摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1507-1511 Host-Guest Complexes of Cucurbituril with the 4-Methylbenzylammonium Ion, Alkali-metal Cations and NH4+ Rudiger Hoffmann, Wilhelm Knoche" and Christian Fenn Fakultat fur Chemie , Universitat Biele feld, 0-3350I Biele feld, Germany Hans-Jurgen Buschmann Deutsches Textilforschungszentrum Nord-West, 0-47798Krefeld, Germany Kinetic measurements indicate the existence of two different complexes of cucurbituril with 4-methylbenzyl- ammonium ions: (i) an association complex, where the ammonium group binds to one of the polar portals of cucurbituril and the hydrophobic part extends into the solvent, and (ii) an inclusion complex, where the hydro- phobic part extends into the cavity of cucurbituril. This complex is used as an indicator for the binding of simple monovalent cations to cucurbituril.Stability constants are reported for the 1 : 1 complexes of cucurbituril with different cations. In 1905, Behrend et al. reported on the isolation of a com- pound from the condensation of urea with glyoxal and formaldehyde1 which was later shown by Mock to be a macropolycyclic compound whose molecular structure resembles a pumpkin. Thus, Mock gave the compound the trivial name cucurbituril and then went on to study its recep- tor properties.' Cucurbituril (Fig. 1) is a nonadecacyclic cage compound of a relatively rigid structure. The two carbonyl- fringed portals at the upper and lower side of the molecule have a diameter of 4 A. The internal cavity has a diameter of ca.5.5 A, and the distance of the portals is 6 A. The interior of the molecule represents a hydrophobic region, whereas the two portals are hydrophilic. Consequently the hydrophobic organic moiety of organic ammonium ions extends into the interior, and the ionic part coordinates to one of the planes spanned through the negatively polarized carbonyl groups. Dissociation constants of complexes of cucurbituril with various organic ammonium ions have been determined from thermodynamic and kinetic mea~urements,~.~ and catalytic activity in 1,3-dipolar cycloadditions has been reported. Recently the reaction of cucurbituril with alkaline cations was studied, and a 1 : 2 complex was observed.6 In this contribution we report upon a spectrophotometric and kinetic investigation of the binding of the 4-methylbenzylammonium ion (MBAH+) to cucurbituril.When alkali-metal or ammonium salts are added to the solu- tions, we observe a competitive binding of MBAH' and the alkaline cations, which influences strongly equilibrium and '0 0 0 Fig. 1 Molecular structure of cucurbituril kinetics of the reaction. A thorough analysis of all experimen- tal results leads to a detailed understanding of the complex formation. Cucurbituril is soluble only in very acidic solvents and most studies were performed in 48 : 52 (v/v) formic acid- water mixtures. Therefore we used the same solvent in our experiments. Experimental Cucurbituril was prepared as described by Behrend et al.' and Mock et al.' 4-Methylbenzylamine was converted to the hydrochloride by dissolving the amine in ether and intro- ducing HC1 gas into the solution.The precipitate was washed with ether, dried and used without further purification, All other chemicals are commercially available (grade p.a.). All experiments were performed in 48 : 52 (v/v) mixtures of formic acid and water. In this solvent the maximum solubility of cucurbituril is 3.0 x mol dm-3. Stock solutions were prepared from cucurbituril, 4-methylbenzylamine hydro-chloride (MBAHCl) and the alkali-metal chlorides. Stock solutions of the complex were obtained by dissolving 7.0 x lop3mol dm-3 cucurbituril plus 8.0 x mol dm-3 MBAHCl. In all experiments the temperature was controlled within & 0.2 "C, and stock solutions were thermostatted before use.For recording the UV spectra and the kinetic measure- ments a Kontron Uvikon 860 spectrophotometer (double- beam operation with the same solvent in both cuvettes) was used. Absorbances are measured with an error of k0.002. Since the reactions are slow, the kinetics of the complex for- mation were studied by mixing stock solutions of cucurbituril and MBAHCl in a spectrometer cuvette. The reverse reaction was studied by mixing stock solutions of the complex and alkali-metal chlorides. The progress of the reaction was mon- itored at the wavelength 1 = 271.5 nm. The results were digi- tally averaged over at least three measurements. In most experiments one of the reactants is in large excess, and the reaction proceeds according to pseudo-first-order kinetics.In those cases where the reaction proceeds under second-order conditions, the change of absorbance was fitted to the corresponding equation a exp(-t/z) (1)A=A,+ 1 + b exp(-t/7) A, is the absorbance at equilibrium (t p 7).The time constant z of the reaction is called relaxation time. If the reaction pro- ceeds according to pseudo-first-order kinetics, the term 'b exp(-t/z)' in eqn. (1) may be neglected, a represents the amplitude of the relaxation effect and l/z = /'cobs. That means, z is obtained in different ways from the change of absorbance, depending on the order of the reaction studied. Close to equi- librium all reactions proceed according to first-order kinetics, and therefore z has the same meaning (and the same depen- dence on concentration) for first- and second-order reactions.A,, z and the amplitude a/(l + b) of the relaxation effect will be discussed in this contribution. Results Fig. 2 shows spectra of solutions of cucurbituril and 4- methylbenzylamine hydrochloride in 48 : 52 formic acid- water mixtures, where the same solvent is used as reference. The measurements are restricted to the range R > 250 nm, since below this wavelength the solvent absorbs too strongly. For cucurbituril a broad absorption band is observed at R < 300 nm [spectrum 01. This absorbance does not appear for other compounds with the same -N-CO-N-structural unit (e.g.tetramethylurea or 1,3-dimethylimidazolidin-2-one),and we cannot attribute it to a certain electronic transition. The 4-methylbenzylammonium ion (MBAH') shows the weakly structured a-band of the aromatic ring [spectrum (a)]. When cucurbituril is added to the solution, this absorbance increases and the band is better resolved. At high concentrations of cucurbituril we observe the well resolved spectrum of a complex with a narrow peak t 250 290 qnrn Fig. 2 Spectra of solutions containing cucurbituril and Cmethyl- benzylamine. Lo = 8.0 x mol dm-3 for (a)-@) and C, = 0 (a), 1.6 x (b),9.7x (c),7.0 x (4, 1.6 x mol dm-3 (e).cf)is the spectrum of 7.0 x mol dm-3 cucurbituril. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 at R = 271.5 nm.The absorbances of 4-methylbenzylamine and the complex differ most strongly at this wavelength, which therefore is used to determine the equilibrium constant of the complex. For the evaluation we use the abbreviations C and L+ for cucurbituril and the ligand MBAH', respectively. Assuming the simple complexation reaction k C+L+ LCL+ (2)k' the concentrations at equilibrium are determined by the total concentrations Co and Lo and by the stability constant K: [C] + [CL'] = c, (3) [L'] + [CL'] = Lo (4) The absorbance of the solution (compensating the absorb- ance of the solvent) is given by A7= ECCCI + EJ-L+] + ECL[CL+] K and cCL are evaluated by fitting these equations to the experimentally obtained absorbances. In Fig. 3 the contribution of the free ligand L+ and the complex CL+ to the absorbance is plotted against the logarithm of the concentration of cucurbituril at constant concentration of 4-methylbenzylamine, and the expected sigmoidal curve is obtained.The spectrum of the complex (insert in Fig. 3) is calculated by subtracting the absorbance due to free cucurbituril and free ligand from the observed spectrum. At two different concentrations of ligand the same value of K is obtained, guaranteeing the 1 :1 stoichiometry of the complex. Finally K was determined at different tem- peratures. All results are summarized in Table 1. 250 2700.2 l-------I -4 -3 -2 log(Co/mol dm-3) Fig. 3 Absorbance of L+ and CL" us. the logarithm of the total concentration of cucurbituril.The inserts show the spectra of the L+ (left-hand side) and CL+ (right-hand side). [C] is calculated from eqn. (3)-(5). Table 1 Constants evaluated for the binding of MBAH+ to cucurbituril obtained from spectrophotometric measurements at different tem- peratures with Lo = 8.0 x mol dm-' -~ EC EL TpC /dm3iol-' /dm3 mol-' cm /dm3 mol-' cm-' ECL K. /mol dm-3 /dm3 mol-' K,/dm3 mol-' /dm3 mo";' cm-' E, /dm3 mol-' cm-' k, s-l ki /W3s-' 25 1200 21.4 148 400 25' 1400 21.4 148 380 15@ 1150b 41@ 169 l.lb 1.6* 35 1700 21.3 148 355 230 1490 385 169 2.2 3.3 45 1500 21.4 148 error +200 *0.2 335 310 & 20 & 50 1160 f200 380*20 169 f1 3.2 +20% 8.6 *20% a Lo = 1.6 x 10.' rnol dm-'. Evaluated for both 8 x lO-*and 16 x lo-' rnol dm-3.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 In a previous communication6 it was shown that two alkali-metal cations can bind to a cucurbituril molecule. In order to study this complexation we added alkali-metal chlo- rides and ammonium chloride to solutions of cucurbituril, but no change in absorbance was observed. This means that the complexes formed have the same absorbance as the free cucurbituril molecule. Therefore we used reaction (2) as indi- cator for the formation of an alkali-metal complex, i.e. we added the salts to solutions containing both cucurbituril and 4-methylbenzylamine. As an example Fig. 4 shows the titra- tion curve for NaC1. Now we have competing equilibria, which are described by the reaction scheme CL' + 2Me+=C + L+ + 2Me+ eCMef + L' + Me'eCMe,'' + L+ (7) where Me' stands for the monovalent cations.The absorb- ance of the solutions is given by A -= &C{[C] + [CMe'] + [CMe,"]} + E~[L+]+ cCL[CL+]a (8) since the absorption coefficients of the complexes are equal to that of cucurbituril, as stated above. All absorption coeffi- cients and the association constant of CL+ are already known, and by fitting this equation to the experimental data we may evaluate the stability constants of the two alkali- metal cation complexes: [CMe'] K1 = [C][Me'] (9) [CMe,, '1K2 = [CMe '3 [Me '3 Assuming that the concentration of the 1 :1 complex CMe+ is negligible ([CMe+] + C0),the best fit yields the dashed curve in Fig.4 with strong deviations from the experimental points. On the other hand, neglecting the 1 :2 complex CMe2,+ in the fitting procedure ([CMe,2' J 4 C,) leads to the dotted curve in Fig. 4, where the deviations are much Table 2 Association constants of complexes of cucurbituril with monovalent cations at 25 "C and ionic radii of Me' Kl/dm3 mol-' K,/dm3 mol-' rlA Li + 170 <3 0.76" Na' 1450 60 1.02' K' 560 <20 1.38' Rb+ NH4 + 410 170 <1 t3 1.52' 1.70' Ref. 7; ref. 8; ref. 9. smaller, but they are still systematic. The residual plots demonstrate that for the dashed and the dotted curve the deviations are in opposite directions. Therefore the fitting has to take into account both complexes. This leads to the full curve in Fig.4, which agrees well with the experimental points. The dashed curve shows the largest deviations, i.e. the 1 : 1 complex contributes more strongly to the absorbance than the 1 :2 complex, and correspondingly the association constant K2 has a large error. For the other cations an acceptable fit is obtained even if the 1 : 2 complex is neglected, and therefore for those ions only a lower limit of K, can be estimated (see Table 2). The kinetics of the complexation between cucurbituril and MBAH' were studied by mixing different solutions of cucurbituril with solutions containing 8.0 x rnol dm-3 MBAH', and the progress of the reaction was observed at 2 = 271.5 nm. In all experiments the absorbance changed in a single relaxation effect according to a first- or a second-order reaction, i.e.the relaxation time is obtained by fitting eqn. (1) to the absorbance. For equilibrium (2) the relaxation time z is given by [C] and [L'] are the concentrations at equilibrium, which are calculated from eqn. (3)-(5). According to eqn. (11) the plot of 2-' us. ([C] + [L']) yields a straight line, with inter- cept k' and slope k'K. The straight line in Fig. 5 shows the best fit of the experimental results to eqn. (ll), taking into account that the ratio between slope and intercept has to be equal to K. The disagreement is obvious, and thus the reac- tion scheme has to be expanded. The stoichiometry of the complex is unambiguously determined by the spectrophoto- metric measurements at equilibrium, and thus the simplest possibility to expand equilibrium (2) is to assume two differ- ent 1 :1 complexes: an association complex CL,+, which is formed very quickly without a change in absorbance, and an k :-I t1 I-3 -2 -1 log( [NaCl],/mol dm-3) Fig.4 (a)Absorbance of solutions containing Lo = 8.0 x lop4mol dm-3 and C, = 7.0 x lop3 mol dm-3 us. the logarithm of the con- centration of NaCI. The curves are obtained by fitting eqn. (8) to the experimental points considering both complexes (full line) and with the assumption that complex CNa+ (dashed line) or CNa,'+ (dotted line) may be neglected. (b) Deviations between experimental points and the best fit assuming that complex CNa+ (0)or CNaZ2+ (A) may be neglected.01 I I 0 0.5 1.o ([C] + [L+])/l 0-2 mol dm-3 Fig. 5 t-l us. ([C] + [L']) at 25°C. The full curve is calculated with the constants given in Table 1. The dashed curve shows the best fit of eqn. (1 1) to the experimental points with the restriction : slope/ intercept = K. - 3. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 inclusion complex CLi*, the formation of which is observed in the rate-determining step. This is described in the equi- o.olllibrium C+L+ e,CL,+ CL, + for which the relaxation time is given by: Now the equilibrium is determined by two stability constants K and ECL relate to the corresponding constants of the associ- ation and inclusion complexes by the equations : K = K,+ Ki (16) E, K, + E~Ki ECL = K,+ Ki Furthermore, we may assume that the absorption coefficient of CL,' is equal to E~ + EL, since we do not observe a fast change in the absorbance when the two solutions are mixed.In eqn. (13) K, can be calculated from K and Ki, and thus the two constants kf and Ki have to be obtained by fitting the equation to the relaxation times. The curve in Fig. 5 shows that the experimental results are well described by eqn. (13). The intercept of the curve and the limiting slope for small concentrations yield ki and kfK,, respectively, and K, deter-mines the curvature. The kinetic measurements have been performed at different temperatures, and all constants evalu- ated are included in Table 1. When salts are added to the solutions of cucurbituril and 4-methylbenzylamine, the relaxation time changes in a char- acteristic way, as shown in Fig. 6 for KCl.To describe this behaviour, we have to include the complex formation of cucurbituril with Me+ ions, which leads to the scheme C + L+ + Me+ CL,+ + Me' The measurements are restricted to relatively low concentra- tions of KCl, where according to the values of K, the concen- tration of the 1 :2 complex CMe22+ may be neglected. Assuming again that the formation of the inclusion complex CLi+ is the rate-determining step of the overall reaction, scheme (18) leads to the very complicated equation Xi -[CL,'] *-[Me']] Xi I c +--I v)--. ye 0.005--I I I -3 -2 -1 log( [KCI],/mol dm-3) Fig.6 Reciprocal relaxation time of the reaction of cucurbituril with MBAH' in the presence of KC1 us. total concentration of KCI. C, = 7.0 x lo-' mol dm-3;Lo = 8.0 x mol dm-3; T = 25°C. for the relaxation time. In this expression xj, xk are defined as the difference between the actual concentrations and the con- centrations at equilibrium of speciesj, k. The ratios xj/xk are calculated from the stability constants. The rate constant kf has already been determined by eqn. (12). Thus k& is the only parameter, which can be varied to fit eqn. (19) to the experimental value of z. The best fit leads to the curve in Fig. 6, which describes the experiments well. The derivation of eqn. (19) and the analytical expressions for xj/xkas well as all experimental results are given in ref.10. Discussion The spectrophotometric titrations clearly indicate the exis- tence of a 1 : 1 complex of cucurbituril with MBAH'. According to the results summarized in Table 1 the stability constant of this complex does not depend on the temperature within experimental error. However, we observe a systematic change in its absorption coefficient cCL,which strongly hints at the existence of two isomeric structures of the complex. Two different complexes are also necessary to explain the concentration dependence of the relaxation time: one complex, which is in fast pre-equilibrium with cucurbituril and MBAH', and a second slow one, the formation of which can be followed spectrophotometrically. No fast change of absorbance is observed when solutions of cucurbituril and MBAH are mixed, and therefore the absorption coefficient + of the first complex has to be equal to the sum of the absorp- tion coefficients of cucurbituril and MBAH'.The formation of the second complex is connected to an increase of absorb- ance and leads to a well resolved spectrum of the chromo- phore of MBAH'. This indicates that in the first complex the environment of the chromophore is not changed compared to that of the free MBAH', whereas the chromophore is desol- vated in the second complex. These arguments lead to the identification of the first complex as an association complex, where the charged ammonium group of the ligand binds to the six carbonyl groups in one of the portals of cucurbituril and where the chromophoric aromatic group still extends into the solvent.The second complex is an inclusion complex, where again the ammonium group binds to six carbonyl groups but where the chromophore is desolvated and extends into the cavity of cucurbituril. This is shown schematically in Fig. 7, which includes three water molecules. According to X-ray crystallographic studies these molecules are located in the cavity of cucurbituril." Since the two complexes are of the same stoichiometry, the titration yields only the overall stability constant K, as defined in eqn. (16). The concentration dependence of the relaxation time in Fig. 5 is characterized by the limiting value J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 + CH3 Fig. 7 Formation of the association and the inclusion complex of cucurbituril and MBAH+ of l/z at low concentrations, the limiting slope and the curva- ture. The good agreement between the calculated curve and the experimental values confirms the reaction scheme pro- posed, since we fit only the two constants k' and Ki to those three features. (K, is determined by K and Ki.) The measure- ments are evaluated for different temperatures, and the absorption coefficients E~ of the inclusion complex are calcu- lated with the values of Ki and E, = cC + E~ as stated above. A further confirmation of the existence of two different com- plexes is that E~ does not depend on temperature, in contrast to cCL,which is evaluated assuming the single-step reaction, eqn.(2). The complex formation between cucurbituril and MBAH + is reduced, when an alkali-metal salt or NH4C1 is added to the solution. Obviously the added cations form complexes with cucurbituril, and thus they compete with MBAH' for the binding sites in the carbonyl planes. The absorbance of cucurbituril is not altered by the binding of the cations, and their binding constants have to be determined using MBAH' as an indicator. The results show that a 1 :1 complex is favoured. Owing to the repulsion of two equally charged ions through the nearly empty cavity of cucurbituril a 1 : 2 complex is formed only at very high concentrations of salt. The binding constants relate well to the crystal radii of the ions: Na' binds much stronger than Li', and for ions of larger size the complexation decreases monotonically (see Table 2).The proposed mechanism indicates that the simple cations bind to cucurbituril in the same way, as MBAH' is bound in the association complex. This is confirmed by the fact that the stability constant of the association complex of MBAH' is equal to that of the NH4+ complex. The reaction rate of the formation of the MBAH' inclu-sion complex is reduced when small amounts of salt are added. This dependence is expected, since MBAH+ can form an inclusion complex only with the free cucurbituril molecule. However, the reaction rate increases on further addition of salt, as shown in Fig. 6. Eqn. (19) allows for this increase by considering a pathway in scheme (18), where the ligand reacts with CMe' to form the inclusion complex.Thus also the dependence of the reaction rate on the salt concentration can well be described. Summarizing, it may be said that in this contribution results are presented which allowed us to determine stability constants and to deduce the mechanism for the formation of association and inclusion complexes of cucurbituril. These studies were performed in water-formic acid mixtures at a constant composition. For a more detailed discussion the specific solvation of cucurbituril in this solvent has to be con- sidered. Therefore the studies are extended to aqueous solu- tions of acid at relatively low concentration. The authors are indebted to the Fonds der Chemischen Industrie for the financial support of this work. References 1 R. Behrend, E. Meyer and F. Rusche, Liebigs Ann. Chem., 1905, 339, 1. 2 W. L. Mock, W. A. Freeman and N-Y. Shih, J. Am. Chem. SOC., 1981,103,7367. 3 W. L. Mock and N-Y. Shih, J. Org. Chem., 1986,51,4440. 4 W. L. Mock and N-Y. Shih, J. Am. Chem. SOC., 1989,111,2697. 5 W. L. Mock, T. A. Irra, J. P. Wepsiec and M. Adhya, J. Org. Chem., 1989,54,5302. 6 H-J. Buschmann, E. Cleve and E. Schollmeyer, Inorg. Chim. Acta, 1992,193, 9. 7 R. M. Izatt, J. S. Bradshaw, S. A. Nielsen, J. D. Lamb and J. J. Christensen, Chem. Rev., 1985,85,271. 8 R. D. Shannon and T. C. Prewitt, Acta Crystullogr., Sect. B, 1969, 25,925. 9 D. H. Aue, H. M. Webb and M. T. Bowers, J. Am. Chem. SOC., 1976,98,318. 10 R. Hoffmann, Ph.D. Thesis, Universitat Bielefeld, 1993. 11 W. A. Freeman, Acta Crystallogr., Sect. B, 1984,40,382. Paper 3/07417J; Received 16th December, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001507

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1513-1522 General Thermodynamic Analysis of the Dissolution of Non-polar Molecules into Water Origin of Hydrophobicity Miguel Costas Departamento de Fkica y Quimica Tebrica, Facultad de Quimica, Universidad Nacional Autonoma de Mexico, Mexico D.F. 04510,Mexico Bengt Kronberg and Rebecca Silveston Institute for Surface Chemistry, P.O.Box 5607, S-114 86 Stockholm, Sweden The Gibbs energy, enthalpy, entropy and heat capacity of transfer from the pure non-polar liquid into water are analysed in detail. It is found that if the combinatorial contribution to the Gibbs energy and entropy of transfer is subtracted from the experimental values, all non-polar solutes in water behave in a universal manner, i.e. all of their thermodynamic transfer functions can be studied with their molecular surface area as the only parameter.This is illustrated with the alkylbenzene series, for which experimental Gibbs energies of transfer in a wide temperature range have been obtained recently. A new interpretation scheme for the thermodynamic transfer functions is presented and contrasted with that due to Privalov and Gill. It is considered that water molecules around the solute undergo a relaxation process which lowers the Gibbs energy, enthalpy and entropy of the system and is responsible for the large heat capacity of transfer. This relaxation process is described here using a two-state model for water molecules obtained from first principles. The negative relaxation contribution to the Gibbs energy promotes solubility, but is overcome by a large positive contribution arising from the creation of a cavity in water and the large differences between solute-solute, water-water and solute-water interactions. The origin of hydrophobicity lies then in the high cohesive energy of water. The proposed interpre- tation scheme is used to (a) predict the solubility of alkanes in water, (b) understand the origin of the solubility minimum appearing in aqueous solutions of non-polar solutes, (c) rationalize the experimental finding that the enthalpy of transfer becomes zero in a narrow temperature range for many non-polar solutes, (d)discuss the significance of entropy of transfer vs.heat capacity of transfer plots often used to understand the nature of the hydrophobicity of non-polar solutes and proteins, and (e) account for the expected change in sign (with temperature) of the water proton NMR chemical shifts discussed earlier in the literature.An understanding of the molecular behaviour underlying the low solubility of non-polar substances in water is a prerequi- site for the rationalization of a wide variety of phenomena such as the formation of micelles and biological membranes, the stability and thermal denaturation of proteins, and the effect of solvent changes in chemical reactions. The ther- modynamic functions i.e. Gibbs energies, enthalpies, entropies and heat capacities, associated with the transfer of a non-polar solute from its pure gas or liquid state into water have been studied experimentally for some time.Most of these data are available only at a single temperature (298 K) or in small temperature intervals. Over the years, these data have been interpreted in several different and often conflicting The most recent and complete effort to understand the dissolution of non-polar solutes into water is due to Pri- valov and Their interpretation scheme, however, is not free of inconsistencies and conceptual problems which have been discussed in detail and hence the current view of the so-called hydrophobic effect or hydrophobic interactions still stands in need of amendment. Motivated by the availability of accurate Gibbs energies of transfer, and the derived enthalpies, entropies and heat capacities, for a series of alkylbenzenes in the 273-563 K temperature range,' we present here a new thermodynamic analysis of the dissolution of non-polar substances into water which we believe rep- resents a step towards a satisfying explanation of this pheno- menon.Liquid to Water Thermodynamic Transfer Functions for Alkylbenzenes Using inverse high-performance liquid chromatography (HPLC) the Gibbs energies of transfer from the pure hydro- carbon liquid into water for benzene, toluene, ethylbenzene and propylbenzene have recently been measured.' In ref. 9, the alkylbenzene series was chosen over the alkane series since the latter was not resolvable by the refractive index detector used in the experimental HPLC set-up. The reli- ability of the alkylbenzene data has been established from calculating solubilities and heat capacities of transfer, which are in good agreement with direct measurements.' The need for correcting thermodynamic transfer functions for the com- binatorial effects has been clearly shown recently." The non- combinatorial part of these Gibbs energies (ATG) from ref.9 are shown in Fig. 1. It has been noticed6." that for many non-polar solutes the values for their enthalpies and heat capacities of transfer are proportional to the surface area of the solute molecule; it has also been shown5" that for gases ranging from Ne to butane, the gas to water entropy of trans- fer at 298 K and at the same mole fraction is proportional to the solute surface area.In most cases, these surface areas have been calculated from the known spatial structure of the solute and, assuming each water molecule occupies an area of 9 A2, represented by the number of contacting water mol- ecules.12 An alternative is to use the surface areas which are extensively employed in group contribution models for organic mixtures.' These areas are obtained from Bondi's methodi4 and, taking methane as reference, are reported for many non-polar molecules13 in the form A, = qSACH4,where the subscript s identifies the solute and ACH4= 2.9 x lo5 m2 mol-'. Using the q, values for the series of alkylbenzenes (2.1035 for benzene, 2.5690 for toluene, 3.0345 for ethyl- benzene and 3.5000 for propylbenzene), ATG in Fig. 1 can be expressed per solute area and the results are plotted in Fig.2. It can be seen that the Gibbs energies of transfer for the four J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 6o I iI ref. 13); we chose to alter the original q, value13 as this gave a better collapse. In a sense, benzene has then been forced to belong to the alkylbenzene series, although its behaviour appears to be slightly different. This approximation, however, has no consequences for the general conclusions regarding the dissolution of non-polar solutes in water presented in this work. ATG (in mJ m-2) for all alkylbenzenes in Fig. 2 can be fitted to a polynomial, the result being ATG = -82.549 + 0.79481 T -(1.6507 x lOP3)T2+ (1.1981 x 10-6)T3 (1) with a correlation coefficient of 0.971.Using eqn. (1) the 2o F lot ii entropy of transfer TATS = -ThATG/hT, the enthalpy of transfer AT€€ = ATG + TATS and the heat capacity of trans- fer TATC, = -T2S2A:G/hT2 are easily obtained and are shown in Fig. 3 for the experimental temperature range (273- 363 K) used in ref. 9. Unique or master curves for ArH and TArC, have been found previously for non-polar solutes.6*' In contrast, for ATG and TATS, whose examination is com- plicated by standard-state choices, common curves for a series of solutes have never been reported. The unique curve 260 300 340 380 for ArG in Fig. 2 is the result of having subtracted from the TIK experimentally obtained Gibbs energy of transfer its com- Fig. 1 Experimental non-combinatorial liquid to water Gibbs binatorial part, determined according to the Flory-Huggins expressiong and producing AFG (in kJ mol-') in Fig.1. This energy of transfer against temperature for the alkylbenzenes series : asbenzene (a),toluene (O),ethylbenzene (m) and propylbenzene (0).has a substantial effect on the resulting AyG (kJ mol-') Data taken from ref. 9. described in detail in ref. 9 and 10. It follows then that TArS solutes in Fig. 1 collapse into a single or master curve in Fig. 2, i.e. ATG is also proportional to the solute surface area. Note that (i) if the solute surface areas are taken from ref. 12, as has been common the collapse of the ATG curves in Fig. 1 is less satisfactory than that seen in Fig. 2 and (ii) the qsvalue for benzene used here differs from that in ref. 13 (2.0724) and is the result of taking a constant difference between members of the alkylbenzene series (qCH2= 0.4655 in 60 50 40 NI E 30 7EG-SdU 20 10 0 i Fig.2 Experimental non-combinatorial liquid to water Gibbs energy of transfer expressed per unit solute area against temperature for the alkylbenzenes series. Code as in Fig. 1. derived from this unique curve for ATG is also non-combinatorial. With this, AFG (in mJ m-2) in Fig. 2 and 3 and the derived ArH, TATS and TAYC, in Fig. 3 are a reflection only of solute-solvent and solute-induced solvent- solvent interactions. Clearly, Fig. 3 expresses the fact that it is possible to make a general analysis of the dissolution of alkylbenzenes in water without studying each individual member of the series.Furthermore, as will be discussed below, the interpretation scheme presented in this work can be applied to any non-polar solute. 140 120 100 00 N 6o E 7 E 40 4 20 0 -20 -40 2 1 300 340 300 T/K Fig. 3 Experimental non-combinatorial liquid to water transfer functions, AFX, against temperature for the alkylbenzenes series. (a) TATC,, (b)AYG, (c)AFH, (6)TATS. ATG is from eqn. (1) and AFH, TAFS and TAFC, were obtained from AFG (see text). 35 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 New Interpretation Scheme At the molecular level, one can imagine that the transfer of the non-polar solute from its pure liquid state (L) to water (W) has many components.A reasonable division of the transfer process is given by the following components: (1) removal of a solute molecule from the pure liquid, thus breaking solute-solute contacts, (2) closing the cavity therein, (3) creating a cavity in water, breaking H-bonds, to accom- modate the solute molecule, (4) re-formation of H-bonds between water molecules around the solute and creation of solute-water contacts and (5) rearrangement or relaxation of water molecules around the solute. The experimental finding -----I-that AwCP is much greater than the heat capacity of the pure has almost unanim~usly~~"~~~' liquid ~olute,~>~ ' been inter- preted (an exception is that given in ref.15) to be a reliable indication of the rearrangement of water molecules around the solute, i.e.that Arcp is mainly caused by water solvating the non-polar solute. Then, although in principle, all five components of the transfer process described above have a contribution to Arcp, it appears reasonable to assume that those corresponding to components (1)-(4) are very small compared with that from component (5), i.e. that the experi- mental AYC, and its temperature dependence are due only to component (5). Based on this, the proposed interpretation scheme is illustrated in Fig. 4. Here, the L and W transfer has been divided into two steps introducing an intermediate hypothetical solution state termed unrelaxed water (URW). It is considered that the transfer L to URW contains com- ponents (1)-(4) above, while the transfer URW to W involves only component (5).Then Arcp = AzRwCp; further, it is assumed that AFS = AERwS. This equality is tantamount to considering that the non-combinatorial entropy changes due to components (1) and (2) and those due to components (3) and (4) cancel each other. It is conceivable that an entropic contribution may occur in this L to URW step in Fig. 4.If it exists, this contribution would arise from a non-zero balance between the entropy changes associated with components (3) and (4)and, given that Arcp = ATRWCp,it should be tem- perature independent. However, it would be difficult to extract this entropic contribution accurately without resorting to molecular models describing in detail com-ponents (3) and (4).This is avoided with the aim of main- taining simplicity. For the other thermodynamic functions X (G and H), AYX = A,URWX+ ArRwX, where AERwX is the L com ponen ts (1) -(4) components(1) -(5) \URW w Fig. 4 Interpretation scheme for analysis of the dissolution of non-polar solutes into water. The liquid (L) to water (W) transfer is divided into two steps introducing an intermediate solution state rep- resenting unrelaxed water (URW). Components 1-5 are described in detaii in the text. 1515 contribution to X due to the rearrangement of water mol- ecules around the solute. Clearly, in this scheme AFRWG= AFRWHat all temperatures. Hence, it is considered that the temperature dependence of the L to W transfer is all con- tained in the URW to W step in Fig.4. Finally, note that the scheme in Fig. 4 can accommodate different views as to which is the precise balance (in number and energy) between the H bonds broken during the introduction of the solute into water and those H bonds re-formed around the solute. In fact, the scheme can also accommodate the view, discussed in detail in ref. 7, that the introduction of the solute into water does not involve any H-bond breaking. The URW to W transfer in Fig. 4 can be calculated using a two-state model for water molecules in the immediate vicinity of the solute that is described in detail in the Appendix. The resulting equations for the thermodynamic transfer functions are ArRWG= -NRT ln(A/B) -NAhC1 -(l/B)] (2) TAERwS= NRT ln(A/B) + NAh[l -(l/A)] (3) ArRWH= NAh[(l/B) -(1/A)] (4) ArRwC, = (NAh2/RT2)[(A -1)/A2] (5) with A = 1 + exp[ -Ah(p -p,)]; B = 1 + exp(Ahp,) (6) p = l/RT; p, = 1/RT, (7) Eqn.(5) was first presented in ref. 11 and used to model heat capacities of transfer from the gaseous state to water. The parameters N, T, and Ah in eqn. (2)-(7) have the following meanings. N is the number of water molecules affected by a solute molecule and which have two possible energy states accessible, T, is the temperature at which both states are equally populated and Ah is the energy difference between those two states. The latter is interpreted as being the energy required for the geometrical rearrangement that an H-bonded water molecule undergoes in the vicinity of the solute molecule; as such, Ah can be thought of as the change in the energy of a water molecule in this relaxation process.With this, the URW state in Fig. 4 represents a mixture of non- polar solute and water, where the water molecules in the vicinity of the solute have undergone the energy changes implied by components (3) and (4) above, but have not yet reached their equilibrium (lower) energy. A number of con- ceptual models have been presented to describe and visualize the condition of water molecules around a solute molecule. These involve aggregates of water molecules that have been called icebergs,' flickering clusters'6*' or clathration shells similar to those found in crystalline hydrates.' * In contrast, in the interpretation scheme presented here there is no need to invoke any kind of water aggregates or structures, but rather it can only be assumed that water molecules around the solute have, because of their spatial position, two acces- sible energy states. In this sense, the two-state model used here is (a) conceptually similar to that developed by Nemethy and Scheraga' to describe water molecules surrounding the solute in terms of modified energy levels and (b)in agreement with the suggestion by Miller and Hildebrand'" that when a non-polar solute is introduced into water, H bonds are 'deactivated' to an extent depending upon the total surface of the solute.Other alternative approaches are that due to Hvidt, who assumed that water molecules in the neighbour- hood of the solute form a cooperative unit with different ther- modynamic properties from those of normal water? and the modified hydration shell H-bonding model due to M~ller,~.'~ where a distinction is made between the H-bonding enthalpy in bulk water and that in the hydration shell around the solute. Numerical Values of Ah, T,,,and N The use of the interpretation scheme in Fig. 4and eqn. (2)-(5) requires knowledge of the N, T, and Ah parameters. Of these, Ah and T, are solute independent, i.e. their values are charac- teristic of water as a solvent. On the other hand, the N parameter is proportional to the surface area of the solute.Since AfRWS = 0, ATS = ArRwS and Ah and T, can be obtained using ArS experimental data for any non-polar solute or series of them, we have here arbitrarily chosen to use benzene ATS data. From eqn. (3) 1/T, = 1/T + (R/Ah)ln[C/(l -C)] (8) where C = [TATS/N -RT ln(A/B)]/Ah (9) and where a = N/A, is a constant and TATS (in mJ m-2) is obtained from eqn. (1).In this work N has been taken equal to N,, where N, is the number of water molecules in direct contact with the solute (first hydration layer). Amongst the different ways of estimating N, ,we have chosen that reported in ref. 12. The N = N, criterion receives support from NMR20 and computer simulations,2' which indicate that it is essentially the first hydration layer that is significantly affected by the solute. Using N, = 26.7 for benzene,22 the numerical value of a = 43.77 x mol of water per m2 of solute was calculated.For a given Ah, eqn. (8) was then solved at each T using a fixed-point iteration method.23 This was done in the 273-363 K range (of 5 K intervals) producing an average T, value for each given Ah. This procedure was repeated for different Ah values until a minimum of the func- tion E = 1[ATSIN (calc) -AFS/N (exp)12 was found. The Ah and T, values thus obtained were 5.32 kJ mol-' and 220 K, respectively. Note that any N value for benzene can be chosen to fit Ah and T, to the experimental data. Our calcu- lations showed that as N increases i.e. as more than one water layer is assumed to be affected by the solute, Ah decreases.The values for Ah and T, obtained here differ from those in ref. 11 (6.5 kJ mol-' and 370 K, respectively); this might be due to the different fitting procedures employed. Temperature Dependence of the URW to W Transfer Using the Ah and T, values obtained above, Fig. 5 shows the temperature dependence of ArRWX/N for X = G, H, TS (in kJ mol-') and C, (in J K-' mol-l) from eqn. (2) to (5) indi-cating the experimentally accessible temperature range. Note that ArRwX/N is expressed per mol of water molecules being affected by the presence of the solute. Using eqn. (10) and its analogue for G, H and Cp,the A:RwX expressed in mJ m-2 and mJ K-' m-2, i.e. per m2 of solute, are easily obtained and are shown on the right-hand axes in Fig.5. Since the parameters Ah and T, are characteristic of water and hence unique, the curves in Fig. 5 are independent of the chemical structure of the non-polar solute (alkanes, alkenes, alkyl- benzenes, etc.) and of their size, i.e. the URW to W transfer functions are universal. A comparison between Fig. 5 and 3 reveals that there are two types of 'universality' of the trans- fer functions. First, the URW to W and the L to W (S and C,) transfers are universal for any non-polar solute; secondly the L and W (G and H) transfers are universal only for a J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 r I h 1.0 g-s N E W I-I 0.5 E1 uQ232a c I ha)4--100 N E Y 7 E -200 33 a 0 100 200 300 400 500 600 700 800 TIK Fig. 5 Unrelaxed water to water transfer functions AL,X/N from eqn.(2)-(5) against temperature. (a) AGwCp/N, (b) TAG,S, (c) ATRwG, (d) A&, H. The experimentally accessible temperature range (R) is indicated. Calculations were performed using Ah = 5.32 kJ mol-' and T, = 220 K. Since Ah and T, are characteristic of water and hence unique, AGwX/N are universal for any non-polar solute (see text). On the left-hand side axis ALwX/N are expressed per mole of water molecules. Using eqn. (lo), ALwX/N are expressed per m2 of solute on the right-hand side axis. given set of non-polar solutes of the same chemical nature as, for example, the alkylbenzene series in Fig.3. Thus, the L to URW transfer in Fig. 4 is also universal in the second sense. In other words, the ATX functions will have the same tem- perature dependence for any non-polar solute. When com- paring between two families of non-polar solutes, e.g. alkylbenzenes and alkanes or when comparing amongst members of a given family, ATX (X = G and H)values will differ only by a constant whose value depends on the ALRWX for that family of solutes and the N value for a given solute. The experimental temperature dependence of AFX emerges only from the relaxation of water molecules in the solute/ water interface or, more precisely, from a change in the rela- tive populations of the two states accessible to water molecules around the solute.Note that the above general statements on the dissolution of non-polar solutes in water are valid only if the ArX with X = G and TS are non- combinatorial, i.e. they are based on the fact that when the combinatorial contributions to G and TS are taken off the resulting AFX are only proportional to the non-polar surface area as Fig. 3 illustrates for the alkylbenzenes. We expect that any other family of non-polar solutes will behave in an analo- gous manner. Application to Alkylbenzenes Data In this section the interpretation scheme proposed above is applied to the alkylbenzenes L to W transfer data and a general description of the dissolution of non-polar solutes into water is given. This description is then contrasted with that produced by the analysis scheme reported in ref.6. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Proposed Scheme In order to apply the above described scheme to any family of non-polar solutes of the same chemical structure, it is necessary to determine AzRWG= AyRWH.According to Fig. 4, AiRWG= ATG -ATRWGso that AYRWGcan be easily evalu- ated. For alkylbenzenes, using eqn. (1) for ArG and eqn. (2) for AFRWGin the experimental temperature range, AyWG= 63.1 f0.05 mJ mP2. The fact that AFRWGhas the same value at all temperatures is in accordance with the scheme in Fig. 4 and is a consequence of the practically perfect fitting of eqn. (3) (see below) to ATS described above. To calculate the transfer functions for any solute within a family of com-pounds, it is necessary to know the N value for that solute.For any non-polar solute i, N can be approximated by Ni = N,(benz)qi/qb,,, = 26.7 qJ2.1035 (11) so that for toluene, ethylbenzene and propylbenzene, N values are 32.6, 38.5 and 44.4, respectively. Fig. 6 and 7 show the thermodynamic transfer functions for benzene and propylbenzene, indicating their contributions according to the interpretation scheme in Fig. 4. Here, the experimental ArX from eqn. (1) and the calculated ATR,X from eqn. (2)-(4) have been transformed from being expressed per solute area to kJ mol-' of solute molecules through the use of eqn. (10) and its analogue for G and H. As already indicated by the transformation from Fig. 1 into Fig. 2, it is clear from Fig.6 and 7 that the difference in magnitude between the ArX in kJ mol-' for benzene and propylben- zene is a consequence only of their different surface areas. In other words, both solutes have the same fundamental behav- iour in water, the magnitude of the thermodynamic functions expressing that behaviour as being proportional to their surface areas. Following the scheme in Fig. 4, ATG and AYH in Fig. 6 and 7 are composed of two contributions of opposite sign: a large positive contribution (AFRWGand AFRWH)and a nega- tive contribution (A:Rw G and ATRwH). The positive contri- butions arise essentially from the large differences among 260 300 340 380 420 TIK Fig. 6 Thermodynamic transfer functions against temperature for benzene, indicating their contributions according to the interpreta- tion scheme in Fig.4. (a) AyWG= AywH, (b) AFG, (c) AFH, (6) ALwG, (e) TAFS = TAGw& (f) ALwH. The experimental AFX (0)are from eqn. (1). AERwX are from eqn. (2)-(4) with Ah = 5.32 kJ mol-', Tm= 220 K and N, = 26.7. AYRWG= AFG -ArRwG = 38.5 kJ mol-. 1517 loo80 00 L1-1-80260 300 340 380 420 TIK Fig. 7 Thermodynamic transfer functions against temperature for propylbenzene indicating their contributions according to the inter- pretation scheme in Fig. 4. (a)AFwG = AFRWH,(b)Arc, (c) AFH, (d) ArRWG,(e) TAFS = TAERwS, (f)ALwH. The experimental AFX (0)are from eqn. (1).ALwX are from eqn. (2)-(4) with Ah = 5.32 kJ mol-', T, = 220 K and N, = 44.4 A,URWG= ArG -AERwG = 64.05 kJ mol-'.solute-solute, solute-water and water-water interactions which are reflected in components (1)-(4). All of these inter- actions are attractive, as clearly shown by the so-called work of adhesion between alkanes and water and the work of cohe- sion of water and of alkanes." Amongst these interactions the water-water interaction is, by far, the largest and hence it is possible to state that the large magnitudes of ArRWGand ApWH are, primarily, a consequence of the high cohesive energy of water. The negative contributions in Fig. 6 and 7 are due to the relaxation of water molecules around the solute [component (5)] which lowers the enthalpy, Gibbs energy and entropy of the system and which is described here by the two-state model discussed above.The resulting positive AFG in Fig. 6 and 7 is then due to IAFRWGI> IArKWGI. This is in contrast with the interpretation2.' 7924 that owing to a lack of AFH -TATS compensation, 'water structuring' leads to a positive AFG. In fact, as seen in Fig. 6 and 7 enthalpy-entropy compensation occurs between A&WH and TArRwS producing a relatively small negative AFRw G. This compensation is in agreement with the general finding4d that when an 'ordering' process decreases rapidly with increasing temperature (in this case the water relaxation process), enthalpy+ntropy compensation occurs and the resulting change in Gibbs energy is small. In summary, the low solubilities of non-polar solutes in water are due to the positive A,URWGcontribution to the Gibbs energy.This contribution to the Gibbs energy overcomes that arising from the water relaxation process which stabilizes the non-polar solute-water system and promotes solubility. Ana- logous interpretations have been given before using different descriptions or conceptualizations of the state of water mol- ecules around the non-polar solute: in ref. 4(d) and (e) in terms of water ordering or structuring, in ref. 4(a) and (b)in terms of iceberg formation and in ref. 6 in terms of hydration. Using eqn. (S), Arc, = AYRWC, for any non-polar solute can be predicted and it is shown in Fig. 8 together with the 1518 400 I\: 0 100 1 -I 01 """"'1' I '('I' 260 300 340 380 420 TIK Fig. 8 Liquid to water heat capacity of transfer expressed per solute area against temperature for benzene and toluene.Experimental points are from ref. 22 (0)and from Arcp= -TbZArG/6T2 (@) with AYG given by eqn. (1). Full line is AFC, = AZwC, from eqn. (5) with Ah = 5.32 kJ mol-l and T,= 220 K. experimental values for the alkylbenzenes obtained from eqn. (1) and those reported in ref. 22 for benzene and toluene (expressed per unit solute area). Fig. 8 indicates that there is a reasonable agreement between both sets of experimental data. Since the AFC, from eqn. (1) was obtained as a second derivative of AFG it is concluded that the primary data are of good quality, as already indicated in ref. 9. The calculated Arc, in Fig. 8 has the correct temperature dependence, i.e.AFC, decreases strongly with increasing temperature. Com- parison between the experimental and calculated Arc, shows that the prediction from eqn. (5) is quantitatively correct only at low temperatures. At 298 K eqn. (5) gives rea- sonable Arc, predictions (in J K-' mol-l): 222 us. 238 or 271 us.225 & 5 for ben~ene,~~,~~ 305 or 263 & 13 for t~luene,'~.~~ us. 318 f13 for ethylben~ene,~' 369 us.320 391 & 25 for pr~pylbenzene,~~ 274 us. 360 30 for cyclo- andhe~ane,~'286 DS. 400f 70 for ~entane~~ 333 us. 440 & 45 for he~ane.~' Note that for the last two solutes Arcp was obtained from only two experimental ATH values.25 It appears then that eqn. (5) with the Ah and T, parameters obtained above can be used to obtain reasonable estimations of AFCpat 298 K for any non-polar solute.Comparison with the Privalov and Gill Interpretation Scheme Another effort to understand the dissolution of non-polar solutes in water has been presented recently by Privalov and Gill6 (PG). Fig. 9A shows the experimentalg L to W transfer functions for benzene ArXx. For X = G and TS, these func- tions correspond to the ArX used here without having sub- tracted their non-combinatorial part, i.e. AFGx = -RT In x where x is the solubility expressed as mole fraction. The cal- culated AFGx in Fig. 9A were obtained using6 ATGx = AFH -TAFS, = AFC,(T -TH) -TArC, ln(T/TJ (12) where Arcpis taken as a constant equal to its value at 298 K and TH and T, are the temperatures where ATH = 0 and AFSx = 0, respectively.Their values from ref. qa) are TH = 293 K and T, = 413 K and Arc, = 238 J mol-' K-' from ref. 22. If AFCp is taken as a function of temperature,22 the results are qualitatively similar to those in Fig. 9A.6a In the PG interpretation scheme a hypothetical compact state (CS) is defined such that the L to W transfer is divided into two steps, L to CS and CS to W, the latter transfer being termed J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 f ,,'if) '~a l . l . / ' l j ~I'I~~'~*40 -40 400 500 300 400 500 TIK Fig. 9 A, Experimental and calculated thermodynamic transfer functions against temperature for benzene: (a) ATGx, (b)ATH, (c) TATS,. Experimental AFX, values (0)are from ref.9 and are expressed on the mole fraction basis, i.e. without subtraction of the non-combinatorial part (see text). Full lines are ATX, , calculated using eqn. (12). B, Contributions to ATXx according to the interpre- tation scheme in ref. 6: (a) AFG = AFH = ATH(Ts), (b) AFG,, (c) ATH, (d) AZG, (e) TATS, = TAZS, (f)AZ H. The compact state (CS) to water (W) transfers are the hydration contributions to X. hydration.6 The properties of this hypothetical compact state are essentially those of a compressed hydrocarbon gas at T,.6aFig. 9B shows the two contributions to ArXx. Here, given the definition of the CS state, AyG = AyH = ArH(T,) and AFS = AFC, = 0. A comparison between Fig. 6 and 9 shows that both the PG analysis scheme and the one presented here are able to reproduce the behaviour of the L to W transfer in the tem- perature range where there are experimental data.However, it is in the interpretation of this transfer and particularly in its decomposition into different contributions and their physical meaning where the two schemes differ considerably. Eqn. (12) indicates that the PG interpretation scheme is based on the existence of two temperatures (TH and T,) of a universal nature that describe the thermodynamic properties of the dissolution of non-polar solutes into water. In ref. qb), it is stated that at T, water does not solvate the non-polar solute, i.e. T,is the temperature at which the solute molecules are not hydrated. Hydration occurs at T < T, and as a process is defined as the dissolution of the non-polar solute in the compact state into water.6b Clearly, according to this defi- nition, hydration is constituted by components (3)-(5) above.The numerical value of T, and its physical significance have been recently examined,'** concluding that the interpretation of T, as the temperature where hydration ceases cannot be sustained. On the other hand, as discussed below, TH appears to be the same for a family of solutes of the same chemical nature. As in Fig. 6 with ArG, Fig. 9 indicates that AFGx is also the result of the balance between two contributions of opposite sign. However, since the hydration term (AZ G) con-tains contributions from cavity formation, re-formation of water-water H bonds around the solute, and van der Waals interactions, as well as others arising from the behaviour of water molecules around the solute, it is difficult to under- stand the physical origin of the positive AFGx value.Note that according to Fig. 9(b), AZG stabilizes the solute-water system and promotes solubility, i.e. in this sense AZG plays the same role as ArRWG.It is not clear, however, which com- ponent of hydration is causing this phenomenon. The differences between the two interpretations go beyond the mere definition of the contributions to the L and W transfer. One of the properties of the CS state is that AFS = 0 and hence ArSx = AZS = AS (hydration). On the other hand, since hydration is believed to vanish at T, then AFS, J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 should be zero at T > T,. What is then the origin of the positive AFSx = AZS at T > T’ calculated from eqn. (12) and seen in Fig. 9? Furthermore, what is the physical meaning of ATGx and AZG decreasing with increasing tem- perature for T > T, in Fig. 9? This situation contrasts with that displayed in Fig. 5 for the AcRwX functions which asymptotically approach zero at high temperatures, a behav- iour which is expected from the physical meaning of the URW to W transfer. It appears then that the PG interpreta-tion scheme is not free of inconsistencies. On the other hand, the interpretation scheme proposed here seems to be consis- tent and, in addition, does not rely on the existence of the temperature T,.Predictions using the New Interpretation Scheme The generality of the proposed scheme and the simplicity of its application lead to the following quantitative and qualit- ative predictions regarding the dissolution of non-polar solutes into water. In all calculations the values for Ah and T, obtained above were used. Solubility of Alkanes The solubility, x, of any non-polar solute i can be calculated from 1519 -8 t i -1 0 0. -1 2 + % -C -1 4 A A -1 6 -1 8 0.028 0.030 0.032 0.034 0.036 KIT Fig. 10 Predicted (full lines) and experimental solubilities for the alkanes. Experimental values are from ref. 27: pentane (O),hexane (+), heptane (A) and octane (A). Calculations were performed using eqn.(13) and the interpretation scheme in Fig. 4. lowers AyG/T (increases the solubility). More precisely, as T decreases, it is the increasing importance of the water relax- In(x) = -(A~G/RT) -[ln(Vw*m/V&)+ 1 -(v~*~/v;)]ation process that makes possible the dissolution of larger quantities of solute into water. A similar conclusion was first (13) where the term in square brackets on the right-hand side arises from the Flory-Huggins expression for the com-binatorial Gibbs energy. In eqn. (13), Vy.Oo/V$is the ratio of the partial molar volume of solute i at infinite dilution in water to the molar volume of pure water, and AFG = A:RWG + AZRWG, with ArRWG being given by eqn. (2). In using eqn. (13) for the alkanes, (a) the N values were obtained using eqn.(11) with qi from ref. 13, (b) Vw,m/V; =f[(VP/V&)],where f=(V~~/V~,,J 0.913 from ref. 9 and Vp are the molar = volumes of the pure solutes calculated at each temperature using the Martin equation,26 (c) V&= 18 cm3 mol-’ at all temperatures and (d) AyRWGwas fitted so as to reproduce the experimental solubility of pentane at 293 KZ7This gave AFWG = 67.3 mJ m-’, i.e. only slightly greater than that for the alkylbenzenes, as intuitively expected from the less favourable interaction between alkanes and water than that between the aromatic rings of the alkylbenzenes and water. The calculated solubilities for pentane to octane in the 273-363 K temperature range are shown in Fig. 10, together with the experimental values.z7 It can be seen that the predictions from eqn.(13) are in fair agreement with the experimental values. The proposed interpretation scheme can readily explain the solubility minimum appearing in aqueous solutions of non-polar solutes. Fig. 11 shows A?G/T as a function of 1/T, indicating its two contributions ALURWG/T and ArRw G/N T from eqn. (2). Since both of these functions are independent of’ the surface area of the solute, Fig. 11 is a universal plot for a given family of solutes of the same chemical nature where, as seen above, A:RWG is a constant. The term in square brackets in eqn. (13) is solute and temperature dependent but its value does not change the general physical picture arising from Fig. reached by Shinoda and FujiharakVb using the iceberg forma- tion concept.For different solutes, ArRwG/T increases and AFRWG/T decreases with 1/T proportional to the solute surface area. Fig. 11 predicts that for a large solute (large N value) the maximum AyG/T will be bigger than for a smaller solute, i.e. the solubility minimum will occur at a lower mole fraction x, as found e~perimentally.~~ According to Fig. 11, this solubility minimum will appear at the same temperature for any solute within a given family of equal chemical nature compounds. It is the weak temperature and solute depen- dence of the term in square brackets in eqn. (13) that dis- places the solubility minimum towards slightly higher temperatures as the available dataz7 indicate.0.2 L-I Y NI E 7 E G3. -“I I I I I-0.4L I I 0.000 0.002 0.004 0.006 11, i.e. the solubility minimum, corresponding to the KIT maximum in Fig. 11, is the result of the balance between the Fig. 11 AyG/T (a)as a function of 1/T, indicating its two contribu- two contributions of opposite sign; as the temperature tions, AyWG/T(b)and AgWG/NT(c), from eqn. (2). This is a uni-decreases the negative ArRw G/T increases in magnitude and versal plot for a given family of solutes of the same chemical nature. 1520 L to W Enthalpies of Transfer for Non-polar Solutes The L to W transfer for any solute is simply calculated as AFH = AtRWH+ AzRwH, with ArRwH given by eqn. (4) and A:RWH = 63.1 and 67.3 mJ mP2 for the alkylbenzenes and alkanes, respectively.Fig. 12 shows the predicted ATH for these two families of non-polar solutes together with the available experimental data.25 For a given family of solutes of the same chemical structure, AFH = 0 at a given tem- perature. In ref. 6(a) this temperature was denoted as THand considered to be equal for all non-polar solutes. However, it is clear from Fig. 12 that TH changes from one family of solutes to another; it is because the THvalues are close (295 K for alkylbenzenes and 285 K for alkanes) that THappeared as a universal temperature for a variety of solutes. A unique value of TH for a family of solutes is clearly a consequence of AFRWH= -ArRWH at the same temperature for all of the solutes in that family.Different TH values amongst unlike solute families arise from different AFRWHfor each group of solutes. The predicted ATH in Fig. 12 are not in quantitative agreement with experiment. Also, THfor alkanes is predicted to be lower than that for alkylbenzenes, while the opposite is found experimentally. On the other hand, the qualitative pre- dictions for the alkylbenzenes are correct, i.e. as the surface area increases from benzene to propylbenzene, AFH becomes more negative for T < TH and more positive for T > TH. This is the result of ATCp increasing with surface area, as predicted by eqn. (5). For alkanes, at T < THthe experimen- tal AFH for pentane is more negative than that for hexane. Here, the lack of sufficient and more accurate experimental data prevents any definitive conclusion.In summary, Fig. 12 shows that the ATH predictions for the alkylbenzenes are better than those for the alkanes; a possible reason for these poor predictions for the alkanes is that AFw= 67.3 mJ m-2 was fitted (previous section) to a single solubility point for pentane (at 293 K), and hence this value might not be very accurate. 4 2 r I-fo -2 d -4 -6 t -1 0 280 290 300 310 T/K Fig. 12 Predicted (full lines) and experimental liquid to water enth- alpies of transfer for the alkylbenzenes series, pentane and hexane. Experimental values are from ref. 25 : benzene toluene (O),eth-ylbenzene (W)y propylbenzene (a),pentane (A) and hexane (A). Cal-culations were performed using the interpretation scheme in Fig.4. For each family of solutes of the same chemical nature AYH becomes zero at a given temperature. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Significance of AP against ArcpPlots It has been found that when ATSx, i.e. the change in entropy without having subtracted its combinatorial part, is plotted against Arcpfor a series of solutes at a given temperature, a straight line is ~btained.~~-~' This empirical observation has been used to discuss the nature of the hydrophobicity both of non-polar molecules and proteins. Using the PG interpreta-tion scheme, i.e. eqn. (12), the slope of such straight lines pro- vides a value for T,. Since according to Fig. 4 the experimental values of ATSIN and AFCp/N are universal, i.e.they are the same for any non-polar solute, a plot of AFS against Arcp(in mJ rn-' K-') at a given temperature is composed of only one point representing all non-polar solutes, as indicated in Fig. 13(a).The more familiar AYS us. Arcpemerges from Fig. 13(a) by simply using the solute surface areas, A,, for any set of solutes. This is shown in Fig. 13 for the alkylbenzenes and alkanes. In this way, the single point in Fig. 13(a) is transformed into a set of points in Fig. 13(b) and (c). Combining Fig. 13(b) and (c) produces Fig. 13(d). The experimental points in Fig. 13(d) are clearly in a straight line of zero intercept; this is the result of ATS and Arc, being proportional to the solute surface area and hence they are 'scaled up' by the same amount for each solute.The origin and physical significance of this plot can be seen easily using the interpretation scheme presented here. From eqn. (3) and (5) AcRwS/N = AyS/N = mArRWCp/N = mATC,/N (14) with m = [RTA/AhI2[ln(A/B)/(A -l)] + RTA/Ah (15) Eqn. (14) and (15) indicate that a plot of AFS against ATCp (in kJ mol K-') should be a straight line of zero intercept and that its slope is independent of what set of solutes is used to build the plot. The existence of this linear relationship implies that both AFS and Arcpreflect the same physical process. The nature of this process is revealed by eqn. (15), 0 NI E 7 -0.1 Y 0 7 'E -0.2 O$1 e-OOO -I2 d 0 r I Y 7 -0.1-0 E2 -0.2 z -0.3 0 0.2 0.4 0.6 0 0.2 0.4 0.6 AKC,/mJ K-' m-2 AyC,/kJ mol-' K-' Fig.13 Entropy against heat capacity for the liquid to water trans- fer of non-polar solutes at 298 K. Following the scheme in Fig. 4, AFS/N us. AFCJN is composed of only one point (a)representing all non-polar solutes. Different N values for each solute produces a set of points [(b)and (c)]; the solutes used were the alkylbenzenes (a)and the alkanes (0).When both entropy and heat capacity of trans- fer are scaled up, AFS us. Arcpis a straight line (d)with zero inter- cept, whose slope is independent of the set of solutes used and is a function only of the parameters governing the relaxation of water molecules (Ah and T,). The straight line in (d)is eqn.(14) with rn = -0.353 from eqn. (15); (d)also indicates the result of using the molar basis, AFSx,for the alkylbenzenes (m) and alkanes (0). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 which shows that at a given temperature, the numerical value of the slope is a function of only the parameters governing the relaxation that water molecules undergo in the vicinity of the solute, i.e. Ah and T, . On going from Fig. 13(a) to (6)it is possible not only to scale up ATS using the As values but also to convert the AFS to the often used AFSx through AFSx = AFS -R[ln(Vy,"/V&) + 1 -(V/w*co/V&)] (16) where, as above, VT,"/V$ = 0.913 [(Vp/V&)] with Vp for alkylbenzenes and alkanes taken from ref. 26. The result of using eqn.(16) and the corresponding points in the AFSx against AFC, plot is shown in Fig. 13(d). Combining eqn. (14) and (16) AFS, = -R[ln(VT*"/V&) + 1 -(Vy*"/V&)] + rnAFCp (17) Both terms on the right-hand side of eqn. (17) are solute dependent, i.e. they are a function of solute volume and surface. For a series of solutes the AFSx against Arcpplot appears to be a straight line [see Fig. 13(d)] only because the first term in eqn. (17) varies smoothly with solute volume. Fitting a straight line to these data in Fig. 13(d) is then not formally justified and produces, according to the interpreta- tion scheme in Fig. 4, a slope and an intercept without any clear-cut significance. Proton NMR Chemical Shifts Shifts of the water proton magnetic resonance signal for mix- tures of non-polar molecules in water cannot be measured owing to the extremely low solubility of these solutes. Proton NMR results for alcohols in water show a downfield shift which implies a 'structure-promoting' role of the alkyl chain of the alcohol molecule^.^^ On the other hand, for alkyl- ammonium chlorides in water3* an upfield shift is observed implying a 'structure breaking' behaviour.Both of these results have been taken as indirect evidence that in a non-polar solute-water system the chemical shift should change sign as a function of temperat~re.~"~Recently, Muller reported that none of the existing models for hydrophobicity are able to predict such a behaviour and hence proposed a modified hydration shell H-bond model which is able to account for the expected change in sign of the chemical shift^.^ In his model, this is accomplished through consider- ation of the fractions of broken H-bonds in bulk water and the hydration shell.Using the interpretation scheme pre- sented here it is also possible to account for a change in sign of the chemical shift. Following ref. 7 the chemical shift can be written as A6 = D[afRW -bfURWl (18) where D is a proportionality constant, fRw and f"RW are the fractions of relaxed and unrelaxed molecules around the solute, and a and b are the corresponding downfield chemical shifts. As such, a is the shift due to the presence of the solute and the relaxation process, while b reflects only the former and hence a > b.The fractions, fRw andf,,,, are easily calcu- lated, their expressions being given in the Appendix [eqn. (1.8)]. Fig. 14A shows these fractions in a wide temperature range, indicating also the experimental temperature range used here (273-363 K). Since this temperature interval is above T,,fURW >fRw . Using eqn. (18) with a/b = 2.5, A6/Db is plotted against temperature in Fig. 14B, showing the expected change from positive shifts at low temperatures to negative shifts at high temperatures. 1521 1.o I 1 II I I I dI \ -0.41 l , , , 280 320 360 400 TIK Fig. 14 A, Fraction of (a) unrelaxed vnw)and (b) relaxed uRw)water molecules around the solute, calculated according to the expressions given in the Appendix. The experimental temperature range used here (R)is indicated.B, Predicted proton NMR chemical shift, A6/Db, from eqn. (18), which shows the expected change of sign with temperature. Here, D and b are constants (see text) and a/ b = 2.5. Conclusions The new interpretation scheme proposed here is able to rationalize the experimental thermodynamic properties for the dissolution of non-polar molecules into water. If the com- binatorial contribution to the Gibbs energy and entropy of transfer from L to W is subtracted from the experimental values, all non-polar solutes in water can be treated with their molecular surface area as the only parameter. The origin of the hydrophobicity lies in the high cohesive energy of water, being counteracted by the relaxation of water mol- ecules in the neighbourhood of the hydrophobic solute.The proposed scheme includes a two-state model for water mol- ecules in the vicinity of the solute. This model is simple to use and allows the prediction of data in the literature and the rationalization of several concepts often used in the field. The results presented here provide a firm basis to re-examine and hopefully advance the understanding of more complex phe- nomena such as micellization, adsorption of surfactants on non-polar surfaces and the stability and denaturation of pro- teins. We gratefully acknowledge the support of Seminario Acade- mico Hector Sobol (Facultad de Quimica, UNAM and ICI de Mexico), the Consejo Nacional de Ciencia y Tecnologia and The Swedish Institute.R. Silveston thanks The Founda- tion for Surface Chemistry Research for their financial support. We extend our sincere appreciation to Donald Pat- terson, Jorge Hernandez, Maria Luisa San Roman and Mar- garita Isabel Bernal for their valuable comments on the manuscript, to Vicente Talanquer for his comments on the two-state model and to Gabriela Marmolejo for her inspiring support during our work. Appendix Two-state Model for Water Molecules Consider N water molecules in the first hydration shell which have two accessible energy states, c1 and E,, such that c2 -c1 = Ah > 0. Molecules in levels 1 and 2 are in their relaxed (RW) and unrelaxed (URW) states, respectively.Ah is then the change in energy of a water molecule undergoing the relaxation process. Assuming that the molecules are indepen- dent and distinguishable, the partition function for the system is given by Q = qN (1.1) where q is the molecule partition function 4 = exp(-&) + exp(-Pc2) = 1 + exp( -j?Ah) (1.2) with B = 1/RT.The fraction of molecules in each energy level is then N,/N = 1/[1 + exp(-BAh)] N ,/N = exp(-BAh)/[ 1 + exp( -PAh)] (1.3) Using eqn. (1.3) when T --+ 0, N,/N + 1 and N2/N -+ 0 while when T -+ co,N,/N = N,/N + 4.It is convenient to move the equal population temperature from co to some finite tem- perature, T, . This is accomplished by rewriting eqn. (1.2) as q = 1 + c exp(--BAh) (1.4) where c is a constant whose value can be found from Nl/N = 1/[1 + c exp(-&,Ah)] = 4 (1.5) giving c = exp(B,Ah) (1.6) with /?, = l/RT,.Eqn. (1.4) then becomes q = 1 + exp[ -Ah(B -831 (1.7) with the fraction of molecules at each level being N,/N =fRw = 1/{1 + ~xPC-WB -8m)I) From eqn. (1.8), when T -+ 0, N,/N -+ 1 and N,N +0, while when T -+ co, N,/N --+ 1/(1 + c) and N,N -+ c/(l + c). The thermodynamic functions (at zero pressure) are found from eqn. (1.1) and (1.7) as AG = -RT In Q; AS = -(6AG/6T),, AH = AG + TAS; AC, = (6AH/6T)p,N (1.9) In eqn. (1.9) the reference state is that where T = 0, i.e. the state where all water molecules are in energy level 1. As such, AX(T) (X= G, S, H and C,) is the change in X associated with the passage of water molecules from the relaxed to the unrelaxed energy level when the temperature increases from T = 0 to T.It is then convenient to change the reference state to that where T = co. Using AX in eqn. (1.9), this is accomplished through AETwX = AX -lim AX (1.10) T+ r+) producing eqn. (2)-(5) in the text where we have written RW as W for simplicity. ARX(T) in eqn. (1.10) is the change in J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 X associated with the passage of water molecules from the unrelaxed to the relaxed energy level when the temperature decreases from T = GO to T. References 1 H. S. Frank and M. W. Evans, J. Phys. Chem., 1945,13,507. 2 W. Kauzmann, Adv. Protein Chem., 1959, 14, 1. 3 C. Tanford, The Hydrophobic E#ect : Formation of Micelles and Biological Membranes, Wiley-Interscience, New York, 1980.4 (a)K. Shinoda and M. Fujihara, Bull. Chem. SOC.Jpn., 1968, 41, 2612; (b) K. Shinoda, J. Phys. Chem., 1977, 81, 1300; (c) A. Hvidt, Acta Chem. Scand., Ser A, 1983, 37, 99; (d) D. Patterson and M. Barbe, J. Phys. Chem., 1976, 80, 2435; (e) R. Silveston and B. Kronberg, J. Phys. Chem., 1989,93,6241. 5 (a) K. W. Miller and J. H. Hildebrand, J. Am. Chem. SOC.,1968, 90, 3001; (b)J. H. Hildebrand, J. Phys. Chem., 1968,72,1841; (c) J. H. Hildebrand, Proc. Natl. Acad. Sci. USA, 1979,76, 194. 6 (a) P. L. Privalov and S. J. Gill, Adv. Protein Chem., 1988, 39, 191; (b)P. L. Privalov and S. J. Gill, Pure Appl. Chem., 1989,61, 1Oq7. 7 N. Muller, Acc. Chem.Res., 1990,23, 23. 8 B. Kronberg, M. Costas and R. Silveston, J. Solution Chem., sub-mitted. 9 R. Silveston and B. Kronberg, J. Chromatogr., 1994,659,43. 10 (a) B. Lee, Biopolymers, 1991, 31, 993; (b) K. A. Sharp, A. Nicholls, R. F. Fine and B. Honig, Science, 1991, 252, 106; (c) K. A. Sharp, A. Nicholls, R.Friedman and B. Honig, Biochem-istry, 1991,30,9686. 11 S. J. Gill, S. F. Dec, G. Olofsson and I. Wadso, J. Phys. Chem., 1985,89,3758. 12 R. B. Hermann, J. Phys. Chem., 1972,76,2754. 13 H. V. Kehiaian, J-P. E. Grolier and G. C. Benson, J. Chim. Phys., 1978, 75, 1031. 14 A. Bondi, J. Phys. Chem., 1964,68,441. 15 A. Ben-Naim, in Water, A Comprehensive Treatise, ed. F. Franks, Plenum Press, New York, 1973, vol. 2, ch. 11. 16 H. S. Frank and W-Y. Wen, Discuss. Faraday SOC.,1957,24133. 17 G. Nemethy and H. A. Scheraga, J. Chem. Phys., 1962,36,3401. 18 D. N. Glew, J. Phys. Chem., 1962,66,605. 19 N. Muller, J. Solution Chem., 1988, 17, 661. 20 P. 0. Eriksson, G. Lindblom, E. E. Burnell and G. J. T. Tiddy, J. Chem. SOC.,Faraday Trans. I, 1988,84,3129. 21 A. Wallquist, J. Phys. Chem., 1991,95, 8921. 22 G. I. Makhatadze and P. L. Privalov, J. Chem. Thermodyn., 1988,20,405. 23 S. D. Conte and C. de Boor, Elementary Numerical Analysis, McGraw Hill, New York, 2nd edn., 1972, 24 (a) E. Tomlinson, Znt. J. Pharmacol., 1983, 13, 115; (b) W. Melander, D. E. Campbell and C. Horvath, J. Chromatogr., 1978,158,215. 25 S. J. Gill, N. F. Nichols and I. Wadso, J. Chem. Thermodyn., 1976,8,445. 26 CDATA: Database of Thermodynamic and Transport Proper- ties for Chemistry and Engineering. Department of Physical Chemistry, Prague Institute of Chemical Technology, 1991. 27 Hydrocarbons with Water and Seawater. Solubility Data Series, ed. D. G. Shaw, Pergamon Press, Oxford, 1989, vol. 37, parts I and 11. 28 J. M. Sturtevant, Proc. Natl. Acad. Sci. USA, 1977,74,2236. 29 R. L. Baldwin, Proc. Natl. .4cad. Sci. USA, 1986,83,8069. 30 K. P. Murphy, P. L. Privalov and S. J. Gill, Science, 1990, 247, 559. 31 (a) M. D. Zeidler, in Water, A Comprehensive Treatise, ed. F. Franks Plenum Press, New York, 1973, vol. 2, ch. 10; (b) W-Y. Wen and H. G. Hertz, J. Solution Chem., 1972, 1, 17; (c) F. Franks and J. E. Desnoyers, Water Sci. Rev., 1985, 1, 171. 32 R. Bhanumathi and S. K. Vijalakshamma, J. Phys. Chem., 1986, 90,4666. Paper 3/02930A; Received 24th May, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001513

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1523-1532 Micellar Aggregates of Sodium Glycocholate and Sodium Taurocholate and their Interaction Complexes with Bilirubin-IXa Structural Models and Crystal Structure Maria D'Aiagni" Centro di Studio per la Chimica dei Recettori e delle Molecole Biologicamente Attive del C.N.R., c/o lstituto di Chimica, Universita Cattolica , Largo F. Vito I,00 168 Roma , Italy Lucian0 Galantini and Edoardo Giglio" Dipartimento di Chimica, Universita di Roma 'La Sapienza ', P.le A. Moro 5,00185Roma, Italy Enrico Gavuzzo lstituto di Strutturistica Chimica 'G. Giacomello ' C.N.R., C.P. no. 10,00016 Monterotondo Stazione, Roma, Italy Lucio Scaramuzza lstituto di Teoria e Struttura Elettronica e Comportamento Spettrochimico dei Composti di Coordinazione del C.N.R., C.P.no. 10,00016Monterotondo Stazione, Roma, Italy Sodium glycocholate (NaGC) and taurocholate (NaTC) have been studied by means of X-ray and circular dichro- ism (CD) measurements, using bilirubin-lXa (BR) as probe molecule, together with potential-energy calculations. Helical models for the micellar aggregates of NaGC and NaTC were inferred from crystal structures solved by X-ray analysis. Since it is known that chiral molecules, micellar aggregates and macromolecules select prefer- entially or exclusively one of the two enantiomeric conformers of BR, CD spectra of BR in submicellar and micellar aqueous solutions of NaGC and NaTC were recorded as a function of pH and BR concentration in order to verify these helical models and the enantioselective ability of the bile salt monomers and micellar aggre- gates.Potential-energy calculations supported the CD experimental results and provided reasonable bile salt-BR interaction models. The behaviour of NaGC and NaTC is compared with that of sodium deoxycholate (NaDC), previously studied. The CD spectra of the bile salt-BR systems seem to allow characterisation of the typical structure of the bile salt micellar aggregates. Helical structures have been proposed for NaDC and rubid- ium deoxycholate (RbDC), and proved by means of wide-and small-angle X-ray scattering, '-'EXAFS,4*5 NMR,2,6-8 EPR' and CD8-" measurements together with potential- energy calculation^.^^^*^^^^ Subsequently, some studies on sodium glycodeoxycholate (NaGDC),' 'sodium taurodeoxy- cholate (NaTDC),7*'0-12 NaGC'' and NaTC14 were accom- plished following the strategy used for NaDC and RbDC." Again, helical models were found in crystals and their ability to describe aqueous solutions of micellar aggregates for NaGDC and NaTDC was checked. Although some evidence was collected in favour of these models, no definitive proof about their consistency was reached.We previously used probe molecules to investigate the nature of the interactions between bile salt and probe, and to obtain information on the structure of the micellar aggre- gate~.~*'*~-'~BR, a bile pigment of biological interest,16 gives rise in solution to an equal number of two interconverting enantiomers in dynamic equilibrium having left ( -) and right (+) handed chirality (LBR and RBR, respectively). BR is characterized by a 'ridge tile' conformation stabilized by six intramolecular hydrogen bonds (Fig.l), and has a structure which was determined mainly from X-rayI7-' andNMR20-23 measurements. The enantiomers interconvert by breaking and remaking the six intramolecular hydrogen bonds. The dynamic equilibrium is disrupted by a preferential interaction of BR with a chiral molecule, macromolecule or molecular assembly. In this case, BR exhibits optical activity, giving rise to a bisignate CD Cotton effect, on the basis of the exciton coupling theory,24 typical of dipole-dipole coupling between the two pyrromethenone chromophores of one BR molecule.Many interaction complexes of BR with a wide variety of compounds were investigated by means of CD.25 It was observed that the complexation of NaDC with BR in water produces a bisignate CD Cotton effect,26 which was explained later by invoking a chiral structure of the helical micellar aggregates of NaDC.27 Bisignate CD spectra were also recorded for the complexes of BR with RbDC, NaGDC and NaTDC in water.'' More recently, the influence of pH and BR concentration on the CD spectra of submicellar and micellar NaDC-BR aqueous solutions was investigated.8 Potential-energy calculations provided some NaDC-BR interaction models which were checked by CD and NMR measurements. The aim of this paper is to obtain information on the struc- ture of the NaGC and NaTC micellar aggregates, since NaGC and NaTC (Fig.1) are the most abundant and the most carefully studied conjugated bile salt in humans, respec- tively. We resort to structural models, observed in crystals, in order to represent and to verify the micellar aggregates in aqueous A possible model, composed by bile salt molecules arranged in a helix with 2, symmetry, is avail- able for NaGC.13 NaTC is known to form three crystalline phases.28 We crystallized the two most stable, belonging to the triclinic and monoclinic systems. The crystal structure of the triclinic phase has been solved previou~ly.'~ The struct- ural unit is a bilayer which is unlikely to grow in solution by addition of molecules one at a time.On the other hand, several experiments have suggested that the bile salts self- associate in a non-cooperative continuous manner in water and at low ionic strengths, whereas sometimes a cooperative association of a large number of molecules occurs, especially at high ionic strength^.^' In the case of NaTC, for example, a progressive aggregation can be inferred from reliable experi- mental data obtained by means of surface tension and trans- lational diffusion coefficient measurement^.^^ Thus, a helix C(18) C(21) 4 O(32) C(18) C(21) 0(33) 5% 0(34) O(35) Fig. 1 Atomic numbering and enantiomeric conformers of BR (top). Broken lines represent intramolecular hydrogen bonds. Atomic num- bering of NaGC (middle) and NaTC (bottom).Hydrogen atoms are omitted. fulfils both types of growth, since it is a one-dimensional open system containing monomers with unsaturated forces at the end-points if the solvent molecules are disregarded." The helix can grow by a stepwise addition of monomers or by welding of helices. Therefore, the structural unit found in the NaGC crystal is compatible with the experimental results, whereas that found in the triclinic NaTC crystal is less prob- able. For these reasons we solved the structure of the NaTC monoclinic crystal, hoping to obtain a reliable model for the micellar aggregates, and investigated by means of CD mea- surements and potential-energy calculations the complexes of BR with NaGC and NaTC to commence a study of the tri- hydroxy bile salts.Experimental Materials and Methods NaGC and NaTC (Sigma) were twice crystallized from a mixture of water and acetone. BR (puriss., cu. 99%, Fluka) was used. High-performance liquid chromatography (HPLC) and spectroscopic measurements indicate that the com-mercial sample contains at least 96% of bilirubin-IXa. Trizma base (reagent grade) and phosphate buffer (pure reagent) were purchased from Sigma and Carlo Erba, respec- tively, and were used in order to adjust the pH. BR was J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 added to NaGC and NaTC aqueous solutions by using 0.01 mol dm-3 aqueous NaOH. All the measurements were accomplished after 30 min of preparation. CD spectra were recorded on a JASCO J-500A spectropol- arimeter at 25°C by using quartz cells with pathlengths of 0.1, 0.2,0.5, 1.0 and 2.0 cm and by flushing with dry ultrapuri- fied nitrogen before and during the experiments.A slit program that gives a wavelength accuracy better than 0.5 nm was used. The instrument was calibrated with androsterone (1.69 x mol dm-3 in dioxane) on the basis of a molar ellipticity [OlZo4 11 180 degree cm2 dmol-'. = Potential-energy calculations were carried out on Vax 8530 and MicroVax I1 computers by means of programs written in our laboratory. Suitable single crystals of monoclinic NaTC were grown from a solution containing water, acetone and diethyl ketone by vapour diffusion of acetone into the solution. Intensity X-ray data were collected at room temperature by means of a Syntex P2, automatic four-circle diffractometer, equipped with a graphite monochromator, using Cu-Ka radiation (A = 1.5418 A).Results and Discussion Crystal Structure of Monoclinic NaTC Unit cell parameters were determined by least-squares refine- ment of the angular setting of 15 selected reflections. Crystal data: 2NaC2,H4,N0,S + 11.5H20 + C,H,O, f, = 1340.6, monoclinic, C2, a = 40.204 (lo), b = 7.665 (2), c = 23.042 (7)A, jl = 92.77 (2)", I/ = 7092 (3) A3, 2 = 4, D,= 1.26 g ~m-~, Do= 1.28 g (by flotation in a chloroform-benzene chloride solution), p = 14.31 cm-', mp 499 K. A total of 4977 unique reflections with I > 2.5o(I) were considered observed and used in the calculations. They were collected at a variable and appropriate speed in o scanning mode to a maximum 28 of 138".Background counts were taken for a time equal to that of the scan. The intensities of three standard reflections, monitored every 100 reflections, remained essentially constant throughout data collection. The data were corrected for Lorentz and polarization effects, but not for absorption. The structure was solved by using the MULTAN pr0gram.j' The refinement was carried out by means of the program SIR-CAOS.32 The final atomic coordinates and equivalent isotropic thermal parameters, together with their estimated standard deviations (esd) in parentheses, have been deposited. The atom labelling in accordance with Fig. 1 and 2, where bond lengths and bond angles are given.Atomic numbering from O(1) to O(12) refers to oxygen atoms of water molecu1es.t Scattering factors and anomalous disper- sion coefficients were taken from ref. 33. The hydrogen atoms were generated at the expected positions, except for the water, hydroxylic and acetone hydrogens which were not taken into account in the calculations. A full-matrix least- squares refinement was performed with anisotropic tem-perature factors for all the non-hydrogen atoms, except for C(66), O(7) and the atoms of the acetone molecule, which appeared in the Fourier syntheses with electron densities lower than those of similar atoms, and during the refinement exhibited some abnormally high anisotropic temperature factors. O(7) was refined isotropically. C(66) was refined iso- tropically with fixed atomic coordinates, whereas the acetone atoms were introduced with fixed atomic coordinates and t Final fractional coordinates and equivalent isotropic thermal parameters of the non-hydrogen atoms have been deposited with the Cambridge Crystallographic Data Centre.Details are available from the Editorial Ofice. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0s 00 oc ON C( 1)-C( 10)-c(9) C(5)-C( 10)-C( 19) C( 12)-C( 13)-C( 17) C(14)-C(13)-C(18) C(31)-S(32)-0(34) O(33)-S(32)-0( 35) C(36)-C(45)-C(44) C(4O)-C(45)-C(54) C(47)-C(48)-C(52) C(49)-C(48)-C(53) C( 66)-S(67)-0(69) 0(68)-S(67)-0(70) Q 111.6 109.8 1 16.4 113.3 107.9 109.4 11 1.4 110.3 118.0 113.5 107.7 11 1.9 Fig.2 Final bond lengths (A)and bond angles (degrees) found for the two taurocholate anions of the asymmetric unit. The average esds are 0.012 A and 0.7"with maximum values of 0.021 A and 1.3". isotropic thermal parameters. An occupancy factor of 0.5 was assigned to O(7) and to the acetone atoms. The location of the acetone atoms is very approximate owing to their occu- pancy factor of 0.5 and to a possible positional disorder, sup- ported by the low electron density of the corresponding peaks observed in the Fourier syntheses. However, the inclu- sion of acetone, located near a special position, was crucial for fitting some strong low-angle reflections and for obtaining a satisfactory convergence in the refinement.The atomic coordinates of the hydrogens were not refined, and their thermal parameters were taken to be equal to the isotropic ones of the parent atoms. The function minimized was w( I F, I -I F, I )2 with w = (sin 6/A)2.Several other weighting schemes were checked, but the results were worse. The final agreement factors R and R, converged to 0.074 and 0.083, respectively. S was found to be 0.68. The torsion angles of the side chain and ring D are re- ported in Table 1. The two NaTC anions are characterized by a conformation of ring D intermediate between the half- chair and the C(13) en~elope.~' The C( 13)-C( 17)-C(20)- C(21), C( 13)-C( 17)-C(20)-C(22), and C( 17)-C(20)-C(22)- C(23) torsion angles are always confined within a narrow range around -60 and 180", respectively, as established by energy calculation^.^^*^^ The deviation from planarity of the amide group (assuming that the three bonds to the nitrogen atom lie in a plane) is slight.The rotation around N(29)-C(30) seems to be almost completely free, since the values of C(24)-N(29)-C(3O)-C(31) vary from about -160 to 120" in all the conjugated bile salts studied so far.'0~1'*13*14From these results it emerges that several permissible conforma- tions can be achieved in order to satisfy packing and energy requirements in the crystals and in solution. The coordination of the sodium ions and oxygen atoms of water molecules is visible in Fig. 3. The sodium ions are hexacoordinated, with a distorted octahedral coordination.They are engaged in strong Coulombic interactions with the SO, -groups and ion-dipole interactions with water mol- ecules. Moreover, the structure is further stabilized by a J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Torsion angles of the NaTC side chain and ring D together with A and 4,,,;esds in parentheses” atoms torsion angleldegrees atoms torsion angle/degrees C(13)-C( 17)4(20)-C(21) C( 13)-C( 17)-c(20)-c(22) C( 17)4(2O)-C(22)-C(23) C(20)-C(22)-C( 23)-C(24) -59.0 (10) 176.6 (6) 178.8 (7) -170.5 (7) C(48)-C(52)-C(5S)-C(56) C(48)-C( 52)-C( 55)-C( 57) C( 52)-C( 55)-C( 57)-C( 58) C(55)-C(57)-C(58)-C(59) -67.7 (10) 166.5 (7) 61.2 (10) -164.6 (8) C( 22)-C(2 3)-C( 24)-O( 28) 43.6 (12) C( 57)-C( 58)-C( 59)-O(63) 66.2 (14) C(22)-C( 23)-C( 24)-N(29) -138.1 (9) C( 57)-C( 58)-C( 59)-N( 64) -116.8 (11) C(23)-C(24)-N(29)4(30) 178.8 (9) C(58)-C( 59)-N( 64)-C( 65) -169.2 (12) C(24)-N(29)-C(3O)-C(3 1) 0(28)-C(24)-N(29)-C(30) N( 29)-C( 30)-C(3 1)-S( 32) C( 30)-C( 3 1 )-S( 32)-O( 33) C(30)-C(3 l)-S(32)-0(34) 94.4 (11) -2.9 (15) 63.3 (10) 51.2 (9) -71.2 (9) C( 59)-N(64)-C( 65)-C( 66) 0(63)-C(59)-N(64)-C(65) N(64)-C(65)-C( 66)-S(67) C( 65)-C(66)-S(67)-0(68) C( 65)-C(66)-S(67)-0(69) -120.1 (13) 7.8 (20) -176.6 (8) 52.3 (8) -73.2 (8) C(30)-C(3 l)-S(32)-0(35) 171.4 (8) C(65)-C( 66)-S(67)-0( 70) 168.6 (8) C( 17)-C( 13)-c( 14)-C( 15) C( 13)-C( 14)-C( 15)-C( 16) C( 14)-C( 1 5)-C( 16)-C( 17) C( 15)-C( 16)-C( 17)-C( 1 3) C( 16)-C( 17)-C( 13)-C( 14) 45.9 (7) -34.1 (8) 8.8 (9) 18.8 (8) -38.8 (7) C( 52)-C(48)-C(49)-C(SO) C(48)-C(49)-C(5O)-C( 51) C(49)-C(5O)-C(5 1)-C( 52) C(50)-C(5 1)-C(52)4(48) C(Sl)-C(52)-C(48)-C(49) 46.3 (8) 3.9 (9) -31.3 (8) 24.1 (9) -42.3 (8) Aldegrees 11.9 24.7 4Jdegrees 46.2 47.4 ” The values of the torsion angles are in accordance with the convention of Klyne and Pre10g.~~ The phase angle of pseudo-rotation, A, and the maximum angle of torsion, 4m,are calculated according to Altona et ~ 1 .~ ~ complex network of hydrogen bonds. Those formed by water molecules are shown in Fig. 3. Two different units can be recognized in the crystal packing (Fig. 4). The first one is a 2, helix, very similar in size and shape to that of NaGC,I3 with an approximate cross-section, perpendicular to the helical axis, which is ellipsoidal if the sodium ions and the water molecules are not taken into account (Fig. 5).The helix presents a polar outer surface around the end regions of the semimajor axis and can be permeable to the aqueous solvent since the separation between two anions related by a b translation along the helical axis is sufficiently large. The most protruding outside apolar groups are the C(18) and C(19) methyl groups. The helix is stabilized by the hydrogen bonds O(25).. aO(35) = 2.741(13), O(26). -vO(28) = 2.785(9), and 0(27)...0(28)= 2.744(13) A, in addition to some of those reported in Fig. 3. Two adjacent anions along the helix are arranged in antiparallel mode and are linked by head-to-tail hydrogen bonds together with a Na(2).. eO(34) Coulombic interaction. The second unit (Fig. 6) resembles the first one in size and shape, although a two-fold rotation axis replaces the two-fold screw axis. The main differences are the side chain conformation (see Table 1) and the location of the hydrated sodium ions, which in the second unit are more embedded between the O(25) hydroxylic group and the sulfonate ion. Two more hydrogen bonds (not shown in Fig. 3) stabilize this unit. They are O(61)-* aO(68) = 2.834(12) and O(62)-. .0(68) = 2.905(13) A. The two facing antiparallel anions are linked by head-to-tail hydrogen bonds, together with a Na(1). -0(70) Coulombic interaction. The crystal packing resembles that of NaGCI3 character- ized by hydrophilic and hydrophobic channels.The hydro- philic channel, centred on a two-fold screw axis at a = 1/4 and c = 1/2, is filled with sodium and sulfonate ions and water molecules, which give rise to strong polar interactions, and is a locus of aggregation of the two types of structural unit. The hydrophobic channel, centred on a two-fold rota- tion axis at a = 0 and c = 0, is filled with acetone molecules, which give rise to van der Waals interactions with the C(18), C(19), C(53)and C(54) methyl groups of NaTC and improve the crystal stability. CD Spectra of NaGC and NaTC with BR It was decided to investigate the enantioselective complex- ation of BR to NaGC and NaTC and to compare the CD results with those of NaDC.8 The spectra of the bile salts, recorded at natural pH within the pH range 6-7, are shown in Fig.7. The trend of Fig. 7(a)and (4,corresponding to a concentration that is certainly below the critical micellar con- centration (c.m.c.), is monotonic and gives rise to molar ellip- ticity values that are always positive or zero up to 250 nm for both NaGC and NaTC. No CD spectrum was observed at higher wavelengths in the region where the absorption of BR occurs. A negative dichroic band appears and becomes deeper upon increasing the concentration, in agreement with the behaviour of NaTDC” and at variance with that of NaDC and sodium ~holate.~~ These findings seem to confirm a non-cooperative continuous self-association of the micellar aggregates, as was observed by means of other techniques for bile salts in water and at low ionic strength.*’ CD spectra at various pHs of NaGC and NaTC below the c.m.c., with BR at a concentration of 4.6 x mol dm-3, are shown in Fig.8. The low BR concentration ensured that BR was present mainly as monomer, because of its well known tendency to associate upon increasing concentration and as a function of pH.39-41 The spectrum of BR with NaGC does not change appreciably within the pH range 7.3- 9.3 (Fig. 8). Its typical profile could indicate a very small pref- erential selection of RBR, as in the case of BR with NaDC.8 Similar results were acquired for BR and NaTC. Only two spectra are shown as examples [Fig. 80 and (g)]. The attempt to investigate the effect of the BR concentration increase as a function of pH in aqueous solutions of NaGC and NaTC was unsuccessful, since BR precipitates at a pH within the range 7-8, beginning from a concentration of ca.lo-’ mol dm-3. CD spectra were recorded only at pH = 9. As an example, the spectrum of BR with NaTC is shown in Fig. 8(h). It is similar to the other ones in this figure and to those recorded for BR with NaDC under nearly the same conditions. Therefore, it seems reasonable to suppose that the NaGC, NaTC and NaDC anions behave likewise in forming the corresponding complexes with BR. This is supported by J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Fig. 3 Projection on the uc plane of the sodium ions and oxygen atoms of water molecules with their coordination sphere.The distances corresponding to ion-ion, ion-dipole and hydrogen-bonding interactions are given in A,together with their esds in parentheses. the geometry of the NaDC anion-BR interaction complexes, found by means of potential-energy calculations, where the NaDC side chain is weakly engaged with BR (see Fig. 5 of ref. 8). The three bile salt anions are different at the end of the side chain and are practically equal in the remaining part, with the exception that one hydrogen atom of C(7) in NaDC is replaced by one hydroxylic group in NaGC and NaTC. Thus, their complexes with BR could have comparable inter- action energy and those with RBR could be more stable than those with LBR. The behaviour of BR at low concentration (4.6 x loA6mol 0 dm-3) in the presence of NaGC and NaTC micellar aggre- gates is very similar and is shown in Fig.9. The CD spectra are characteristic of a preferential selection of the LBR enan- tiomer at pH values below ca. 7.9. Again, the trend follows that of BR with NaDC, even though in this case the dichroic bands invert at more alkaline pH (about 9, see Fig. 7 of ref. 8). The CD spectra progressively change upon increasing pH, showing that the enantioselective ability of the NaGC and NaTC micellar aggregates is greater with the BR biacid than with the BR dianion and, also, is greater than that of the monomers at pH < 8 (cf: Fig. 8 and 9). The change in the profile of the spectra could be due to an intramolecular coup- ling between the electric transition dipole moments of the two pyrromethenone chromophores in the BR dianion different ow from that in the BR biacid and/or to an intermolecular Fig.4 NaTC crystal packing viewed along b dipole-dipole coupling between BR dianions. The CD 0s 00 Fig. 5 NaTC 2, helix projected (a) along a direction perpendicular to the helical axis, and (b) along the helical axis. A thicker line rep- resents an anion nearer to the observer. Thin full lines indicate ion-ion and ion-dipole interactions. Broken lines indicate hydrogen bonds. spectra of Fig. 10A, compared with those of Fig. 9, show that in the case of NaGC the increase of the BR concentration (3.40xlo-' mol dm-3) is roughly proportional to that of the ellipticity values in the pH range 7.2-9.4.A weak positive 0s Fig. 6 NaTC unit with a two-fold rotation axis projected (a) along a direction perpendicular to the two-fold rotation axis, and (b) along the two-fold rotation axis. The meaning of the lines is as in Fig. 5. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 A/nm Fig. 7 CD spectra of aqueous solutions of NaGC (full line), (a) 1.01 x mol dm-3, pH 6.06;(b) 1.011 x mol dm-j, pH 6.70;(c) 5.059 x mol dmF3, pH 6.98;and NaTC (broken line), (d) 1.02 xlo-' mol dm-3, pH 6.52;(e) 2.500 x mol dm-3, pH 6.33;cf) 5.001 x mol dm-3, pH 6.10.(a) refers to molar ellip- ticity [8] x1O-j. 0.5-I I I I I I 0.25-ca ? 0.0--0.25 -360 380 400 420 440 460 480 500 520 A/nm Fig. 8 CD spectra of BR (4.6xlov6 rnol dm-3) with NaGC (1.00x mol dm-'), (a)pH 7.30;(b) pH 8.10;(c) pH 8.35;(d) pH 9.02;(e) pH 9.32;and with NaTC (1.00x10-3 rnol dmW3), cf) pH7.41;(9)pH 9.06.(h) refers to BR (1.61xlow5mol dm-3) and NaTC (1 .OO x 10-mol dm -3), pH 9.07.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Fig. 9 CD spectra of BR (4.6 x loe6 mol dmP3) and NaGC (full line) with molar concentration, (a) 5.012 x lo-’, pH 7.28; (b) 5.010 x lo-’, pH 7.64; (c) 5.002 x lo-’, pH 8.30; (d) 5.005 x pH 8.60; (e) 4.999 x lo-’, pH 9.53; and NaTC (broken line) with molar concentration, cf) 5.007 x pH 7.35; (9)4.997 x lo-’, pH 8.13; (h)4.995 x lo-’, pH 9.12 band centred at 490 nm appears at pH x 7.7 [Fig. 10A(b)]. This band becomes more pronounced and is shifted to the blue upon increasing the pH.Previously, the increase in absorption at about 500 nm was observed in concentrated aqueous alkaline solutions of BR and attributed to an aggre- gation proce~s,~~*~~ which causes a weak intermolecular elec- tric dipole coupling. A similar behaviour is noted for NaTC in Fig. 10A, where only three spectra are shown for compari- son in the same interval of pH. A fourth spectrum at pH 11.60 [Fig. 10AG)] suggests that little change occurs in the bile salt-BR system above pH 9. The effect of a further increase of BR concentration (3.182 x mol dm-3) is dis- played only for NaGC in Fig. 10B, since NaTC precipitates. No sensible influence of BR concentration is observed on the pH value at which the dichroic bands invert, at variance with the NaDC-BR system.’ Poteotiai-energy Calculations The NaGC micellar model was inferred from the tetragonal NaGC crystalI3 and was used to calculate the potential energy of the interaction complex with LBR and RBR in the biacid form, and to check indirectly the validity of the model, without taking into account the water molecules of the solvent.Six NaGC molecules were considered in a right-handed rectangular framework OXYZ and were kept fixed. The atomic coordinates of the first molecule (xl,y,, zl)were obtained from the x, y, z (in A) of Table 1 of ref. 13 as follows: x1 = x -u/4, y1 = y -b/4, z1 = z. Those of the second molecule (x,, y,, z,) are: x, = -xl, y, = -yl, z2 = z1 -42. The other four molecules are generated by trans- posing c for -c in this pair. The hydrogen atoms were put at the expected positions (C-H = 1.08 A, N-H = 1.00 A), except those of the hydroxylic groups, which are inside the helix and interact very weakly with BR, and those of the methyl groups, treated as one atom.The atomic coordinates of BR, in the ‘ridge tile’ conforma- tion of the biacid form,” are reported in Table I1 of ref. 8 for the LBR enantiomer (those of RBR are obtained by changing z for -2). BR was moved as a rigid body by anticlockwise rotations around OX ($1), OY ($,) and OZ (t/j3), in this order, and by translations along OX (tx),OY (t,) and OZ (tz). B I I I I I I I -10 iI -’ I I 1 I , (g), I 360 380 400 420 440 460 480 500 A/nm Fig.10 A, CD spectra of BR (3.40 x mol dm-’) and NLC (full line) with molar concentration, (a) 4.999 x pH 7.25; (b) 5.032 x lov2, pH 7.73; (c) 5.000 x lo-’, pH 7.92; (6)5.013 x lo-’, pH 8.31; (e)5.004 x pH 8.95; (f) 4.996 x lo-’, pH 9.42; and NaTC (broken line) with molar concentration, (9)5.001 x pH7.23; (h) 4.989 x pH 8.15; (i) 5.008 x pH 9.35; 0’)5.009 x lod2,pH 11.60. B, CD spectra of BR (3.182 x mol dm-3) and NaGC with molar concentration, (a) 4.980 x lod2,pH 7.64; (b)5.028 x pH 8.71; (c)4.990 x pH 11.55. The van der Waals energy was computed by using stan- dard atom-atom semiempirical potentials with coefficients listed in Table I11 of ref. 8, together with an N-N potential.42 The hydrogen-bonding energy was calculated by using a direction-dependent 6-4 function43 with parameters equal to those employed in ref.8 and 38. The six-dimensional para- metric space was explored assuming a cut-off distance of 9 A and giving angular and translational increments progressively decreasing from 10 to 2” and from 0.5 to 0.1 A. The two lowest-energy minima for each BR enantiomer are defined in Table 2. For these minima no contribution of the hydrogen- bonding energy was found. Projections perpendicular (along OY) and parallel (along OZ) to the NaGC helical axis of the interaction complexes with BR (minima A-D of Table 2) are depicted in Fig. 11-14, where only four of the six NaGC mol- ecules used in the calculations are shown. The complexes are Table 2 Most relevant data of the lowest-energy minima calculated for the complexes between the NaGC 2, helix and LBR or RBR" minimum ~~ $2 t,b3 tx t,, t, energy A (LBR) 100 0 -43 -11.70 5.65 1.55 -25.82 B (RBR) C(LBR) D(RBR) 265 160 25 2 15 -22 -45 -40 -53 -11.60 -11.40 -10.10 5.90 6.40 7.40 2.30 2.50 0.75 -25.14 -24.98 -24.10 a The rotation angles, translations and energy are given in degrees, A and kcal mol- ',respectively.rather similar and are characterized by stable apolar inter- actions between BR and the B faces of the bile salt anions. The least-squares planes of the two pyrromethenone moieties of BR form a dihedral angle of about lOO", a value nearly equal to that of the dihedral angle formed by the least- squares plane of ring A with that of rings B, C and D in the steroid skeleton.Therefore, a good fit between BR and the rings A, B, C and D can be achieved. The best interactions arise from apolar contacts involving mainly the methyl groups C(18) and C(19) and the most protruding hydrogen atoms of the NaGC helix, together with the methyl and vinyl groups of BR, which are particularly active. The NaGC-LBR complex is more stable than the NaGC-RBR one (compare the energy of minimum A with B and C with D). Thus, a chiral recognition should occur, at least within the range of the pH values characteristic of the BR biacid form, in accord- ance with the CD spectra [(a) and (b)]of Fig. 9 and 10A, and Fig. 10B(a). The small energy difference between the NaGC- LBR and NaGC-RBR complexes suggests a slight prefer- ow 0 Td-.-.__.Fig. 11 Projection perpendicular (a) and parallel (b) to the NaGC helical axis for minimum A of Table 3. A thicker line represents a molecule nearer to the observer. Broken lines indicate hydrogen bonds. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 r-94 f---..l Fig. 12 As in Fig. 11 for minimum B /-KO PY Fig. 13 As in Fig. 11 for minimum C J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I e Secondaria, and by the Italian Minister0 per 1'Universita e per la Ricerca Scientifica e Tecnologica. References 1 A. R. Campanelli, S. Candeloro De Sanctis, E. Giglio and S. Petriconi, Acta Crystallogr., Sect. C, 1984,40, 631. 2 G. Conte, R. Di Blasi, E.Giglio, A. Parretta and N. V. Pavel, J. Phys. Chem., 1984,88,5720. 3 G. Esposito, E. Giglio, N. V. Pavel and A. Zanobi, J. Phys. Chem., 1987,91,356. 4 E. Giglio, S. Loreti and N. V. Pavel, J. Phys. Chem., 1988, 92, 2858. 5 E. Burattini, P. D'Angelo, E. Giglio and N. V. Pavel, J. Phys. Chem., 1991,95,7880. 6 G. Esposito, A. Zanobi, E. Giglio, N. V. Pavel and I. D. Camp-bell, J.Phys. Chem., 1987, 91, 83. 7 E. Chiessi, M. D'Alagni, G. Esposito and E. Giglio, J. Inclusion Phenom. Mol. Recognit. Chem., 1991,10,453. Na' '+ Fig. 14 As in Fig. 11 for minimum D ential selection of the LBR enantiomer, in accordance with the low ellipticity values of the CD spectra. 8 M. D'Alagni, M. Delfini, L. Galantini and E. Giglio, J. Phys. Chem., 1992,%, 10520.9 M. D'Alagni, M. L. Forcellese and E. Giglio, Colloid Polym. Sci., 1985,263,160. 10 A. R. Campanelli, S. Candeloro De Sanctis, E. Chiessi, M. D'Alagni, E. Giglio and L. Scaramuzza, J. Phys. Chem., 1989,93, 1536. 11 A. R. Campanelli, S. Candeloro De Sanctis, E. Giglio and L. Scaramuzza, J. Lipid Res., 1987,243,483. 12 M. D'Alagni, E. Giglio and S. Petriconi, Colloid Polym. Sci., 1987, 265, 517. 13 A. R. Campanelli, S. Candeloro De Sanctis, L. Galantini and E. Giglio, J. Inclusion Phenom. Mol. Recognit. Chem., 1991,10,367. 14 A. R. Campanelli, S. Candeloro De Sanctis, A. A. D'Archivio, E. Giglio and L. Scaramuzza, J. Inclusion Phenom. Mol. Recognit. Chem., 1991,11,247. 15 A. R. Campanelli, S. Candeloro De Sanctis, E. Giglio, N.V. Pavel and C. Quagliata, J. Inclusion Phenom. Mol. Recognit. Chem., 1989,7,391. 16 Bile Pigments and Jaundice: Molecular, Metabolic and Medical Aspects, ed. J. D. Ostrow, Marcel Dekker, New York, 1986. 17 R. Bonnett, J. E. Davies, M. B. Hursthouse and G. M. Sheldrick, Proc. R. SOC.London, Ser. B, 1978,202,249. 18 G. Le Bas, A. Allegret, Y. Mauguen, C. De Rango and M. Bailly, Acta Crystallogr., Sect. B, 1980,36, 3007. 19 A. Mugnoli, P. Manitto and D. Monti, Acta Crystallogr., Sect. C, 1983,39, 1287. 20 D. Kaplan and G. Navon, Biochem. J., 1982,201,605.The energies of the A-D minima, computed with a cut-off distance of 7 A, are -23.43, -22.72, -22.71 and -21.69 kcal mol- ', respectively. They are comparable with those of the deepest minima of the system NaGDC-LBR (-24.0 kcal mol-') and NaGDC-RBR (-22.6 kcal mol- '), calculated with the same cut-off distance." The energy difference in this case is higher than that for NaGC-BR, in agreement with the greater NaGDC enantioselective ability." On the other hand, the helical micellar models of NaGDC and NaDC, as well as their enantioselective ability, are very similar." Hence, the greater enantioselective complexation of BR with NaDC than that with NaGC is justified (see Fig. 7 of ref.8 and Fig. 9 of this work). The similar behaviour of NaGC and NaTC is easily explained by assuming the two structural units observed in the NaTC monoclinic crystal as models for the NaTC micellar aggregates. Previously, their close resem- blance with the 2, helix of NaGC was emphasized.In partic- ular, the region of the NaGC anion more strongly engaged in the formation of the complex with BR includes many atoms of the rings A, B, C and D, and is practically the same in the NaTC anion. From all of these results it seems reasonable to suppose that BR, on the basis of the ellipticity values of its CD spectra, can act as a probe for the recognition of the structural motifs characterizing the bile salt micellar aggre- gates. Further work is in progress to clarify this hypothesis. This work was supported financially by the Italian Consiglio Nazionale delle Ricerche, Progetto Finalizzato Chimica Fine 21 D. Kaplan and G. Navon, Isr. J. Chem., 1983,23, 177. 22 F. R. Trull, J-S.Ma, G. L. Landen and D. A. Lightner, Isr. J. Chem., 1983,23,211. 23 G. Navon, S. Frank and D. Kaplan, J. Chem. Soc., Perkin Trans. 2, 1984, 1145. 24 G. Blauer and G. Wagniere, J. Am. Chem. SOC., 1975,97, 1949. 25 Y-M. Pu and D. A. Lightner, Croat. Chem. Acta, 1989,62,301. 26 J. H. Perrin and M. Wilsey, J. Chem. SOC.,Chem. Commun., 1971, 769. 27 M. Reisinger and D. A. Lightner, J. Inclusion Phenom., 1985, 3, 479. 28 J. Lon Pope, J. Lipid Res., 1967, 8, 146. 29 M. C. Carey, in Sterols and Bile Acids, ed. H.Danielsson and J. Sjavall, Elsevier/North-Holland Biomedical Press, Amsterdam, 1985, ch. 13, p. 380. 30 J. P. Kratohvil, W. P. Hsu, M. A. Jacobs, T. M. Aminabhavi and Y. Mukunoki, Colloid Polym. Sci., 1983, 261, 781. 31 P. Main, S.J. Fiske, S. E. Hull, L. Lessinger, G. Germain, J. P. Declercq and M. M. Woolfson, MULTAN 80. A System of Computer Programs for the Automatic Solution of Crystal Struc- tures from X-Ray Difraction Data, Universities of York (England) and Louvain (Belgium), 1980. 32 M. Camalli, D. Capitani, G. Cascarano, S. Cerrini, C. Giaco- vazzo and R. Spagna, SIR-CAOS: User Guide, Instituto di Strutturistica Chimica CNR C. P. n. 10, 00016 Monterotondo Stazione, Roma, 1986. 33 International Tables for X-Ray Crystallography, The Kynoch Press, Birmingham, 1974, vol. IV. 34 W. Klyne and V. Prelog, Experientia, 1960, 16,521. 35 C. Altona, H. J. Geise and C. Romers, Tetrahedron, 1968,24, 13. 36 E. Giglio and C. Quagliata, Acta Crystallogr., Sect. B, 1975, 31, 743. 1532 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 37 38 39 E. Giglio, in Inclusion Compounds, ed. J. L. Atwood, J. E. D. Davies and D. D. MacNicol, Academic Press, London, 1984, vol. 2, p. 207. M. DAlagni, A. A. DArchivio and E. Giglio, Biopolymers, 1993, 33, 1553. R. Brodersen, Acta Chem. Scand., 1966,20,2895. 41 42 43 M. C. Carey and A. P. Koretsky, Biochem. J., 1979,179,675. N. G. Parsonage and R. C. Pemberton, Trans. Faraday Soc., 1967,63, 311. B. R. Brooks, R. E. Bruccoleri, B. D. Oldson, D. J. States, S. Swiminathan and M. Karplus, J. Comput. Chem., 1983,4,187. 40 P. E. Hansen, H. Thiessen and R. Brodersen, Acta Chem. Scand., Part B, 1979,33,281. Paper 3/05912J;Received 1st October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001523

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1533-1535 Quartz Crystal Microbalance Study of the Adsorption of Ions onto Gold from Non-aqueous Solvents Andrew P. Abbott," David C. Loveday and A. Robert Hillman Chemistry Department, University of Leicester, Leicester, UK LE1 7RH This work investigates the adsorption of ions onto a gold electrode surface using an electrochemical quartz crystal microbalance. It was found that in anisole solutions adsorption of a monolayer of tetrafluoroborate anions or tetrabutylammonium cations occurred at different potentials, whereas in ethanol only tetra-butylammonium cations were adsorbed. The implications that this has upon electron-transfer processes are discussed. The use of non-polar solvents has become widespread in elec- trochemistry because of their wide potential windows, chemi- cal inertness and high solubility for organic electroactive species.' High conductivity has been achieved in such media using quaternary ammonium salts as electrolytes.2 This has led to their use in studying solvent effects on electron-transfer kinetics by electrochemical method^.^,^ Recent work by Fawcett and Fedurco' has shown that the ionic radius of quaternary ammonium ions used as an electrolyte has a marked effect upon the kinetics of electron transfer in a range of non-aqueous solvents.It is suggested that this is caused by the adsorption of the ions at the electrode surface which would present a barrier to electron transfer. The measure- ment of ion adsorption is difficult, particularly by the usual method of measuring electrode capacitances. The aim of the present work is to investigate ion adsorp- tion using an electrochemical quartz crystal microbalance (EQCM).This technique measures the resonant frequency variation (An of a quartz crystal oscillator from its base value cfo)that accompanies a change in mass (AM) attached to the crystal. When the additional mass is small and rigidly coupled,6 (AflHz) = -(2/Pv)f;(AM/g cm -2, (1) where p is the density of the quartz and v is the wave velocity in the quartz. The EQCM is an extremely sensitive device for measuring in situ processes at the electrode/solution interface with submonolayer resolution and has found a variety of electrochemical application^.'.^ Although this technique has been extensively applied to the study of polymer films, very little work has been carried out on the adsorption of ions onto electrodes.One example of this, however, was work by Deakin et aL9 which studied the adsorption of bromide and iodide onto gold and found that the weight gain corres-ponded well to a complete close-packed monolayer of bromide. We are interested in studying the specific adsorption of ions onto gold from electrolytes with different characteristics, as determined by solvent relative permittivity, acidity and electron-donating ability. In this work, we describe the adsorption of tetrabutylammonium tetrafluoroborate (TBABF,) from anisole (E = 4.3), ethanol (E = 24.3) and meth- anol (E = 32.6). Experimental Anisole (Aldrich, 99%) was distilled at reduced pressure under a nitrogen atmosphere.Ethanol and methanol (BDH, AnalaR) were used as received. Tetrabutylammonium tetra- fluoroborate (TBABF,) (Fluka, puriss, electrochemical grade) was used as received. The EQCM instrumentation and cell configuration have been described elsewhere. lo All of the experiments described were performed in potentiostatic mode controlled by an Oxford electrodes modular potentiostat. Data were acquired and stored by an IBM-ATX computer using a Keithley series 570 data acquisition workstation and a Hewlett-Packard 5334B frequency counter. Gold-coated 10 MHz AT-cut quartz crystals were supplied by International Crystal Manufacturing Company (Oklahoma City, OK, USA).The crystals had a 900 A thick gold layer deposited on the surface in a keyhole shape with a central disc (area, 0.211 cm') sensitive to mass changes. All potentials given are with respect to the Ag I AgBF, reference electrode" which was in contact with 0.1 mol dm-3 TBABF, ,separated from the bulk solution by a glass frit. All solutions were bubbled with argon for at least 10 min before use. Results and Discussion Fig. l(a) shows a plot of weight gain against potential for a gold electrode in a 0.1 mol dm-3 TBABF, in anisole solution at 23°C at a sweep rate of 1 mV s-'. Note that the results presented here are from a single scan, unlike those previously presented by Deakin et al.' which were an average of several scans.When the potential was scanned from -0.2 to +0.7V there was an increase in mass which can be attributed to the adsorption of a negatively charged species onto the gold elec- trode. This could either be BF4- ions or the triple ion [TBA(BF,),] -. While the concentration of free BF,- ions is small (about mol dm-3), the repulsive force between the positively charged electrode and the TBA ion would prob- + ably cause dissociation of the triple ion somewhere close to the electrode surface. When the potential was scanned in the negative direction the adsorbate was desorbed to a potential of cu. -0.3 V, whereupon the electrode again started to increase in mass. This was ascribed to the adsorption of TBA' ions {or, less likely, the triple ion [(TBA),BF,]+). When the potential was swept in the positive direction from -1.2 V the adsorbate was again desorbed from the electrode surface.The adsorp- tion processes are seen to reach a maximum on both the positive and negative potential scans. Before proceeding to interpret the data, we make some comments on possible experimental artefacts. First, the observed frequency (mass) changes are not a consequence of impurity adsorption. Experiments employing less pure electrolytes resulted in monotonic mass increases that were (a)independent of poten- tial and potential scan direction and (b) quantitatively differ- ent (cu. lo00 ng cm-2 on the timescale of the experiment in Fig. 1) to the mass changes for 'clean' electrolytes.Secondly, the discrepancy in the weight change at the start and finish of I I I I I I -1200 -800 -400 0 400 800 E/V vs. Ag 1 AgBF, 5-a 40 -1200 -800 -400 0 400 800 E/V vs. Ag IAgBF, Fig. 1 Mass change us. potential for a gold electrode in a 0.1 mol dm-’ solution of TBABF, in anisole at 23 “C at a sweep rate of (a) 1 and (b)10 mV sP1 the experiment is small and probably due to temperature changes during the long timescale of each run (>1 h). The observation that the drift is so small is proof of the stability of the system. Knowing the ionic radius of the two adsorbed ions (BF,-= 2.02 A and TBA+ = 4.13 A ’), and assuming each ion occupies a box corresponding to its diameter, d, the mass of an adsorbed monolayer, M, ,can be calculated : M, = (d-2M,)/N, (2) where M, is the molar mass and N, is Avogadro’s number.The corresponding masses of an adsorbed monolayer of TBA’ and BF,- are 59 and 89 ng cm-’, respectively. These agree well with the values of ca. 50 and 90 ng cm-’ shown in Fig. l(a).This means that in media of such low relative per- mittivity, at potentials positive and negative of the potential of zero charge (pzc), the electrode is covered with a close- packed layer of specifically adsorbed electrolyte ions. Fig. l(b)shows a repeat of the above experiment at a faster scan rate (10 mV s-I). Pronounced hysteresis is observed between the forward and reverse scans. The adsorption and desorption of the charged species from the electrode surface reflect an equilibrium between the electrostatic and disper- sion interactions of the ion and the electrode and the solva- tion interactions of the ion and solvent molecules.The adsorption and desorption processes are further complicated by the ion-ion interactions in media of low permittivity. Close to the electrode surface, ion aggregates will dissociate due to the electric field. The ions with a charge opposite to J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 that of the electrode will experience a large electrostatic inter- action and hence be strongly attracted to the electrode. The desorption of the ion is difficult since the solvation of a single ion is thermodynamically unfavourable owing to the low sol- vation energy.Thus the ion will remain adsorbed until the concentration of counter-ions in the double layer at the electrode/solution interface becomes significant, when it can form an ion pair and be solvated, i.e. when the potential is closer to the pzc. Two interesting points arise from the above results. First, the minimum in the mass-potential plot at both scan rates occurs at ca. -0.3 V. This minimum in the mass-potential plot occurs in this region, irrespective of the potential from which the scan starts. It can be inferred that this must be the pzc of the electrode. Secondly, the above result has signifi- cant consequences for electrochemical experiments carried out in such media. For redox processes where El/’ < -0.8 V the electrode will be covered with a close-packed monolayer of TBA+ ions.This will block the electrode and any electron transfer will have to occur over a distance of cu. 8.3 A. This will lead to a decrease in apparent rate constants for pro- cesses studied in non-polar media, as observed by Fawcett and Fedurco.’ It would also lead to a marked electrolyte effect on the apparent rate constant of electron-transfer pro- cesses. The same would also be true, although to a lesser extent, of redox processes carried out at El/*> 0.5 V, where BF,-blocks the surface. Ion adsorption could also present problems for the study of phase transformation reactions such as metal deposition. Recently, a large amount of work has been carried out on the deposition of metals from aromatic and polyaromatic sol- vents (E < 5).’,’’ Metal ions with a relatively positive reduction potential, such as copper and zinc, could be depos- ited easily.Metal ions such as titanium and tungsten with deposition potentials < -1 V appeared to be reduced,” but no macroscopic deposits could be obtained. The above obser- vations would account for this anomaly because, although the metal ions could be reduced to the corresponding zero valence state, they could not be incorporated into the metal lattice because the electrode would be covered with a layer of quaternary ammonium ions. If the above ideas are correct then similar anomalies will also be observed for other com- monly used electrochemical solvents such as tetrahydrofuran, dichloroethane and dichloromethane.Fig. 2 shows a plot of mass gain of a gold electrode against potential in a 0.1 mol dm-3 TBABF, in ethanol solution. In this case, no mass change was observed on a potential sweep -20 1 -1000 -800 -600 -400 -200 0 200 E/V vs. Ag IAgBF, Fig. 2 Mass change us. potential for a gold electrode in a 0.1 mol dm-’ solution of TBABF, in ethanol at 23°C at a sweep rate of 5 mV s-l J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 12 -7 8-5 UJ 5.4-f a 0--1200 -1000 -800 -600 -400 -200 0 200 E/V vs. Ag 1 AgBF, Fig. 3 Mass change us. potential for a gold electrode in a 0.1 mol dm-3 solution of TBABF, in methanol at 23 "C at a sweep rate of 5 mV s-' from 0 up to +0.7 V. On sweeping the potential in a negative direction from 0 V, however, an increase in mass of the elec- trode was again observed.These observations can be inter- preted as showing no adsorption of BF,- over the range of positive potentials scanned, which probably results from a stronger solvation of the anion by the solvent molecules, making the desolvation/ion adsorption process more difficult. At potentials negative of -0.6 V the adsorption of TBA' is again observed with a coverage of ca. one monolayer. This shows that even in higher relative permittivity solvents, such as ethanol (E = 24.3), the specific adsorption of quaternary ammonium ions could still produce experimental artefacts. Fig. 3 shows a plot of mass gain of a gold electrode against potential in a 0.1 mol dm-3 TBABF, in methanol solution.Results similar to those observed in ethanol were obtained. Note, however, that the mass increase corresponds to only half of one monolayer. This suggests that the higher the polarity, the greater the solvation of the electrolyte ions and the lower the extent of adsorption on the electrode surface. The results of this work support the ideas of Fawcett and Fedurco that ions are adsorbed onto electrode surfaces from low relative permittivity solvents containing electrolytes. This has large implications for the study of electron-transfer kinetics from such solvents, as the apparent rate constants will be affected not only by the solvent, but also by the separation of the electroactive species from the electrode owing to the presence of adsorbed electrolyte ions.Conclusion In solvents of low relative permittivity, quaternary ammon- ium electrolytes are found to be adsorbed on a gold electrode surface forming a close-packed monolayer. We have no evi- dence for the adsorption of triple ions of either charge type. This adsorption process appears to be only partially reversible and this may result in experimental artefacts when measuring electron-transfer kinetics. It was found that in ethanol and methanol anion adsorption did not occur, but cation adsorption occurred in both solvents, although to a lesser extent in methanol. This work was funded by a grant from the Leicester/ Loughborough Research Fund. The authors would like to thank Mrs. G. Lonergan for her help with the experiments. References 1 A. P. Abbott, Chem. SOC.Rev., 1993,22,435. 2 A. P. Abbott and D. J. Schiffrin, J. Chem. Soc., Faraday Trans., 1990,86,1453. M. J. Weaver, Chem. Rev., 1992,92,463, and references therein. W. R. Fawcett, Lmgmuir, 1989,5,661. W. R. Fawcett and M. Fedurco, J. Phys. Chem., 1993,97,7075. G.Z. Sauerbrey, 2. Phys., 1959,155,206. D. A. Buttry, in Electroanalytical Chemistry, ed. A. J. Bard, Marcel Dekker, New York 1991, vol. 17, p. 1. 8 M. R. Deakin and D. A. Buttry, Anal. Chem., 1989,61, 1147A. 9 M. R. Deakin, T. T. Li and 0.R. Melroy, J. Electroanal. Chem., 1988,243, 343. 10 S. Bruckenstein and M. Shay, Electrochim. Acta, 1985,30, 1295. 11 A. P. Abbott, E. E. Long, A. Bettley and D. J. Schiffrin, J. Elec-troanal. Chem., 1989,261,449. 12 A. P. Abbott, A. Bettley and D. J. Schiffrin, J. Electroanal. Chem., 1993,347,153. Paper 3/07103K; Received 1st December, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001533

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1537-1540 Glass Transition of Liquid-crystalline 4-Alkoxyphenyl and 4-Cyanophenyl 4=(2,4=Dialkoxybenzoyloxy)Benzoates Shunsuke Takenake and Hiroshi Yamasu Department of Materials Science and Engineering, Faculty of Engineering, Yamaguchi University, Ube Yamaguchi 755,Japan Some 4-alkoxyphenyl and 4-cyanophenyl 4-(2,4-dialkoxy)benzoates show a nematic phase and a glassy phase at low temperatures. The glass-transition phenomena were examined as a function of the sweep rate in both heating and cooling processes. The derivatives intrinsically experience two kinds of transition process in the nematic phase. Two transitions were observed when the sweep rate was <1 K min-', and these overlapped when the sweep rate was >1 K min-'.The transition temperatures show an interesting dependence on the chain length of three alkoxy groups; i.e. lengthening the lateral alkoxy group at position 2 lowers the glass- transition temperature (T,) while lengthening both alkoxy groups at the terminal positions increases Tg. The thermodynamic parameters are discussed in terms of the McMillan theory of the glass transition. Pure compounds sometimes exhibit a glassy phase when re- crystallization does not occur as they are cooled from the molten state. Some liquid-crystalline materials are also known to exhibit glassy phases corresponding to pre-states such as cholesteric'*2 and nematic phase^.^-^ A nematic glassy phase is also formed by a eutectic mixture.' The for- mation of a glassy phase is usually very undesirable in liquid- crystal cells that are driven electrically.In pure compounds, however, the observation and thermodynamic consideration of the glassy phase is not easy since in many cases re-crystallization precedes the transition. In such cases, the glassy phase is usually observed only during rapid cooling of the sample. In this paper, we describe the thermal properties of the liquid-crystalline compounds shown in Table 1. The derivatives have a long alkoxy group in the lateral position so that these form only a nematic phase mono-tropically. However, the lateral alkoxy group prevents re- crystallization during the cooling process and makes observation of the glassy phase easy. The results will be dis- cussed in terms of the McMillan theory of the glass tran- si tion.' Experimental The materials were synthesized according to the method described previously.' The purity and structures were con- firmed by HPLC, NMR and elemental analysis.The thermal properties were determined by a Seiko-denshi SSC-5200 work station differential scanning calorimeter Table 1 Liquid-crystalline compounds studied R2 R' RZ R3 17O OC4H9 OCBH17 'aH, 7O 0C6H 13 OCBH17 7O OC4H9 'sH17 'aH 17' 0C6H 13 CN 17O oclOH21 OCEHl7 C12H250 OC4H9 OClzH25 C12H250 oc1 OH21 OCl 2H25 'aH 17O H OCaH, 7 (DSC) and a Nikon POH polarizing microscope fitted with a Mettler FP-5 heat controller. The DSC thermograms were calibrated with indium (99.9%, mp 439.8 K, AH = 28.59 mJ mg-I).Results and Discussion The transition temperatures and latent heats for compounds 1-7 are summarized in Table 2. The melting points in Table 2 were obtained during the heating process of the virgin sample, as were the N-I transition temperatures. The liquid-crystalline phase showed a typical schlieren texture and it was identified as a nematic phase. 8 has a fun-damental skeleton of the present series, and the structure may keep a rod-like nature. 8 is known to exhibit a mesophoric sequence of the crystal-smectic C-smectic A-nematic-isotropic type. Therefore, the N-I transition temperature is high. As is evident from a comparison of 1-8, a lateral substituent at position 2 reduces the N-I transition temperature markedly.In addition, the presence of a lateral substituent tends to prevent the formation of smectic layers. The effect of a lateral substituent on the mesomorphic properties has been already discussed.' The DSC thermograms for 1and 4 are shown in Fig. 1 and 2. Fig. 1 shows the DSC thermograms for 1. In the cooling processes the thermograms showed an exotherm at 342 K (0.5 K min-') due to the I-N transition. The same transition occurred at ca. 342 K for cooling rates of 2, 5 and 10 K min-', and ca. 341 K for cooling rates of 20, 25 and 30 K min-' (the temperature was obtained from the onset of the exothermic peak). As shown in Fig. l(a), the exothermic Table 2 Transition temperatures (T/K) and latent heats (kJ mol-') for 1-7 compound K N I AHm,, * 344 (. 342).19.9 -311 * 337 * 27.6 -324 (* 312) -34.2 -348(* 315). 14.4 * 347(* 322). 27.5 -342 * 342 31.5 * 318 * 322 * 34.7 -357 * 461 AH,, 0.8 0.7 0.3 0.1 0.3 0.6 0.4 Parentheses indicate a monotropic transition. K, N, and I indicate crystal, nematic and isotropic phases, respectively. " This compound undergoes smectic C-smectic A and smectic A-nematic transitions at 417 and 436 K also, respectively." J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 glass20852800< 2025 I U I /A/ 10U "V198 227 256 285 315 343 TIK 0 -750 z $-1500 n -2250 N -3000 I I 1 in the thermograms indicate the sweep rate (K min-'). peaks tend to become broader with increasing cooling rates.The small change in the transition temperature is due to instrumental error. The DSC thermograms in Fig. l(a) show a remarkable deviation of the base line around 240 K due to a glass tran- sition of the nematic phase. Therefore < values were obtained from the peak of the differential curve, according to the McMillan method.' The values decreased gradually with increasing cooling rate (Fig. 3). During the heating process, a glass-N-crystal transition can be observed in Fig. l(b). The glass transition accompa- nied a small endotherm arising from the enthalpy relaxation when the heating rate was slow, e.g. 2-10 K min-'. The endotherm is more pronounced for terminal groups with higher carbon numbers, e.g. 6 and 7.Interestingly, two kinds of glass transition were observed at 230 and 247 K when the heating rate was 1 K min-', while only one kind was observed when the heating rate was > 1 K min-'. The deviation from the baseline was larger for the former than for the latter. This shows that this compound intrinsically has at least two kinds of glassy phase, and the transitions merge when the sweep rate is >2 K min-'. Recrystallization occurred whatever heating rate was used ; recrystallization occurred at 263 K for a heating rate of 2 K min-' and at 298 K for a heating rate of 30 K min-'. The endothermic peak around 340 K arises from melting of the crystalline phase formed during the heating process. Inter- estingly, a similar deviation of the baseline to the glass tran- sition is observed during the recrystallization process.A 1850 1325 800 275 I I I I I I 213 237 261 285 309 333 TIK -1 00 -575 32 -1050 n -1 525 glass I-2000 213 237 261 285 309 333 T/Vlib Fig. 2 DSC thermograms for 4: (a) cooling, (b) heating. The numbers in the thermograms indicate the sweep rate (K min-'). similar phenomenon was observed for related compounds with a long substituent in the lateral position of liquid- crystalline molecules.' ' The DSC thermograms for 4 exhibited a similar feature (Fig. 2). During the cooling process the I-N transition occurred at 318-317 K for a cooling rate of 1-25 K min-'. The onset of the transition peak was almost independent of the cooling rate, while the peak became broader with increas- ing cooling rate.During the heating process, the monotropic nematic phase recrystallized during heating at any rate, but the transition temperature increased with the heating rate. A distinct glass transition for 1 was not observed even for heating and cooling rates of 1 K min-'. Fig. 3 shows plots of the % against sweep rate. For 1 T value varies from 240 K (sweep rate 1 K min-') to 246 K (30 K min-') in the heating process and from 240 K (sweep rate 1 K min-') to 235 K (30 K min-') in the cooling process. The characteristics of the heating and cooling processes are almost symmetric with respect to the 240 K line. We propose that is proportional to the sweep rate within the tem- perature range studied.The temperature dependence of 3 was obtained from the slope of the plots, giving 0.27 and 0.23 for the heating and cooling processes, respectively. Similar results were obtained for 2-5. The features for 6 and 7 are somewhat different. In the heating process, the temperature dependence of the < value on the sweep rate is 0.07, while for the cooling process it is 0.5. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I ^^^I .== 1220p *** sweep rate/K min-' sweep rate/K min-' sweep rate/K min-' 2250 46r Y h' 242 0 10 20 30 0 10 20 30 40 0 10 20 30 40 sweep rate/K min-' sweep rate/K min-' sweep rate/K min-' 260(g)l .-240-2401 230 0 10 20 30 40 sweep rate/K min -' Fig. 3 Tgus.sweep rate for compounds: (a)1, (b)2, (c) 3, (d) 4, (e) 5,(I)6, (9)7. Upper and lower plots correspond to the heating and cooling processes, respectively. Extrapolation of the plots for the heating and cooling pro- cesses in Fig. 3 give the 'average' transition temperatures for heating and cooling rates of 0 K min-(Table 3). This table shows some interesting trends in connection with the molecu- lar structure. The ratios of 5 and the sweep rate (T, in K min-') in the heating and cooling processes are symmetrical for 1-5, but those for 6 and 7 are asymmetrical, i.e. the Tg values for the heating process are almost independent of the rate, while those for the cooling process change markedly with T,. Generally, the ratio of <to Tmpis ca. 0.7 due to the similarity in the entropy change between the melting and the glass-transition processes.' As can be seen from Table 3, the T$Tmpvalues for 1-5 fall in this category, while those for 6 and 7are significantly greater than 0.7.Kirov et al. have demonstrated that the values for sub- stituted two-ring compounds show a good correlation with their molecular weights.6 We attempted to correlate q with the carbon numbers of both lateral and terminal alkyl chains Table 3 Thermodynamic parameters for the glass transition compound Tmp heating cooling Tgb </TmP 1 344 0.27 0.23 240 0.70 2 331 0.23 0.23 234 0.71 3 324 0.27 0.23 225 0.69 4 348 0.2 0.2 244 0.70 5 347 0.23 0.23 231 0.67 6 342 0.07 0.5 268 0.78 7 318 0.07 0.5 250 0.79 is the sweep rate.'Values extrapolated from Fig. 3. (Fig. 4). This figure clearly shows the role of the alkoxy group in the glass transition, viz. the lateral alkoxy group reduces the transition temperature, probably due to an increase in the molecular breadth. On the other hand, the terminal alkoxy groups increase the transition temperature. Therefore, these results indicate that the glass transition is dependent on the molecular shape rather than the molecular weight. As mentioned above, both Tmpand % for the cyano com- pound, 4, are higher than those for the other derivatives. However, there is no fundamental difference in the ratio of q to Tmpcompared with the ratios for the other derivatives. The present results are discussed in terms of the McMillan model for the glass (g) transition of the nematic phase (N);' g eN (1)l-x x dx/dt = (1 -x)nk (k, T/h)exp(AS/R)exp(-AH/RT) (2) where k is the transmission coefficient for activation, k, is Boltzmann's constant, h is Plank's constant, R is the gas con- stant, AS is the entropy of activation, AH is the enthalpy of activation and n is the order of the reaction.The solution of eqn. (2) gives : (3) Eqn. (3) indicates that a plot of log[Ti/(dT,/dt] US. 1/< is linear, the slope giving the enthalpy of activation for the glass 220 4 6 8 10 12 carbon number 220 4 6 8 10 12 carbon number Fig. 4 Tgus. the carbon numbers of the terminal (a)and lateral (b) alkyl chains ern rnc rn rn 3.0 400 410 420 4 1 420 440 460 lo5 KIT, lo5 KIT, w .5.0-4.0-rn rn 4.0-*rn *m 't 4 3.0.-3.0 .I.,.I. 400 410 420 390 400 410 420 4d~ 1O5 KIT, 1O5 KIT, Fig.5 McMillan plots for: (a)1, (b) 3, (c) 4 and (d) 7. The plots for the left and right sides in each figure are for the heating and cooling processes, respectively. transition. Such plots are shown in Fig. 5. Interestingly, the plots for the cooling and heating processes are symmetrical, except for compounds 7 and 6. In order to clarify the effect of the conditions of the glass transition such as the cooling rate and an anneal, the values were examined under the various conditions (Table 4). The results in Table 4 show that the Tg values obtained for the heating process are completely inde- pendent of the cooling conditions and the anneal of the glassy state.Therefore, the symmetric and asymmetric fea- tures in Fig. 5 are due to the intrinsic nature of the molecules. McMillan' and Tsuji et aL2 calculated the activation enth- alpies for the glass transition of glycerol and a steroid deriv- ative by using eqn. (3) and obtained reasonable values. In the present case, however, Fig. 5 shows a remarkable non-linear behaviour in both heating and cooling processes and does not follow eqn. (3). The following facts are noteworthy in connection with the abnormal features in Fig. 5. Table 4 Effect of cooling rates on the Tgvalues in the heating process for 1 heating rate T,"/cooling rate T,b/cooling rate 1" 23011 230110 247 2471 2 24112 241110 5 24215 243110 243/1od 10 244110 244110 20 245120 2451 10 30 246130 246110 The values were rounded to the nearest whole number." Samples prepared by cooling from 373 to 193 K at the cooling rates indicated. Samples prepared by cooling from 373 to 193 K at a cooling rate of 10 K min-'. 'Two kinds of transformation could be observed. The glassy state was annealed for 2 h at 233 K before it was heated. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 As can be seen from Fig. 1 and Table 4,l shows two kinds of glass transition when the heating rate is 1 K min-'. A similar trend is observed for 2 and 5. The glass transition temperatures for 2 and 5 are 229 and 245, and 230 and 237 K, respectively. These results indicate that the transition in the low-temperature region is almost independent of the chain length of the lateral alkoxy chain, and the 3 in the high-temperature region decreases with increasing chain length.As a result, the difference between values becomes small with increasing chain length of the lateral alkoxy chain. This apparent separation of the glass transition was not observed in 6 and 7, where the alkoxy chains at the terminal and lateral positions are significantly longer. Sorai et al." reported that N-(4-n-pentyloxybenzylidene)4'-n-butylaniline showed two kinds of glassy phases of a smectic G phase, and the transitions were attributed to a freezing of the intrinsic molecular modes of the layer structure and anisotropic translational self-diffusion parallel and perpen- dicular to the long molecular axes.This explanation cannot be applied to the split in the glassy transition in the nematic phase. We assume that the freezing of motional freedom for the terminal and lateral alkoxy groups is responsible for the glass transitions in the low- and high-temperature regions, respec- tively, and the 5 values observed for the high sweep rates are a thermodynamical average of the two transition tem-peratures. Conclusions In the 4-substituted phenyl 4-(2,4-disubstituted benzoyl- 0xy)benzoate systems the substituent at position 2 affects the observation of the glass transition. Two kinds of glass tran-sition were observed when the sweep rate was slow, and only one kind when the sweep rate was fast.Chain elongation at both terminal alkoxy groups increases the transformation temperature, while elongation of the lateral groups reduces it. The formation of two kinds of glass phase in the nematic phase is quite rare. This work was supported by Grant-in aid (no. 04640504) from the Ministry of Education, Science and Culture, Japan. References 1 K. Adachi, H. Suga and S. Seki, Bull. Chem. SOC.Jpn., 1969, 41, 1073; 1970,43,1961; 1971,44,78. 2 K. Tsuji, M. Sorai and S. Seki, Bull. Chem. SOC.Jpn., 1971, 44, 1452. 3 M. Sorai and S. Seki, Bull. Chem.SOC.Jpn., 1971,44,2887. 4 M. Sorai and S. Seki, Mol. Cryst. Liq. Cryst., 1973,23, 299. 5 N. Kirov, M. P. Fontana and F. Cavatorta, Mol. Cryst. Liq. Cryst., 1979,54,207. 6 N. Kirov, M. P. Fontana and N. Affanassieva, Mol. Cryst. Liq. Cryst., 1982,89, 193. 7 J. Cognard and C. Ganguillet, Nol. Cryst. Liq. Cryst. (Lett.), 1978,49, 33. 8 J. A. McMillan, J. Chem. Phys., 1965,42, 3497. 9 S. Takenaka, Y. Masuda, M. Iwano, H. Morita, S. Kusabayashi, H. Sugiura and T. Ikemoto, Mol. Cryst. Liq. Cryst., 1989, 168, 111. 10 D. Demus, H. Demus and H.Zaschke, Flussige Kristalle in Tabellen, VEB Deutscher Verlag fur Grundstoff Industrie, Leipzig, 1976. 11 S. Takenaka and H. Yamasu, Mol. Cryst. Liq. Cryst., in the press. 12 M. Sorai, K. Tani and H. Suga, Mol. Cryst. Liq. Cryst., 1983,97, 365. Paper 3/07032H;Received 26th November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001537

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1541-1545 Catalytic Combustion of Methane :Copper Oxide supported on High-specific-area Spinels synthesized by a Sol-Gel Process Nolven Guilhaume" and Michel Primet Laboratoire d 'Application de la Chimie a I'Environnement, Unite Mixte CNRS -Universite Claude Bernard, 43 Boulevard du I I Novembre 1918,69622Villeurbanne Cedex, France Three spinels (MgAI,O, , ZnAl,O, and CaA120,) have been synthesized using a sol-gel process coupled with supercritical drying. After calcination at 1000 K, specific areas as high as 300 m2 g-' can be preserved. Deposi- tion of copper oxide on such carriers leads to the formation of catalysts active in methane combustion. The catalytic activity is strictly proportional, at low conversion levels, to the number of Cu2+ ions deduced from FTIR measurements of carbon monoxide adsorption.By comparison with spinels prepared by impregnation of alumina with nitrate salts, the use of a sol-gel process allows an increase in the dispersion state of the active phase. Spinel oxides such as ZnA120, exhibit interesting physical properties and are potentially useful as supports for hydro- carbon combustion catalysts based on transition-metal oxides : CuO deposited on ZnAl,O, is very active for methane combustion and is almost unaffected after ageing tests at 1340 K.' In comparison, CuO deposited on A1,0, has a higher activity in its fresh state, but shows severe deac- tivation after ageing under the same conditions. This has been explained by the fact that, because of the structure of ZnAl,O,, CuO particles cannot enter the spinel lattice, which is supposed to act only as a dispersing agent. In the case of CuO,/Al,O,, deactivation has been shown to be due to a solid-state reaction between the active phase and the support, leading to inactive copper aluminates.The dispersion of CuO on ZnA120, is mostly limited by the low specific surface area of the support (9 m' g-') which therefore disfavours the catalytic activity of the active phase in its fresh state compared with CuO/Al,O,. This is the reason for the investigation of new preparation methods in order to obtain supports with high specific surface areas. We report here the preparation of high-specific-area spinels MgA1204, ZnA120, and CaAl,O, by the sol-gel process.They were obtained as aerogel powders by supercritical extraction of the solvent2 which preserves the structural properties of gels such as high porosities and large surface areas. Their thermal behaviour under ageing at high tem- perature was also studied. These aerogels were used as sup- ports for the preparation of combustion catalysts based on copper oxide as the active phase. Their activity for methane combustion was compared with that of similar CuO/MgAl,O, and CuO/ZnAl,O, catalysts whose supports have been prepared by solid-solid reactions. Experimental Physicoc hernial Characterizations BET areas were measured by nitrogen adsorption at 77 K on samples previously evacuated under vacuum at 573 K.Powder XRD patterns were recorded with a D 500 Siemens diffractometer using monochromatized Cu-Ko! radi- ation. The patterns were recorded from 3 < 28/degrees < 70 with a scan rate of 1.2" min-'. The patterns were compared with JCPDS reference data for phase identification. Elemental analyses were obtained from the 'Service Central d'Analyses du CNRS'. IR spectroscopy studies of CO adsorption on the CuO/ spinel catalysts were performed with an FTIR spectrometer (Bruker IFS 110). The spectral range observed was 4000-lo00 cm-' and the resolution was set to 4 cm-'. The samples were pressed into thin discs of known weight (between 25 and 35 mg), and introduced into a cell allowing in situ treatment. The samples were pretreated at 773 K under flowing oxygen for one night, evacuated at 773 K under vacuum and cooled at room temperature in UQCUO.The background spectra were recorded (100 scans), then CO (40- 45 Torrj-) was introduced and spectra were recorded at differ- ent contact times (0, 1, 4 and 20 h).The samples were evacuated under vacuum and spectra recorded at room tem- perature after 10 s, 15 min, 1 and 3 h evacuation. Preparation of the Supports The synthesis and manipulation of alkoxide precursors were performed under an inert atmosphere, using standard Schlenk tubes and vacuum-line techniques. Solvents were dried and distilled before use. Drying of the gels was achieved in an autoclave under supercritical conditions.' The aerogels obtained were fired in a flowing stream of air at 973 K for 3 h (heating rate :2.5 K min -I).MgA1204 This oxide was prepared from the mixed alkoxide MgAl,(O-Bu), as described in ref. 3, with a slightly modi- fied procedure : magnesium turnings and aluminum powder (1 :2 atomic ratio) were refluxed in butanol until all the metals were consumed. After cooling, the reacting mixture was filtered and the concentration of the solution was adjust- ed to 0.15 mol I-' by addition of butanol. Hydrolysis of the OBu groups was achieved at room temperature by slow addi- tion of the stoichiometric amount of water diluted in butanol (5% volume). The gel obtained was dried under supercritical conditions (593 K, 60 atm). Analysis: found: Mg, 15.31; Al, 35.73; C, 0.92; H, 1.54.Mg/Al = 0.476. MgA120, requires: Mg, 17.08: Al, 37.93; Mg/Al, 0.50. ZnA120, Commercial Al(OPr'), was dissolved in refluxing isopro- panol. The solubility of the product was rather poor. Zinc acetylacetonate (1 : 2 molar ratio) dissolved in acetone was added and the mixture was refluxed for 2 h to yield a clear solution. After cooling, the stoichiometric amount of diluted water (5% in isopropanol) was added slowly and the gel obtained was dried under supercritical conditions (523 K, 50 t 1Torr = 101325/760 Pa. atm). Analysis: found: Zn, 34.45; Al, 27.85; C, 0.58; H, 0.69; Zn/Al = 0.51. ZnA1204 requires: Zn, 35.66; Al, 29.43; Zn/Al, 0.50. CaA1204 Metallic calcium (40.14 mmol) was suspended in 100 ml 2- methoxyethanol with two crystals of HgCl, .The mixture was refluxed overnight to yield a pale yellow solution. After cooling, Al(OPr'), (81 mmol) was added and the mixture was refluxed again for 30 min, giving a clear, pale yellow solution. This solution was hydrolysed at room temperature with the stoichiometric amount of water diluted in 2-methoxyethanol. The gel was dried at 603 K and 58 atm (supercritical conditions). Analysis: found: Ca, 26.12; Al, 35.59; C, 1.54; H, 1.70. Ca/Al = 0.49. CaAl,04 requires: Ca, 25.36; Al, 35.59; Ca/Al, 0.50. Ageing of the Supports The samples were aged by firing at 1273 K under flowing air for 24 h. Preparation of the CuOlspinel Catalysts Catalysts consisting of 5 wt.% CuO deposited on the spinel were prepared by impregnation of the supports by an aqueous solution of the required amount of copper nitrate, evaporation of the solvent under reduced pressure, followed by calcination in flowing air at 773 K for 2 h.Analysis: 3.78, 3.85 and 3.62 wt.% Cu (or 4.73, 4.82 and 4.53 wt.% CuO) for CuO/MgA1204,ZnA1204 and CaA1204 ,respectively. Catalytic Activity Measurements The catalytic activity was measured under isothermal condi- tions, in the range 623-1023 K. The temperature was increased by steps of 50 K and the catalysts kept at each temperature for 3 h. In order to avoid overheating during the reaction, the reactant mixture consisted of diluted methane and oxygen in nitrogen (1 vol.% CH,, 4 vol.% 0,, N, as balance), with a total flow rate of 6.4 dm3 h-'.Carbon mon- oxide, carbon dioxide and unreacted methane were separated on a Carbosieve S (60-80 mesh) chromatography column. CO and CO, were methanized, on an Ni/MgO catalyst maintained at 753 K, and analysed by flame-ionization detec- tion. The catalyst (500 mg) was loaded into a U-shaped quartz reactor and calcined again at 673 K under flowing oxygen for 1 h prior to the test. Blank experiments with an empty reactor showed the absence of methane conversion up to 873 K. Beyond this temperature, the methane conversion was 0.4% at 923 K and 2% at 973 K. Comparison of the catalysts was performed using the Ts0,defined as the tem- perature corresponding to 50% methane conversion. Results and Discussion Preparation and Characterization of the Supports Preparation of the solids by the sol-gel process starting from metal alkoxides allowed us to obtain oxides with large surface areas compared with more conventional preparation methods: ZnAl,O, was only 9 m2 g-' when prepared by impregnation of high surface area alumina with zinc nitrate.' MgA1204 could be obtained with a surface area of 60 m2 g-' when prepared with an improved method4 of exfoliation of the solid also prepared by impregnation of alumina with magnesium nitrate. Besides the sol-gel process, a high-temperature aerosol decomposition process has been described' which allows the preparation of MgA1204 with a J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 specific area of 250 m2 g- '. Finally, MgAl,O, was prepared with an area of 290 m2 g-' by a sol-gel method starting from a modified Mg/Al double alkoxide, M~A~,(OBU),(PEG),~ with supercritical drying of the gel.This method seemed to be the most efficient to obtain solids with high specific areas. X-Ray Diflraction The aerogels obtained after supercritical drying were com- pletely amorphous to X-rays. After calcination at 973 K, the XRD patterns of the magnesium and zinc spinels (Fig. 1) showed the main lines of MgAl,O, and ZnAl,04. They are very broad (FWHM = 1.85" in 26 and 1.55-1.66' in 28, respectively), indicating very small crystalline particles. The CaAl,04 spinel was still amorphous at this temperature. The same supports prepared by impregnation of alumina with magnesium6 or zinc nitrates' had to be calcined at 1273 K for 24 h and 4 days, respectively, in order to form the spinel phases, and still some zinc oxide was present together with ZnA1204 after this treatment.' The sol-gel process allows mixing of the precursors at the molecular scale in solution, by formation of double alkoxides [the alkoxides MgAl,(OEt), and CaAl,(OEt), are known and characterized]., The hydrolysis/gelification reactions, which start building the oxide network by hydrolysis of the alkoxo groups, preserve the homogeneity of the precursors.The desired spinel phases can thus be obtained at much lower temperatures than in the case of solid-solid reactions. Specijic Areas The three spinels obtained by sol-gel methods had high spe- cific areas compared with those prepared by impregnation of alumina with metal nitrates (Table 1).Drying the gels under 200 1 5 100 .AB 10 20 30 40 50 60 2Oldegrees Fig. 1 XRD patterns of the aerogel samples after calcination at 973 K. (a)MgAl,O,; (b) ZnAl,O,; (c) CaAl,O,. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Comparison of the BET areas (m2 g- ') of the supports pre- pared by sol-gel methods (before and after ageing) and by impregna- tion of commercial alumina (102 mz ggl)' support impregnation sample aerogel aerogels after ageing MgA1Z04 ZnAl,O, 49 9 296 147 27 59 CaAl,O, - 230 1 supercritical conditions is one way of preserving the high porosity of the gels and providing solids with high surface area. It affords very homogeneous powders' which can react at low temperatures to form the oxide network, and to keep higher specific areas.Effect of Ageing tbe Supports After ageing the supports by firing them at 1273 K for 24 h, the three spinels were well crystallised: the XRD patterns of the samples (Fig. 2) present all the characteristic diffraction lines of the spinel phases. No other phase than ZnAl,O, was detected. In the pattern of MgA1204, two very small peaks can be attributed to the two main lines of MgO. All the dif- fraction lines of CaA1,0, were present, but also some small peaks could be attributed to the phase Ca,Al,,O,, . No CaO was detected. This ageing procedure led to a severe decrease in the spe- cific surface area (Table 1).The loss is not proportional to the initial area : the zinc aluminate appears more thermostable than the other spinels. The decrease in specific area is particu- v)c 5 5008 0 10 20 30 40 50 60 70 29/degrees Fig. 2 XRD patterns of the aerogel samples after ageing at 1273 K. (4MgAl20, (+,MgO); (b)ZnAlz04;(4 CaAl,O, (V,Ca,AI,,O,,). larly important in the case of CaAl,O,, this solid having a poor resistance to thermal sintering. CuOFpineI Catalysts 5 wt.% CuO was deposited on each spinel by an impregna- tion procedure. The XRD patterns of the three catalysts remained unchanged after the calcination step at 773 K per-formed to form the copper oxide: no diffraction lines of CuO can be detected, suggesting a good dispersion of the active phase on the supports.The specific areas of the catalysts were smaller than those of the supports alone (cf: Tables 1 and 2). It is known' that when aerogels are immersed in a liquid, they collapse immediately because the large-scale structure of the network is weak and easily collapsed by capillary pres- sure. After drying, the resulting xerogel has lost a portion of its larger pores, but the micropores and mesopores that produce the high surface area remain, in a more rigid network. This could explain why, during the impregnation step, a decrease in specific area was observed. Here again the calcium aluminate is the least resistant to pore collapse. Despite the above-mentioned collapse, the aerogel-based catalysts had large surface areas compared with those pre- pared by solid-solid reactions (Table 2).Catalytic Activity Fig. 3 shows the methane conversion as a function of the catalyst temperature for various CuO/spinel samples. Experi- ments performed with MgAl,O, alone showed that the con- version does not exceed a few per cent of ca. loo0 K. Results from previous work on CuO/ZnAl,O, and CuO/MgAl,O, have been included for comparison. Table 2 Comparison of the BET areas (mZg-') of the CuO-based catalysts supported on carriers prepared by sol-gel methods and by impregnation of a commercial y-Alz03 CuO/spinel prepared sample CuO/aerogel by solid-solid reaction CuO/MgAl,O, CuO/ZnAl,O, CuO/CaAl,O, 227 108 103 50 10 - 00 80 60 40 20 0 600 700 800 900 1000 1100 T/K .,Fig.3 Catalytic activity for methane combustion. CuO/ MgAl,O, aerogel; A, CuO/ZnAl,O, aerogel; 0, CuO/CaAl,O, aerogel; U, CuO/MgAl,O, 16; A, CuO/ZnAl,O, .' Carbon dioxide was the single oxidation product. The activity of the two CuO/MgAl,O, (high and moderate spe- cific areas) were identical. In the case of CuO/ZnAl,O,, the activity for CH, combustion was strongly improved for the catalyst supported on ZnAl,O, aerogel, as can be seen with the decrease in Ts0of ca. 55 K. CuO/CaAl,O,, despite its high specific area, had the poorest activity. This could be caused by a reaction between this support and CuO, which cannot be detected by XRD since the samples are amor- phous. These results suggest that the dispersion of CuO was already optimum when deposited on MgA120, having a spe- cific area of 49 m2 g-', and that an increase in the specific area of the support does not improve the dispersion state of the active phase.In the case of CuO/ZnAl,O,, the specific area of the support prepared by solid-solid reaction was very low (9 m2 g-I), and CuO could be detected in the XRD pattern of the catalyst, indicating that rather big crystallites of copper oxide were formed. When the same loading (5 wt.% CuO) was deposited in ZnA120, aerogel, no diffraction lines of copper oxide could be detected. The higher dispersion of CuO on this support is responsible for the better activity of this catalyst. IR Spectroscopy of CO Adsorption After oxygen pretreatment at 773 K, desorption at the same temperature and cooling at 298 K, carbon monoxide was admitted onto the three samples under a pressure close to 40 Torr.Whatever the support, a v(C0) band between 2105 and 2120 cm-'was observed, in addition to the bands due to gaseous carbon monoxide. The intensity of the v(C0) bands increases slightly with contact time, but most of the sites able to chemisorb CO are covered as soon as the CO is intro- duced. Table 3 shows the position and intensity of the v(C0) band as a function of time. The intensity of the previous bands are expressed in absorbance units referred to 1 g cata- lyst, with a CuO content normalized to 1 wt.%. Note that the wavenumber of the v(C0) bands does not vary when the CO coverage changes, suggesting that adsorption occurs on ionic species.Upon desorption at room temperature, the absorbance of the v(C0) bands of the three samples decreases quickly during the first 15 min, but only slowly for longer evacuation times (Table 4). The absorption of CO, studied by IR spectroscopy, is a very useful tool for analysis of the surface properties of sup- ported active phases, either in a metallic or an ionic The position of the v(C0) band depends on the oxidation state as well as on the surroundings of the adsorption centre; its intensity is proportional to the number of accessible adsorption sites.' ' According to literature data, and as far as ionic oxidized copper species are concerned, it seems that Cu2+ and Cu' ions lead to well separated v(C0) bands only when these ions are located in a zeolitic structure.In a recent paper, Sarkany Table 3 v(C0) band position and intensity (absorbance units referred to 1 g CuO/spinels catalysts and normalized to 1 wt.% CuO) after CO adsorption at 25 "C, at several contact times absorbance sample v(CO)/cm-' t=O t= 1 h t=4h t=20h CuO/MgAl,O, CuO/ZnAl,O, CuO/CaAl,O, 2110(m) 2120 (s) 2105 (w) 1.25 3.22 0.59 1.64 3.685 0.77 1.895 4.13 0.92 2.47 4.74 1.29 s = strong, m = medium, w = weak. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Absorbance of the v(C0) bands (absorbance units referred to 1 g catalysts and normalized to 1% CuO) after different times of evacuation under vacuum time of evacuation sample 10 s 15 min lh 3h ~ ~~~~ CuO/MgAl ,04 CuO/ZnAl,O, 2.26 4.29 1.555 3.28 1.35 2.91 1.18 2.575 CuO/CaAl,O, 0.80 0.22 0.12 0.07 et al.' investigated CO adsorption onto excessively ion- exchanged Cu/Na-ZSM 5.They observed v(C0) bands at 2180 and 2157 cm-' attributed to CO adsorption onto Cu2+ and Cu+, respectively. Upon hydrogen reduction at ca. 773 K, the formation of metallic copper was evidenced by a v(C0) band close to 2108 cm-'. In the case of copper-containing species deposited on more conventional supports such as silica or alumina, CO adsorp- tion led to v(C0) bands which are not so different according to copper oxidation state. For metallic copper, CO adsorp- tion leads to the formation of v(C0) bands in the spectral range 2110-2063 cm-', according to the nature of the support and to the CO ~overage.'*'~-'~ In the case of sup-ported CuO, a main v(C0) band close to 2125 cm-' was assigned to Cu2+ ions belonging to the cupric oxide lattice.l4-I8 Cu+ ions have not been definitively identified by an FTIR study of CO adsorption.Several authors proposed to assign a v(C0) band at ca. 2115 cm-' to CO bonded to Cu ions.' 9-2+ In the present study, the position of the v(C0) bands, i.e. 2110, 2120 and 2105 cm-' for CuO/MgAl,O,, CuO/ZnAl,O, and CuO/CaAl,O, , respectively, is the spec- tral range previously assigned for Cu2+ and Cu+ species. However, according to the preparation method of the cata- lysts (calcination in air at 773 K) and the oxygen pretreat- ment at 773 K before IR measurements, it is unlikely that the surface copper ions could be in a reduced state (Cu' species).A reduction of Cu2+ into Cu' by CO at room temperature might occur as pointed out in ref. 14. Nevertheless such a reduction is expected to lead to changes in v(C0) of adsorbed CO, and to the formation of CO, . In the present study, such changes were not observed. Finally, in a previous study per- formed on CuO/Al,O, catalysts," CO adsorption led to a v(C0) band at 2120 cm-', whereas UV-VIS spectroscopy measurements evidenced the presence of Cu2+ ions. As a consequence, we assume that the v(C0) bands observed in the present work can be attributed to carbon monoxide adsorbed onto surface Cu2+ ions.Correlation between Catalytic Activity and F TIR Spectroscopy As in a previous study,I8 a correlation between the catalytic activities in methane combustion (low conversion levels, i.e. ~30%conversion) and the absorbance of the v(C0) band (after 15 min evacuation) was established. The conversion levels as well as the v(C0) absorbance were referred to 1 g catalyst and normalized to 1 wt.% CuO (Fig. 4). The linear relationship shown in Fig. 4 suggests strongly that the active sites in methane combustion are the same as those responsible for CO adsorption. The more active the catalyst, the more intense the v(C0) bands. For the three aluminate supports prepared by the sol-gel process, the catalytic activity does not follow the specific area of the carrier: CuO/ZnAl,O, and CuO/CaAl,O, have similar BET areas (108-103 m2 g-') whereas they exhibit strong differences in catalytic behaviour. It must be postu- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 oxide followed by IR spectroscopy. Nevertheless, it seems that the dispersion of the active sites is more dependent on 6-the nature of the support than on its BET area. The nature and number of superficial groups present on the carrier 5-appears to be a key factor for the active-phase dispersion, and therefore for the catalytic activity. 4-Finally, it must be stressed that aluminate-based aerogel supports with high specific area are rather sensitive to 3-thermal sintering, probably because the small crystallites sizes favour sintering at high temperatures.In that case also, the 2-sensitivity towards sintering depends strongly on the alumin- ate type and seems not to be related to the specific area of the starting material. 0.0 1.o 2.0 3.0 References absorbance (arb. units) 1 M-C Marion, E. Garbowski and M. Primet, J. Chem. SOC.,Fig. 4 Correlation between absorbance of v(C0) bands (after 15 Faraday Trans., 1991,87,1795..,min evacuation) and catalytic activity of CuO/spinels. 2 S. J. Teichner, in Aerogels, ed. J. Fricke, Springer-Verlag, Berlin, CuO/MgAl,O, aerogel; A, CuO/ZnAl,O, aerogel; @, 1986, p. 22. CuO/CaAl 0, aerogel. lated that surface properties of the support, for instance the presence of functional groups, are very important for the dis- persion of the active phase.Further investigations of the surface properties of these aerogel supports should give infor- mation about the surface functional groups available for the anchoring of the active phase. Comparison of the two CuO/ZnAl,O, catalysts prepared by either the sol-gel process (present study) or by the solid- solid reaction’ shows that the support preparation by a sol-gel method leads to a drastic improvement of the cata- lytic activity. For instance, at 723 K, the methane conversion (normalized to 1 wt.% CuO) increases from 1.9 to 6% on going from the ZnAl,O, on alumina support to the same carrier prepared by sol-gel. At the same time, the BET area of the support increases by a factor of 10 (Table 2).In that case, the catalytic activity does not strictly follow the BET area of the support, suggesting that surface properties of the carrier are strongly dependent on the preparation procedure. Conclusion The sol-gel process coupled with supercritical drying is an efficient method for preparation of high specific area spinels at low temperatures. Copper oxide deposition onto these aerogel aluminates leads to catalysts active in methane com- bustion. As far as ZnAl,O, is concerned, the catalytic activity is strongly improved by the use of a carrier prepared by a sol-gel process and exhibiting a large specific area. Catalytic activity is directly governed by the dispersion state of the active phase as shown by specific adsorption of carbon mon- 3 0.Varnier, P. Bergez, N. Hovnanian and L. Cot, in Comptes-Rendus de I’Ecole d’itt! Sol-Gel 91, Oliron, 15-20 Septembre 1991, vol. 2, p. 569. 4 S. D. Peter, E. Garbowski, V. Perrichon and M. Primet, unpub- lished results. 5 W. R. Moser and J D. Lennhoff, Chem. Eng. Commun., 1989,83, 241. 6 M-C. Marion, PhD Thesis, Universite Claude Bernard Lyon 1, France, 1990, no. 02.90. 7 D. C. Bradley, R. C. Mehrotra and D. P. Gaur, in Metal Alkox- ides, Academic Press, London, 1978, p. 300. 8 C. J. Brinker and G. W. Scherer, in Sol-Gel Science, 1990, Aca-demic Press, London, p. 501. 9 N. Sheppard and T. T. Nguyen, in Advances in Infrared and Raman Spectroscopy, ed. R. J. H. Clark and R. E. Chester, Heyden, London, 1978, vol. 5, p. 67. 10 A. A. Davydov, Infrared Spectroscopy of Adsorbed Species on the Surj-ace of Transition Metal Oxides, Wiley, New York, 1984. 11 M. C. Kung and H. H. Kung, Catal. Rev., 1985,27,425. 12 J. Sarkany, J. L. d’Itri and W. M. N. Sachtler, Catal. Lett., 1992, 16, 241. 13 J. A. Dalmon, M. Primet, G. A. Martin and B. Imelik, Surf. Sci., 1975,50,95. 14 G. M. Millar, C. H. Rochester and K. C. Waugh, J. Chem. SOC., Faraday Trans., 1991,87,1467. 15 J. W. London and A. T. Bell, J. Catal., 1973,31, 32. 16 G. M. Millar, C. H. Rochester and K. C. Waugh, J. Chem. Soc., Faraday Trans., 1991,87,1477. 17 M. A. Kohler, N. W. Cant, M. S. Wainwright and D. L. Trimm, J. Catal., 1982, 117, 188. 18 M-C. Marion, E. Garbowski and M. Primet, J. Chem. SOC., Faraday Trans., 1990,86,3027. 19 Y. A. Lokhov, V. I. Zaikovskii and A. A. Solomennikov, Kinet. Catal., 1982,23,348. 20 A. A. Efremov and A. A. Davydov, Kinet. Catal., 1983,24, 1005. 21 Z. Chajar, M. Primet, H. Praliaud, M. Chevrier, C. Gauthier and F. Mathis, Catal. Lett., in the press. Paper 4/00728J; Received 7th February, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949001541

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

1547J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1547-1551 Two Members of the ABC=DGR Family of Zeolites: Zeolite Phi and Linde D Karl Petter Lillerud" and Rosemarie Szostakt Department of Chemistry, University of Oslo, P.O.Box 1033 Blindern, N-0315 Oslo, Norway Alicia Long Georgia Tech Research Institute , Georgia Institute of Technology, Atlanta , Georgia 30332,USA Linde D and phi, members of the ABC-double six-ring (ABC-DGR) family of zeolites, have been studied by X-ray diffraction. Broadening of specific lines in the X-ray powder diffraction pattern is observed which is consistent with the existence of faulting within the crystals. Both X-ray powder diffraction patterns can be simulated by introducing double six-member-ring sheet structural faults into the AABBCC stacking sequence of chabazite.These zeolites can be synthesised from a potassium-sodium-containing gel system in the absence of organic ami nes. A chabazite-like material identified as Linde D was first reported by Breck and Acara in 1961.' This zeolite crys- tallises from silica-rich gels in the presence of both sodium and potassium. Though the reported adsorption properties of Linde D differed from that of pure samples of chabazite, no additional studies on the nature of this phase have been undertaken. s2 Phi is the name given to another patented aluminosilicate phase. The X-ray powder diffraction pattern, chemical com- position and adsorption properties were reported by Grose and Flanigen in 1978.3 This zeolite was prepared from sodium aluminosilicate gels in the presence of tetra-methylammonium cations.A similar phase was prepared by Jacobs and Martens, and Franco et al., using tetra-ethylammonium instead of tetramethylammonium cation^.^,^ In addition to the organic amine cations, potassium cations were also present in their synthesis Franco et al. further characterised this material. They reported the nature of the diffraction lines. Both broad and sharp peaks were found to be present, an indication that this material may contain a significant amount of structural fault^.^ They acknowledged the similarity between the X-ray powder dif- fraction patterns of Phi and chabazite and suggested that phi contains offretite-like zeolite intergrown with the chabazite structure.Recently, Lob0 et al. prepared several materials fol- lowing altered recipes of both Franco et al. and Jacobs and Martens. They conclude that their material, which they also called phi, did not represent a pure phase but a physical mixture of the two zeolites offretite and chabazite.6 We have synthesised materials from inorganic gel systems with chemical composition, X-ray powder diffraction pattern and IR spectra matching those reported in the literature for zeolites D and phi.'*',* In order to identify the structural phases that these materials represent, we have simulated their diffraction patterns using the DIFFaX program.' High-temperature X-rai diffraction, 29Si NMR and IR spectros- copy were also used to characterise these materials.From our studies, we conclude that neither Linde D nor phi represent physical mixtures of phases or intergrowths of offretite and chabazite. These materials constitute faulted members of the ABC-D6R or chabazite family of zeolites. Experimental Si02/A1203= 6-28; Na20/Na20 + K20 = 0.8-0.95; Na20 + K20/Si02= 0.4-0.55; and H20/A1203 = 250-400. While Ludox HS from DuPont and sodium metasilicates from Fisher Scientific or Sigma Chemicals have been used suc-cessfully as a silica source; aluminium hydroxide from Pfaltz and Bauer was the preferred source of alumina for producing the pure phi-type phase. KOH and NaOH were both from Fisher Scientific. Crystallisation occurs within 3 days at 100°C. Chemical analysis of crystals obtained from gels with Si02/A1203 of 15 indicate a crystal composition with Si02/A1203= 4.46, Na/K = 2.1 and Na + K/AI = 1.For samples prepared in a more aluminium-rich system (Si02/A1203= 10) the Si02/A1203 ratio determined from 29Si NMR is found to be 4.0. The range over which phi-type materials crystallise is compared with those reported in the literature for this zeolite and summarised in Fig. 1. Linde D can be prepared using the following batch compositions as claimed in the patent:'26.18NaOH :4.62KOH :28Si02 :2Al(OH), :250H20 and L 1 25 --/Jacobs' phi -'phi' ,20 -this work a-9 '15-v) phi patent 10 --/France's phi Linde D -Breck -I 5-F 13 The phi-type zeolite discussed in this study crystallises from 0.4 0.5 0.6 0.7 reactive gel mixtures within the following composition range : R20/Si0 Fig.1 Ranges of SiOJAI,O, and hydroxide content over which Present address : Center for Catalysis and Separation Science, zeolites Linde D and phi have been claimed to crystallise (seetext for Clark Atlanta University, Atlanta, Georgia, USA. references) 24.7NaOH :2.74KOH : 28SiO, : 2Al(OH), : 250H,O. The source of silica was Ludox LS40 (DuPont) and the source of alumina was aluminium hydroxide from Aldrich. Reagent grade sodium and potassium hydroxide were purchased from Eka Nobel. The Si02/A1,03 ratio determined from NMR is found to be 4.4, falling within the range claimed in the patent. A typical synthesis procedure for both phi and Linde D is as follows: sodium hydroxide and potassium hydroxide were weighed and dissolved in water, the aluminium hydroxide powder was dissolved in this solution and Ludox was added to the resulting basic solution.Stirring was brief and only used to ensure adequate mixing. 30 ml capacity Teflon-lined autoclaves were charged with the resulting gel to three-quarters of their capacity. The autoclaves were sealed and heated without stirring for 1 to 3 days. After crystallisation was complete, the autoclaves were rapidly quenched by placing them under flowing cold water until they attained room temperature. The solid was filtered and washed with copious amounts of deionized water before X-ray analysis. The highly crystalline chabazite (zeolite K-G) used for comparison was prepared by optimising the procedures ini- tially reported by Barrer and Baynhum." The batch com- position that produced the best fault-free chabazite was: 5K20 : 5Si0, : Al,O, :600H,O.Crystallisation was com-plete after 11 days: 5 days at 90°C and 6 days at 150°C. Offretite was prepared from the batch composition :0.48KOH : 4.32NaOH :0.48TMAOH :6Si0, :2Al(OH), :400H,O using the same reagents and methods as the phi synthesis reported above. The tetramethylammonium hydroxide used was from Fluka. A crystallisation temperature of 110°C was used and crystalline product results after 3 days. A trace amount of erionite intergrowth was observed in this sample, but, it did not influence the results of this study.Physical mixtures of offretite and chabazite were prepared by carefully weighing out the desired amount of each phase and mixing the phases by grinding them together. The XRD pattern was run several times using different sample holders and packing methods. Differences between spectra were minimal. Analytical Methods A Siemens D500 powder diffractometer equipped with a ger- manium primary monochromator to ensure strictly Cu-Ka, radiation, an automatic aperture slit and a Bhuler high- temperature sample holder for measurements under con-trolled atmosphere and temperature were used in this study. The identification of faulted phases from the powder XRD may be even more difficult than identifying physical mixtures. Treacy, Newsam and co-workersg have developed a com-puter program (DIFFaX) to simulate diffraction patterns of such phases.Their work shows that the pattern derived from faulted materials is not a linear combination of the patterns of the end members. Their methodology was used in this work. IR spectra were recorded on a Perkin-Elmer model 225 grating spectrometer and on a Bruker model 88 FTIR spec- trometer covering the regions 4O00 to 350 cm-'. Pellets were prepared using 10 wt.% zeolite in dry KBr and pressed to obtain thin transparent wafers. The ,'Si NMR spectra were recorded on a Bruker CXP-200 pulse Fourier-transform NMR spectrometer oper- ating at 39.7 MHz. The spectrometer was fitted with a magic- angle spinning probe with a D-poly(methylmethacry1ate) rotor spinning at 3.0 kHz.Using a 30 degree pulse angle (4.1 p) with 6.5 s repetition time, 8 x lo3 data points were recorded over a spectral width of 20 kHz. 3000 scans were recorded per spectrum and 10 Hz line broadening was J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 applied. Chemical shifts were determined relative to external Si(CH3),, . Results and Discussion The major difference between the synthesis of a phi-type zeolite and Linde D appears to be the SiO,/Al,O, ratio of the gel. The crystallisation of Linde D is claimed for potassium/sodium-containing batch compositions of SiO,/Al,O, ratio of 28. By lowering this ratio and lowering the hydroxide content of the gel, we have observed the forma- tion of a phi-type phase.Phi-type zeolites readily crystallise from inorganic-only reaction mixtures over a wide range of Si02/A1,03 ratios from 6 to 28. This differs from phi prepared in the presence of organic amines.' This can be seen in Fig. 1. The Na,O/Na,O + K,O and Na,O + K20/Si02 ratios in the inorganic system border the crystallisation fields identified for Linde D.',, Though Breck reports the synthesis of D at gel Si02/A1203 ratios of 7, few further details are provided., Both ranges reported for Linde D are shown in Fig, 1 for completeness. Crystallisation of both phi and Linde D occurs from 24 and 72 h, respectively, at 100°C. Crystallisation of phi-type materials from organic containing systems as described by Franco et al. requires 8 to 13 days at similar crystallisation temperatures.' By adjusting the synthesis conditions, other phases are observed.These include offretite and phillipsite in both inorganic and organic synthesis systems. A more detailed summary of batch compositions examined and crys- talline products resulting from the template-free system are provided in Table 1. At a SiO2/A1,O, ratio of 28 and low potassium content in the gel, Linde D is formed exclusively. In the absence of pot- assium cations, faujasite readily crystallises. Increasing the amount of potassium in the gel results in the crystallisation of an offretite-type phase (Linde T) along with Linde D. The X-ray powder diffraction pattern of this mixture of Linde T and Linde D is similar to the diffraction pattern reported by Lob0 et al.for the material they claim as phi.6 Lowering the SiO2/A1,O3 ratio of the gel from what is claimed in the Linde D patent produces the phi-type phase. At lower SiO,/Al,O, Table 1 Batch compositions and products at crystallisation tem- perature of 100-110°Cafter 1 to 3 days, H,O/AI,O, = 250-400 SiO,/AI,O, Na/Na + K R,O/SiO, product 28 0.8 0.49 Linde T, Linde D 28 0.85 0.46 Linde D 28 0.9 0.5 Linde D 15 0.9 0.4 phi15 0.9 0.4 phi, faujasite 15 1 0.47 faujasite15 0.9 0.3 amorphous12 0.9 0.52 phi12 0.9 0.45 phi12 0.9 0.4 phi12 0.8 0.46 phi, (phillipsite) 12 0.8 0.4 phi, (phillipsite) 12 0.9 0.3 amorphous10 0.9 0.52 phi, (trace of phillipsite) lo" 0.9 0.6 phi, (faujasite) 6 0.9 0.52 phi6" 0.9 0.52 faujasite6b 0.9 0.51 phillipsite6 0.7 0.45 phillipsite Sodium aluminate as alumina source.Catapal B as alumina source. J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 ratios, phillipsite is occasionally observed as a secondary phase. At Si02/A1203 ratios of 10, long crystallisation times (2 weeks) do not result in the conversion of phi to phillipsite or to any other phases indicating no interdependence between these phases. The source of alumina is critical for the formation of the phi-type phase in this system. Successful crystallisation occurs when highly soluble aluminium hydroxide is used as the aluminium source. Using sodium aluminate encourages the formation of faujasite and catapal B results only in the crystallisation of phillipsite. With time, any low-crystalline faujasite formed does disappear.Low hydroxide contents fail to produce any products within 3 to 5 days. Longer crys- tallisation times were not explored. Elevated temperatures generally result in producing phillipsite. Offretite, or Linde T, forms in potassium-rich systems at the higher temperatures. The X-ray powder diffraction pattern for Linde D is shown in Fig. 2(a)and is compared with that of template-free phi in Fig. 2(b).The d-spacings (indicated as solid lines) claimed in the respective patents are included to confirm the identity of the materials reported here. The presence of the low-angle reflection around 11.6 A in the X-ray powder diffraction pattern represents a characteristic of zeolite phi that is present to a lesser extent in the diffraction pattern of Linde D. This low-angle diffraction peak in Linde D is not always observable under normal conditions encountered for the X-ray data collection.The conditions used to obtain the XRD patterns of Linde D are critical in the characterisation of this material at the low angles. The intensity of the low- angle peaks in zeolite diffraction patterns is strongly depen- dent on the amount of water adsorbed in the zeolite. Also, when the purpose of the X-ray powder diffraction data is only to identify the presence of crystalline phases, it is common to use fixed slits that expose more than the mounted sample at low angles. This results in an underestimation of the intensity of the low-angle reflections.In this study, we have used an automatic divergence slit in recording the dif- 100 r 15 20 25 30 35 fraction patterns represented here and the observed inten- sities are corrected with the l/sin 8 function. The X-ray powder diffraction patterns shown in Fig. 2 were recorded at 200°C with minimal water content in the sample, as deter- mined from water loss experiments using thermal analysis techniques. The presence of two phases has been suspected by others to explain the XRD pattern of zeolite phi.6 The broadness of only 1 lines (hexagonal indexing) in combination with a single uniform morphology observed for the crystal agglomerates of these samples, makes this possibility unlikely.To illustrate the difference between physical mixtures of offretite and cha- bazite and zeolites phi and D, two physical mixtures contain- ing different proportions of pure offretite and chabazite were used in this study. The first mixture composition chosen con- tains 5 wt.% offretite. Such a composition would match the weak-intensity reflection appearing around 11.6 A in the powder diffraction pattern of Linde D. The X-ray powder dif- fraction pattern of Linde D would then be expected to match that of the major phase, chabazite, more closely since the most intense reflection found in offretite appears at 11.4 A. The second physical mixture for this study is conservatively chosen to be that of 30% offretite-70% chabazite. With this amount of offretite, the intensities of the first two reflections in the phi diffraction pattern would be approximated.The X-ray powder diffraction pattern of Linde D, template-free phi and the two physical mixtures at 5% and 30% offretite in chabazite are shown in Fig. 3 and 4 and the d-spacings for phi and the two physical mixtures are shown in Table 2. What is immediately obvious from the diffraction patterns is the variability in the FWHM in several of the reflections in the X-ray diffraction pattern of Linde D and zeolite phi. Broadening of certain reflections is generally indicative of materials containing faults. The zeolites beta and ZSM-20 are two well known examples of materials con- taining stacking faults and exhibiting X-ray powder diffrac- tion patterns consisting of combinations of broad and sharp I A 5 10 15 20 25 30 35 B 1,I I I (b! nw5 10 15 20 25 30 35 26/degrees 5 10 15 20 25 30 Fig.2 Comparison of the reported diffraction patterns for zeolites 28jdegrees (a) Linde D and (b) phi with the patterns obtained in this study. Fig. 3 Comparison of the diffraction patterns of A, (a) Linde D and Dashed lines indicate spacings and intensities reported in the patents. (b)5% OFF-95% CHA and B, (a) phi and (b)30%OFF-90% CHA 60 50 40 30 20 10 LA. A I 0 5 10 15 20 25 0 5 10 15 20 25 2O/degrees ZO/degrees Fig. 4 Simulated X-ray powder diffraction patterns for a series of randomly faulted chabazites. The numbers indicate O/O disorder.Experimentally determined patterns for (a)chabazite, (b) Linde-D, (c) phi-I and (a) phi-I1 are also shown. reflection^.^^"*'^ What is more striking is the number of major reflections in chabazite and offretite that are not present in either zeolite phi or Linde D and reflections observed in these materials that are absent in the physical mixtures. The most notable difference is the absence of a strong reflection at 28 = 25" that is a low-intensity broad band in phi (Fig. 3). The intense reflection at 28 = 26" in phi appears as a very weak intensity reflection in the physical mixture. The doublet at 28 = 31" in the mixture shown in Fig. 3 appears as one intense reflection in Linde D. This is a further indication that both zeolites are not simple physical mixtures of these two components.The IR stretching frequencies for all materials are com- pared in Table 3. Vibrations unique to offretite around 775 and 575 cm-I are present in physical mixtures containing offretite and in Linde T but are absent in the samples of Linde D and phi. Those vibrations more characteristic of chabazite appear at 760 and 510 cm-'. The 760 and 510 cm-' bands appear in similar positions in Linde D. The 760 cm-' band is absent in phi but the 510 em-' band is observed, an indication of some uniquely different character about the phi structure. Comparing the IR spectra of phi with offretite, chabazite and their mixtures further substan- tiates the absence of offretite in the material. A similarity does exist between phi, Linde D and chabazite.It can therefore be suggested that these materials are members of the ABC-D6R family of structures. The com- binations of broad and sharp reflections are a result of faults contained within the structure and would arise from stacking disorders in the double six-ring AABBCC sequence. Such a conclusion was also reached by Franco et al. though they considered the faults to be due to offretite phase^.^ The J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Comparison of the X-ray powder diffraction data for phi and mixed phases if chabazite (CHA) and offretite (OFF) phi" 5% OFF/95% CHA 30% OFF/70% CHA 11.78 (mbr) 9.368 (msbr) 11.39 (vw) 9.33 (m) 11.40 (m) 9.339 (m) 8.076 (vw) 7.112 (w) 6.878 (vs) 6.903 (vw) 7.537 (w) 7.061 (vw) 6.906 (w) 6.598 (m) 6.293 (vw) 5.558 (wbr) 5.039 (vs) 5.555 (vw) 4.997 (vs) 4.677 (w) 5.717 (vw) 5.556 (vw) 5.321 (vw) 4.999 (vvs) 4.673 (vw) 4.319 (sbr) 4.330 (m) 4.555 (vw) 4.326 (m) 3.975 (m)3.893 (w) 3.988 (vw)3.873 (m) 3.989 (vw) 3.873 (w) 3.811 (vw) 3.609 (vwbr) 3.750 (w) 3.443 (s) 3.578 (s) 3.454 (vw) 3.578 (s) 3.455 (vw) 3.237 3.184 (vwbr) 3.118 2.932 (vs) 3.235 (vw) 3.178 (mw) 3.119 (vw) 2.934 (s) 3.301 (vw) 3.233 (vw) 3.176 (w) 3.150 (w) 2.934 (m) 2.888 (s) 2.889 (m) 2.604 (s) 2.845 (vw) 2.778 (vw) 2.682 (w) 2.612 (w) 2.843 (mw) 2.780 (vvw) 2.680 (w) 2.612 (vw) 2.579 (vw) 2.501 (mw) 2.352 (vw) 2.312 (vw) 2.579 (vw) 2.502 (w) 2.353 (vw) 2.296 (mw) 2.304 (vw) 2.303 (vw) ~~~ Phi is in NH, form, CHA and OFF in K and Na/TMA forms, respectively.Data collected at room temperature under similar levels of hydration. a This work. 29Si NMR of phi exhibits a classic pattern for a material con- taining equivalent tetrahedral (T) sites which further rules out the presence of an offretite intergrowth or second phase. Linde D also displays a similar pattern. Structures with single Table 3 Comparison of the IR vibrations between 900 and 300 cm-' of chabazite (K) and reported chabazite-like materials (R,D), zeolite phi and physical mixtures of offretitwhabazite and offretite-erionite (T) zeolite R" chabazi te-like G" chabazite D" chabazite-like T" offretite-erionite phib 30% OFF-70% CHAb 5% OFF-95% CHAb " Ref.6; 'this work. wavenumber/cm 738 (w) 678 (w) 625 (m) 508 (mw) 452 (m) 426 (m) 720 (w) 696 (wsh) 632 (m) 515(m) 460(m) 408 (m) 755 (wsh) 711 (w) 631 (m) 513 (m) 459 (m) 415 (m) 771 (w) 718 (w) 623 (mw) 575 (w) 467 (ms) 433 (ms) 410 (vwsh) 730 (w) 775 (w)760 (vwsh) 685 (w) 725 (w)720 (wbr) 630 (m) 630 (mw)630 (m) 575 (vw) 511 (m) 510 (w) 465 (ms)460 (m) 460-370 (mbr) 435 (ms)440-350 (mbr) 410 (wsh) J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 six-member-rings, such as offretite, would give rise to inequi- valent T sites within the structure changing the 29Si NMR pattern. An additional peak at low 6 would be observed. Merlino has addressed the stacking of the zeolites with six- member rings.' Stacking sequences for six-member rings can be ABC at one extreme and AABBCC at the other.Interme- diate between these are the AABAAB sequences of offretite, AABAAC of erionite and ABBACC of TMA-E. What remains unobserved is the mixture of two double six-member rings followed by a single ring. A stacking sequence such as AABBC appears to be an unlikely possibility." The intro- duction of the single six-ring in an AABBCC double six-ring stacking sequence introduces significant strain in the crystal lattice and therefore would not be expected to occur. Calcu- lation of the a dimension for the unit cell of phi (a = 13.8 A, c = 5 A) confirms the lack of single rings as the unit-cell dimensions favour the double-ring type stacking AABB (a = 13.75 A) of gmelinite rather than the AAB of the mixed double-single ring systems (a = 13.29 A) of offretite.Thus it is highly unlikely that faults within the phi structure are due to the presence of single six-member rings. From our results we can conclude that Linde D and zeolite phi do not represent physical mixtures of offretite and chaba- zite but represent members of the ABC-D6R family of materials. These phases contain stacking faults more prob- ably due to random stacking of double six-ring units rather than the presence of single six-ring units as found in offretite. The unit-cell values, and the IR and NMR spectra further substantiate the absence of single six-member rings. The pres- ence of faults in an AABBCC-type structure gives rise to the combination of broad and sharp bands in the X-ray powder diffraction pattern and to the unique reflections not found in a pure fault-free chabazite.High-resolution electron micro- scope imaging of these materials is presently underway to increase understanding of the nature of the faulting in the phi and D phases. Linde D and zeolite phi differ from one another and from chabazite by the number of stacking faults they contain. The X-ray powder diffraction patterns of these materials compare well with the simulated diffraction patterns based on increas- ing amounts of faulting in the chabazite structure. This is shown in Fig. 4. One difference observed between the calcu- lated and the actual XRD pattern is that the 003 reflection in the simulated pattern, which is based on the c-axis for CHA, shifts to a higher angle in the actual material.Such compres- sions along the c direction are also observed in more heavily faulted ABC-D6R family materials. l4 Compression or relax- ation of the lattice around the fault in the structure is not taken into consideration in constructing the simulation. Modification of the synthesis conditions e.g. by changing the Si02/A1203 ratio or hydroxide content of the gel pro- duces materials that may contain the same d-spacing as reported for D and phi with lineshapes that differ signifi- ~ant1y.l~Because of the large variability which can occur from the presence of faulting in such materials, it becomes difficult to assign the names 'Linde D' or 'phi' to any highly faulted material based on reported d-spacing position and intensity alone.Such difficulties must be considered when characterizing fault-containing materials. In the case of Linde D and phi, we can conclude that both are presently ill- defined members of the chabazite family of struct~res.'~ The authors thank Ms. Anne Horn for obtaining the IR spectra of these materials, Dr. M. Stocker for obtaining the NMR spectra and Norsk Hydro for use of their Biosym licence. RS is grateful to the Royal Norwegian Council for Scientific and Industrial Research for financial support. References 1 Br. Pat., 868,846, 1958. 2 D. W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974, pp. 290,291. 3 R. W. Grose and E. M. Flanigen, US Pat., 4,124,686, 1978. 4 P. A. Jacobs and J. A. Martens, Stud. Surf. Sci. Catal., 1987, 33, 15. 5 M. J. Franco, J. Perez-Pariente and V. Fornes, Zeolites, 1991, 11, 349. 6 R. F. Lobo, M. J. Annen and M. E. Davis, J. Chem. SOC., Faraday Trans., 1992,88,2791. 7 J. A. Martens, M. Tielen, P. A. Jacobs and J. Weitkamp, Zeo-lites, 1984, 4, 98. 8 J. A. Martens and P. A. Jacobs, Zeolites, 1986,6,98. 9 J. M. Newsam, M. M. J. Treacy, W. T. Koetsier and G. B. Deruyter, Proc. R. SOC. London, A, 1988, 420, 375; M. M. J. Treacy, J. M. Newsam and M. W. Deem, 1991,433,499. 10 R. M. Barrer and G.W. Bayhnum, J. Chem. SOC.,1956,2882. 11 M. M. J. Treacy and J. M. Newsam, Nature (London),1988,332, 249. 12 J. M. Newsam, M. M. J. Treacy, D. E. W. Vaughan, K. G. Strohmaier and W. J. Mortier, J. Chem. SOC., Chem. Commun., 1989,493. 13 S. Merlino, Proceedings of the Sixth International Zeolite Con- ference, ed. D. Olson and A. Bisio, Butterworths, Boston, 1984, p. 747. 14 K. P. Lillerud and R. Szostak, in preparation. Paper 3/04912D; Received 13th August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001547

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1553-1557 BOOK REVIEWS Reviews in Computational Chemistry. Volume 3. By K. 6. Lipkowitz and D. 6. Boyd. VCH, New York, 1992. Pp. xvi + 271. Price DM 128, f53.ISBN 3-527-89619-8. This is the third volume in an on-going series whose purpose is to provide a resource to help readers keep abreast of devel- opments in computational chemistry. As such, these volumes are designed to be broadly representative of the field. The author of each chapter was asked to provide a mini-tutorial on the computational methods in addition to the usual review article data and references. This volume contains four chapters and a lengthy appendix. Chapter 1 by Tamar Schlick, ‘Optimization Methods in Computational Chemistry’, provides a detailed review of the mathematics of optimization.It contains a general discussion of methods and algorithms, with many references, and also provides several examples of computational chemistry prob- lems. Molecular modelling of deoxycytidine and water cluster geometries are presented. Chapter 2 by Harold A. Scheraga, ‘Predicting Three- dimensional Structure of Oligopeptides’, deals with methods for the prediction of three-dimensional structures of oligopep- tides. It begins by describing how to construct a model of an oligopeptide chain, including potential functions and opti- mization strategies, and it then covers methods for solving the multiple-minimum problem. Applications to simple systems follow. The methods are then extended to larger polypeptides and proteins.25 1 references are cited. Chapter 3 by Andrew E. Torda and Wilfred F. van Gun- steren, ‘Molecular Modelling Using Nuclear Magnetic Res- onance Data’, describes another approach to molecular modelling that uses NMR data. Modelling of experimental NMR data is covered first. This is followed by methods for the adjustment of the three-dimensional coordinates of a molecule to achieve the best agreement between the experi- mental data and known chemical and energetic properties. Chapter 4 by David F. V. Lewis, ‘Computer-Assisted Methods in the Evaluation of Chemical Toxicity ’, deals with computer-assisted methods for the evaluation of chemical toxicity. Methods discussed include QSAR, pattern recogni- tion, computer modelling, and knowledge-based systems.Cytochrome P-450is used as a specific example in conjunc- tion with the COMPACT (computer-optimized molecular parametric analysis of chemical toxicity) method of toxicity investigation. Comparisons of other computer-based systems are also included. A reference list of 227 citations is given. The Appendix is a compendium of software for molecular modelling. Programs that are designed for personal com-puters, minicomputers, workstations, mainframes, and super- computers are all listed, and some of their features are described. In addition, listings of quantum chemistry soft- ware, databases of molecular structures, and molecular graphics and allied methods are given. Said to be the most complete listing of sources of software for computational chemistry that is available, it is a valuable resource.The volumes in this review series, including this one, provide the reader with a compact guide to the world of com- putational chemistry and will be valuable additions to many literature collections. P.C. Jurs Received 25th November, 1993 ~~~~~~ Ion Exchange and Solvent Extraction. Volume 11. Ed. J. A. Marinsky and Y. Marcus. Marcel Dekker, New York, 1993. Pp. xiv + 375. Price $195.00. ISBN 0-8247- 8472-3. In this, the latest volume in an important series, the main topic likely to be of interest to chemists faced with practical problems is the uptake of metals by natural colloids, which are mixtures of hydrous oxides, aluminosilicates, and humic substances, in some cases alive as algae, fungi etc.Chapter four relates metal uptake to the surface structures of such materials, while Chapter six deals with metal-humate inter- actions. These chapters should appeal to those concerned with relevant phenomena in the natural environment, as well as to those who want to exploit natural humic materials in chemical processing. The remaining four chapters treat, from different points of view, the thermodynamics of equilibria between on the one hand, electrolyte solutions, usually in water, but in some cases in, for example, water-acetone mixtures, and on the other, ion exchangers of all kinds, including inter alia resins, inorga- nic exchangers and polyelectrolytes.Chapter one attempts a ‘classical’ thermodynamic approach based on formulae for the activity coefficients of the species in the two phases. This is necessarily exceedingly complicated, and Chapter two makes use, instead, of a three-parameter model, which is able to correlate large quantities of data, but is essentially empiri- cal. Chapter three focuses on the processes at the surface of the solid phase, which are discussed in terms of double-layer theory and of the chemical interactions between the solid surface and the dissolved ions. Chapter five follows a Gibbs- Donnan approach, in which an electrical potential difference between the phases is balanced by a ‘swelling pressure’ in the exchanger phase. These four chapters are by no means easy to follow.In part this is due to the recondite nature of the subject, but the pres- entation adds to the reader’s difficulties. The authors appear to be writing for specialists well versed in the earlier liter- ature, and the editors have not attempted to help the non- specialist, for instance by highlighting the principal features and achievements of each approach and making comparisons between them. In particular in Chapter five, there is no list of symbols and no rigorous definition of the basic concept of swelling pressure. The aim of each of the four chapters is, of course, to explain the results of measurements of ion-exchange equi- libria. Extensive data in this area have long been available and there has been much theorising, but fresh efforts can be justified on the grounds of progress in statistical and com- puter techniques. The authors in all cases demonstrate excel- lent agreement between their theories and the data, though it would be an arduous task to work through the detail of their papers in order to assess the validity of their claims.H. A. C. McKay Received 30th November, 1993 Physico-Chemical Properties of Selected Anionic, Cationic and Nonionic Surfactants. By N. M. Van Os, J. R. Haak and L. A. M. Rupert. Elsevier, Amsterdam, 1993. Pp. viii + 608. Price US $245.75, Dfl. 430.00. ISBN 0-444-89691-0. Although it is difficult to get too enthusiastic about a com- pilation of physico-chemical parameters, the authors have produced an extremely useful and extensive tabulation of the physico-chemical properties of surfactants.It is organised into three chapters; an anionic surfactants (alkyl sulfates, alkane sulfonates and alkylarene sulfonates), cationic sur-factants (alkyl trimethyl ammonium and alkyl pyridinium salts), and non-ionic surfactants (alkyl polyoxyethylene glycol ethers and alkyl phenol (ethylene oxide) ethers. There is an extensive index, which occupies almost a third of the volume. This reflects the thoroughness rather than any redundancy, and does considerably help the reader to quickly access the information sought. The authors have provided a much needed service for those working in the field of surfactants as the compilation goes beyond that of existing works, such as the compilation of cmc’s by Mukerjee and Mysels.However, I suspect that at a price of $245.00 access to the volume may be in many cases restricted just to some libraries; this would be a pity. The authors have not attempted to make this a com-prehensive compilation. The parameters tabulated include c.m.c. krafft temperatures, aggregation number, cloud points, micelle radii, and a range of thermodynamic and other rele- vant parameters. There is no attempt at uniformity of presen- tation, but this does not detract from it’s value. It is difficult at times to decide whether the contents reflects the authors particular needs and interests, or what was readily available in the literature. There are a number of minor criticisms the authors may like to consider, if they ever have the enthusiasm to produce a second edition.The non-ionic phase diagrams included are most welcome, and it was disappointing not to see some for the other surfactant types. There are a few limited references to mixed surfactants. Although this would have added considerably to the length, it is an important area which could have been given more attention. There was a disappointing lack of data on interfacial tension and sur-factant adsorption (from surface tension, surface quasi-elastic light scattering and reflectivity). There were also important classes of surfactants, such as the dialkyl chain surfactants, not included. The practise of including a key to the experi- mental method used in the derivation of some parameters is welcomed, and should have been adopted throughout.Read- ability would have been improved in places if some of the tabulations were simply replaced by a functional form. I was left with the impression that there were not many recent references (say within the last five years), that may just reflect the current nature of publications in this field. However, in conclusion I believe that this is a compilation that most groups working in the field would wish to have access to. J. Penfold Received 20th December, 1993 The World of Physical Chemistry. By K. J. Laidler. Oxford University Press, Oxford, 1993. Pp. xii + 476. Price (Hardcover) f55.00.ISBN 0-19-855597-0. ~ ~~ This book describes the historical development of the main branches of physical chemistry.There are individual chapters on thermodynamics, kinetic theory of gases and statistical mechanics, chemical spectroscopy, electrochemistry, chemical J. CHEM. SOC. FARADAY ’TRANS., 1994, VOL. 90 kinetics, colloid and surface chemistry and quantum chem- istry. Crystallography and several other aspects of structural chemistry are notable omissions in the coverage. The book is written by an eminent reaction kineticist, who has also main- tained throughout his life a strong interest in historical aspects of chemistry; he has published several papers in this area. In these days of domination in physical science by the ultra-specialist and of a general neglect in undergraduate chemistry courses of historical and philosophical aspects, it is refreshing to read a book covering the generalities of physical chemistry, to see how its main facets developed in previous ages and to learn about the people involved in both scientific and personal aspects.In addition to chapters covering the development of the various branches of physical chemistry, there are three introductory chapters on ‘The origins of physical chemistry’, ‘Communication in the physical sciences’, and ‘The growth of the physical sciences’. The first of these covers, inter alia, the difficulty of precise definition of physical chemistry (and indeed of physics and chemistry) and, by an interesting comparison of two eminent reaction kinet- icists (Eyring and Norrish) of the author’s acquaintance, the disparate nature of physical chemists as a breed.In the chap- ters covering developments in particular areas, the contribu- tions of our famous forebears are described in modern terms (and occasionally also in terms used by them at the time). The use of modern terms will make for easier understanding by the reader; those who have read original papers written in the nineteenth century will appreciate the difficulty of under-standing the scientific language of the day. Unfortunately, there are many errors, typographic and otherwise, in the modern terms presentation. I fear that some of these errors reflect inadequate proof reading; additional reading by a col- league, particularly one knowledgeable about modern nota- tions could, I believe, have avoided many of the errors.However, the errors should not detract too much from the general value of the book in its concern with physical chem- ists as people. The historical aspects are well researched and the book is very extensively referenced. In particular substan- tial use has been made of the Dictionary of Scientific Biog- raphy. Both within the main chapters and in an Appendix, there are potted biographies of the many interesting persons (some better known than others) encountered within the book. A few of these are still alive; this is particularly notice- able in the reaction kinetic chapter (where the author’s exper- tise is greatest). Many of the biographies make fascinating reading.One could speculate how some of these scientists might have fared in the present scientific climate; would they have had such broad interest both within physical chemistry and outside science altogether (politics, religion, etc.)? Despite some unfortunate technical errors in the book, I believe that Keith Laidler has made a unique and useful con- tribution to the literature of physical chemistry and I hope that the book will be read widely by physical chemists, chem- istry teachers, students and scientific historians. J. Lee Received 26th January, 1994 ~~ Cambridge Monographs on Atomic, Molecular and Chemical Physics. Photodissociation Dynamics. By R. Schinke. Ed. by A. Dalgarno, P. L. Knight, F. H. Read and R.N. Zare. Cambridge University Press, Cambridge, 1993. Pp. xv + 417. $89.95. ISBN 0-521-38368-4. This is a very nice book. It is clear, well written and broadly based. The intention is not to give a historical description of the understanding of photodissociation dynamics but to present the state of the art of this domain. The content of the book may be broadly described as the application of exact J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 and approximate methods of quantum mechanics to the problem. This means that several aspects (in particular the description of experimental techniques) are out of the scope of this book, but experiments are not absent because most of the results of calculations are compared to experimental results.The bibliography is impressive with 700 references, mostly recent, which cover the period of rapid explosion in the field due to the introduction of laser techniques. A brief description is given of the detailed content of this book. The book is divided into 16 chapters, covering 379 pages. The first chapter is an introduction to photo-dissociation, describing the main features and the various interests of this subject. The second chapter treats the quantum description of light absorption, being restricted to the case of single photon processes, and using a classical description of the electromagnetic field. The Born-Oppenheimer approximation is also discussed. The following two chapters present the general treatment with time inde- pendent and time-dependent methods, from basic equations to numerical methods.The classical (i.e. trajectory) descrip- tion of the dynamics is made in Chapter 5. The limiting cases of direct and indirect photodissociation as well as the inter- mediate case are presented in Chapters 6-8, with a descrip- tion of the reflection principle, of resonances and recurrences and of diffuse structures. The question of the vibrational dis- tribution is the focus of Chapter 9 while the rotational dis- tributions are fully described in two long Chapters (10 and 11). The last five chapters treat rather rapidly a series of more specific topics : dissociation of van der Waals molecules, photodissociation of vibrationally excited states, emission spectroscopy of dissociating molecules, non-adiabatic tran-sitions in dissociating molecules and real-time dynamics of photodissociation. This long list gives an impressive view of the content of this book.However, a few subjects concerning the theoretical understanding of photodissociation are not fully described. For instance, this is the case for the anisotropy of the angular momentum distribution of the fragments. A few pages of Chapter 11 discuss the vector correlations, but most of this field is not treated: the word ‘polarization’ is absent from the index, and the seminal van Brunt and Zare paper of 1968 does not appear among the references. To conclude, I am sure that this book will be extremely useful. It will be of interest for a great variety of readers, from graduate students to senior scientists working with molecules.I am not aware of any comparable book on this subject and I think that by its vast content and unified presentation, it will be an almost necessary introduction to photodissociation. J. Vigue Received 26th January, 1994 Biomoleculrr Spectroscopy Part A. Advances In Spectroscopy. Volume 20. Ed. by R. J. H. Clark and R. E. Hester. John Wiley and Sons Ltd, Chichester, 1993.Pp.xxi + 383.Price f120.00.ISBN 0-471-93806-a. The application of spectroscopic methods to the problems of molecular biology, biomolecular spectroscopy, requires the combined expertise of spectroscopists and of their colleagues in the biomedical sciences. This volume describes the work of such groups, usually by their principals, and particularly using various forms of infrared and Raman spectroscopy.It is very noticeable that site-directed mutagenesis is starting to play a large part in many of these spectroscopic and mecha- nistic studies. The absorption of light by the rhodopsins responsible for mammalian vision initiates a photoreaction with a number of intermediates, the dynamics of which cover femtoseconds to seconds. The experimental study of these processes have involved isotope-enriched magic-angle spinning solid-state NMR studies and resonance Raman (RR) measurements. The processes can be slowed by cooling. A third technique involves investigations of the retinal materials using FTIR difference spectroscopy of the membranes.This elegant tech- nique has enable Siebert and his group to show that for bovine rhodopsin a critical link appears to be the 9-methyl group of the chromophore, which somehow transmits (a form of steric trigger) the information of retinal isomerization to the cytosolic loops, causing corresponding conformation changes. Halophilic bacteria such as Halobacterium halobium contain a remarkable light-driven proton pump bacte-riorhodopsin, which enables the organism to synthesize ATP. Infrared difference spectroscopy has been especially useful here, and enables the identification of specific molecular alter- ations vital to proton pumping, and has demonstrated the importance of aspartic acids to the function. Ultraviolet resonance Raman spectroscopy (UVRR) of proteins and related model compounds are reviewed by Spiro’s group.Since an earlier review in 1986 there have been significant improvements in the laser techniques available for these studies, particularly to enable the use of lower laser powers, so avoiding saturation of the signals. UVRR can be used to probe the subtle features of protein tertiary and sec- ondary structures in solution, e.g. to determine helical content, and to probe hydrogen bonding. To achieve this a detailed understanding of the spectra, particularly of the con- stituent chromophores-aromatic amino acid groups (histidine, phenylalanine, tyrosine, tryptophan), of proline, of amide groups, and of simple models such as N-methylacetamide is needed.Studies of hemogloblin dynamics, cytochrome c, myoglobin, bacteriorhodopsin, and of the enzymes ketosteroid isomerase and Cu, Zn-superoxide dismutase are reviewed. FTIR spectroscopic studies of enzyme-substrate complexes have become possible with the advent of modern sensitive FTIR instruments. Similarly RR and FT-Raman studies have benefitted from improvements in laser technology and optoelectronics. Whilst the intensities of FTIR bands are less than the RR bands, and water interferes with the infrared, FTIR is not limited to chromophore-containing groups. Enzyme systems described are serine proteases, nicotinamide adenine dinucleotide linked dehydrogenases, triosephosphate isomerase, phospholipase A, , and ketosteroid isomerase.These spectra techniques can also be used to study protein dynamics. Bakers’ yeast cytochrome c peroxidase (CCP) is a mito- chondrial heme protein that catalyses the reduction of hydrogen peroxide by ferrocytochrome c. Also the gene for CCP has been isolated, cloned, and the protein expressed in Escerichia coli. The latter has enabled delicate structure- function relationships to be established via RR spectroscopy using site-directed mutagenesis techniques. The recombinant protein CCP (MI) and a series of its mutants (Trp -+ Phe; His52 + Leu; Arg48 -,Leu, Lys; His 181 -+ Gly; Asp235 -, Asn; Trpl91 +Phe) have been studied. The mutations allow the coordination of the iron atom [as F~(III) or F~(II)]by surrounding ligands such as histidines and water to be exam-ined in great detail.Combined with EPR and magnetic sus-ceptibility measurements this gives information on the coordinations and spin states at different pHs. It is found that variations in the surrounding side chains of the heme not only alter the spin and coordination state of the heme iron, but also destabilize the delicate architecture of the protein which maintains the mechanical coupling between the proxi- mal and the distal residues. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Tryptophan is a useful chromophore which occurs in many proteins, in those which do not contain it, then genetically inserted tryptophans can be used as probes without signifi- cant biological and structural modification.The applicability and control of this technique, the measurement of fluores- cence decay lifetimes of the tryptophan residues, and suitable exchanges for tryptophan residues are reviewed. In L-lactate dehydrogenase, succinyl coenzyme A synthetase, glutamine- binding protein, D-a-amino acid transaminase and lac per-mease, the replacement of intrinsic tryptophans to create single tryptophan proteins has been described. Conversely, replacement in non-trytophan containing proteins, or in multi-tryptophan protein where all tryptophans have been replaced, can potentially be used to link biological events, such as catalysis and regulation in enzymes, with shape changes at defined points in proteins of known X-ray struc- ture. The modification experiments are both beautiful and elegant ! Calmodulin (a protein which activates binding of actin and myosin in smooth muscle) has been modified by replacing phenylalanine-99 with tryptophan to help study the multiple calcium binding behaviour.In L-lactate dehydroge- nase the tryptophan fluorescence shows that the closure of the loop of polypeptide carrying the tryptophan probe over the active site of the protein limits the maximum rate of the enzyme. The conformation of the protein at various stages of the reaction cycle in phospholipase A, has been studied using replacement of the single tryptophan by phenylalanine. If flu-orotryptophan is used then ”F NMR can be applied, but here the perturbation of the enzyme function is much greater.Raman microscopy and Raman micro-spectroscopy of single whole cells, with the possibility of spatially resolved monitoring of molecular processes inside a single cell, has become possible with the advent of spectra from samples less than a micrometer spatial resolution. However, current instrumentation limits measurements to cells containing a high concentration of RR scattering material, specific foreign bodies such as crystal-like cellular inclusions, or a very highly condensed protein-DNA complex. Techniques described are confocal Raman spectrometers, UV Raman spectrometers, and Raman microscopes. Great care is needed in the choice of the appropriate spectrometer configuration and also if photochemical cell damage and fluorescence artefacts are to be avoided.Studies so far have been on bacterial cells, eukaryotic cell organelles, algae and plant cells, and animal cells, all showing that a surprising number of possibilities exist for useful measurements. Molecules adsorbed on metal surfaces can show an unusually large Raman cross-section, the change in optical properties near the surface can be detected with the surface- enhanced Raman spectroscopy (SERS) technique. For bio- logically significant molecules the application is about 15 years old, the subject shows particular promise for studying nucleic acids and antitumour drugs, with the study of drug distribution in single living cells, of eye lens pigments in normal and cataractous human eye lens extracts, and influ- enza virus-inhibitor interactions being the most important applications.The technique is likely to become important in biotechnology and in medicine, and where the analysis of large numbers of samples is needed. The final chapter concerns the use of FTIR measurements of the various CH, vibrational modes in phospholipids to try to quantify the conformational disorder in biological mem- branes. This is a new approach which has advantages over X-ray, ESR and NMR measurements, but which needs an order of magnitude gain in infrared sensitivity to be properly exploitable. John Maher Received 26th January, 1994 ~~ ~ Biomolecular Spectroscopy Part B. Advances in Spectroscopy. Volume 21. Ed. by R. J. Clark and R. E. Hester.John Wiley and Sons Ltd, Chichester, 1993. Pp. xix + 344. Price f110.00.ISBN 0-471-93832-7. In this volume, the focus is largely on ultra-fast molecular dynamics. Time-resolved infrared spectroscopy (TRIR) measurements are now possible for events on the picosecond timescale: much of this work has been done in Hochstrasser’s labor- atory. Theory and techniques are described, and the nano to sub-picosecond infrared studies on various proteins so far studied are reviewed. The technique reveals subtle details of the bound CO in the heme-CO complex. For bacte-riorhodopsin the experiments show that it is possible to examine the protein dynamics and the coupling of the chromophore motion to the protein on the picosecond time- scale at ambient temperature.Work on photosynthetic bac- teria is also described. The photoisomerization of the retinylidene chromophore triggers a series of reactions in retinal proteins. The reasons for the particular cis-trans and trans-cis photoionization pathways, which excited state, S, or T,, is involved, why a particular chain length of the conjugated chain is used, and how the protonated Schiff-Base linkage affects the excited- state, are all of fundamental importance. To help answer these questions the excited states of retinoids and aldehyde homologues can be studied by time-resolved, electronic absorption spectroscopy and RR spectroscopy. The tech- niques can also be used to study carotenoids and chlo- rophylls both free and bound in pigment-protein complexes.Carotenoids act both as photo-protectors (preventing forma- tion of singlet oxygen), and as light harvesting materials. For the former the 154s function is selected by the reaction centre probably because of its unique TI-state isomerization property. For the light harvesting function, carotenoids absorb visible light and then transfer this energy to chlo- rophyll with a singlet energy transfer via the 2A, (S,) state. For the chlorophylls changes in the bond orders of the macrocycle on excitation, and intermolecular interactions affect the excited-state properties. Time-resolved RR (TR3) and UVTR3 spectroscopy of pro- teins involves the use of mixing flow cell systems to investi- gate transient species. Combining optical measurements with TR3 enables a check on the species present during the intense laser illumination of the TR3 experiment.An interesting arti- ficial cardiovascular system was used to enable the accumula- tion of spectra of reaction intermediates of cytochrome c oxidase with 0, over a long time using limited amounts of enzyme. The TR3 techniques have been applied to studies of various heme-oxygen systems, e.g. cytochrome P-450, horse radish peroxidase, and cytochrome c oxidase. TR3 has been used to study how ligands such as CO approach the heme iron when they bind to myoglobin. The CO entry probably goes by way of protein rearrangements as distinct from an ‘open’ structure intermediate. TR3 was also used to examine how the quaternary structure of hemoglobin changes on binding/dissociation of CO and how a structural change in the heme ligation in the a-sub-unit can be communicated to the /?-sub-unit. Cytochrome oxidase (CcO) has been estimated to account for 90% of the biological 0, reduction on earth! Much of our knowledge of the dynamics of this system comes from the ‘flash-flow’ techniques first applied by Gibson and Green- wood in 1963.Measurement of transient species by electronic absorption, MCD, TR3, FTIR, TRIK, and RR are reviewed, together with the systems cytochromes aa3, ba,, and 0.The reviewers, (W. H. Woodruff, R. B. Dyer and 0.Einarsdottir) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 present a mechanism for the photodissociation, thermal recombination and thermal dissociation of CcO-CO.This chapter illustrates how difficult it is to speculate on the microscopic mechanism of many metal-enzyme systems, and alongside of this how challenging it is to design experiments to test a mechanism. The theory and measurement of vibrational Raman optical activity (RAO) is reviewed in Chapter 5, its application to biomolecular conformational problems in amino acids, pep- tides, proteins, carbohydrates and nucleosides in aqueous media is described. ROA provides a unique method to inves- tigate the turns, loops and other types of local structure in proteins and small peptides, as well as providing ‘fingerprints’ for complex carbohydrates. In Chapter 6, VCD (vibrational circular dichroism) is described, this com-plements both ECD and RAO, particularly the latter tech- nique. Empirical correlations of VCD spectral features can be made with e.g., the secondary structures of proteins. Instru- mental techniques and sample preparation are described, the latter is critical for VCD since D,O is used for most of the protein measurements. Measurements on polypeptides, oligo- peptides, and nucleic acids are described. Finally the application of X-ray absorption edge spectros- copy (XAS) to the characterization of transition metal sites in biological systems is reviewed. Data for vanadium sites in amavadin and bromoperoxidase, molybdenum in nitro-genase, iron in ferritin and hemosiderin are discussed. In these two volumes, 20 and 21, of the Advances in Spec- troscopy series, the editors, R. J. H. Clark and R. E. Hester have assembled a fascinating summary of the application of (mostly) vibrational spectroscopic studies to biochemistry and bioinorganic chemistry. The articles also provide valu- able summaries of the plethora of new techniques which have arisen as a result of the application of the powerful tri- umvirate of computers, lasers and Fourier transforms ! I suspect that many chemists in other fields of study could find ideas here for applications outside of biochemistry. John Maher Received 26th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949001553

出版商:RSC    

年代:1994

数据来源: RSC