrical double layer on the negatively charged faces. This, of course, makes face-face aggregation even less likely, as explained above, but will actually enhance face- edge bonding. A similar trend is seen for the case of bentonite which has not been aggregated. This observation implies that, even without forming discrete aggregates or clusters, the same type of order, or bonding, exists throughout the suspension. It is certainly the case that at pH 1.1, the suspension appears to be a slightly thickened gel, and this may also be true at pH 1.9.Therefore, the same arguments, either for face-face or for face-edge bonding between clay particles, may be proposed. It is interesting that all the measurements of yield stress in an unaggregated suspension of sodium bentonite are greater than those made in the flocculated case. One reason may be that, whilst the variation in yield stress as a function of pH does derive from the type of bonding within the aggregates, the total amount of resistance to flow will be less than for an unaggregated suspension where there may be more long-range order.In other words, the suspending liquid between the aggregates, once these have been formed, contains fewer particles per unit volume than the unaggregated dispersion. This fluid will make only a limited contribution to the yield stress, though it may still be capable of transmitting stress through the suspension. In contrast to this, the unaggregated bentonite suspension will be more uniformly dispersed, and may offer a greater resistance to flow, even if the individual bonds between the networked particles are considerably weaker than those bonds which exist only within an aggre- gate. Finally, we may consider the results presented in Fig.9. Here the values of G’/G’’for the aggregated sodium bentonite are shown as a function of pH. The ratio G‘/G‘‘is the ratio of the storage modulus to the loss modulus of the entire suspen- sion. In this case, there will be some contribution to G’ from the suspending liquid, so that G’/G’’is in fact a measure of the amount of rigidity, compared to viscous or fluid-like behav- iour, of a mixture both of the aggregates and of the liquid containing weakly bound, unaggregated, clay particles. Immediately it may be noticed that the graph takes the same form as Fig. 5, with a rapid transition from a high value for G’/G” of ca. 10.2below pH 4.5,to a lower value of ca. 8.9-9.1 above pH 4.5.As with the measurements of fractal dimen- sion, essentially only these two values of G’/G”are found over all the pH range studied.Making the assumption that the contribution to G/G” from the suspending fluid remains roughly constant, then differences in G’/G’’ should relate to differences in the viscoelastic behaviour of the clusters, as a function of pH. Whilst it is clear that the absolute variation in G/G” is not large, Fig. 9 shows the change is outside the bounds of experimental error, and is clearly pronounced at ca. pH 4.5.It may also be reasoned that the contributions of the suspending fluid, to the elastic and viscous moduli, serve to limit the measurable difference in G’/G’’for the two struc- tures; the change in G’/G” seen in Fig. 9 is likely to increase with increasing volume fraction of the bentonite.However, the differences in rigidity of clusters formed either side of pH 4.5,still appears to be measurable. If the previous deductions about the two likely structures over these different pH ranges are applied, then it would appear that the band-like structure has greater rigidity, whilst the card-house structure is more prone to viscous behaviour, and is less rigid. This is, perhaps, what one would inherently expect from two such dissimilar structures, and these measurements serve to support this view. What is more important is that this last result confirms the idea of a card-house, rather than a weak, band-like struc- ture, for the aggregates with a lower fractal dimension. Conclusions The work presented here has shown that for very dilute sus- pensions of sodium bentonite, the formation of two types of floc is possible. One is a tightly bound, band-like structure, with a fractal dimension of ca. 3.0,whilst the other is similar to a house-of-cards, with a lower fractal dimension of ca.1.8. The ability to form one or other of these structures depends simply on the pH of the suspending medium. At the cross- over point of ca. pH 4.3,there is also a minimum in the yield stress of the suspension, with respect to shear flow. In addi- tion to these points, there is a clear difference in the dynamic rigidity of the two structures, with the close-packed structure being more rigid than the open structure. Future work will involve the study of other clays.In partic- ular, the previous investigations’ of Herrington and Midmore into kaolinite may now be extended to include static light scattering, together with rheological aspects, of the aggregates. Changes in the structure of the aggregates by other external factors which might operate during their growth, such as the application of a shear field, will also be considered. The authors are grateful to the SERC for its financial support of this work. Thanks are also due to Dr. B. R. Midmore for useful discussions, and to Mr. C. N. Malde for help in oper- ating the Bohlin rheometric apparatus. References 1 T. M. Herrington and B. R. Midmore, Colloids Surf., 1993, 70, 199. 3 D. A. Weitz, J. S. Huang, M. Y. Lin and J. Sung, Phys.Rev. Lett., 1984,53, 1657. 3 M. von Smoluchowski, Phys. Z., 1916,17,585. 4 M. von Smoluchowski, 2. Phys. Chem., 1917,92, 129. 5 H. Reerink and J. Th. G. Overbeek, Discuss. Faraday SOC., 1954, 18, 74. 6 D. L. Swift and S. K. Friedlander, J. Colloid Sci., 1964, 19,621. 7 R. H. Ottewill and J. N. Shaw, Discuss. Faraday SOC., 1966, 42, 154. 8 P. Meakin, Phys. Rev. Lett., 1983,51, 1119. 9 A. A. Potanin, J. Colloid Interface Sci., 1993, 157, 399. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2093 10 D. Schaeffer, J. E. Martin, P. Wiltzius and D. S. Cannel, Phys. 24 T. M. Herrington and B. R. Midmore, J. Chem. Soc., Faraday Rev. Lett., 1984, 52, 2371. Trans. I, 1989,85,3529. 11 D. A. Weitz, J. S. Huang, M. Y. Lin and J. Sung, Phys. Rev.25 N. K. Tovey, Cambridge University Engineering Department Lett., 1985, 54, 1416. Technical Report CUED/C-SOILS/TRS, 324.4/5, 197 1, Cam-12 M. Y. Lin, R. Klein, H. M. Linsay, D. A. Weitz, R. C. Ball and bridge.P. Meakin, J. Colloid Interface Sci., 1990, 137, 263. 26 P. Smart and N. K. Tovey, Electron Microscopy of Soils & Sedi-13 G. J. von Dongen and M. H. Ernst, Phys. Reo. Lett., 1985, 54, ments, Oxford Science Publications, Oxford, 1981, p. 15. 1396. 27 Th. F. Tadros and A. Hopkinson, Faraday Discuss. Chem. SOC., 14 A. Weiss and R. Frank, 2. Naturforsch., 1961,86, 141. 1990,90,41. 15 A. W. Flegmann, J. W. Goodwin and R. H. Ottewill, Proc. Br. 28 M. K. Packman and Th. F. Tadros, Colloids Surf.,1992,67,265. Ceram. SOC., 1969,13,31. 29 Th. F. Tadros, W. Liang, B. Costello and P. F. Luckham, Col- 16 S. K. Nicol and R. J. Hunter, Aust. J. Chem., 1970, 23, 2177. loids Surf, 1993,79, 105. 17 B. Rand and I. E. Melton, J. Colloid Interface Sci., 1977,66, 308. 30 P. A. Theisen, Z. Elektrochem., 1942,48,675. 18 U. Brandenburg and G. Lagaly, Appl. Clay Sci., 1988,3,263. 31 P. A. Theisen, 2. Anorg. Chem., 1947,253,161. 19 G.Lagaly, Appl. Clay Sci., 1989,4, 105. 32 H. van Olphen, Clays, Clay Miner., 1956,4, 204; 1956,6, 196. 20 H. van Olphen, in An Introduction to Clay Colloid Chemistry, 33 J. Schweitzer and B. R. Jennings, J. Colloid lnterface Sci., 1971, Wiley, New York, 2nd edn., 1977, ch. 5. 37, 443. 21 M. Y. Lin, H. M. Linsay, D. A. Weitz, R. C. Ball, R. Klein and 34 D. A. Weitz and M. Oliveria, Phys. Rev. Lett., 1984,52, 1433. P. Meakin, Proc. R. SOC.London, A, 1989,423, 71. 35 T. M. Herrington, B. R. Midmore and A. Lips. J. Chem. SOC., 22 J. G. Rarity, R. N. Seabrook and R. J. G. Carr, Proc. R. SOC. Faraday Trans., 1990,86,2961. London, A, 1989,423,89. 23 S. D. T. Axford, T. M. Herrington and B. R. Midmore, Colloids Surf.,1992,69, 73. Paper 3/07002F; Received 24th November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949002085

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2095-2098 Direct Observation of Aluminium Guest Ions in the Silicate Phases of Cement Minerals by 27AI MAS NMR Spectroscopy Jsrgen Skibsted and Hans J. Jakobsen Department ofChemistry, University ofAarhus , DK-8000 Aarhus C, Denmark Christopher Hall Schlumberger Cambridge Research, PO Box 153,Cambridge, UK CB3 OEL The principal mineral phases in Portland cements are the impure forms of the calcium silicates, Ca,SiO, and Ca,SiO,, known as alite and belite, in which the silicates are modified in composition and crystal structure by incorporation of guest ions such as Mg2+, A13+ and Fe3+. This work reports the first direct evidence for guest- ion substitution in alite and belite by the observation of Al substitution employing 27AI magic-angle spinning (MAS) NMR.For both minerals A13+ is observed to substitute for Si4+ and it is shown that 27AI MAS NMR can be used to quantify the amounts of Al substitution in the silicate phases of ordinary Portland and oilwell cements at levels well below 1 wt.%. The 27AI quadrupole coupling parameters and isotropic chemical shift (6) have been determined for the unique At guest-ion site in belite, which exhibits the most deshielded chemical shift (6 = 96.1) yet reported for a tetracoordinated Al bonded to four oxygens. Portland cements used in the construction industry usually contain 50-70 wt.% alite (Ca,SiO,), 5-25 wt.% belite (Ca,SiO,), 0-15 wt.% tricalcium aluminate (Ca3A1,0,) and 5-15 wt.% ferrite [Ca,(Al,Fe,-,),O, ;0 < x 5 0.71.The sili- cate phases, alite and belite, are the main hydraulic com- ponents responsible for strength development during hydration. Although both silicates can exist in several crys- talline modifications, metal-ion impurities incorporated into the lattice during manufacture normally stabilize the mono- clinic forms M,, MI, and MI,, of alite and strongly stabilize the monoclinic P-form of belite.' However, the presence of such ions can greatly affect the physical and chemical properties of cements. In Portland cements MgO, A1203 and Fe203 are the main oxide impurities of both silicates. Typical concentrations of Al,O, are 1 wt.% in alite and 2 wt.% in belite.' Experimental methods for characterising the nature and quantity of such impurities are essential for understand- ing the material properties of Portland cements, especially the variation in the hydrational reactivity which controls strength development.High-speed '7Al MAS NMR spectroscopy3 has proven a powerful tool in studies of a range of different materials since the 27Al isotropic chemical shift (6) distinguishes clearly between A1 in tetrahedral (AIIv) and in octahedral (AlvI) coordination., Furthermore, the 27Al quadrupole coupling constant (C,) and asymmetry parameter (q)reflect the electric field gradients (EFGs) at the nuclear site and therefore provide additional information about the elec- tronic environments at the A1 nucleus, as recently shown for some aluminate phases of cement materials., This work describes the nature of the incorporation of A1 as an impurity ion in laboratory samples of alite and belite and in an ordi- nary Portland and an oilwell cement as studied by 27Al MAS NMR.Experimental The laboratory samples of alite and belite were provided by Aalborg Portland A/S, Denmark. Powder X-ray diffraction (XRD) of the two samples showed that the alite has a mono- clinic form (MI and/or M,,,) and the belite the monoclinic /?-form. X-Ray fluorescence (XRF) analysis revealed an A1203 content of 1.1 f0.1 wt.% for the alite and 0.7 f0.1 wt.% for the belite sample. Furthermore, XRF showed the presence of minor quantities of MgO, Fe203, Mn,03, P20, and K20 in both samples. The phase compositions for the commercial ordinary Portland and oilwell cement were estimated from a modified Taylor-Bogue using the data from bulk chemical analysis.Solid-state 27Al MAS NMR spectra were obtained on Varian XL-300 (7.1 T) and UNITY 400 (9.4 T) spectrometers equipped with home-made 'A1 background-free' cross-polarisation (CP)/MAS probes for 4 and 7 mm 0.d. Si3N, rotors7 and employing spinning speeds up to 16 kHz. Single-pulse excitation using an rf field strength yB1/2n = 60 kHz, a 0.7 ps pulse (l5O flip angle), and a relaxation delay of 1 s ensured reliable quantitative result^.^.^ The Al,O, contents of the samples were deter- mined from the intensities of the central transition using weighed samples and the "A1 MAS NMR spectrum of syn- thetic Ca,A120, as standard.The results are compared with the data from XRF and X-ray microprobe analysis. "A1 Chemical shifts (6) are reported in ppm relative to an external sample of 1.0 mol dmV3 AlC13.6H20. Simulations of the solid-state 27Al MAS NMR spectra were performed on the SUN Sparc 10 work stations of the UNITY 400 spectrometer using a general software package developed in the Aarhus laboratory for simulation of MAS/variable-angle spinning (VAS) NMR spectra of quadrupolar nuclei." Results and Discussion The high-speed "Al MAS NMR (9.4 T) spectrum of the belite sample is shown in Fig. l(a) and reveals two central transitions for A1 incorporated into the belite. In the Al,, chemical shift range (ca. 100-50 ppm) the edge at 91 ppm, and the singularities at 77 and 56 ppm, characterize the quad- rupolar lineshape for a single A1 site.The Al,, shift range (ca. 20 to -10 ppm) displays an asymmetric resonance with a centre of gravity at -0.6 ppm. The lineshapes of the simu- lated spectrum in Fig. l(b) are in excellent agreement with the experimental spectrum. The simulation employs quadrupole coupling parameters corresponding to a single Al,, site with the optimized parameters C, = 7.1 & 0.1 MHz, q = 0.33 f0.05 and 6 = 96.1 0.5 and to a gaussian dis- tribution of EFG tensor elements for the Al,, resonance. The optimized simulation of the asymmetric resonance for the Al,, site is obtained by summation of a set of simulated quadrupolar lineshapes corresponding to a gaussian distribu- tion of the V,, and V,, EFG tensor elements (I V,, I >, I V I >, V,, 1 ) as recently performed for an amorphous glass.' lyyThis procedure gives 6 = 10.1 and the mean quadrupole coupling J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 120 100 80 60 40 20 0 -20 -40 -60 -80 6 Fig. 1 (a) 27Al MAS NMR spectrum of A1 guest ions in belite recorded at 9.4 T using a spinning speed v, = 15.5 kHz (4 mm rotor) and 101 680 scans. (b)Simulation of the partly overlapping lineshapes of the Al,, and Al,, resonances in (a) using an Al,, : Al,, intensity ratio of 58 :42. For the single Al,, site the simulation employed the optimized 27Al quadrupole coupling parameters and isotropic chemi- cal shift in Table 1. For the asymmetric resonance from the Al,, site (centre of gravity at -0.6 ppm) a set of simulated quadrupolar line- shapes corresponding to a gaussian distribution of the V,, and V,, EFG tensor elements is employed (see text).parameters c, = 4.5 MHz and ij = 0.4 for the Al,, site employing a gaussian distribution function with a halfwidth of 0.75vzz(= 0.75CQh/eQ)for the V,, and V,, tensor elements. Quantitative 27Al MAS NMR,**' using the combined inten- sity for the central transitions from Al,, and Al,,, and a syn- thetic sample of Ca,Al,O, as standard, gives a bulk Al,O, content of 0.7 wt.% for the belite sample, which is in excellent agreement with the result from XRF analysis (see Table 1). The observation of a single Al,, resonance for belite demon- strates that A1 substitutes for Si,, because the crystal struc- ture for /3-Ca,Si04 shows that all Si atoms are equivalent.', We note that the value 6 = 96.1 determined for the Al,, site, although close to the shifts observed for calcium alum in ate^,^ represents the most deshielded chemical shift yet reported for tetracoordinated A1 bonded to four oxygen^.'^ This high- frequency chemical shift further supports substitution of A1 for Si into the crystal structure for /3-Ca,Si04 (Si-0 bond lengths of 1.61-1.65 A)', since we would expect the A1-0 bonds for the A1 guest ion in belite to be shortened relative to the Al,,-O bond lengths in calcium aluminates (1.73-1.81A).'. A less ionic character is therefore predicted for the A1-0 guest-ion bonds in belite which should lead to a shift to higher frequency consistent with earlier observations.' The absence of octahedral sites or vacancies in the crystal structure for /3-Ca,Si04 suggests that the Al,, resonance b..k= 500 400 300 200 100 0 -100 -200 -300 6 Fig.2 27Al MAS NMR spectra of monoclinic alite illustrating that A1 substitution for Si,, is the dominant mode for the incorporation of A1 in alite. (a) Experimental spectrum at 9.4 T (v, = 15.6 kHz, 4 mm rotor, 61440 scans) with (b)expansion of the central transition for the Al,, resonance. (c) Expansion of the central transition in the experimental spectrum at 7.1 T (v, = 8.0 kHz, 7 mm rotor, 8192 scans). The asterisk in (a)indicates the Al,, resonance observed in the alite sample; this corresponds to 3% of the total aluminium intensity.originates from a separate aluminate phase. Furthermore, the distribution of quadrupole coupling parameters observed for the Al,, resonance [Fig. l(b)] strongly indicates that the addi- tional phase is amorphous or contains slightly different Al,, sites. This observation is consistent with a recent electron mi- croscopy study of Al,O,-doped belites, where an amorphous Al-rich grain-boundary phase was observed between the belite grains.16 On this basis we propose that A1 for Si,, sub-stitution is accompanied by charge-balancing oxygen vacancies as earlier proposed by Regourd et al.' 27Al MAS NMR spectra (central transition) of the alite sample (Fig. 2) show an Al,, quadrupolar broadened line- shape [from ca.88 to 30 ppm at 9.4 T, Fig. 2(a) and (b)] which further broadens at 7.1 T [ca. 90-20 ppm, Fig. 2(c)]. The centre of gravity is about 75 ppm at both field strengths. The lineshape bears no resemblance to the well known C,, q-dependent quadrupolar lineshape for a single A1 site. However, it can be ascribed to the superposition of several lineshapes originating from A1 substituting for different Si tetrahedral positions in alite. Simulations of the lineshapes at 7.1 and 9.4 T in Fig. 2, using a set of quadrupolar lineshapes, indicate that the quadrupole coupling parameters and the isotropic shifts are in the range 3.5 5 CQ/MHz 5 7.0, 0.1 5 q 5 0.7 and 80 5 6 5 91 for the A1 guest-ion sites in alite. The observation of several Al,, sites is fully consistent with the broadening observed in the 29Si MAS NMR spectrum" and the crystal structure" of monoclinic alite which contains 18 different Si,, sites in the asymmetric unit for the MI,, form.Table 1 Optimized 27Al MAS NMR data (C,,q and 6) for A1 guest ions in belite and alite and A1,0, contents for belite, alite and for these phases and Ca,Al,O, of an oilwell and ordinary Portland cement wt.% A1,0, sample CQ/MHz v 6IPPm 27Al MAS NMR" XRF or X-ray microanalysis belite Al,, Alm 7.1 _+ 0.2 4.5' 0.33 f0.05 0.4' 96.:01z0.5 ] 0.7 & 0.1 0.7 fO.lb alite oil well cement ca. 3.5-ca. 7' 0.1 5 q 5 0.7' 80 5 6 5 91' 0.9 f0.1 1.5 f0.2 1.1 fO.lb 1.5 f0.1 (alite)' ordinary Portland cement silicate phases 1.1 f0.2 1.7 0.1 (belite)' I Ca,Al,O, ~ phase 1.9 f0.2 I a From the intensity of the central transitions using a 27Al MAS NMR spectrum of synthetic Ca,Al,O, as standard.From XRF analysis. 'Parameters corresponding to a gaussian distribution of the V,, and V,, EFG tensor elements (see text). Estimated range for the C,, q and 6 values from simulation of the central transition at 7.1 and 9.4 T using a set of quadrupolar lineshapes. From X-ray microanalysis of the alite and belite phases in unground clinkers from the same source. Not determined. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 XRD investigations of doped Ca,SiO, have suggested that besides replacing Si,, , A1 can substitute for octahedrally coordinated Ca or vacancies.,' Although a low-intensity Al,, resonance [Fig.2(a)], observed at 4 ppm and constituting 3% of the total intensity, may arise from this type of substitution, the present study shows that A1 substitutes almost exclusively for tetrahedral Si sites in alite. Furthermore, an examination of the Ca-0 bond lengths in the monoclinic MI,, form of alite demonstrates that substitution of A1 for Ca would entail excessively long A1-0 bonds. Thus, again charge-balancing is most likely achieved by creation of one oxygen vacancy for each pair of substituted A1 ions. We note that the A1,0, content of 0.9 wt.%, determined for the alite sample from the intensities of the central transitions in Fig. 2(4, is in excellent agreement with an XRF analysis (Table 1).Furthermore, this value is a typical A1203 impurity level for alites in normal Portland cements.' Direct observations of A1 substitution in the silicate phases of production cements are illustrated by the NMR spectra of an oilwell and an ordinary Portland cement in Fig. 3. Oilwell cements are Portland cements with characteristically low bulk Al,O, : Fe203 ratios (typically < 1.0) and are used in well construction as slurries which are pumped between the wellbore wall and steel casing.'.6*21 For these cements the tricalcium aluminate phase (Ca,Al,O,) is usually absent and A1occurs mainly in the ferrite phase [Ca,(Al,Fe, -J205; 0 < x5 0.71 which is extremely difficult to detect by 27Al MAS NMR because of its high Fe3+ content.The strong similarity of the NMR spectra for the oilwell cement [Fig. 3(a)] and the synthetic alite [Fig. 3(b)] demonstrates the great potential of 27Al MAS NMR to provide detailed information on A1 substitution in the silicate phases of cements. Consider- ing the phase composition of the oilwell cement (70 wt.% alite, 7 wt.% belite and 20 wt.% ferrite from a modified Taylor-Bogue we expect that its NMR spec- trum [Fig. 3(a)] also includes a minor A1 contribution from the belite phase. Quantitative 27Al MAS NMR shows that a total A1 content corresponding to 1.5 0.2 wt.% Al,O, is incorporated into the alite and belite phases of the oilwell cement. By direct X-ray microanalysis of unground clinker from the same source, we obtain comparable Al,03 contents for the two phases (see Table 1).For the ordinary Portland cement (74 wt.% alite, 7 wt.% belite, 10 wt.% tricalcium aluminate and 6 wt.% ferrite2V6) the MAS spectrum [Fig. 3(c)] shows an asymmetric peak at 79 ppm superimposed on a broad resonance extending from CQ. 100 to -40 ppm. These resonances are assigned to A1 in alite-belite and to the tricalcium aluminate phase, respec- tively. Contrary to the well defined quadrupolar lineshapes observed for the two non-equivalent Al,, sites in synthetic Ca,Al,O, ,3*5 the tricalcium aluminate resonance in Fig. 3(c) is featureless. This is ascribed to the incorporation of impu- rity ions in the Ca3Al,0, lattice in Portland cements,' resulting in distortions in the local environments of the AlO, tetrahedra.Fig. 30 illustrates that the lineshape of the broad tricalcium aluminate resonance can be simulated using a gaussian distribution of the elements for the two EFG tensors for Ca3Al,06 ., Deconvolution [Fig. 3(d)] of the spectrum of the ordinary Portland cement, employing the simulated 27Al spectra of alite [Fig. 3(e)] and tricalcium aluminate [Fig. 301, leads to a quantification of Al,O, of 1.1 f0.2 wt.% and 1.9 +_ 0.2 wt.% for the alite-belite and tricalcium alumin- ate phases, respectively. The latter quantity corresponds to 4.9 f0.5 wt.% Ca3A1,0, which we consider an improved value for the calcium aluminate content compared with the somewhat uncertain estimate (10 wt.%) obtained from the Taylor-Bogue We emphasize that high-speed spinning is a prerequisite for the observation (and thereby ll! 1 ~60""'40-4 IA \ i\ /AT------I'..'I.'~'I....I'..'I''"~l'.I I.'..250 200 150 100 50 0 -50 -100 -150 6 Fig. 3 27A1 MAS NMR spectra (9.4 T, v, = 15.8 kHz,4 mm rotor) of an oilwell and an ordinary Portland cement. (a) Experimental spectrum of the oilwell cement (26 750 scans). The close similarity of the expanded Al,, lineshape in (a) and that observed for the alite sample (b) from Fig. 2 demonstrates that A1 is incorporated exclu- sively into the silicate phases (mainly alite) of the oilwell cement. (c) Experimental spectrum of the ordinary Portland cement (20736 scans) showing overlap of resonances from (i) A1 guest ions in alite (and belite) and (ii) the tricalcium aluminate phase.(d) Optimized simulated spectrum for the Portland cement obtained by summation of the simulated lineshapes for Al in (e) the alite phase and cf)the tricalcium aluminate phase. The simulated alite spectrum (e) is obtained by addition of a set of quadrupolar lineshapes with C,, q and 6 values in the range 3.5 < Co/MHz < 7.0, 0.1 < q < 0.7 and 80 < 6 < 91. The simulated tricalcium aluminate spectrum cf) employs addition of a set of quadrupolar lineshapes for each of the two All, sites in Ca,A120, using a gaussian distribution of the V,, and V,, EFG tensor elements similar to the simulation in Fig. l(b). The C,, q and 6 data reported for the two All, sites in synthetic Ca,AI,O, 3*5were used as mean values for these parameters and a gaussian distribution function with a halfwidth of O.lV,, for the V,., and V,, tensor elements were employed for both A1 sites.quantification) of undistorted central transitions for the broad resonances from the Ca,Al,O, phase in Portland cements. In conclusion, our results demonstrate that A1 prefer- entially substitutes for tetrahedrally coordinated Si in the sili- cate phases, alite and belite, of these cements. MAS NMR represents an elegant and very sensitive tool for 2098 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 detailed characterization and quantification of the discr sites for this guest ion in cement and similar minerals. A1 9 10 D. Massiot, C. Bessada, J. P. Coutures and F.Taulelle, J. Magn. Reson., 1990,90,231. J. Skibsted, N. C. Nielsen, H. Bildsse and H. J. Jakobsen, J. The use of the facilities at the University of Aarhus ivdR Laboratory, sponsored by Teknologistyrelsen, The Danish 11 Magn. Reson., 1991,95,88; J. Am. Chem. SOC., 1993,115,7351. C. Jager, G. Kunath, P. Loss0 and G. Scheler, Solid State Nucl. Magn. Reson., 1993, 2, 73. Research Councils (SNF and STVF), Carlsberg-fondet, and Direktrar Ib Henriksens Fond, is acknowledged. We thank Aarhus University Research Foundation for equipment grants and the Danish Natural Science Research Council for financial support to J.S. (J. No. 11-9160). 12 13 14 K. H. Jost, B. Ziemer and R. Seydel, Actu Crystallogr., Sect. B, 1977,33,1696. J. F. Stebbins, in Handbook of Physical Constants, ed.T. Ahrens, American Geophysical Union, Washington DC, 1993. D. Muller, W. Gessner, A. Samoson, E. Lippmaa and G. Scheler, Polyhedron, 1986,5779. 15 D. Muller, W. Gessner, A. Samoson, E. Lippmaa and G. Scheler, J. Chem. SOC., Dalton Trans., 1986, 1277. References 16 C-J. Chan, W. M. Kriven and J. F. Young, J. Am. Ceram. SOC., H. F. W. Taylor, Cement Chemistry, Academic Press, London, 1990. H. F. W. Taylor, Adv. Cem. Res., 1989,2, 73. J. Skibsted, H. Bildsse and H. J. Jakobsen, J. Magn. Reson., 1991,92,669. D. Muller, W. Gessner, H-J. Behrens and G. Scheler, Chem. Phys. Lett., 1981, 79, 59. 17 18 19 1988,71, 713. M. Regourd, M. Bigark, J. Forest and A. Guinier, in Proceedings of the Fifth International Symposium on the Chemistry of Cement, Cement Association of Japan, Tokyo, 1969, vol. I, p. 44. J. Hjorth, J. Skibsted and H. J. Jakobsen, Cem. Concr. Res., 1988, 18, 789. F. Nishi, Y.Takeuchi and I. Maki, 2. Kristullogr., 1985, 172, 297. J. Skibsted, E. Henderson and H. J. Jakobsen, Inorg. Chem., 1993,32,1013. T. B. Bergstrom, C. Hall and K. L. Scrivener, Adu. Cem. Res., 1991/92,4, 141. H. J. Jakobsen, P. Daugaard and V. Langer, J. Magn. Reson., 1988,76, 162. 20 21 T. Hahn, W. Eysel and E. Woermann, in Proceedings of the Fifth International Symposium on the Chemistry of Cement, Cement Association of Japan, Tokyo, 1969, vol. I, p. 61. Well cementing. Developments in Petroleum Science, No. 28, ed. E. B. Nelson, Elsevier, Amsterdam, 1990. A. Samoson and E. Lippmaa, Phys. Rev. B, 1983,28,6567. Paper 4/00906A; Received 15th February, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002095

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2099-2106 Physicochemical and Catalytic Properties of Polyaniline Protonated with 12-Molybdophosphoric Acid M. Hasik and A. Pront Department of Materials Science and Ceramics, Academy of Mining and Metallurgy, 30459 Krakow, Mickie wicza 30, Poland J. Pozniczek and A. Bielanski Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, 30-239 Krako w, Niezapomina jek , Poland Z. Piwowarska, K. Kruczala and R. Dziembaj Faculty of Chemistry, Jagiellonian University, 30-060 Krako w, lngardena 3, Poland Using acid-base-type doping of polyaniline with H,PMo, 2040,two series of catalyst samples were prepared. In series SI the doping and polymerization were carried out simultaneously, leading to a uniform distribution of the dopant over the whole volume of the polymer. In series SII the doping was achieved by protonation of already formed polyemeraldine base with H,PMo, 2040.Since this process must involve diffusion in the solid matrix, the surface becomes enriched in heteropolyanions (Keggin units) due to their limited diffusivity.The examination of physicochemical properties of the samples by XP, EPR and FTIR spectroscopies indicated that the Keggin units incorporated into the polymer matrix retain their identity and represent catalytically active centres. The insertion of the dopant species increases the electrical conductivity of the polymer by several orders of magnitude. The catalytic activity was tested in ethyl alcohol conversion and both the products of acid-base [C,H, and (C,H,),O] and redox (CH,CHO) type reactions were observed.Owing to the surface enrichment of the SII series of cata-lysts their catalytic activity was much higher than that of samples of the SI series nominally doped to the same level. In both types of catalysts the selectivity to the redox reaction product (CH,CHO) was greatly enhanced in comparison with unsupported H,PMo, 2040. Heteropolyacids (HPA) and their salts have been extensively studied because of their interesting catalytic properties.' Sig- nificant research effort has been directed towards the entrap- ment of heteropolyanions in suitable polymeric matrices with the main goal of preparing a new type of polymer-supported catalyst for various applications in heterogeneous and elec- trocatalysis.This entrapment can be conveniently achieved if electroactive conjugated polymers are used as host matrices. Two approaches to this problem can be used: (i) Hetero- polyanions can be incorporated into the growing polymeric matrix during electrochemical or chemical polymerization of a suitable monomer. In this case the growing polymer chains are of a cationic nature and the inserted heteropolyanions serve as charge-compensating species. This approach has .~~~been used by Keita et ~21 and Bidan et in the electro- ~22.~9~ chemical preparation of heteropolyanions containing poly- aniline and polypyrrole. (ii) Alternatively, heteropolyanions can be incorporated into the polymer matrix oia the so-called doping reaction.In this case the process of polymer matrix formation and the process of incorporation of hetero-polyanions into this matrix are separated in time. In other words heteropolyanions are introduced into the already formed, neutral solid polymer. The doping process consists therefore of the transformation of neutral polymer chains into polycations and the simultaneous incorporation of het-eropolyanions. This doping reaction can be oxidative in its t Also Department of Chemistry, Technical University of Warsaw, 00-664 Warszawa, Noakowskiego 3, Poland. nature, as in the case of p~lyacetylene,~or acid-base, as in the case of polyaniline.* Taking into account the large size of heteropolyanions and their limited diffusivity into the bulk of the solid polymer, it is expected that doping will be limited to the surface or near-to-surface layer of the polymer; this has been proven e~perimentally.~ Previously7 we demonstrated that polyacetylene doped with 1Zmolybdophosphoric acid exhibited a high catalytic activity in ethyl alcohol conversion compared to the unsupported crystalline 12-molybdo-phosphoric acid studied under the same experimental conditions.The overall increase in catalytic activity is accom- panied by a significant change in the redox activity over the acid-base one. Polyaniline is another example of a polymer host which can accommodate anions originating from 12-molybdo-phosphoric acid. This can be achieved either by poly- merization of aniline in the presence of H,PMo,,O,, in one step" or by doping, i.e.the protonation of the poly-emeraldine base with H,PMo,,O,, in a two-step pro-cedure.* Our preliminary experiments with H,PMo~~O~,- doped polyaniline have shown that distinct differences exist in the catalytic behaviour of the samples obtained by these two methods." The catalyst prepared by the latter method exhibited much higher activity in the ethyl alcohol conver- sion than the catalyst prepared according to the former method, despite the fact that both had similar surface areas. In order to explain these differences the present study of the physicochemical properties of our catalysts was undertaken. FTIR, XP and EPR spectroscopies as well as electrical conductivity measurements were applied. Catalytic experi- ments were used in this case as an additional method of char- acterizing surface properties of the samples.Experimental Preparation of Polyaniline Protonated with 12-Molybdophosphoric Acid Polyaniline can be rendered conductive either by the oxida- tive or acid-base doping according to the scheme: 1 ti -2e 2 Polyaniline in the form of polyleucoemeraldine can be oxi- datively doped by withdrawal of electrons from its n-bonding system. This oxidation process results in electrical charge introduction into the polymer matrix and its transformation into an organic conductor. If one electron per two polymeric units of polyleucoemeraldine is withdrawn, the polymer reaches the oxidation state of polyemeraldine, which is stable in air both in its conducting polyemeraldine salt form and in its insulating polyemeraldine base form.The base can easily be converted into the salt form by protonation with a suffi- ciently strong acid.12 The anions of the protonating acid are simultaneously incorporated into the polymer, thus neutral- izing the positive charge of polymer chains. As already stated, this doping of the polyemeraldine base with dodecamolybdo- phosphoric acid can be achieved either in a one-step or a two-step process. Both were used in the present study. One-step Procedure Polyaniline protonated with H,PMo, 2040was prepared in a one-step procedure using a modification of the method described in ref.10. In all preparations the amounts of aniline and H,PMo,,O,, were fixed (8.25 and 2.60 mmol, respectively) whereas varying amounts of the oxidant were added (increasing amounts from 1.29 to 6.88 mmol). Typically aniline and H,PMo120,, were dissolved in 50 ml of acetoni- trile. Then ammonium persulfate dissolved in 1.5 ml of H20 was added dropwise. The reaction mixture was kept at room temperature for 24 h with constant stirring. Reaction was ter- minated by pouring the reaction mixture to 250 ml of acetone which caused immediate precipitation of a black powder. The powder was then separated by centrifugation and repeatedly washed with acetone until the filtrate became colourless. Polyaniline-H,PMo, 2040prepared in such a manner con- tains significant amounts of sulfur as determined by elemental analysis.Evidently the HSO, ions created upon the reduction of the persulfate ion in the acidic medium are co- inserted with heteropolyanions into the polymer matrix. These hydrogenosulfate anions can conveniently be removed from polyaniline by prolonged washing with water. Washing results in effective removal of HSO, ions via deprotonation, whereas heteropolyanions remain essentially intact. Because of the HSO, co-insertion mentioned above, the content of heteropolyanions in the polymer matrix must be dependent on the concentration of the oxidant in the reaction medium. More persulfate will create more HSO, and thus fewer sites will be available for the protonation with heteropolyanions.This is indeed the case. The heteropolyanion content in the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 polymer matrix decreases markedly with increasing oxidant/ aniline ratio (see Table 1). In the subsequent text all samples prepared in the one-step procedure are denoted as SI, (n = 1, 2, 3, . . .). Two-step Procedure In the two-step procedure the polyemeraldine base was first synthesized and then protonated with 12-molybdophosphoric acid. The polyemeraldine base was prepared by condensation-polymerization of aniline in aqueous HCl using (NH,),S208 as the oxidant. The polymerization under these conditions leads to polyemeraldine hydrochloride, which must be then converted into polyemeraldine base by deprotonation with a suitable base (NH,).In a typical prep- aration 10.2 g (0.1093 mol) of aniline was dissolved in 125 ml of 1.5 mol 1-' HCl. Then 12.5 g (0.0548 mol) of ammonium perchlorate was dissolved in 125 ml of 1.5 mol I-' HCl and added dropwise to the aniline-HC1 solution. The reaction was carried out at 0-1 "C for 4 h as recommended in ref. 13. The black precipitate obtained was filtered off, washed with water and subsequently with methanol and diethyl ether until in each washing the filtrate was colourless. The polymer was then vacuum dried to constant mass and then deprotonated with 3% NH, aqueous solution for 2 h. Deprotonated poly- emeraldine was dried to constant mass and washed again with water, methanol and diethyl ether in a Soxhlet appar- atus.This final washing is essential for total removal of all oligomeric species still extant in the system. The polyemeraldine base was then protonated in acetonitrile-HPA solutions. Note that for a given HPA : polyemeraldine ratio in the protonating medium a partition equilibrium of HPA between two phases (polyemeraldine and acetonitrile) is easily estabilished. Thus the protonation level can conveniently be varied by changing the HPA : polyemeraldine ratio (Table 2). The samples pre- pared according to the two-step procedure are abbreviated as SII,(n = 1,2,3...). Characterization of Unprotonated and Protonated Pol yerneraldine The samples of polyemeraldine base and polyemeraldine protonated with 1Zmolybdophosphoric acid were subjected to elemental analysis.Nitrogen, carbon, hydrogen, molyb- denum and sulfur were determined using classical methods. The specific surface area of the obtained powders was mea- sured using the BET method on a Carlo Erba Sorpty 1750 apparatus. Thermogravimetric (TG and DTG) and differential thermal analyses of the samples were carried out with a Mettler Thermoanalyser TA-2 using Al,O, as the standard substance. Table 1 Elemental analysis and conductivity of C6H4.5N(H3PMo12040)yprepared in the 'one-step procedure' involving oxidation of aniline in the presence of 12-molybdophosphoric acid aniline : persulfate empirical formula conductivity sample molar ratio based on N : Mo /S cm-' SI, SI2 SI3 SI4 SI5 1.2: 1 4.8 : 1 6.4 : 1 2.0 : 1 3.2 : 1 c,H,,~N(HPA),,,,,~ C,H4~5N(HPA),,,,3 C6H4,5N(HPA),,l,, C,H4~5N(HPA)o.056 C6H4,5N(HPA)o,075 7.0 x 10-4 1.8 x 1.9 x 1.3 X 1.7 x lop3 a HPA = 12-molybdophosphoric acid.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Protonation of polyemeraldine base with 12-molybdophosphoric acid ~~ ~ HPA :polyaniline molar ratio in the protonating empirical formula conductivity sample medium'" based on N : Mo /S cm-* SII 0.0025 C6H,,sN(HPA)o,oo24b 8.2 x SII, 0.0050 C,H,,,N(HPA),,,,,, 6.1 x loa6 SII 0.010 C6H4,sN(HPA)O~O077 6.4 x lo-, SII, 0.015 C6H,,5N(HPA)o,o13 3.6 X SII5 0.020 C6H,~SN(HPA)o~o,5 2.3 X SII, 0.025 C6H,~SN(HPA)o,o16 2.8 X lo-, SII , 2.000 C,H,,SN(HPA)o,o,7 7.0 x lo-, a Concentration of the doping solution 0.001 mol 1-'.HPA = H,PMo,,O,, * The electrical conductivity of the protonated polyaniline was measured on pressed pellets (4 t cm-') using a classical four-probe method. Since the conductivity of polyaniline depends on the water vapour pressure in air, the samples were dried in vacuum and the conductivity was also mea- sured in vacuum. FTIR spectra were recorded on a Digilab FTS-60 spec- trometer in transmission mode using KBr pellets. The X-ray photoelectron (XP) spectra were taken on powders of polyaniline base or of polyaniline protonated with H,PMo,,O,, pressed onto a copper mesh. XP spectra recorded on an ESCA 100 (VSW Manchester) using Mg K, radiation of 1253.6 eV under pressure lower than 5 x lo-' mbar. This low pressure was achieved in the case of proto- nated polyemeraldine after 24 h of evacuation at 27 "C. As an internal standard C 1s = 284.5 eV was applied.In the course of recording the spectra the samples were cooled to -123"C to improve the quality of the spectra. EPR spectra were recorded by means of a computer-controlled spectrometer operating in the X-band with 100 kHz modulation. DPPH was used as the g-factor standard (g = 2.0036). EPR spectra of the polyemeraldine base and of polyemeraldine protonated with H,PMo,,O,, at room and at liquid-nitrogen temperatures were recorded. Additionally the EPR measurements were carried out while conducting the catalytic reaction on sample SII, in the specially designed reactor that was placed in the spectrometer cavity at elevated temperatures (175-240 "C) and after annealing the reaction mixture to -170°C.Catalytic Experiments As in the previous st~dies,~,'~ conversion of ethyl alcohol was used as the catalytic test reaction. The experiments were carried out in the same pulse micro- reactor in which 0.3 g of the catalyst mixed with 0.3 g of powdered quartz was placed. Before the catalytic experiments the samples were standarized by heating them for 30 min at 50, 100 and 150°C and for 1 h at 200°C. The sample pre- pared in the one-step procedure was only weakly catalytically active and in order to obtain measurable amounts of pro- ducts it was necessary to carry out catalytic testing at 300 "C. On the other hand the sample obtained by the two-step pro- cedure was much more active and the catalytic test could be carried out at 240"C, ensuring better thermal stability of the polymer matrix.Results The experiments were carried out using three samples: the undoped polyaniline base (PA), polyaniline doped with 21G1 heteropolyacid using the one-step procedure (SI,) the com- position of which corresponded to the formula C6H,~,N(HPA),,,,, (36.9 wt.% of H3PMo,,0,,) and poly- aniline doped with HPA (SII,) obtained in the two-step pro- cedure of the composition C6H,~,N(HPA),,,,, (35 wt.% H,PMo,,O,,). The specific surface areas of samples PA, SI, and SII, were 20.1, 20.2 and 24.7 m2 g-',respectively. The results of TG, DTG and DTA analyses are shown in Fig.1 Both SI, and SII, exhibited a mass loss between CQ. 50 and 150°C characterized by a DTA peak at 100°C. This was interpreted as the desorption of molecules of adsorbed (or possibly occluded) solvent molecules or adsorbed water ii IL I I I I I I I 50 100 200 300 400 500 600 Tl" C iii-4-cad I1 1 I I 50 100 200 300 400 500 600 T/OC Fig. 1 Thermogravimetric (i), differential thermal (ii) and differential gravimetric (iii) analyses of: (a) unsupported H,PMO~,O~~f26.3 H,O, (b)undoped polyemeraldine base, (c) HPA-doped polyaniline in a one-step procedure, SI,, (6)HPA-doped polyaniline in a two- step procedure SII,. Heating rate 10°C min-', sample weight 50 mg. vapour. In this period SI, lost 7.5% of its mass and sample SII, only 4.3%.Both samples exhibited practically constant mass between 200 and 270-280°C, above which temperature slow decomposition of the polymer was recorded. FTIR spectra of the samples prepared in both one- and two-step procedures for a given protonation level are essen- tially indistinguishable. In Fig. 2 IR spectra of samples of the SII series are presented and compared with those of the poly- emeraldine base PA and the crystalline heteropolyacid. With an increasing protonation level the modes originating from heteropolyanions grow in intensity and become dominant for higher protonation levels. "-'Note that the two bands which are diagnostic of Keggin structural units are clearly observed in heteropolyanions incorporated into the polymer : v(M-0,) at 954 cm-' and v(x-0,) at 1067 cm-' [Fig. 2(d)].The v(M-0,) band is not obscured by peaks originat- ing from polyaniline chains, whereas the v(X-0,) band strongly overlaps with polyaniline bands but it can be clearly identified. The most intense HPA band, i.e. v(M-OO,-M) is also present. Coulombic interactions between hetero-polyanions and the protonated polymeric support induce a shift in M-Oc-M vibrations by ca. 10 cm-' [compare Fig. 2(d)and 2(e)]. The changes in the modes associated with the poly-emeraldine chain are essentially the same as observed for polyaniline protonated with inorganic acids such as HCl or H2S04 and correspond to the benzenoid and quinoid sequences of the polyaniline chain.' The doping-induced 1140 cm- 'band characteristic of the charged (protonated) chain increases markedly with the level of protonation.Additional information about the protonation of poly- emeraldine with HPA can be extracted from N 1s XP spectra. Fig. 3 and 4 show deconvoluted N 1s XP spectra of the pris- tine undoped polyemeraldine base and the pristine sample SII,. The deconvolution was carried out assuming in each case the same weighting of Gaussian (75%) and Lorentzian (25%) functions. Under these conditions the binding energy 1494.0i\1300.8 I15rO 829.4 154.2 I\ 1613.1dln I L I I I 400 600 800 -1001 0 1200 1400 1600 wavenumberfcm-' Fig. 2 FTIR spectra of the SII series: (a) undoped polyemeraldine base PA, (b) sample SI12, (c) sample SI14, (d) sample SI16, (e) H,PMo,,O,, J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I,I ,*,,I,, IIII,,,I,.III.II,,,,, ,,.I,,,III~II .,.IIIII,II.II,I,I~III.I,,I,,I,,I,,, 408 403 398 393 EbW Fig. 3 N 1s XP spectrum of polyemeraldine base deconvoluted into the imine (398.4 eV), amine (399.5 eV) and protonated nitrogen atom (401.9 eV) contributions (Eb)of N 1s in the imine state (==N-) has been obtained within a very narrow range of 398.4 If: 0.1 eV. Similarly the Eb values of the amine state (-NH-) have been determined within the range of 399.4 &0.1 eV. The values are higher than those obtained in ref. 19 but very similar to those found by Kang et ~1.~'The third peak at 401.9 eV must be ascribed to a protonated nitrogen species (symbol N').The data con- cerning N 1s spectra of undoped polyemeraldine and the heteropolyacid-doped samples are collected in Table 3. Note that the analysis of the N 1s spectra of poly-emeraldine doped with H3PM~12040 requires first the sub- traction of Mo 3p3/2 from the N 1s envelope. This molybdenum peak is expected at ca. 396 eV as the standard value in the pure metallic state is 392.3 eV.21 The second line of the Mo 3p doublet lies 17.4 eV to higher energy, i.e. way above the N 1s envelope. The parameters of Mo 3p3,2 were practically constant in all the samples investigated (Table 3), independent of their pretreatment. Fig. 5 and 6 show the deconvolution of the 0 1s XP spectra of undoped polyemeraldine and sample SII, as pre- pared.Analysis of all spectra listed in Table 4 allowed us to distinguish between three different states of oxygen atoms. The fractional peak at 530.5 k0.1 eV, which was present only in the case of heteropolyacid-doped polyemeraldine, has been ascribed to oxide species as the one showing the lowest binding energy. This value is very close to frequently quoted data (530.4 eV) for MOO,. 22,23 The second 0 Is peak was characterized within the range 532.05 0.15 eV in all samples investigated. It is ascribed to surface OH- anions and its binding energy is similar to the value of 531.80 eV found in ln(OH), .24*2s The fractional 0 1s peak showing the highest E, (533.55 &0.15 eV) and also appearing in the spectra of all the samples investigated is ascribed to oxygen atoms in IIIIII,II I, (I,,,...IIII.III.,ILIII IIIIIIIIII,II 11,,1,,,,,,,,, 406 402 398 394 EbIeV Fig.4 N 1s XP spectrum of SII, deconvoluted as in Fig. 3 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 N 1s XPS data for the polyemeraldine base and polyaniline doped with 12-molybdophosphoric acid proportion (%) sample E, "+)lev -N= PA as prepared PA activated" 401.9 401.2 47 50 SI, as prepared SII, as prepared SII, activated" 400.6 400.5 400.6 401.6 401.6 401.8 33 41 40 SII, after catalytic reaction 400.6 401.7 48 "Activation under the conditions of catalytic reaction. water molecules. The literature data for E, in H20 give 533.20 eV.24.26 Table 4 presents the share of fractional peaks in the total intensity of 0 1s XPS signals for all the samples investigated.Fig. 7 shows the Mo 3d5,,doublet obtained for SII,(two- step preparation) comprising peaks at 235.6 and 232.5 eV. lss~***m*,l IIIII*I!.(II I I I I IIIII1II.III,I.IIIIIII,II,IIII'IIII,,,,,,,,(,,,,,,,,, I 542 537 532 527 Eb/eV Fig. 5 0 1s XP spectrum of polyemeraldine base deconvoluted into peaks corresponding to the oxygen atoms in OH-groups (532.05 eV) and in H,O molecules (533.55 eV) ~I.~III1II~IIIII.I~I~IIIIIIIII~IIIIIIIII~II.IIIII.~IIIIIII.I)III..I.I,I'I., "' 540 535 530 525 Eb/eV Fig. 6 0 1s XP spectrum of SII, deconvoluted into peaks corre- sponding to oxide oxygen atoms (530.55 eV), oxygen atoms in OH-groups (532.05 eV) and in water molecules (533.55 eV) Table 4 0 1s XPS data for the polyemeraldine base and polyanil- ine doped with 1Zmolybdophosphoric acid proportion (%) sample O2-OH-H2O ratio -NH- N+ -NH- :-N- J% (Mo 3P5,2)/ev 42 11 0.9 44 6 0.9 47 20 1.4 397.0 41 18 1.o 396.9 40 20 1.o 396.9 35 17 0.7 396.9 In the case of the one-step preparation (SI,) the analogous values were 235.3 and 232.2 eV.The analytical peak of Mo 3d5,2 is located at 232.6 eV for MOO,, 22923i27 at 232.5 eV for A12(MOO& 28 and Na2Mo0, 2H20,29therefore no Mo is present in oxidation states lower than +6 in the above experiments. Fig. 8 and 9 show the results of catalytic measurements using SI, and SII,. Undoped polyemeraldine (PA) did not exhibit any catalytic activity.The main product of catalytic I I ,,,.,,),, ,.,,,,.I'*".''<,~IIII~.IIIIIIII.II.I(..IIIIIIIIIIIIIII1I(IIII1,IIIIIII 244 239 234 229 EbW Fig. 7 Mo 3d5,, XP spectrum of SII, deconvoluted into a doublet (235.6 and 232.5 eV) 2.5 \ \ \ \ \2.0 \0) dI 0 F\-g1.5 m 5 'c E 1.0 3 0.5 PA as prepared -37 63 PA activated" -72 28 0.1 SI, as prepared 45 32 24 1 5 10 15 20 ene; ., no. ofpulsesSII, as prepared 52 36 12 SII, activated' 66 24 10 Fig. 8 Results of catalytic experiments at 300 "Cfor SI, :x ,ethyl-unreacted alcohol; (---)deficit of alcohol SII, after catalytic reaction 53 33 14 acetaldehyde; 0, as a function of the pulse number. Amounts of products are "Activated under the conditions of catalytic reaction.expressed as the equivalent amount of alcohol. 2104 :::;I I 15 10 15 20 25 no. of pulses Fig. 9 Catalytic experiments at 240 "C for sample SII,: x , ethyl-acetaldehyde; 0,ene; 0,diethyl ether; ., unreacted alcohol; (---) deficit of alcohol as a function of the pulse number. Amounts of pro-ducts are expressed as in Fig. 8. ethanol conversion was acetaldehyde which is produced in the redox-type reaction. The yield of ethylene in the case of sample SI, and ethylene and diethyl ether in the case of sample SII,, products of acid-base type reaction, is low. Note that in our previous studies in which we studied poly- aniline supported H,SiW 120,0the acid-base activity was predominant, acetaldehyde being a minor product.8 No hydrogen was detected in our catalytic experiments, although separate experiments have shown that an amount of hydro- gen corresponding to the amount of acetaldehyde produced (dehydrogenation of ethyl alcohol) could be detected chro- matographically in our apparatus.No hydrogen was detected in the case of unsupported H3PMo120,, and most probably 3.0.-I \ \a-2.5 \P, \tI z \ \5 2.0.-\ t 5 .4-2 1.5-3 G 1.o ,r 0.5 0.1 5 10 15 20 no. of pulses Fig. 10 CatH,PMo,,O,, alytic experiments . Symbols as in Fig. 9. at 240 "C for unsupported J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 hydrogen evolved in the course of acetaldehyde formation is immediately used for the hydrogenation of the carbonaceous deposit, the product of side-reactions (oide infra).Water present in the reaction products gave a diffuse peak in the gas chromatograph that was unsuitable for quantitative determi- nations. The sum of the amounts of alcohol necessary to produce acetaldehyde, ethylene and (in case of SII,) diethyl ether as well as the unreacted alcohol detected in the pulse leaving the reactor was always smaller than the amount of alcohol intro- duced on the catalyst in one pulse (3.6 x lo-, g). A similar effect was observed in the case of polyacetylene' and p~lypyrrole'~ supported H,PMo, 2040as well as polyaniline-supported H,SiW120,, .8 This might be due to the irreversible penetration of alcohol into the bulk of cata- lyst and/or to the oligomerization of ethylene and the forma- tion of coke.This deficit of alcohol was highest in the initial pulses, reaching 80-70% of the introduced alcohol, but rapidly decreased to ca. 50% after 20-25 pulses. The course of the catalytic reaction at 240°C in the case of unsupported H,PMo,,O,, catalyst is shown in Fig. 10. Discussion Tables 1 and 2 show that by changing the preparation condi- tions the amount of heteropolyanions incorporated into poly- emeraldine matrix can be varied to a significant extent. Generally the one-step procedure results in a higher HPA content, but a small overlap of the doping ranges exists for the one-step and two-step methods. It is therefore possible to compare samples with very close nominal HPA contents pre- pared by the two different methods: C,H,.,N(HPA),.,,, (one-step) and C,H,~,N(HPA),~,,, (two-step).In the one-step procedure doping is effected simultaneously with the poly- merization of aniline, and the dopant may be evenly distrib- uted throughout the whole volume of the polymer. On the other hand, in the two-step procedure the doping is necessar- ily connected with the chemisorption of heteropolyacid at the surface and subsequent diffusion into the bulk, which cannot be fast and may stop completely without reaching an even spatial distribution. Hence, the formation of samples with lower contents of heteropolyacid but concentrated mainly on the surface and in the near-to-surface layer is expected. Additional information indicating an uneven distribution of HPA in the two-step series samples can be extracted from the dependence of the conductivity on the heteropolyanion content in the polymer matrix.If the conducting (i.e. protonated) phase is limited to the surface and its penetration into the bulk of the polymer is almost negligible, percolation- type conductivity behaviour is expected with a low perco- lation threshold. This is indeed the case. For the two-step series samples a clear percolation threshold exists for a com- position of ca. C,H,,,N(HPA),~,,, (Fig. 11). Some information concerning the structure of the doped and undoped polymers or at least the chemical structure of the surface layer could be obtained from XPS measurements. First, deconvoluted XP spectra of the undoped poly-emeraldine base contained N 1s peaks not only at 398.4 and 399.5 eV corresponding to the imine (=N-) and amine (-NH-) nitrogen atoms but also a peak at 401.9 eV ascribed to protonated nitrogen atoms (symbol N') (Table 3).The existence of a small number of protonated nitrogen atoms can be ascribed either to incomplete deprotonation in the transformation from polyemeraldine hydrochloride to polyemeraldine base or possibly to the dissociation of chemi-sorbed water in accordance with the 0 1s XP spectrum (Table 4). This spectrum can be deconvoluted into two peaks: one, at 33.55 eV, is characteristic of adsorbed water molecules J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 -* I -3 YOt 00 0 0 Table 5 product catalyst H,PMo 12040unsupported H,PMo 2040aunsupported doped polyaniline, SII, doped polyaniline, SI 0 0.01 0.05 0.10 Y Fig. 11 Conductivity us. HPA content (y) in the samples C6H4.5N(HPA),: 0,one-step preparation; x ,two-step preparation and the other, at 532.05 eV, corresponds to oxygen atoms in OH -ions. In the strictly stoichiometric polyemeraldine base, the structure of which is shown schematically in the previous section, the molar ratio of -NH- : =N-is equal to unity. Considering the fact that a certain number of imine nitrogen atoms in the polyaniline sample had been protonated we would expect an increase of this ratio beyond unity. However, the opposite is observed (Table 3): the concentra- tion of imine nitrogen atoms exceeds that of amine atoms.A plausible explanation for this is the assumption that in the polyemeraldine base as prepared by the above method (or at least in its near-to-surface layer accessible for XPS measurements) strict stoichiometry is not preserved and there is an excess of quinoid groups with respect to benzenoid groups if one compares it with the stoichiometry of perfect polyemeraldine. Assuming that in the course of partial proto- nation protons were localized on imine atoms we can calcu- late the hypothetical amine : imine nitrogen atom ratio in the sample which is not protonated at all: z = [-NH-]/([=N-] + "'1) = 42/(47 + 11) = 0.72 The values in parentheses refer to the proportions of the par- ticular species in the sample from XPS measurements (Table 3).Doping of the polyemeraldine base with H3PMo,,04, results in an increase in the proportion of protonated nitro- gen atoms with increasing HPA content. Thus the HPA Brernsted acidity is the main source of N+.The protonation occurs preferentially on imine sites since the N+ intensity grows at the expense of the -N= peak. Again, as in the case of the undoped sample, we can determine the oxidation state of the polymer being protonated. For example, in the case of SII,, the observed -NH- :-N= ratio (assuming proto- nation of only imine sites) is: z = 41/(41 + 18) = 0.70 i.e. practically the same as in the polyemeraldine base dis- Yield per pulse calculated per g H3PMo,,04, and expressed as the amount of C,H,OH (g x cussed above.The analogous value calculated for sample SI, is 0.85, indicating that the one-step preparation of doped polyemeraldine results in the formation of a product, the structure of which is closer to that shown in the scheme (vide supra). The structural difference between SI, and SII, does not give rise to any essential difference in their FTIR spectra. The spectra also show that heteropolyanions (structural Keggin units) preserve their identity when they are dispersed in the polymer matrix. Despite the fact that the specific surface areas of both SI, and SII, are similar, their catalytic behaviour is very differ- ent. At 240"C, a temperature that ensures satisfactory thermal stability of the polymer (Fig.l), in contrast to sample SII, sample SI, was catalytically inactive and it was neces- sary to heat it to 300°C in order to obtain a measurable reaction yield. Table 5 shows that the total catalytic activity calculated per g of heteropolyacid for SII, is 5.6 times greater than that of unsupported H3PMo,,04, in comparable conditions. On the other hand, dispersion of heteropolyacid in the one-step preparation (SI,) resulted in a decrease of specific activity to ca. 58% of that of unsupported H3PMo,,04,. This shows that in the SII, catalyst the Keggin units are more accessible and/or more active than in the unsupported acid, probably owing to their higher concentration on the polymer surface.On the other hand, the one-step procedure results in the dis- persion of HPA over the whole volume of the polymer, which in turn leads to a distinctly lower concentration of HPA at the catalyst surface. From the catalytic point of view the most striking property of both catalysts, independent of the differences in their activ- ity, is the very high selectivity to acetaldehyde, the product of the redox-type reaction, while the products of acid-base-type reaction, i.e. ethylene and diethyl ether, are formed with very low selectivity. Table 6 shows that the redox selectivity of polyaniline-supported H,PMo,,O,, is more than three times higher than that found in the case of the unsupported hetero- polyacid and is also much higher than in the case of polyacetylene-and polypyrrole-supported H3PMo 2040.For polyaniline-supported H,SiW ,2040only very weak redox activity was observed.8 This is not unexpected since the molybdenum in the HPA is much more easily reduced than the tungsten. We expected to detect a distinct reduction in the amount of molybdenum in the samples of the catalyst after the catalytic reaction. However, the XP spectra of such samples gave only the XP signals for Mo 3d,,! at 396.9 eV characteristic of MoV* atoms. This could be ascribed either to the real absence of reduced molybdenum at the surface of the working catalyst or, more probably, to the oxidation of reduced Mo atoms when the sample was exposed to air when it was removed from the reactor.Note that any catalytic model describing redox reactions must include oscillations between the higher and the lower oxidation states of the active centre. One can conclude that in the case of ethanol oxidation to acetaldehyde the reoxidation step of surface centres is very fast and spontaneous. used for formation of the given sum of T/"C CH4 C2H6 C2H4 (C2H5)20 CH3CH0 products 240 0.39 0.76 0.32 1.45 320 0.32 0.28 4.67 1.35 2.10 8.12 240 0.93 0.94 6.08 7.95 300 1.18 traces 3.55 4.73 14.Data from ref. ,, 2106 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 6 Selectivity to particular products after 20 pulses of ethanol (%) catalyst T/"C CH4 C2H6 C2H4 (C2HS)20 CH,CHO ~~ H3PMoI2O4, unsupported 240 -H,PMo,,O,," unsupported 320 3.7 doped polyaniline, SII, 240 -doped polyaniline, SI 300 -doped polyacetyleneb 230 -doped polypyrrole" preheated in He 320 - -25.7 51.7 22.6 3.3 53.5 15.5 24.1 -11.4 11.6 77.0 --25.0 75.0 -16.4 27.8 55.8 -45.5 3.5 51.0 doped polypyrrole" preheated in air 320 - -58.4 13.5 28.1 " Data from ref.14. Data from ref. 7 This problem was studied in a more detailed way by EPR spectroscopy. The EPR signal of HPA-doped polyaniline depends on the content of the heteropolyacid in the matrix. All samples of the SI series exhibited only a narrow signal with g = 2.003, characteristic of the presence of free radicals in the polyaniline matrix. In the SII series the signal de- 2 3 4 5 B. Keita, L. Nadjo and J. P.Haeussler, J. Electroanal. Chem., 1988,243,481. B. Keita, K. Essadi and L. Nadjo, J. Electroanal. Chem., 1989, 259, 127. B. Keita, D. Bouazis and L. Nadjo, J. Electroanal. Chem., 1988, 255, 303. G. Bidan, E. M. Genies and M. Lapkowski, J. Electroanal. pended on the content of HPA, and for low contents of HPA a weak but distinct signal which could be ascribed to MoS+ appeared. However, for higher contents the Mo5+ signal was no longer present. Since SII, was highly protonated, its EPR spectra both in the pristine state and after its use as a catalyst revealed only one signal at g = 2.003 which was ascribed to 6 7 8 Chem., 1988,251,297. G. Bidan, E. M. Genies and M. Lapkowski, J. Chem. SOC.,Chem. Commun., 1988,533. J. Poiniczek, I. Kulszewicz-Bajer, M. Zagorska, K.Kruczala, K. Dyrek, A. Bielanski and A. Pron, J. Catal., 1991,132, 311. M. Hasik, J. Poiniczek, Z. Piwowarska, R. Dziembaj, A. Biel-anski and A. Pron, J. Mol. Catal., submitted. the presence of unpaired spins in the n system of the polymer backbone. A very similar signal, although much weaker, was observed in the pristine polyemeraldine base. Catalytic reaction strongly influences the EPR signal of the catalysts. Joint EPR-catalytic experiments were carried out with SII, in a small reactor situated in the EPR spectrometer cavity. The catalytic reaction took place at 175-240 "C and the EPR measurements were taken at the reaction tem- 9 10 11 12 13 I. Kulszewicz-Bajer, M. Zagorska, A. Pron, D. Billaud and J. J. Ehrhardt, Mater. Res. Bull., 1991,26, 163.A. Pron, Synth. Met., 1992,43,277. M. Hasik, A. Pron, I. Kulszewicz-Bajer, J. Pozniczek, A. Biel-anski, Z. Piwowarska and R. Dziembaj, Synth. Met., 1993, A. G. MacDiarmid and A. Epstein, Faraday Discuss. Chem. SOC., 1989,88,3 17. Y. Cao, A. Andreatta, A. J. Heeger and P. Smith, Polymer, 1989, 55-57,972. perature as well as at -170"C immediately after the reaction was interrupted (without exposing the sample to air). EPR spectra recorded at 240 "C during the reaction exhibited only a signal at g x 2. However, at -170°C a poorly resolved signal of MoS+ (g < 2) became visible. Obviously its shape 14 15 30,2305. J. Poiniczek, A. Bielanski, I. Kulszewicz-Bajer, M. Zagorska, K. Kruczala, K. Dyrek and A. Pron, J. Mol. Catal., 1991,69,223. C. Rocchiccioli-Deltcheff, R.Thouvenot and R. Franck, Spectro-chim. Acta, Part A, 1976, 32, 587; C. Rocchiccioli-Deltcheff, M. Fournier, R.Franck and M. Thouvenot, Znorg. Chem., 1983, 22, was influenced by molecules of the reacting species adsorbed on the catalyst. When air was allowed into the reactor the EPR parameters of the sample changed, giving a better defined signal with g1 = 1.95 and gll = 1.88. Note that the observed g values are different from those of Keggin and close to those observed for MOO, or to those 16 17 18 207. C. Rocchiccioli-Deltcheff, M. Amirouche, G. Herve, M. Four-nier, M. Che and J. M. Tatibouet, J. Catal., 1990, 126, 591. C. Rocchiccioli-Deltcheff, M. Amirouche and M. Fournier, J. Catal., 1992, 138, 445. J. Tang, X.Jing, B. Wang and F. Wang, Synth.Met., 1988, 24, 213. of MoS+-grafted samples.,, This last observation may suggest that the catalyst may undergo some decomposition under catalytic conditions. A detailed understanding of this phenomenon requires more detailed studies involving other spectroscopic techniques such as 31PMAS NMR studies.34 19 20 21 22 23 J. Yue and A. J. Epstein, Macromolecules, 1991,24,4441. E. T. Kang, K. G. Neoh and K. L. Tan, Polym. J., 1989,21,873. K. T. Ng and D. M. Hercules, J. Phys. Chem., 1976! 80,2095. D. D. Sarma and C. N. R. Rao, J. Electron. Spectrosc. Relat. Phenom., 1980,25.R.J. Colton, A. M. Guzman and J. W. Rabelais, J. Appl. Phys., 1978,49,409. Conclusions 24 C. D. Wagner, D. A. Zatko and R. H. Raymond, Anal. Chem., 1980,52, 1445. Protonation of polyaniline with 12-molybdophosphoric acid is a convenient route for preparing a new type of catalyst that is highly selective towards the formation of acetaldehyde in ethyl alcohol conversion. The catalytic activity of this catalyst is much higher in those samples in which the polymerization of aniline and the protonation reaction are separate in time.In this case catalytically active centres are located only on the polymer surface and are more accessible for alcohol mol- ecules. 25 26 27 28 29 30 31 K. Kishi and S. Ikeda, Bull. Chem. SOC. Jpn., 1973,46,341. V. I. Nefedov, D. Gati, B. F. Dzurinskii, N. P. Sergushin and Y. W. Salyn, Zh. Neorg. Khim., 1975,20, 2307. P. Gajardo, D. Pirotte, C. Defosse, P. Grange and B. Delmon, J. Electron. Spectrosc. Relat. Phenom., 1979,17, 121. T. A. Patterson, J. C. Carver, D. E. Leyden and D. M. Hercules, J. Phys. Chem., 1976,80, 1702. S. 0.Grim and L. J. Matienzo, Znorg. Chem., 1975,14,1014. C. Sanchez, J. Livage, J. P.Launay, M. Fournier and J. Jeannin, J. Am. Chem. SOC., 1982,104,3194.R.Fricke and G. Ohlman, J. Chem. SOC., Faraday Trans., 1986, This work was financially supported by KBN (Committee for Scientific Research of Poland), grant no. 20898 91/p01 and p02. 32 33 34 82, 263. G. Centi, J. Lopez Nieto, C. Iapalucci, K. Bruckman and E. M. Serwicka, Appl. Catal., 1989,46, 197. C. Louis and M. Che, J. Phys. Chem., 1987,91,2875. R. Thouvenot, C. Rocchhicciolo-Deltcheff and M. Fournier, J. References Chem. SOC., Chem. Commun., 1991, 1352. 1 M. Misono, Proc. 10th Int. Congr. Catal. Akademiai Kiado, Budapest, 1993, vol. A, p. 69. Paper 4/01411A; Received 9th March, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949002099

出版商:RSC    

年代:1994

数据来源: RSC