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J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 721-726 721 Spin Trapping of Radicals formed upon Irradiation of Organobromine Compounds with Low-energy X-Rays Valentin E. Zubarevt and Alexander Halpern lnstitut fur Nuklearchemie , KFA Julich , 52425 Julich, Germany Solutions of several organobromine compounds in methanol or benzene were irradiated with Mo-Ka, p X-rays (€ = 17.79 keV). In contrast with y-rays, such low-energy photons interact predominantly (ca. 89-95%) with bromine causing Auger cascades and eventually the formation of highly excited moieties, which are possible precursors of free radicals. The irradiation was followed by the detection of free radicals through spin-trapping and EPR spectroscopy. Radicals from both the solvent and the solute were identified and quantified.The yield of radicals normalized to unit energy deposited by secondary electrons was studied as a function of the solvent and the compound structure. In benzene, this yield is very low, indicating that the organobromine moieties largely escape fragmentation, since their excess energy is easily channelled into collective excitation of the n-electrons of benzene molecules. In methanol two factors contribute to a more abundant formation of radicals: (a) the lack of a 'deactivation ' mechanism of excited moieties which now tend to decay more freely, yielding aryl radicals and (b) the in situ radiolysis of methanol, yielding additionally methoxy and hydroxymethyl radicals. The in situ radiolysis follows the high-LET pattern.As the interaction of photons of energy up to 30 keV with atomic systems is nearly completely photoelectric it causes inner-shell ionization of the atoms affected. The creation of a vacancy in an inner-shell of an atom triggers an Auger cascade with the emission of, on average, several Auger and Coster-Kronig electrons of low energies. The process results in multiply charged ions and in a high density of electrons in the proximity of the photoelectric event. Thus, two physical processes, acting concomitantly, contribute to the molecular consequences of the Auger cascade :(a)effects associated with the high positive charge and its neutralization and (b) radi-ation effects due to the short-range electrons emitted (in situ radiolysis).The relative importance of these two processes depends on the system under study (aggregation state, molec- ular size and structure etc.),but regardless of the mechanism of action, extensive local molecular effects are anticipated.' Most experimental studies of the molecular consequences of the Auger effect in condensed phases have been based upon measurements of stable products in organic molecules or strand breaks in DNA. Alternatively, they may be based upon the analysis of the intermediate species, e.g. free rad- icals. However, at ambient temperature most of the radicals disappear within the duration of an experiment, so that the steady-state concentration is too low for EPR detection. This limitation can be overcome by using nitrone or nitroso com- pounds as scavengers.Both of these, referred to as spin traps, react rapidly with a variety of short-lived radicals to form relatively long-lived adducts which can be identified by EPR spectroscopy.2-4 In the present study, solutions of several organobromine compounds in methanol or benzene were irradiated with Mo-Ka, p X-rays (I? = 17.79 keV) followed by the detection of free radicals through spin-trapping EPR spectroscopy. Radicals from both solute and solvent were identified and quantified. An important feature of the experiments is that such low-energy X-rays are absorbed preferentially by bromine, whereas there is little absorption by other elements. This selectivity contrasts with y-rays which interact indis- criminately with the sample.For comparison, similar solu- tions were irradiated with 6oCo y-rays. Experimental Materials and Sample Preparation All reagents were purchased from Aldrich, except the spin trap N-benzylidene-tert-butylamine N-oxide (PBN) which was synthesized by one of us (V.E.Z.). They were of the highest grade and were used without further purification. Aliquots of a 1 mol 1-' organic bromide and 0.1-0.5 mol 1-' PBN in methanol, or 0.05 mol 1-' of a spin trap 2,4,6-tri- tert-butylnitrosobenzene (BNB) in benzene, were degassed by repeated freeze-pumpthaw cycles, sealed in Suprasil quartz tubes (id = 2 mm) and used for irradiation. The effective volume of the sample under irradiation was 40 pl. Irradiation Procedure and Dosimetry The primary beam from an Mo-anode tube (Siemens AgMo 61), operated with a Kristaloflex 4 generator (voltage 20 kV, current 60 mA) was used.Samples were placed in a quartz Dewar flask with cold water and irradiations were carried out at 0°C. The low-energy part of the continuous X-ray spectrum was fully absorbed by quartz (3.5 mm) and water (3 mm), so that practically only the intense Ka (17.44 keV) and KB (19.61 keV) characteristic lines, in the ratio 0.84 : 0.16, reached the samples. The cross-sections for the weighted mean energy of these two lines (17.79 keV) were used in the calculations. They were estimated by interpolation of the data in ref. 5 and 6. Radiation exposure was measured with a small (0.02 cm3) air ionization chamber for low-energy X-rays (PTW Freiburg) attached to a calibrated dosemeter PTW DL4.The photon fluence per rontgen in the quasi-monoenergetic X-rays was 4.5 x lo9 photons cm-* R- ',t and the exposure rate at the point at which the photons entered the liquid sample was 11.4 kR min-' (2.94 C kg- ' min-'). The yields of radicals were determined from the initial linear part of the dose curves (spin-adduct concentration us. irradiation time). Numerical data in Tables 2 and 3 (later) refer to 1 min irradiations with X-rays, which corresponds to the target sample being subjected to lOI3 photons. t Visiting scientist, on leave from the Chemistry Department of Moscow State University. t 1 R = 2.58 x C kg-'. 722 EPR Measurements The EPR spectra were recorded at -6.5 "C on a Varian E-9 X-band spectrometer operating at 100 kHz modulation fre- quency, supported with an HP 9285A microcomputer to save the spectra on a magnetic tape. A ruby monocrystal cemented inside the cavity served as a reference.Magnetic scans were calibrated using a solution of a freshly prepared sample of Fremy's salt. The spectra were first run at 0.4 G modulation amplitude, avoiding saturation. Absolute spin concentrations were estimated by integrating the spectra run at 2 G modulation amplitude twice using the Varian ESR-935 acquisition system. Solutions of known concentra- tion of the stable aminooxy radical TEMPO (tetramethyl- piperidine-N-oxyl) in methanol or benzene served as standards. The relative contribution of individual radicals to the overall spectrum was determined using a computer program described in ref. 7.The hyperfine splittings (hfs) were elucidated by a comparison of the simulated and experimen- tal spectra. Results The spectra observed while irradiating pure methanol or a solution of LiBr in methanol are a composite of the PBN adducts of methoxy and hydroxymethyl radicals and hydro- gen atoms. The spectra of the methanol solutions of phenyl bromide, 2-bromopyridine, 3-bromopyridine, bromotoluene or 5-bromopyrimidine exhibit, in addition, lines belonging to the respective aromatic radical. For example, Fig. 1 shows the spectrum obtained from irradiation of 1 mol 1-' 3-bromopyridine and 0.1 mol I-' PBN in methanol.The EPR parameters are summarized in Table 1. The total yields of the trapped radicals (the sum of radicals from methanol and an aromatic solute) and the relative contribution of individual species are given in Tables 2 and 3. Table 1 EPR parameters of the PBN spin adducts in X-or y-irradiated solutions of RBr in methanol RBr radical a,/G aH BIG AH/G ~ ~~ none H 15.75 8.43 0.5 CH,OH 15.15 3.52 0.9 CH30 14.38 3.0 0.5 LiBr" H 15.85 8.55 0.5 CH,OH 15.4 3.6 1.05 CH3O 14.5 3.2 0.55 Br 15.05 3.05 0.45 15.0 2.92 0.55 14.87 2.8 0.45 15.12 4.75 0.55 CH, CH, 14.75 2.85 0.5 -The increase in uN stems from the increased polarity of the solution. J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 Fig. 1 Simulated spectrum of 1 mol 1-' 3-bromopyridine and 0.1 mol I-' PBN in CH30H; best fit to the experimental spectrum. Table 2 Total yield of trapped radicals in 1 moll-' RBr-CH,OH \ [ ,N-O']/10-6 rnol I-' [PBN] = [PBN] = [PBN] = RBr 0.1 rnol 1-' 0.2 rnol 1-0.5 mol 1-' none 2.1 -2.3 LiBr 7.2 -7.4 11.4 11.3 12.8Q Br 11.0 11.5 13.4 QBr 6.2 7.8 9.8QrBr CH, 10.2bBr Br 7.8cH3tYcH'I CH, 8.3 Table 3 Relative contributions of individual species RBr H(%) CH,OH(%) CH,O(%) R(%) C6H5Br 4 25 44 27 C,H,NBr 4 22 49 25 C,H ,N,Br 4 25 49 23 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Differential absorption of 17.79 keV X-rays in 1 moll-' RBr in methanol RBr sample thickness/g cm - (pe,/p)/cm2 g- none 0.161 0.517 LiBr 0.184 4.32 C,H,Br 0.178 4.02 C,H,NBr 0.178 4.02 C,H,N,Br 0.178 4.03 (p,,/p)/cm2 g-': H, 0.012; C, 0.24; N, 0.48; 0,0.85; Br, 42.0; Li, 0.012.The irradiation of 0.05 mol I-' BNB in pure benzene for up to 10 min did not produce any visible spectrum. The spectra observed when a 1 mol 1-' solution of phenyl bromide or 3-bromopyridine in C6H6 or C6D6 in the pres- ence of 0.05 mol 1-' BNB were irradiated exhibited only one adduct for each solute, independent of the solvent. These were aminooxy radicals (1) But 'But 1 where R = C6H5 (uN = 9.75 G, u,(o, p) = 2.75 G, uH(~)= 0.75 G) or CSH,N (multiplet, not all lines resolved). In the perdeuteriobenzene system the adducts with deuteriated phenyl radicals (triplet, uN = 9.5 G, AH = 1.5 G) were not present, indicating that the radicals detected stemmed from RBr, not from the solvent.The yield of the trapped phenyl radicals was 0.8 x mol I-' in C6H6 and 1.1 x mol I-' in C6D6, and that of the pyriminidyl radical 0.5 x mol 1-l in both sol-vents (1 min irradiation). Discussion Methanol Systems Interpretation of these results requires some knowledge of the initial photon interaction processes, of how the imparted energy leads to the formation of radicals, and how they are eventually converted into PBN adducts. As will be shown later, RBr contributes 89% of the total absorption. Scheme 1 gives a qualitative account of the elementary physical pro- cesses which follow the photoelectric event in RBr (R is an aryl radical). This scheme is adapted to the specific needs of the present study which was biased towards the observation RBr CH OH (thehalization, solvation) e-[R Br]'.-.a ring R autoionization es-, H, CH,O, CH,OH cleavage (slow electrons) Scheme 1 absorption (Yo) total absorption (%) C N 0 Br 8 17.4 -82.2 -55 1.9 -8.8 89.4 51 2.3 -8.8 88.9 51 2.2 0.17 8.8 88.9 51 2.2 0.3 8.8 88.7 of the radical species, not other intermediates (ions, excited states) or stable products.The left-hand side illustrates the processes associated with the charge following the Auger cascade; the right-hand side shows the in situ radiolysis (see Introduction).All primary radicals react with PBN in competition with methanol and RBr ; the respective reactions will be discussed later. Absorption Characteristics of 17.9 keV Photons: Initial Photon Interaction Processes In Table 4 the sample thickness, the mass energy absorption coefficients, and the percentage of photons absorbed in the sample are given. From the elemental composition of the material and the partial absorption coefficients, the differen- tial absorption in individual component elements was also calculated. The dominant mode of photon interaction is with bromine, although the ratio RBr : CH,OH in a 1 mol 1-' solution is 1 : 24.5. To understand the attractiveness of using low-energy X-rays note that for 1.25 MeV prays the mass energy absorption coefficient (pe,,/p) is 0.046 cm2 g-' for 1 mol I-' PhBr-CH,OH, and that these photons interact with the component atoms with relative probabilities that correl- ate well with the ratio of the number of electrons from each element (21% with C, 13% with H, 24% with 0,41% with Br).Thus, reducing the energy of the incident photons from the MeV to the keV region increases the interaction coeffi- cient significantly and drastically changes the topography of absorption. Low-energy electrons set in motion by photons traversing an absorber and interacting with the component atoms are the main agent through which the radiation effects arise. We estimate the number of electrons released in the bulk sample, and the energy they carry, by a two-step procedure.First, knowing the number of photons entering the target sample (1013),the percentage absorption and the contributions of the individual component elements to the total absorption (Table 4), we obtain the number of photoelectric interactions with each type of atom in the sample (Table 5). Then, we evaluate the number of photoelectrons and Auger electrons emitted per interaction from each element in question, and their energy. The product of these two numbers, summed over all elements, gives the total number of secondary electrons and Table 5 Number of photoelectric interactions with components of 1 rnol I-' RBr-CH,OH (bulk sample) no. of interactions x lo-', RBr C N 0 Br total x ~~ ~~~~~ ~ none 0.14 -0.66 -0.8 LiBr 0.10 -0.48 4.92 5.5 C,H,Br 0.12 -0.45 4.53 5.1 C,H,N,Br 0.11 0.0087 0.45 4.53 5.1 C,H,N,Br 0.11 0.015 0.45 4.52 5.1 724 their total kinetic energy.(Previously,' we applied the same computational procedure to estimate the electron energy deposited in brominated DNA upon irradiation with low- energy X-rays.) Interaction of an X-ray photon with C, N and 0 yields one photoelectron per interaction which carries away a large frac- tion of the initial photon energy. The energy of photoelec- trons (EpE= 17.79 -E, keV, where E, is the binding energy of the K shell) is 17.51 keV for C, 17.38 keV for N and 17.26 keV for 0.Moreover, since only 1s and 2s orbitals are avail- able in the electronic structures of these elements, and the radiative relaxation is very low (<0.3%), it is assumed that one Auger electron arises for each interaction. The energy of Auger electrons, estimated by the 2 + 1 approximation,' is 276.6, 393.4 and 515.9 eV for C, N and 0,respectively.The case of bromine is more complex. First, the K, L and M shells take part in the absorption in the ratio 0.864 : 0.1 16 :0.016.5 Thus, per interaction, there will be 0.864, 0.116 and 0.016 photoelectrons of 4.32, 16.0 or 17.5 keV kinetic energy, respectively. Secondly, and more impor- tantly, a large number of the Auger transitions are allowed, down to N orbitals. Thus, regarding cascades in many atoms one would find quite different electron spectra. Humm and Charlton' developed an algorithm to determine the Auger electron spectrum following photon absorption in bromine, which took into account electrons from every allowed tran- sition and rendered an accurate physical picture.Here we did not follow this computational route, assuming that its accu- racy is of minor importance for the interpretation of the experimental results which themselves represent averages over many different cascades. Instead, we adopted a simpli- fied procedure which accounts only for the predominant elec- tronic processes of formation of Auger electrons, but ignores subshell effects and Coster-Kronig transitions. In this way, we obtain, per photoelectric interaction in Br, 0.35 KLL, 1.30 L,MM and 1.33 M shell Auger electrons of energy 9.96, 1.38 and 0.23 keV, respectively (fluorescent yields : 0.60 for the K shell and 0.016 for the L shell; we ignored the re-absorption of the fluorescent X-rays within the sample).This procedure gave a smaller number of electrons per interaction, but it accounted for their total energy, which is our main concern. Thus, we consider a spectrum of electrons of energy ranging from 0.23 keV (Br M shell Auger electrons) to ca. 17.5 keV (photoelectrons from C, N and 0).Since these elec- trons traverse distances from 11 nm to some 7000 nm, they do not escape from the sample (size: 2 mm x 14 mm), but dissipate their energy within it. (This illustrates another feature of low-energy X-rays, namely that electron equi-librium exists even in the absence of photon equilibrium.) From the calculations based on the above data the follow- ing picture emerges.The irradiation of 40 p1 of methanol for 1 min with 17.79 keV X-rays releases 1.39 x 10" photoelec-trons of energy 17.51 keV, 6.58 x 10" photoelectrons of energy 17.29 keV, 1.39 x 10'' Auger electrons of energy 0.277 keV and 6.58 x 10'' Auger electrons of energy 0.516 keV. Hence, 1.4 x 1013 keV of electron energy is deposited. For 1 rnol 1-' RBr-CH30H solution (40 pl, 1 min irradiation), the number of photoelectrons would be : 0.12 x 10l2 of energy 17.506 keV, 0.45 x 10l2 of energy 17.286 keV, 3.90 x 10l2 of energy 4.32 keV, 0.47 x 10l2 of energy 16.0 keV and 0.09 x 10l2 of energy 17.5 keV; the number of Auger electrons would be: 0.10 x 10l2 of energy 0.277 keV, 0.48 x 10l2 of energy 0.516 keV, 1.70 x 10l2 of energy 9.96 keV, 6.41 x 10l2 of energy 1.38 keV and 6.51 x loi2 of energy 0.23 keV.Hence, the electron energy deposited in the sample is 6.1 x 1013 keV (3.6 x 1013 keV from photoelectrons and 2.5 x IOl3 keV from Auger electrons). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 It is interesting to relate these energies with the number of radicals detected. From the data in Table 2 it follows that 5.3 x loi3 methoxy and hydroxymethyl radicals were detected in methanol. In 1 mol 1-' C,H,Br-CH,OH 2.9 x 1014 radicals were detected, of which 2.0 x 1014 were methoxy and hydroxymethyl radicals, the rest being phenyl radicals. Thus, in methanol we detected 0.38 radicals per 100 eV electron energy, and in 1 mol 1-' RBr-CH,OH 0.47 rad- icals (0.33 methoxy and hydroxymethyl plus 0.14 aryl radicals) per 100 eV electron energy.In reality, as follows from competition kinetics, 0.2 mol I-' PBN will trap only 60% of the phenyl radicals produced, the rest reacting with methanol (compare k, and k,, below), so that the corrected value for the overall phenyl radical yield is 0.23 per 100 eV. Note that these radical yields are lower than those observed when irradiating with ,OCo y-rays. This is consistent with the fact that the effectiveness of ionizing radiation of different qualities is determined largely by details of their track struc- ture.'O." In contrast with 6oCoy-rays when the spur separa- tion in the recoil electron track is so large (300 nm) that each spur can be considered as isolated, with low-energy electrons the track consists of a dense cylindrical core of reactive species.This favours their intra-track recombination before radicals have a chance to diffuse into the bulk of the solution and react with the scavenger. A similar effect of decreasing ion yield from liquid hydrocarbons with photon energy in the range 5-30 keV has been reported." Previously, the lower radical yields in water or methanol irradiated with tritium B-particles (e= 5.7 keV) as compared with ,OCo y-rays were ascribed to the different degree of separation or overlap of the spur^.'^,'^ Fate of the Multicharged Ion (Left-hand side of Scheme 1.) The positive charge acquired by bromine (on average 7 +) is redistributed quickly (lo-.15-s) between Br and the aryl moiety. The subsequent neutralization of the charge on bromine by catching electrons from the solvent molecules releases epithermal/thermal elec- trons by autoionization. Simultaneously, during the neutral- ization of the charge on the aryl moiety up to 100 eV of excitation energy is deposited in this moiety. This greatly exceeds the C-Br bond energy (ca. 293 kJ per mol of bonds), and even the atomization energy of the phenyl radical (5112 kJ mol-').15 One would expect such a highly excited moiety to fragment. Deutzmann and Stocklin16 reported that iodo- uracil labelled with a 12,1 Auger emitter efficiently breaks down following neutralization of the charge on the pyrim- idine ring.Destruction of the aromatic ring in iodobenzene and iodotyrosine was also observed."*'' There is no reason to expect that ring cleavage would be less significant in our systems. This may result in the formation of either non-radical products, which of course escape EPR detection, or non-aromatic radicals, the PBN adducts of which, if formed, are concealed in the spectra of CH30 and CH,OH adducts, merely increasing their detected yields. Although it is reasonable to expect that the Auger charge neutralization releases enough excitation energy to bring about the endothermic ring cleavage, the PBN adducts with aromatic radicals contribute appreciably (ca. 25%) to the EPR spectra (Table 3). This may indicate a stabilizing influ- ence of conjugated ring systems (degradation of excitation energy without scission) which prevents fragmentation.Another important event which should yield R' is the inter- action of solvated electrons with RBr (vide infra). Fate of Secondary Electrons; Spin-trapping Reactions Turning now to the right-hand side of Scheme 1 we introduce chemical phenomena into the discussion. Electrons released J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 in the Auger cascades, having been slowed down and sol- vated, cause radiolysis of methanol, eventually giving rise to H atoms, CH,O and CH,OH radicals and es- . These species are then available to react with the solvent and any solute present. The lifetime of a radical with respect to the scavenger reaction is z = (l/k)cs, where k is the rate constant and c, is the concentration of PBN.Since k for the reactions of the radicals in question with PBN are all between 2 x lo7 and 1 x 10' 1 mol-' s-', it is clear that the spin adducts were formed at 2lo-, s, i.e. long after the completion of the spur reactions (< s). The possible out-of-spur reactions in liquid methanol in the presence of PBN and RBr can be summarized as follows: es-+ CH,OH --* CH,O-+ H (1) k, = 8.5 x lo3 1 mol-' s-'; ref. 19 H + CH,OH + H, + CH,OH (2) k, = 2 x lo6 1 mol-' s-'; ref. 19 CH,O + CH,OH -+ CH,OH + CH,OH (3) k, = 2.6 x lo5 1 mol-'s-'; ref. 19 es-+ PBN + 0'-+ PhCH = NC(CH,), (4a) 0'-+ CH,OH -+ OH + CH,OH (4b) k,, = 1.4 x 10" 1 mol-' s-'; ref.20 k,, = 8 x lo8 1 mol-'s-'; ref. 21 H + PBN -+ P, (5) k, = 5 x 10, 1 mol-' s-'; ref. 3 CH,O + PBN -+ P, (6) k6 = 1.2 x lo8 I mol-' s-'; ref. 22 CH,OH + PBN -+ P, (7) k, = 4.3 x lo7 1 mol-' s-'; ref. 23 es-+ RBr --* R + Br-(8) R + CH,OH -+ CH,OH + RH (9) P1-P4 are spin adducts x 0'II PhCH-N-C( CH ,) , where X = H, CH,O, CH,OH or R (aryl radical). For PhBr k, = 8.7 x lo9 1 mol-' s-l (in pr~panol),~~ k, = 8 x lo5 1 mol-' s-',,' k,, = 6.9 x lo7 1 mol-'s-'. For other RBr the rate constants are not known, but they are likely to be close to the above values. Methanol solvates electrons with a solvation time of 11 ps at 293 K,26 so that an appreciable fraction of es- reacts rapidly in the spurs in the ps to ns range (inhomogenous kinetic stage).The yields of solvated electrons, G(es-), at 30 ps, 5 ns and 1 p after energy deposition by fast electrons are 3.4 & 0.3, 2.7 and 2.0, re~pectively.~~,~~The in-spur decay of es- is even faster under high-LET conditions, as in the present investigation. Thus, in reactions (l), (4) and (S), by es-is meant only the fraction of electrons that have escaped from the spurs and subsequently follow homogenous kinetics. In pure methanol solvated electrons produce H atoms and CH,OH radicals [reactions (1) and (2)]. The presence of PBN greatly suppresses these processes, since es- now react much faster with PBN with a rate constant 1.4 x 10'' 1 mol-' s-', ultimately giving CH,OH [reaction (4)]. Also other radical species react with PBN with rate constants 2-3 orders of magnitude greater than with methanol.Hereafter the emphasis is on PBN adducts with aryl rad- icals. As indicated above, these radicals may be initiated by two processes: by the neutralization of the primary multi- charged aryl moiety and by the interaction of es- with RBr. These two phenomena are interdependent, i.e. the number of multicharged ions and of es- alter concomitantly. Let us compare the total radical yields in 1 mol I-' LiBr-CH,OH and 1 mol 1-' PhBr-CH,OH (0.2 mol 1-' PBN) normalized to the same number of photoelectric events in Br, i.e. the same number of ejected secondary electrons, in which case the direct methanol radiolysis [the right-hand side of Scheme 1, reactions (1)-(3)] is independent of the Br target.These yields are 7.4 x 4.53/4.92 x lo6 mol 1-' = 6.8 x mol 1-' and 11.3 x mol 1-', respectively. The difference of 4.5 x lop6 mol 1-', an increase of 40%, represents the contribution of radicals other than those from direct meth- anol radiolysis and must be attributed to radicals from the aryl compound, i.e. resulting from the processes depicted on the left-hand side of Scheme 1 plus reaction (8). Since the contribution of the PBN-Ph adduct was found to be 27% (Table 3), the additional 13% is obviously due to CH,OH radicals from reaction (9). Similar calculations for the 2-bromopyridine system show an increase of 41.6%in the total radical yield, as compared with 25% yield of R, the addi- tional 16.6%being due to CH,OH from reaction (9).The presence of one or three methyl groups in the arene (bromotoluene or bromornesitylene) decreases the yield of the scavenged radicals (Table 2). Since the physical phenomena associated with neighbouring atomic centres in the molecule, such as the energy shifts in the Auger spectra, are far too small to influence the formation of intermediates or products, we assume that the effect is of a chemical nature. However, the alternative explanation that when orbital overlap is more pronounced, the intramolecular charge neutralization is greater should not be ruled out. The significantly higher radical yield from 2-bromopyridine than from 3-bromo-pyridine is indicative in this respect. Benzene Systems The lack of adducts with deuteriated phenyl radicals in RBr-C,D, systems provides important evidence that all detected radicals stem from the organobromine solute and not from the benzene solvent.This makes the data interpreta- tion more straightforward as compared with the methanol systems, as it is apparent that only those processes associated with the Auger charge (left-hand side of Scheme 1) are mani- fest, whereas the amount of in situ radiolysis is too small to be detected. This is obviously so because of the high radi- ation resistance to benzene. We now estimate the electron energy deposited in a sample of 1 moll -' C,H,Br-C,H, subjected to 17.79 keV X-rays for 1 min. The input data are as follows: sample volume 40 pl, sample thickness 0.192 g ern-,, photon flux 1 x loi3 photons, mass energy absorption coefficient 4.11 cm2 g- '.The calculated total absorption of X-rays is 54.6%,of which 95.2%is by bromine and 4.8%by carbon. Applying the same computational procedure as before, the estimated electron energy deposited in the sample is 6.14 x lo', keV. From the measured yield of phenyl radicals, 0.8 x mol 1-', it follows that in a 40 p1 sample 2 x lo1, Ph radicals were trapped. Thus, we detected 0.033 Ph radicals per 100 eV elec-tron energy. We assumed here that BNB trapped all the phenyl radicals while the reaction of phenyl radicals with benzene to form the phenylcyclohexadienyl radical was neg- lible. This assumption is justified, since the rate constant of the reaction C,H, + C,H, is 4.8x lo4 1 mol-I s-~,~'i.e.four orders of magnitude lower than the rate constant for the addition of phenyl radicals to a nitroso scavenger. It is interesting to note that the number of phenyl radicals per unit absorbed energy is a factor seven higher when the bromobenzene target is dissolved in methanol than when it is dissolved in benzene (0.23 :0.03),although an equal amount of electron energy is deposited in the target. This result can be understood in terms of intermolecular energy transfer. The inescapable explanation lies in the ability of benzene mol- ecules surrounding the highly excited bromophenyl moiety [Ph. .Br]* (vide supra) to take away the excess energy and to channel it into collective excitation of their own n-electrons.This deactivation mechanism may prevent the moiety from fragmentation. Note that, as commonly accepted, the radi- ation stability of aromatic compounds depends on the ability of the conjugated ring system to degrade excitation energy without concentration of energy in one bond which conse- quently breaks. In methanol systems the [Ph. * .Br]* moieties do not enjoy this 'protective' deactivation mechanism. A similar observation has been reported17 in a study of the fate of iodobenzene labelled with Auger-emitted '''1 in methanol, hexane or benzene. In the 3-bromopyridine-benzene system a slightly lower yield of the pyridine radical is observed than that of the phenyl radical in the PhBr-benzene system.This may be due to the fact that the replacement of one CH unit by an N atom in the ring increases the electron attraction in the position meta to Br. We do not elaborate upon this result which does not alter the general picture. Summary The following picture emerges from the present work. Low- energy photons used for irradiation of 1 mol 1-l solutions of organobromine compounds in methanol or benzene interact (by the photoelectric effect) predominantly with the com-ponent bromine, eventually forming highly excited species [Re. -Br]: surrounded by solvent molecules. It is important to note that the situation resembles that encountered in photochemistry (direct excitation of a solute), rather than in conventional radiation chemistry (the solvent is excited and energy is then transferred to the solute).The fate of these species depends on the environment. In a medium capable of rapid removal of a large amount of energy, such as n-electron solvents, e.g. benzene, a large frac- tion of highly excited states can relax instantaneously, pre- venting them from unimolecular decay and concomitant formation of aryl radicals. The situation is more complex in methanol systems. The deactivation of highly excited moieties via energy transfer is now inoperative, so that they tend to decay more freely. Moreover, secondary low-energy (high- LET) electrons interact efficiently with methanol (radiation- sensitive molecules, in contrast to radiation-resistant benzene molecules) and the in situ radiolysis becomes important.This follows the high-LET pattern, as testified by lower yields of radicals than those observed upon external irradia- tion with y-rays. The fate of a molecule undergoing the Auger effect, studied here in terms of the yield of radicals formed, was shown to depend on the molecular structure and, above all, on the nature of the medium. This is an aspect that has not been given adequate consideration previously. In particular, the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 radiobiological damage caused by Auger emitters has received wide attention in recent years. 198*30 On the theoreti- cal level, the energy imparted by Auger and Coster-Kronig electrons to a selected volume of a cell is estimated and com- pared with the radiation effect (strand breaks, cell survival) thereon.However, the theoretical methods in use do not account for the physicochemical properties of a specific region of a cell, such as the degree of polarization or of con- jugation (i.e. the presence of polar or non-polar molecules and of delocalized n-bonds). Our results leave little doubt that these are important factors which may modify the final effect. References 1 A. Halpern, in Handbook of Hot Atom Chemistry, ed. J. P. Adloff, P. P. Gaspar, M. Imamura, A. G. Maddock, T. Matsu- mura, H. Sano and K. Yoshihara. Kodansha, Tokyo, VCH, Weinheim, 1992, pp. 550-570. 2 M. J. Perkins, Adv. Phys. Org. Chem., 1980,17, 1. 3 V. E. Zubarev, The Spin-trapping Method (in Russian), Moscow University Press, Moscow, 1984.4 E. G. Janzen and D. L. Haire, Adv. Free Radical Chem., 1990, 253. J. H. Hubbel, Int. J. Radiat. Isotope, 1982,33, 1269. E. Storm and H. J. Israel, Nuclear Data Tables, 1970, A7, 565. U. M. Oeler and E. G. Janzen, Can. J. Chem., 1986,51,1138. H. Menke, W. Kohnlein, S. Joksch and A. Halpern, Int. J. Radiat. Biol., 1991,59, 85. 9 J. L. Hum and D. E. Charlton, in DNA Damage by Auger Emitters, ed. K. F. Baverstock and D. E. Charlton, Taylor & Francis, London, 1988. 10 A. Chatterjee and J. L. Magee, in Radiation Chemistry, ed. Far- thaziz and M. A. J. Rogers, VCH, Weinheim, 1987, pp. 173-199. 11 D. T. Goodhead and H. Nikjoo, Znt. J. Radiat. Biol., 1989, 55, 513. 12 R. A.Holroyd and T. K. Sham, J. Phys. Chem., 1985,89,2909. 13 J. Kroh, B. C. Green and J. W. T. Spinks, Can. J. Chem., 1962, 40,413. 14 A. Halpern, Chem. Phys. Lett., 1984,103, 523. 15 R. T. Sanderson, Polar Covalence, Academic Press, New York, 1983. 16 R. Deutzmann and G. Stocklin, Radiat. Res., 1981,87,10. 17 M. Reiche, A.Halpern and G. Stocklin, Radiochim. Acta, 1988, 43, 191. 18 M. S. Berridge, V. W. Jiang and M. J. Welch, Radiat. Res., 1980, 82,467. 19 D. W. Johnson and G. A. Salmon, J. Chem. SOC., Faraday Trans. 1, 1977, 73,256. 20 V. E. Zubarev, R. Mehnert and 0. Brede, Radiat. Phys. Chem., 1992,39, 281. 21 F. Dainton, I. Janovsky and G. A.Salmon, Proc. R. SOC. London, Ser. A, 1972,327,305. 22 V. E. Zubarev, V. N. Belevskij and L. T. Bugaenko, Moskow Univ. Chem. Bull., 1975,30,28. 23 C. L. Greenstock and R. H. Wiebe, Can. J. Chem., 1982, 60, 1560. 24 W. V. Sherman, J. Phys. Chem., 1966,70,2872. 25 K-D. Asmus and M. Bonifacic, in Landolt-Bornstein, New Series, Springer-Verlag, Berlin, 1984, vol. 13, part b, p. 26. 26 W. J. Chase and J. W. Hunt, J. Phys. Chem., 1975,79,2835. 27 T. Sumiyoshi, K. Tsugaru, T. Yamada and M. Katayoma, Bull. Chem. SOC. Jpn., 1985,58,3079. 28 J. Hunt, in Adv. Radiat. Chem., ed. M. Burton and J. L. Magee, J. Wiley, Chichester, 1976, vol. 5. 29 A. MacLachlan and R. L. McCarthy, J. Am. Chem. SOC., 1962, 84,2519. 30 Biophysical Aspects of Auger Processes, ed. R. H. Howell, V. R. Narra, K. S. R. Sastry and D. V. Rao, American Institute of Physics, Woodbury, NY, 1992. Paper 3/06290B; Received 21st October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000721

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 727-732 Enthalpies of Interaction between Dimethyldioctadecylammonium Bromide Vesicles in Aqueous Solution and either Dipicolinate or Sulfate Anions Michael J. Blandamer, Barbara Briggs, Michael D. Butt, Paul )rA. Cullis and Matthew Waters Department of Physical Chemistry, University of Leicester, Leicester, UK LEI 7RH Jan B. F. N. Engberts Department of Organic & Molecular Inorganic Chemistry, University of Groningen , Nijenborgh 4, 9747 AG Groningen, The Netherlands Dick Hoekstra Department of Physiological Chemistry, University of Groningen , Bloemsingel 10,9712 KZ Groningen, The Netherlands Injection of small aliquots of dipicolinate anions (sodium salt) into an aqueous solution containing dimethyl- dioctadecylammonium bromide (DOAB) vesicles is endothermic at 50 “C, becoming first more and then less endothermic.The injection process is effectively athermal for solutions containing more than equimolar amounts of DOAB and dipicolinate anions. A similar pattern is observed when small aliquots of sodium sulfate(aq) are injected into DOAB(aq). The overall patterns of enthalpy changes are attributed to the vesick dianion interaction which is exothermic and head-group dehydration with bromide ion displacement which is endothermic. Nevertheless, a complexity emerges if the solutions include a buffer which turns out to play a less than passive role. This conclusion is supported by differential scanning microcalorimetry for DOAB(aq) in the presence and absence of HEPES buffer.In aqueous solution, dimethyldioctadecylammonium bromide (DOAB) forms whose structures resemble closed bilayer aggregates formed from phospholipids. Fendler and co-workers’ estimated that the hydrodynamic radius of each vesicle is 80 nm. Cuccovia et aL6 estimated a radius of 65 nm. Vesicles can fuse to form larger vesicles. Fusion is an extremely important physiological process being a key step in, for example, cell fertilisation. Fusion is often initiated by adding a fusogenic agent. For e~ample,~ Ca’+ ions induce fusion between di-n-didodecylphosphate vesicles and phos- phatidylserine liposomes. Fendler and co-workers’ report that SO:-ions induce the fusion of DOAB vesicles. A mechanism was advanced’ in which fusion of two vesicles involves dehydration of head groups and the formation of a trans complex through an SO:-ion which interacts with the head groups on the surfaces of the vesicles.Following estab- lishment of this link, the bilayers coalesce in the area of contact, the two inner aqueous compartments of the vesicles join together and the ellipsoid-shaped assembly reforms to a larger vesicle. In view of the importance of the initial vesicle-SOi-interaction, we decided to study this process using a titration calorimeter with the aim of measuring the associated enthalpy changes. The expectation was that the titration calorimeter would record a series of exothermic pulses when SO:-(aq) aliquots were injected into DOAB(aq). In the event, the recorded trace showed a series of endo- thermic peaks for the first few injections followed by a series of exothermic peaks.However, when the data were corrected for the dilution of Na,SO,(aq) the trace showed a series of endothermic peaks, the intensity of the peaks decreasing with the injection of each new aliquot of SOz-(aq). The dianion of dipicolinic acid7 (DPA, pyridine-2,6-dicar- boxylic acid) was used as a fusogenic agent for didodecyl- dimethylammonium bromide (DDAB) vesicles. This fusogenic agent is more hydrophobic than SO:-anions. In terms of the mechanism proposed by Fendler’ for vesicle fusion, the DPA dianions have structures which would allow these ions to form a bridge between vesicles as the first step in fusion.Again the expectation was, therefore, that addition of DPA to DOAB(aq) would be exothermic. We report that this is not the case. However, these experiments revealed two further complications. The first was not unexpected in the light of our experience in using a differential scanning micro- calorimeter to record the gel-to-liquid transitions.’ There we found that our calorimetric methods could detect variations in vesicle properties associated with various methods of prep- aration. However, adopting standard protocols for vesicle preparation led to entirely reproducible results. The same experience emerged with respect to titration calorimetry involving vesicle solutions. The second complication emerged from the use of buffered solutions in experiments using DPA.Thus both DPA(aq) and DOAB(aq) were buffered at pH 6. The aim was to remove any complications in the recorded heat from dilution of a weak acid. Therefore, in order to facilitate direct comparison we used similar buffered solutions in the experiments using SO:-(aq) as the fusogenic agent. However, we then observed that the results for these experi- ments changed when the buffer was omitted. Nevertheless, addition of SO:-(aq) to DOAB(aq) remained surprisingly endothermic. We discuss the reasons why this pattern is observed rather than the intuitively anticipated exothermic trend. Experimental Materials Following experiments using a number of protocols for the preparation of solutions, satisfactory results were obtained when aqueous solutions containing DOAB vesicles were pre- pared by either of two methods.In the ‘hot-water’ solid DOAB was dissolved in hot water and held at 55°C for 30 min. In the ‘sonicated’ method, DOAB was dissolved in hot water and sonicated for 30 min. The solution was heated to 70°C and held at this temperature for 5 min. The solution was allowed to stand for 1 h before being used to fill the sample cell in the titration calorimeter. In two series of experiments the solutions were prepared using HEPES buffer (Sigma). Aqueous solutions prepared with HEPES buffer at pH 6 contain in associated and disso- ciated forms, H+(aq), Na+(aq) total = 5 x mol dm-3, CH3CO; (as) total = 5 x mol dm-3, and HEPES [N'-(2-hydroxyethyl)piperazine-N-ethanesulfonate, XSO; , total = 5 x mol dmW3].In other words, these constitu- ents are for the most part amphipathic. These concentrations exceed those of DOAB monomers and of the added anions in the titration experiments. Consequently, the vesicular systems are saturated with the HEPES-buffered systems. We return to this point below. Indeed, for reasons which will become clear below we identify the HEPES anion by the symbol XSO,. Calorimetry An Omega titration micr~calorimeter~ (MicroCal Inc., USA) recorded the heat, q, associated with the injection of a dilute solution of a fusogenic agent into DOAB(aq). The sample cell (reservoir) contained 1.4115 cm3 of the DOAB solution at 323 K. A syringe driven by a stepping motor introduced ali- quots of fusogenic solution at predetermined time intervals.In each experiment a sequence of 25 injections was recorded with a time delay of 3 min between each injection. This time delay was sufficient for the calorimeter to bring sample and reference (containing water) cells to the same temperature. If the injection of one aliquot of solution from the syringe was exothermic, the recorded trace showed the rate of heating of the reference cell (recorded in cal s-l) during the time inter- val required for sample and reference cells to attain the same temperature. If the injection was endothermic, the trace showed the rate of heating of the sample cell during the cor- responding time interval. In other words, the raw data com- prised a plot which has a number of extrema showing rates of heating following the injections as a function of time.The experiment was repeated except that the sample cell con- tained only water (or the buffer solution) but no DOAB. The pulsed sequence of injections was repeated and hence the trace recorded the contribution to the injection pattern described above which could be attributed to dilution of the fusogenic agent. The latter trace was subtracted from the first trace to yield the information required in the next stage of the analysis. Using the Omega software (MicroCal Inc.), the pulses were integrated to obtain a plot of heat, q, as a func- tion of injection number. N. A differential scanning microcalorimeter (MicroCal, Inc., USA) was used to record the dependence of relative isobaric heat capacities on temperature for aqueous solutions contain- ing DOAB(aq) in solutions prepared both in the presence and absence of buffer. In each case, the DOAB solutions were placed in the calorimeter and the temperature scanned from 15 to 90 "C.Each solution was cooled to 15 "C and held at that temperature for at least 3 h. A new scan was then recorded over the range 15-90 "C. Results The calorimetric investigation was based on a temperature of 323 K for two reasons. First, with respect to using SO:-(aq) as the fusogenic agent, we found that the results recorded for solutions at 298 K were irreproducible, a consequence of pre- cipitation in the reservoir following injection of SO:-(aq).This conclusion followed visual inspection of the recovered solutions. Secondly this temperature (323 K) is above the temperature' at which the hydrocarbon chains in the DOAB vesicles undergo a gel-liquid-crystal transition, 318 K. In fact, vesicle fusion requires that the hydrocarbon chains are in the liquid-crystalline form. A typical trace is shown in Fig. 1 for a J. CHEM. SOC. FARADAY TRANS., 1994, VOI,. 90 I I -0 50 tirne/rnin Fig. 1 Calorimetric titration of DPA(aq) (20 x mol dm--3) in 5.64 x dm3 aliquots into 1.4115 cm3 of DOAB (aq) (pH 6; 1 x mol dm-3) at 50°C calorimetric titration in which DPA(aq) was injected into DOAB(aq). The reservoir contained DOAB (1 x lov3 mol dm-3) in an aqueous solution at 50°C having pH 6 (recorded at 25 "C) and prepared using the sonication method.The recorded pulses change from endo- to exo-thermic after eight injections. Between each pulse a small length of 'baseline' was recorded showing that the processes responsible for each pulse are effectively complete within 1 min. In other words, no information concerning kinetics of physical/chemical processes was forthcoming. In considering traces of the form shown in Fig. 1, it is often informative to re-express the quantities in molar terms. So, for example, in this experiment there are 1.42 x mol of DOAB in the sample cell. Each aliquot of fusogenic agent adds 0.113 x rnol of this agent into the reservoir. After eight injections (cf. the crossover in Fig. l), 0.904 x mol have been added.DPA and DOAB are roughly equimolar around the 13th injection. At the 25th injection there are 2.72 x mol of DPA in the sample cell. The results illustrated typically by the trace in Fig. 1 were obtained following several preliminary experiments. If the concentration of solution in the syringe was too high, the reaction was over following the first injection. If this concen- tration was too low, the extrema were small, being lost in the noise. Comparison between different titration plots must take into account the compositions of the solutions involved. However, the recorded trace in Fig. 1 includes a contribution from the dilution of the DPA solution into the buffer solu- tion. As noted above, this latter contribution is obtained by repeating the experiment with no DOAB present in the reservoir.In this case, the pulses showed an exothermic dilu- tion. The recorded trace was subtracted from the trace in Fig. 1. The calculated pulses were integrated (OMEGA software, MicroCal) to yield the calorimetric titration curve shown in Fig. 2. At low injection numbers the enthalpy of reaction is endo- thermic becoming more and then less endothermic with increase in injection number. After 25 injections the amounts of DPA and of DOAB in the reservoir are 2.82 x mol and 1.412 x mol respectively, a ratio of ca. 2 :1. The titration curves approach the athermal conditions when slightly more DPA than DOAB is present in the reservoir. The actual molar ratio was difficult to identify precisely but seems in the region of 1.6 :1 DPA :DOAB.The reason for the uncertainty can be traced to the small enthalpies. For a solution more concentrated in DOAB prepared using the sonication method, the endothermic extremum was more J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1.E c I 3 n-z 1.0 W-m 0.6 3 i,d x 0 1 I I I I I 0 10 20 injection number Fig. 2 Calorimetric titration showing dependence of enthalpy of reaction expressed per mol of DPA as a function of injection number; system described in Fig. 1; plot corrected for dilution of DPA intense, Fig. 3. The endothermic maximum was ca. 2.5 kcal (mol DPA)-l. With decrease in concentration of DOAB, the intensity of the endothermic maximum decreased although the scatter on the plots increased, Fig.4. Here the first injec- tion was marginally exothermic. Using freshly prepared solu- tions the patterns shown in Fig. 1-4 were reproducible, although it was difficult to ascertain whether the first injec- tion in the system described in Fig. 4 was marginally endo- or exo-thermic. A similar pattern emerged when SO:-(as sodium salt) was injected into the buffered DOAB solution prepared using the sonication method. The titration plot for a typical experiment is given in Fig. 5 which records the endothermic injection of 5 x lov6dm3 aliquots of SO:-(aq) (22.6 x mol dm-3) into DOAB (1 x mol dm-3). Again it is useful to re- express these details in molar quantities.The sample cell con- tains 1.412 x moles DOAB. Each aliquot of injected solution contained 0.113 x mol SO:-ions. In Fig. 5, the change from endo- to exo-thermic occurs at the sixth injection when 0.678 x mol of SO:-have been injected. At the 25th injection the molar ratio SO:-: DOAB was 2 :1. The corresponding integrated plot, corrected for the dilution of SOi-(aq), shows that it is not possible to define precisely t 0 10 20 injection number Fig. 3 Calorimetric titration of DPA(aq) (20 x mol dmP3) in 10 x dm’ aliquots into 1.4115 cm3 of DOAB(aq) (pH 6; 2 x mol drn-j) at 50°C; plot corrected for dilution of DPA 729 0.3C c Iz o.20 c)-E W0.10 m +. $0 -0.10 inject ion number Fig.4 Calorimetric titration of DPA(aq) (9 x mol dm-3) into DOAB(aq) (pH 6; 0.4 x rnol dm-’) at 50°C;plot corrected for dilution of DPA(aq) the molar ratio at the point where the injection becomes athermal, Fig. 6. Nevertheless, the endothermicity is clear-cut, being more dramatic than in the case of the DPA injections. Consequently, the scatter on the integrated titration curves was less. An interesting indication of the reproducibility is given in Fig. 7. This plot shows the dependence of the enth- alpy of reaction as a function of injection number for three independent titrations using freshly prepared solutions. This plot yields an averaged quantity by noting that at the 25th injection the summed enthalpy of reaction is ca. 22 kcal (mol SO:-)-’.Hence, on average, each injection was endothermic to the extent of 0.8 kcal (mol SO:-)-’.The results summarised in Fig. 5-7 refer to titration calori- metric experiments in which both sulfate and DOAB solu- tions were buffered. We wanted to draw direct comparison with the experiments where DPA(aq) was injected into DOAB(aq), both solutions being buffered to pH 6. However, when the buffer was not used, the intensity of the endo- thermic extrema for SO:-titrations increased dramatically. In Fig. 8 we show the titration plot for a typical run, the DOAB(aq) being prepared using the sonication method. Again the switch from endothermic to exothermic injection is apparent but, as before, the exothermicity is a consequence of dilution of SO:-(aq).A clear indication of the effect of the 15 -10 v)-m 05 z \ P 0 -5 I I I 0 50 100 ti me/m i n Fig. 5 Titration of Na,SO,(aq) (22.6 x lo-’ mol dm-3) aliquots of 5.0 x dm-3 into a solution (1.4115 an3) of DOAB(aq) (1 x lo-’ mol dm-3) at 323 K; dependence of rate of heating follow- ing each injection as a function of time; buffered solution J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 01 LI I I I I I 0 10 20 injection number Fig. 6 Integrated titration curve for the titration of SO:-into DOAB(aq); for details see caption to Fig. 5 buffer is afforded by comparison of the integrated plots in Fig. 7 and 9, both runs reporting the enthalpy changes for solutions containing DOAB (1 x mol dm-3).In both cases the injections were continued until the molar ratio SO:-: DOAB was 2 :1. There is a slight difference between the titration calorimet- ric results for DOAB solutions prepared using sonication and hot-water methods. We attribute this difference to differences 7 20 n IClt z-0E v-m 5 10 i5w I I I I I 1 I 0 10 20 injection number Fig. 7 Summed integrated enthalpies of reaction for three indepen- dent titrations; for details see caption to Fig. 5 I I I 16.67 41.67 time/s Fig. 8 Titration of Na,SO,(aq) (22.6 x mol dm-j) aliquots of 10 x dm3 into DOAB (2 x mol dm-3); unbuffered solu- tions I I I I I I 0 10 20 injection number Fig. 9 Integrated titration curves for the titration of SO:-(aq) into DOAB(aq); for details see caption to Fig. 8.The results for dilution of SO:-(aq) have been removed ;unbuffered solutions. in size and distribution of the vesicles prepared by the two methods described in the Experimental. The results in Fig. 10 show that, on average, each injection is endothermic to the extent of 1.8 kcal (mol SO:-)-', roughly twice that observed in the presence of buffer. The A. I1 I I I I 1 0 10 20 injection number -I I t. -P A. I I I 1 I I 0 10 20 injection number Fig. 10 Summed integrated enthalpies of reaction for independent determinations using SO:-(aq) and DOAB(aq) in unbuffered solu- tions; DOAB(aq) prepared using (a) sonication and (b) hot-water methods J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I\ Inl 20 40 60 TrC Fig. 11 Dependence on temperature of the relative isobaric heat capacity for DOAB(aq), 6C,, monitored using a differential scanning microcalorimeter. The plots have been displaced for clarity on the dC, axis. The curves characterise solutions of DOAB prepared according to four different protocols: see Experimental; (a) hot-water method; (b) sonication method; (c) hot-water method using HEPES buffered solution; (d)sonication method using HEPES buffered solu- tions. significance of the presence of buffer to the properties of DOAB solutions is confirmed in Fig. 11 which records four typical scans obtained using the differential scanning micro- calorimeter .For DOAB(aq) prepared in the absence of buffer using both the hot-water and sonicated methods, two extrema are observed in the DSC scans near 36 and 45°C where [DOAB] = 2 x mol dm-3. When prepared in the pres- ence of buffer only one extremum is observed in these traces near 47°C. In other words, addition of HEPES buffer raises the gel-to-liquid crystal transition temperature ;i.e. stabilises the gel state of the vesicle. Discussion As mentioned in the Introduction, we anticipated that the interaction between vesicle and fusogenic agents would be exothermic. Consequently, several features of the injection plots in Fig. 1, 5 and 8 are of immediate interest. Following each injection, the endothermic pulses are sharply resolved, a short length of 'baseline' being recorded between each pulse.This pattern signals that within a few tens of seconds the solution has recovered an equilibrium state. We cannot comment more precisely on the rate because the time depen- dence is determined by the calorimeter response time. Never- theless, we can be confident that there is no slow (chemical) reaction or process involving either DPA or SO:-ions and DOAB vesicles. The endothermicity decreased with each injection of an aliquot of Na2S04(aq). In one possible model for the process, added SO:-ions displace bromide ions from the double layer. If this was the sole explanation, one might expect a series of pulses of similar magnitude for each aliquot until all the bromide ions had been displaced.The resultant pattern would be a plot with a well defined step from constant endo- thermicity to athermal conditions. This pattern is not observed but, continuing with the essential features of this model, bound SO:-ions clearly modify the energetics of dis- placement of Br- ions by the next aliquot of SO:-ions. Returning to the possible step shape discussed above, the expectation was that, on the basis of charge numbers, each SO:-ion would displace two bromide ions. In these terms it is surprising that the injection becomes athermal at near equimolar amounts of SO:-and Br-. We also note that two processes associated with the head groups on the outside and on the inside of vesicles are not detected. Taken in conjunc- tion with comments concerning the baseline between pulses, we conclude that DOAB vesicles above the transition tem- perature are very leaky allowing SO:-and Br- to pass freely across the vesicle bilayer.The observed pattern can be accounted for in terms of an equilibrium involving SO:-ions which are either bound to vesicles or 'free' in the aqueous solution. The binding reac- tion might therefore involve displacement of bound Br -ions from the vesicles. This process would be characterised by a binding constant and an enthalpy of binding. Moreover, the dependence of 4 on amount of added SO:-(aq) would form a sigmoidal dependence centred around 4 for a solution where the ratio [Na2S04] : [DOAB] is unity. The fact that this pattern is not observed rules out an equilibrium-based model. Rather, the process involves direct replacement of Br- by SO:-, Scheme 1. liquidchains I I I Scheme 1 However, as shown in Scheme 1, the process would be exo- thermic.The dominant endothermic process stems from an accompanying dehydration of the cationic head groups which is also known to be a precursor to fusion." Thus injection becomes athermal when all sites on the vesicle surface are occupied by SO:-ions. Interaction between N'Me, head groups and SO:-ions is exothermic but the endothermicity is the consequence of displacing tightly bound water into the bulk solution and replacement by bound SO:-ions. When small amounts of SO:-ions are added, the boundary layer comprises mainly bromide ions with relatively small numbers of SO:-ions. In other words, the mean separation of SO:-on the surface of the vesicles is large.Nevertheless, the SO:--Br -repulsion means that displacement of further bromide ions and water by added SO:-(aq) is less endo- thermic. This process continues until all bromide ions have been displaced. In other words, there appear to be two processes, one endothermic and one exothermic, which depend on the extent to which bromide ions have been displaced. Each contrib- uting process can be described by the following empirical equation, where i = 1 denotes an exothermic process and i = 2 an endothermic one. AHi = AH: exp(-mi n/N)[N -n)/N] (1) Here AH: represents the enthalpy change for the displace- ment of the first bromide ion by SO:-where AH(obs) = AHl + AH2.Here N represents the number of sites on the surface of the bilayer and n is the number of sites occupied by added ions (cf:injection number). The exponential term through the parameters a1 and a2 produces a characteristic shape to the recorded plot whereas the term linear in (N -n) assures that the plot passes through zero at n = N.We show in Fig. 12 a typical plot which emerges from eqn. (1) for a set of selected parameters. In a process of trial and error we attempted to reproduce as closely as possible the pattern of the recorded J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 closely related to the added fusogenic ions, have already par- tially dehydrated the head groups.I I I I 0 100 200 injection number Fig. 12 Dependence of enthalpy change as a function of injection number, AH, as predicted using eqn. (1) for (a) endothermic process with AHo = -1.5 kcal mol-' and a = 0.4, (b) exothermic process with AHo = 2.0 kcal mol-' and o! = 0.6, and (c) resultant enthalpy change titration plots. Therefore, we reached the conclusion that the endothermic peaks (cf: Fig. 5) in the titration calorimetric experiments following addition of SOt-(aq) to DOAB(aq) are not linked directly to fusogenic processes. In all probability, the fusion process continues in the background to the changes recorded here. It is interesting to note that a reversal in sign of enthalpies of interaction was observed by Jones et al. in their study of interactions between alkylsulfates and the macromolecules, ribonuclease.' 'J' When DOAB(aq) is prepared in the presence of buffer, the gel state is, according to the DSC experiments, stabilised.The presence of a double extremum in the absence of buffer is attributed to melting of patches of vesicles in close proximity (intervesicular) and to the melting of patches unaffected by intervesicular interactions. When buffer is added, the amphi- pathic anions (e.g.XSO, and, marginally, CH,CO,) displace bromide ions from near the vesicle surface. These bound XSO, ions stabilise and insulate the vesicles by weakening intervesicular interactions. Consequently, a single extremum at higher temperatures is recorded in the DSC, Fig. 11. In the titration experiment added SO;-ions displace the bound XSO, ions, Scheme 2.Hence the overall process is less endo- thermic. The associated dehydration of the polar N'Me, head groups is not as dramatic because XSO, ions, being 1 Scheme 2 Overall, the two sets of titration results differ because SOt-(aq) ions are added to DOAB vesicles having quite dif- ferent compositions in their associated double layers. The titration plots describing injection of DPA(aq) into DOAB(aq) are more complicated than observed when the fusogenic agent SO:-was used. However, the production of an endothermic maximum points again to two contributions, one exothermic and one endothermic, eqn. (1) and Fig. 12. However, as the experiments show with buffered solutions using SOi-(aq) injections, the buffer is not passive.We thank the University of Leicester for a travel grant to M.J.B. We thank SERC for a grant to M.D.B. This research was supported under the Molecular Recognition Initiative at the University of Leicester. References 1 J. H. Fendler, Acc. Chem. Res., 1980,13, 7. 2 J. H. Fendler, Chem. Rev., 1987,87,877. 3 T. Kunitake, Angew. Chem., Int. Ed., 1992,31,709. 4 A. M. Carmona-Ribeiro, Chem. SOC.Reu., 1992,21,209. 5 D. Yogev, B. C. R. Guillaume and J. H. Fendler, Langmuir, 1991, 7,623. 6 I. M. Cuccovia, E. Feitosa, H. Chaimovich, L. Sepulveda and W. Reed, J. Phys. Chem., 1990,94,3722. 7 T. A. A. Fonteyn, D. Hoekstra and J. B. F. N. Engberts, J. Am. Chem. SOC.,1990,112,8870. 8 M. J. Blandamer, B. Briggs, P. M. Cullis, J. A. Green, M. Waters, G. Soldi, J. B. F. N. Engberts and D. Hoekstra, J. Chem. SOC.,Faraday Trans., 1992, in the press. 9 T. Wiseman, S. Williston, J. F. Brandts and L-N. Lin, Anal Biochem., 1989,179, 131. 10 L. A. M. Rupert, J. F. L. Breemen, D. Hoekstra and J. B. F. N. Engberts, J. Phys. Chem., 1988,92,4416. 11 M. N. Jones, Chem. SOC.Rev., 1992,21,85. 12 M. I. Paz Andrade, E. Boitard, M. A. Saghal, P. Manley, M. N. Jones and H.A. Skinner, J. Chem. SOC., Faruday Trans. I, 1981, 77,2939. Paper 3/0555OG; Received 15th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000727

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 733-738 Enthalpies of Mixing a Non-ionic Surfactant with Water at 303.15 K studied by Calorimetry Kristian Weckstrom, Kirsi Hann and darl B. Rosenholm Department of Physical Chemistry, Abo Akademi University, Porthansgatan 3-5,20500Turku, Finland The enthalpy of mixing n-octyl penta(oxyethy1ene glycol) (C,E,) with water has been studied at 303.15 K by titration and mixing calorimetry. In the region of the critical micelle concentration (c.m.c.), when liquid surfactant is added to water, a sudden endothermic shift is observed in the differential enthalpy of mixing of C&. The variation of the differential enthalpy with composition in this region gives information on the average size of the non-ionic micelles. Using the mass-action law model, an aggregation number of 41 was obtained for the micel- les.The value is lower than those obtained from recent spectroscopic studies. The thermodynamic quantities of micelle formation of C,E,, and of C&, in water have been obtained. For binary mixtures the enthalpy of mixing was exothermic for all compositions. The apparent and partial molar enthalpies of mixing of the surfactant and the partial molar enthalpies of the water have been calculated. Beyond the c.m.c. the partial molar enthalpies of the two components vary regularly with the composition. A highly variable temperaturexomposition (T, cCE)phase behaviour is shown by the binary systems of n-alkyl oxyethylene oligomeric amphiphiles of the type CH,(CH,), -(OCH,CH,),OH (abbreviated to C,E,), and water.' In the two-component (T, cCE)phase diagrams there are isotropic micellar or liquid phases, two-phase areas and several meso- phase areas.When the hydrocarbon part of the amphiphile is large (n > 8), the liquid-crystalline phases usually occupy extended areas in the phase diagram. However, C,E, amphi-philes with n < 9 (m= 1-6) often have a liquid phase for a wide composition range. At certain temperatures the two components are mutually miscible. The extent to which such C,E, species induce enhanced mixing of oil and water to a thermodynamically stable multicomponent microemulsion has been studied,'., e.g. the influence of temperature, the properties of the oil and the presence of electrolyte on the phase behaviour.The tendency of several types of amphiphiles to aggregate in water depends on the size of the hydrocarbon art.^.^ When n 2 5 micelles begin to form in the solution at a c.m.c., pro- vided the conditions promote the formation of stable struc- tures.'g6 According to extensive spectroscopic studies, the aggregation number of C,E, micelles in water is usually between 20 and 300.7-9 The value depends on the surfactant structure, the amount of surfactant, the temperature differ- ence from the lower consolution boundary (lcb) and, e.g., the presence of electrolyte in the water. In the micelle, the hydro- carbon parts of the solute molecules collectively form the non-polar region, i.e. the core.8 Both the solvent and the oxy- ethylene parts of the surfactants are absent from the micelle core, as indicated by results from solubilization studies.'O.' ' The oxyethylene part interacts mainly through hydrogen bonds with the water."-'4 A recent study by Alami et al., showed that the division of the solution into non-polar regions and surrounding polar continuum exists up to ca. 50 wt.% of surfactant (C,E,, ClOE6 of ClOE,) in water. From thermodynamic measurements information can be obtained on the state of the surfactant and water in binary mixtures, and on the interaction between the two species, e.g. studied the C,E4-water system at 283.15-313.15 K, and they presented the partial molar enthalpies of the two com- ponents over a rather narrow composition range.For the C,E,-water system there is a large amount of spectroscopic data on the ~ize,',~.'~ andstru~ture~.~ the mutual interacti~n'~,~~ of the micelles as a function of tem-perature and composition. The (T,cCE)phase behaviour is well kno~n,'~.'~,~~ including the curve for the c.m.c.,Ig and the effect of several third components on the binary system has been studied.I4 There is a large micellar phase in the binary-phase diagram; two critical points {[T,,c,(CE)] and [T,, c,(CE)]} determine the temperature range suitable for calorimetry: the value of the lcb is 332.5 K,14 and T, of the hexagonal phase is ca. 280 K.22 The extension of the phases shown by the C,E,-water system provides an opportunity to study the interactions leading to micelle for- mation, due to the high c.m.c., and to the complete mutual miscibility.For the long-chain surfactant systems the exten- sion of the liquid-crystalline phases up to high temperatures (ca. 310-350 K) prevents the study of the latter.' The polar interactions determine the enthalpy of mixing and also the stability of the micellar solution, and thus the area of the micellar phase in the (T, cCE)diagram. By using suitable models, valuable information is obtained on the micellar structure as well as on the equilibrium conditions, at the c.~.c.~.'~Results from the above binary system can aid the understanding of the more complex systems containing an amphiphile of the type C,E, .293,14 We have investigated the excess enthalpic properties of the C,E,-water system at 303.15 K by calorimetric methods.We have also studied the variation of the surface tension of the solution with composition in the c.m.c. region. The titration- calorimetry method was used in the c.m.c. region, which gives the excess differential enthalpy of mixing. When analysing the enthalpy data, the micelles were considered to be mono-disperse and with a defined aggregation number. The mass- action law was used to describe micelle formation. The fitting of the model to the calorimetric data by least- squares gave the thermodynamic quantities of micelle forma- tion, the c.m.c. and the aggregation number. For thethe various excess properties of mixing can be determi~~ed.~,'~ There are only a few reported studies on C,E,-water systems.composition range beyond the c.m.c., mixing calorimetry Clunie et all6 and Shinoda17 have studied the Cl,E6-water gave the excess heat of mixing. From the data, the apparent system at 298.15 K. The integral heat of mixing and the enthalpies of C8E, in the mixtures were obtained. Using well partial molal enthalpies of the two components over a broad known relationships, we also obtained the partial molar enth- composition range were reported. Andersson and Olofsson l8 alpies of C,E, and water in the mixtures. The excess enthal- pic properties vary smoothly over the whole composition range, indicating regular changes in the intercomponent interactions. Experimental The surfactant C,E5 was used as received from Bachem Fein- chemikalien AG (CH-4416 Bubendorf, Switzerland), which has high monodispersity with respect to both the alkyl chain length and the oxyethylene chain length, comparatively high purity (98.5 wt.%) and chemical ~tabi1ity.l~ The molecular weight of C8E5is 350.5 g mol-'.At 298.15 K the density of the pure liquid surfactant is 1.0081 g cmP3.l9 Other physical data on C,E5 can be found in ref. 19. The water used in the various experiments was freshly distilled; conductivity <1.2 x iz-' cm-' and surface tension =71.0 mN m-' (303.15 K). In the composition range of C8E5in water corresponding to the c.m.c., a calorimeter similar to the microcalorimeter TAM 2277-205 (Thermometric AB, Jarfalla, Sweden) with a 25 cm3 stainless-steel reaction vessel was used.23 The liquid surfactant was introduced to the sample cell from a gas-tight syringe (Hamilton Bonaduz AG, CH-7402, Bonaduz) through a thin stainless-steel capillary tube with an inner diameter of 0.15 mm.The end of the tube was positioned below the surface of the liquid in the cell. The solution was stirred using a constant stirring speed of 200 rpm with a turbine stirrer. Injection of the pure surfactant was accomplished by means of a microprocessor-controlled motor-driven syringe. The injection rate was 6 mm3 min-'. An experimental series con- sisted of the consecutive addition of small aliquots (10.12 mg) of liquid surfactant to the calorimeter vessel, which initially contained pure water (20.00 g).An LKB 10700 batch microcalorimeter was used for com- positions of C,E5 beyond the c.m.c. Inside the calorimeter in a large carefully isolated rotating drum there were two gold cells, a reaction cell and a reference cell, which were necessary to account for the heats of friction at the liquid-liquid and liquid-cell wall, which occurred during the experiments. In each cell there were two liquid compartments, with maximum filling volumes of 2.5 and 4.5 cm3, which were partially divided by a thin wall. Mixing of the contents in the cells occurred during the rotation of the drum. Samples to be mixed were directly transferred by syringes into the cells, and a corresponding amount of liquid was transferred into the reference cell.In order to obtain thermal homogeneity, the system was allowed to equilibrate for 15 min, after which the calorimetric experiment was started by rotating the drum around its rotation axis; 3-4 cycles of +400" were needed to ensure complete mixing. The signal due to the heat effect in the system was amplified, registered and mechanically inte- grated. The system was calibrated by introducing precise amounts of energy into the sample cell, through a resistance located inside the cell. The heat evolved in an experiment was obtained using established procedures. The surface tension of solutions of C,E, in water (303.15 K) was determined as the maximum pull of a platinum-iridium ring through the surface, avoiding however, complete detachment of the ring from the surface, using a ring-tensiometer balance constructed in our department, but similar to the Sigma 70 (KSV Instruments OY, Helsinki, Finland).Before starting measurements, the tensiometer was calibrated with small weights, A small amount of C,E5 was introduced to the precisely known amount (ca. 40 g) of water with a syringe. The solution was allowed to equilibrate with stirring for 15 min. The determination of the force was then carried out. At the lowest solute molalities (mCE< 3 x mol kg -'), the surface tension exhibited clear equilibration- time dependence, exceeding 15 min. In addition, the time was J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 fixed for the start of the series, and was noted at each force determination. At intervals of 2-3 molalities of C&5 the whole cell compartment containing the aqueous solution was weighed.Precise molalities of C8E5 in water could be calcu- lated, since the variation of the solution weight was known as a function of the time. The surface tension was calculated from the observed maximum force, using established pro- cedures. The c.m.c. of C8E5 in water was obtained as the minimum of the curve representing the surface tension.6 Results There is a smooth decrease in surface tension with concentra- tion of C&5 in the solution. The minimum of the curve of surface tension us. ln(mola1ity) of surfactant occurs at mCE= 7.3 x mol kg-' at 303.15 K (Fig. 1) which represents the c.m.c. (see ref. 6). At compositions above the c.m.c.the tension is nearly constant after a small increase. The shallow minimum in the curve probably indicates the presence of surface-active impurities at low (ca. 1 wt.%) amounts in the liquid surfactant.6 The impurities are solubilized in the micel- les as they form. Whether the non-linear variation of the tension below the c.m.c. is due to the presence of the impu- rities in the solution, or to a property of the system remains unclarified. The surface tension results will be examined further in another p~blication.'~ The excess differential enthalpy of mixing of C8E5,HEEE, was determined by introducing small amounts of the liquid surfactant into the aqueous solution. Owing to the relatively high c.m.c. of C,E5 in water, solutions can be obtained with a final molality of monomers clearly below the c.m.c.Eqn. (1) describes the mixing of a known amount of neat surfactant C,E, (CE) with pure water (w) to form an aqueous surfactant solution (aq. I): %E(l) + nw(l) nCE(aq*I) + nw(aq. I) (1) If the enthalpies of the pure components, HI (i = CE or w), at 303.15 K and atmospheric pressure are chosen as references, then: AH(1)= Hg(I)n(I) (2) where AH(1) is the heat of reaction, HE@) represents the molar enthalpy of mixing of the solution (I), and n(1) [=nCE(I)+ nw(I)] is the total amount of substance in solution (I). The contribution of the initial state is equal to zero since HM(i)= +E(l)(H& -&E) + nw(l)(Hw-Kw)= 0. 60 c 50, z E-..A 40 -30 --10 -8 -6 -4 In(rn ,/rnol kg-') Fig.1 Dependence of the surface tension on the natural logarithm of the molality of C,E, in water at 303.15 K J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 When an exceedingly small amount of neat surfactant [n&(1)] is added to the aqueous solution (cf: the experimental section) containing only monomers eqn. (3) and (4) are obtained : &,(I) + ncE(aq. I) + n,(aq. I) +ncE(aq.11) + n,(aq. 11) (3) AH(I1) = HE(II)n(II) -HzI)n(I) (4) The contribution from the added surfactant is equal to zero according to the definition of standard states. Since the amount of water is kept constant in eqn. (3) and the amount of surfactant is very small, the conditions for differential enth- alpy apply (n, is constant): HFE(II) = (dH'/ancE) = AH^/^;^) (5) In eqn.(5) HEE(II)is the partial molar enthalpy of the sur- factant and AHM= AH(I1).The observed differential enthalpy is thus equal to the difference between the partial molar enth- alpy of the monomer in the aqueous solution (11)and that of the monomer in the pure state, i.e. the partial molar enthalpy of mixing in the monomeric state: HzE(II) = HzE(mon). At compositions well above the c.m.c. the added surfactant essentially all dissolves to form micelles. The same formal dis- cussion applies then as for the pre-micellar solutions; the partial molar enthalpy of mixing surfactant with water to form micelles, HvE(mic), is obtained. Provided that both HFE(mon) and HpE(mic) are roughly independent of the C,E, molality (cf:Fig.2), the partial molar enthalpy of micelle for- mation is given by the difference between the values observed above and below the c.m.c.:23 AHcE(mic)= HFE(mic)-HEE(mon) (6) There is narrow region of mCEwhere a rapid increase of HEE is observed. In this transition region a fraction, aCE, of the added surfactant will dissolve in the water as monomers, while the fraction (1 -aCE) will dissolve to form micelles. The formal process in eqn. (3) is now described by [cf: eqn. (5)]: H&(II)= aC-HFE(mon)+ (1 -acJH&(mic) (7) The observed differential enthalpy is considered to be the sum of the contributions due to the monomer and micellar states, respectively. A series of four calorimetric titrations were conducted in the c.m.c.region of C8E,in water, each consisting of 18 injec--20 I I 1 1 I I I I 0 100 200 300 mcE/l0-3 1 0 10 20 30 mcE/l0-3 mol kg-' Fig. 2 Excess differential enthalpies of solution of C,E, in water as a function of the molality at 303.15 K. There are four separate titra- tion series: series 1, triangles; series 2, squares; (with the curve showing the calculated values); series 3, dots; series 4, crosses. The inset shows the differential enthalpies (series 2) and the partial enth- alpies (in kJ mol-') of C,E, at compositions near to the c.m.c. 735 tions of the neat surfactant into the aqueous solution. In the experiments, the pressure (1 atm) and temperature (303.15 K) were held constant.Of the total number of injections 17 were within the pre-micellar region. A linear least-squares fit of the observed HzE values US. mcE (mcE < 8 x low3mol kg-') yields H& = -(41.56 -112.2 mCE) kJ mol-'. The deviation of the individual enthalpy values from the least-squares line is substantial (f0.5 kJ mol- '). Using titration calorimetry, HFE increases slightly with mcE (below the c.m.c.), and the slope of the least-squares line is 112.2 kJ mo1-'. The change in the differential enthalpy as a function of mCEfor the four series is shown in Fig. 2. A large increase in the observed enthalpy begins at a well defined solute molality (8.2 x mol kg-'). Beyond the region of the increase the differential enth- alpy is almost constant. The dependence of the differential enthalpy on the solution composition in the c.m.c.region can be described by the mass-action law However, certain simplifications must be introduced: First, the micelles which appear at the c.m.c. in the solution are assumed to have a fixed aggregation number N. Secondly, the activity coefficients of both the monomer (ys) and the micelle (yM) are fixed at unity. There- fore, the solution is described as being in an equilibrium state between monomers and micelles. The equilibrium constant, K,, depends on the amounts of the two species and on N, and is given by the following relationships : NCE = (CE)N (8) KN = mCE(mic)/CmCE(mon)lN (9) where wE(i) represents the molality of the monomers (i = mon) and micelles (i = mic), respectively.Within the region of mcE where the surfactant added is distributed between the micellar and monomeric state, the equilibrium constant can be expressed in terms of the fraction a of the monomer species : K, = (1 -a)/~aNm& ' (10) In eqn. (10) mCE is the total molality of the surfactant in the solution. For each titration the value of a at the start and at the end is required. Eqn. (10) is solved by varying N, using the Newton-Raphson numerical method. At the chosen value of N, both K, and AHcE(mic) are optimized using a search routine. Thus in eqn. (11) is obtained, and a comparison with the experimental differential enthalpies is carried out. Allowing the partial molar enthalpy of the monomers at the c.m.c.to be constant [HpE(mon, c.m.c.) = -40.73 kJ mol- '3, the fraction @CE is related to the differential enthalpy by com- bining eqn. (6) and (7): H&(II) = (1 -a,--)AH&nic) + HFE(mon, c.m.c.) (11) where HFE(II)and HFE(mon, c.m.c.) are experimental quan- tities. The fit is given in Fig. 2 as the line through the experi- mental points (series 2). The four different series were fitted separately. The results for N and AHcE(mic) for the series 1-4 are: N = 35 (1); 50 (2); 45 (3); 35 (4) and AHcE(mic) = 16.10 (1); 15.75 (2); 15.75 (3); 16.19 kJ mol-' (4).The average value of AH,,(mic) is 15.9 kJ mol-' and the average aggregation number of the micelles is 41. The ~.m.c.~~ is 8.2 x mol kg-'. For the best fits the standard deviation approached the estimated uncertainty cf the calorimetric measurement (0.07-0.1 kJ mol-').The corresponding Gibbs free energy for the transfer of the surfactant from the monomeric state to the micellar state may then be obtained as: AGCE(miC)= -RTN-' In K, (12) Eqn. (12) represents the Gibbs energy relationship of the mass-action law approach.25 If it is assumed (mCE> c.m.c.) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 C.m.c. values, dissolution enthalpies, enthalpies, free energies and entropies of micelle formation for the surfactants C,E, and C,E, (303.15K) c.m.c. HFE(mon)l H&(mic) AHcE(mic) AGcE(miC)" AGc,(mic)b AS (mi$ AS (mic)b /mol kg-' /kJ mol-/kJ mol-' /kJ mol-' /kJ mol-' /kJ mol-' /J Ks5 mol-' /J Ks5 mol-' C,E, 6.52 x 10-3 -3 1.40 -16.58 14.82 C,E, 8.20 x 10-3 -40.73 -24.83 15.90 a Calculated from eqn.(13). Calculated from eqn. (12). that all monomers are consumed in micelle formation, AGcE(miC) at the c.m.c. (a = 0) is obtained from eqn. (12):26 AGcE(miC)= RT ln(c.m.c.) (13) Eqn. (13) thus corresponds to the pseudo-phase model. The entropy of micelle formation, AScE(mic), is readily obtained through the Gibbs-Helmholz relation and values of these are included in Table 1. The molar enthalpy of mixing C& and water, HE [ct eqn. (l)], was determined over the entire composition range. The calculated enthalpies are listed in Table 2 and shown in Fig. 3. Since CsE, is completely miscible with water at 303.15 K, the mole fraction scale is used.The observed excess solution properties may then be represented as the apparent molar enthalpy of mixing defined as:27 where xCE is the mole fraction of surfactant in the mixture (I). Alternatively, the contribution of C8E5 and water may be represented by the partial molar enthalpies of mixing. These can be obtained in the following way:27 H:E = Ht + (1 -XcEXdH3dXcE) (15) H! = H,M -x,,(dH3dx,,) = (HE -xCE H&)/( 1 -xCE) (16) In eqn. (15) and (16) xCEis the mole-fraction of C,E, in the binary mixture. First, the variation of the enthalpy of mixing with composition (Fig. 3) is described by suitable functions. If Table 2 Excess molar enthalpies of mixing C,E, and water, partial enthalpies and apparent enthalpies of C,E, (303.15 K) 0.002 16 -0.0475 -26.32 0.0094 -21.99 0.00221 -0.0489 -26.30 0.0092 -22.13 0.00229 -0.0529 -26.28 0.0073 -23.10 0.004 19 -0.0943 -25.71 0.014 -22.50 0.00427 -0.0982 -25.69 0.011 -23.01 0.00450 -0.104 -25.62 0.011 -23.20 0.00476 -0.111 -25.54 0.011 -23.34 0.00721 -0.161 -24.82 0.019 -22.27 0.00795 -0.172 -24.60 0.024 -21.64 0.0147 -0.319 -22.72 0.015 -21.72 0.0151 -0.287 -22.57 0.055 -18.99 0.0248 -0.542 -20.14 -0.044 -21.86 0.0485 -0.977 -14.98 -0.26 -20.15 0.0926 -1.494 -8.25 -0.80 -16.13 0.155 -1.679 -3.29 -1.38 -10.83 0.209 -1.754 -1.84 -1.80 -8.39 0.279 -1.753 -1.22 -1.95 -6.28 0.332 -1.690 -0.81 -2.12 -5.09 0.408 -1.572 -0.61 -2.23 -3.85 0.484 -1.399 -0.37 -2.36 -2.89 0.624 -1.058 -0.096 -2.65 -1.69 0.664 -0.998 -0.097 -2.78 -1.50 0.775 -0.706 -0.050 -2.97 -0.91 0.840 -0.500 -0.027 -2.98 -0.59 0.903 -0.313 -0.030 -2.95 -0.35 -12.68 90.7 -12.11 -11.25 92.4 89.6 the entire composition range is represented by two partially overlapping ranges, a standard fourth-order polynomial func- tion can be used.The derivatives in eqn. (15) and (16) were calculated by least-squares fitting the polynomial functions to two sets of enthalpies of mixing, covering the following com- positions: 0.002 < xCE< 0.33 and 0.16 < xCE< 0.91, respec-tively. The curve in Fig. 3 shows the enthalpy obtained from the two polynomial functions. The results for H& and HE are given in Table 2. They are shown graphically in Fig. 4(a)and (b).HE is shown to be positive at values of xCE close to zero in the inset of Fig.qb). The values of partial molar enthalpy of C8E, determined by the two calorimetric methods are similar close to the c.m.c. This is shown in the inset of Fig. 2. From the extrapolation of HE to xCE = 1, we estimate that HE = -2.9 kJ mol-I. Discussion By titration calorimetry enthalpic information can be obtained on both the monomeric and the micellar states of the non-ionic amphiphile in water. At least for C,E, sur-factants with n = 8-12, it is possible to carry out precise experiments below the c.m.c. Olofsson23~28*29 has reported the differential enthalpies in the c.m.c. region at 298.15 K for the surfactants C,E, and CI2Em, rn = 5, 6 or 8. In the mono- meric state the hydrophilic and hydrophobic parts of the molecule are expected to be in full contact with the solvent.The nearly linear variation of ln(c.m.c.) with the length of the hydrocarbon chain indicates an extensive incorporation of the non-polar part in the water structure.6 The oxyethylene part interacts very exothermically with water, whereas it appears that the interaction of the hydrocarbon part with water is endothermic. The observed differential enthalpy of the monomer, H&(mon), is thus a function of the lengths of 0.0 -0.4 1 -0.8 ? 75 -1.2 z -1.6 I#I,I.I.-2.0 0.0 0.2 0.4 0.6 0.8 1 .Q XCE Fig. 3 Excess molar enthalpies of mixing of C,E, and water as a function of the mole fraction of surfactant at 303.15 K. The curve was obtained by fitting two polynomial functions to the experimental values.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0 --10 7Y 3 a (a) l,l,I,j,-30 0.0 0.2 0.4 0.6 0.8 1.0 XCE I”’”’’’’I 0 -1 r I 3, -2 5% \ t -4’ ” ’ ’ ’1 ‘ 1 ’ I 0.0 0.2 0.4 0.6 0.8 1.0 XCE Fig. 4 Partial molar enthalpies of the components as a function of the mole fraction of surfactant in the mixtures: (a) C,E, (apparent molar enthalpies represented by open circles), (b)water. The inset in (b)shows the partial molar enthalpies of water when xCE< 0.05. both parts. An almost linear decrease of this quantity occurs as a function of m, for a constant size of the non-polar part of CI2E, at 298.15 K. It is known that HzE(mon) for a specific surfactant varies significantly with temperature, and values for the partial molar heat capacity have been reported.23 The heat capacity should also be a composite of the heat capac- ities of the two parts of the molecule, which can show quite distinct values and variations with the temperature.In the micelles the strength of the interaction of the oxy- ethylene part of the molecule with water is slightly reduced, resulting in an endothermic enthalpy shift for the micellar state, compared to the monomeric state. This occurs despite the probably exothermic contribution of removing the non- polar part from the water structure into the inside of the micelle. The H&(mic) values also decrease nearly linearly with m,but with a smaller gradient than those of the mono- meric state. Interestingly, the HFE(mic) value for C,E4 falls on the same line as the values for the three C,,E, surfactants.Eventually, the micelle palisade of the different surfactants exhibits certain common properties, which give the regular variation of the partial enthalpy with the length of the polar part. The enthalpy of micelle formation is simply the enthal- pic difference between the two states. When the mass-action law model was used to analyse the differential enthalpy values of C,E, in the c.m.c. region, we assumed that the aggregation number of the micelles and the equilibrium constant do not vary as did Andersson and Olofsson in an earlier study of C,E4 in water.23 For C,E,, the variation of HFEwith mCEis very similar to that of C,E4.The four series of differential enthalpies of C,E, are close to each other as shown in Fig. 2. The break point of HFEdefines the c.m.c.; mCE = 8.2 x mol kg-’, and the value is nearly the same as that obtained from surface tension experi- ments (7.3 x mol kg-’). With the surfactants C8E, (rn = 4 or 5), the endothermic shift with increasing surfactant molality is not very It is therefore possible to deter- mine the average aggregation number of the micelles. The calculated excess enthalpy curves using the values obtained from the model correspond closely to the experimental values.23 Table 1 shows the thermodynamic data for the two sur- factants C,E, (m = 4 or 5) at 303.15 K.The enthalpic values for C,E, were calculated from the available data at 298.15 K, and the heat ~apacities.~~The dissolution enthalpies AH&(mon) and AH&(mic) are included in the table. A com-parison of the results for C,E4 and C,E, reveals the differ- ences in the thermodynamic properties. The dissolution enthalpies both below and above the c.m.c. are, as expected,29 more exothermic for the surfactant with the larger polar part. The calculated enthalpy of micelle forma- tion is slightly larger (1.1 kJ mol-’) for C,E,. As shown in Table 1, the Gibbs energy and the c.m.c. of the two sur-factants both increase slightly when m changes from 4 to 5. Much larger variations of the values are observed when the polar part is kept constant, and the properties are monitored as a function of the length of the non-polar part.6 The calcu- lated entropies of micelle formation for the two surfactants C8E, are almost equal.The present view is that this entropy is mainly determined by the size of hydrocarbon part.28 However, if the oxyethylene part is dehydrated on incorpor- ation of the monomer in the micelle, as mentioned above, this should also contribute to the observed entropy. It appears that the monomeric state and the micellar state of the two surfactants are very similar, since the entropy values coincide. The Gibbs energy and the entropy depend partly (cf:Table 1) on the relationship used [eqn. (12) or (13)]. The value obtained for the aggregation number of the C8E, micelles in water, N = 41, corresponds reasonably well with the value previously deduced from static neutron scat- tering,’ N = 80 (7 wt.% C,EJ at 303.15 K.Binana-Limbele et al.’ also obtained a value of N = 80 (1.7 wt.% C,E,) at 303.15 K in a fluorescence quenching study on this sur-factant. An exact comparison of the values from calorimetry and from static neutron scattering is not possible, since the first value is at the c.m.c. and the latter value is at higher concentration. However, both neutron scattering and fluores- cence quenching indicate only small variations of the micelle size as a function of surfactant concentration (1.7-10 wt.%) at 303.15 K. Although the aggregation number calculated from calorimetry is lower than that from the other data, the result gives further support to the concept of small spherical C8E, (rn = 4 or 5) micelles.In the calorimetric study on C8E4 in water, Andersson and 010fsson~~ observed that N depends on the temperature: at 298.15 K, N = 23 and at 313.15 K, N = 17. The value at the lower temperature differs signifi- cantly from the value of N reported by Frindi et aL3’ [N = 147 at 298.15 K (1.5 wt.% C,E,)]. Thus at a fixed tem- perature calorimetry gives a lower aggregation number for the C,E4 than for the C8E, micelles. The excess enthalpies of mixing C,E, and water obtained in this study are shown in Table 2 and in Fig. 3. The minimum of the Hr us. xCEcurve is located at xCE(min) = 0.225 and Ht(min) = -1755 J mol-’. On both sides of the 738 minimum, the enthalpy of mixing increases smoothly.It approaches zero when the composition approaches either one of the pure liquid states (xi = 1, i = CE or w). Similar trends for enthalpies of mixing were observed for ethylene glycol, polyethylene glycol (PEG) oligomers or polymers, and water (298.15 K),15*'8731 and with 2-butoxyethanol and water (298.15 K).32 At HE(min), the mole fraction of solute in the mixture depends on the degree of polymerisation of the mol- ecule, e.g. for the PEG oligomers in water. The dependence is quite weak, x,,,(min) varies between ca. 0.2 and 0.35. The similarity of curves for enthalpies of mixing of the PEG oli- gomers and the present surfactant indicates, in our opinion, that for these binary systems the variation of the excess enth- alpy is mainly determined by the interaction between oxy- ethylene glycol and water, i.e.the hydrocarbon parts of the surfactants avoid contact with the polar regions. For the PEG 400-water binary system, the calorimetric experiments established that HE(min) increases with increasing tem-perature.', The endothermic shift of H!(min) is nearly 50% when the temperature changes from 278.15 to 353.18 K. The oligomeric state is clearly determined by the average inter- action between solute and water, and the interaction becomes weaker with increasing temperature. The partial molar enthalpies for both C,E, and water, Hy (i = CE or w), are shown in Fig. 4. For the partial enthalpy of C,E,, an increase is seen from the fairly exothermic value of -26.3 kJ mol-' at concentrations near to the c.m.c., to nearly zero at the highest concentration of CBE,.There is a good overlap of the partial enthalpies with the differential enthalpies. For 0.001 < xCE< 0.25, the change of HEE with the composition is especially significant. In this region, the partial enthalpy is dominated by the interaction between the micelles and water. At higher values of xCE,a much slower variation of HEE with the composition is observed. The average surroundings of added surfactant molecules increas- ingly resembles those in the neat surfactant. A similar varia- tion of Hy (i = solute) with xCEhas been found for the PEG 400-water and C,E,-water systems.18 The partial molar enthalpy of water in the C,E,-water mixtures decreases regu- larly from values close to zero at the c.m.c., to = -2.9 kJ mol-' at xCE= 1.A shallow endothermic maximum of Ht occurs close to the c.m.c., which is typical for hydrophobic semi-polar solutes in water. The infinite dilution value of Ht indicates highly polar surroundings of the water molecules in the solution. This can be deduced by comparing the value obtained with the Htvalues observed for binary mixtures of alcohols, with different chain lengths, and water.33 Only minor variations of Ht are observed for 0.6 < xCE< 1. The main consequence of incorporating the small water molecules in the polar regions containing the surfactant oxyethylene parts is the swelling of the structure.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 The authors thank Dr. Gerd Olofsson for much help with the titration calorimetric experiments. References 1 D. J. Mitchell, G. J. T. Tiddy, L. Waring, T. Bostock and M. P. McDonald, J. Chem. SOC., Faraday Trans. 1, 1983,79,975. 2 M. Kahlweit and R. Strey, Angew. Chem., Int. Ed. Engl., 1985, 24,654. 3 M. Kahlweit, R. Strey and G. Busse, J. Phys. Chem., 1990, 94, 388 1. 4 J. E. Desnoyers, G. Caron, R. DeLisi, D. Roberts, A. Roux and G. Perron, J. Phys. Chem., 1983,87, 1397. 5 J. B. Rosenholm, Adu. Colloid Interface Sci., 1992,41, 197. 6 K. Meguro, M. Ueno and K. Esumi, in Nonionic Surfactants. Physical Chemistry, ed. M. J. Schick, Marcel Dekker, New York, 1987, ch.3. 7 W. Binana-Limbelk, N. M. Van Os, L. A. M. Rupert and R. Zana, J. Colloid Interface Sci., 1991, 144,458. 8 E. Alami, N. Kamenka, A. Raharimihamina and R. Zana, J. Colloid Interface Sci., 1993, 158, 342. 9 M. Zulauf, K. Weckstrom, J. B. Hayter, V. Degiorgio and M. Corti, J. Phys. Chem., 1985,89, 3411. 10 T. Sato, Y. Saito and I. Anazawa, J. Chem. SOC., Faraday Trans. I, 1988,84,275. 11 A. D. King Jr., J. Colloid Interface Sci., 1990, 137, 577. 12 R. Kjellander, J. Chem. SOC., Faraday Trans. 2,1982,78,2025. 13 R. E. Goldstein, J. Chem. Phys., 1986,84, 3367. 14 K. Weckstrom and M. Zulauf, J. Chem. SOC., Faraday Trans. I, 1985,81,2947. 15 J-Y. Huot, E. Battistel, R. Lumry, G. Villeneuve, J-F. Lavallee, A. Anusiem and C. Jolicoeur, J. Solution Chem., 1988,17,601. 16 J. S. Clunie, J. F. Goodman and P. C. Symons, Trans. Faraday SOC.,1969,65, 287. 17 K. Shinoda, J. Colloid Interface Sci., 1970, 34,278. 18 B. Andersson and G. Olofsson, J. Solution Chem., 1989, 18, 1019. 19 M. Zulauf and J. P. Rosenbusch, J. Phys. Chem., 1983,37,856. 20 K. Weckstrom, Chem. Phys. Lett., 1985,119,503. 21 N. Chakhovskoy, Bull. SOC.Chim. Belg., 1956,65,474. 22 R. Gaufres, J-L. Bribes, S. Sportouch, J. Ammour and J. Mail- lols, J. Raman Spectrosc., 1988, 19, 149. 23 B. Andersson and G. Olofsson, J. Chem. SOC., Faraday Trans. 1, 1988,84,4087. 24 K. Weckstrom and J. B. Rosenholm, in preparation. 25 A. I. Rusanov, Adv. Colloid Interface Sci., 1993,45, 1. 26 J. B. Rosenholm, T. E. Burchfield and L. G. Hepler, J. Colloid Interface Sci., 1980,78, 1981. 27 1. M. Klotz and R. M. Rosenberg, Chemical Thermodynamics, W. A. Benjamin, Inc., Menlo Park, CA, 3rd edn., 1972, ch. 16. 28 G. Olofsson, Netsu Sokutei, 1992, 19, 76. 29 G. Olofsson, J. Phys. Chem., 1985,89, 1473. 30 M. Frindi, B. Michels and R. Zana, J. Phys. Chem., 1992, %, 6095. 31 S. E. M. Hamam, G. C. Benson and M. K. Kumaran, J. Chem. Thermodyn., 1985, 17,973. 32 W. Siu and Y. Koga, Can. J. Chem., 1989,67,671. 33 S-0.Nilsson, J. Chem. Thermodyn., 1986,18,1115. Paper 3/04492K; Received 27th July, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000733

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 739-743 Metal locyclodextr ins of 6A-(3-Aminopropylam in0)=6~-deoxy=~- cyclodextrint: Their Formation and Enantioselective Complexation of (R)-and (S)-Tryptophan Anions in Aqueous Solution Susan E. Brown, John H. Coates, Christopher J. Easton and Stephen F. Lincoln* Department of Chemistry, University of Adelaide , South Australia 5005,Australia From a pH titration study, the complexation of divalent metal ions (M2+)by 6A-(3-aminopropylamino)-6A-deoxy-~-cyclodextrin (BCDpn) to form the metallocyclodextrins, [M(/3CDpn)12+, is characterized by log(KJdm3 mol-') = 4.22& 0.02, 5.2 f0.1,7.35& 0.04and 4.96f0.08when M2+ = Co2+, Ni2+, Cu2+ and Zn2+, respec- tively, in aqueous solution at I = 0.10 (NaCIO,) and 298.2K.The complexation of the tryptophan anion (Trp-) by [M(/3CDpn)12+ is enantioselective for (S)-Trp-as indicated by log(K,,/dm3 mol-') and Iog(K,,/dm3 mol-') = 4.04& 0.03 and 4.32f0.05, 4.1f0.2 and 5.1 f0.2,and 7.85& 0.07 and 8.09f0.05, where the first and second magnitudes refer to the stability constants for [M(/3CDpn)(R)-Trp] + and [M(/3CDpn)(S)-Trp]+, respec- tively, when Mz+= Co2+,NiZ+ and Cu2+, respectively. The corresponding magnitudes for M2+= Zn2+ are both 5.3 & 0.1, indicating no enantioselectivity. The role of M2+ and other factors affecting complexation and enantio- selectivity are discussed. Natural and modified cyclodextrins exist in single enantio- meric forms and, when acting as host molecules, may prefer- entially complex one enantiomer of a chiral guest to produce two diastereomeric host-guest complexes of differing ther- modynamic stability.1-7 The degree of enantioselectivity varies substantially with the nature of the cyclodextrins and guests. In the case of aromatic guest molecules with polar substituents, this may be understood in terms of a model in which the aromatic moiety enters the cyclodextrin annulus and one or more polar substituents of the guest interact with the hydroxy or other polar groups of the cyclodextrin with varying degrees of inten~ity.'-~*'-l' Thus, R-and S-guests experience different geometric and electrostatic interactions with the cyclodextrin which may generate differing stabilities in the two diastereomeric host-guest complexes. In this study we are particularly interested in the combined effects of the coordinating ability of the metal centres and the chirality of the modified cyclodextrins or metallocyclodextrins on the enantioselectivity between guest enantiomers in host-guest c~mplexes.'~-'~ we showed that In a preliminary rep~rt'~ 6A-(3-aminopropylamino)-6A-deoxy-/3-cyclodextrin(BCDpn) formed a nickel(@ metallocyclodextrin ([Ni(BCDpn)] ),+ which was enantioselective for the (S)-tryptophan anion [(S)-Trp-)] in a ratio of 10:1 over (R)-tryptophan anion [(R)- (Sigma) were dried to constant weight and stored in the dark over P205in a vacuum desiccator prior to use.The enantio- meric purities of (R)-and (S)-Trp were determined to be >99% by HPLC analysis [Pirkle covalent (S)-phenylglycine column] of the respective esters formed with thionyl chloride pretreated methanol at 348 K.These purity limits were used in calculations of error limits of the stability constants char- acterizing the complexation of these enantiomers.Metal per- chlorates (Fluka) were twice recrystallized from water, and were dried and stored over P,O, under vacuum. (Caution: Anhydrous perchlorate salts are potentially powerful oxi- dants and should be handled with care.) All solutions were prepared using deionized water purified with a MilliQ-reagent system to produce water with a specific resistance of >15 Ma cm, which was then boiled to remove CO, . Equilibrium Studies Titrations were performed using a Metrohm Dosimat E665 titrimator, an Orion SA 720 potentiometer, and an Orion 8103 Ross combination pH electrode which was filled with 0.10 mol dm- NaC10,.Throughout a titration a stream of fine nitrogen bubbles (previously passed through aqueous 0.10 mol dm-3 NaC10,) was passed through the titration Trp-] in forming the ni~kel(rr)-6~-(3-aminopropylamino)-6~-solution which was magnetically stirred and thermostatted at deoxy-/3-cyclodextrin-tryptophananion host-guest complex, [Ni(/3CDpn)Trp] +,in aqueous solution. This appears to be the greatest degree of enantioselectivity between chiral guests so far reported for a metallocyclodextrin, and we now examine the effects of the variation of the nature of the metal ion on this enantioselectivity, and make comparisons with related systems.Experimental Preparation of Materials 6A-(3-Aminopropylamino)-6A-deoxy-/3-cyclodextrinprepared as in the literature,', and (R)-, (S)-and (RS)-tryptophan$ 7 B-Cyclodextrin = cycloheptamaltaose. 1Protonated tryptophan, tryptophan zwitterion and tryptophan anion are denoted as TrpH+, Trp and Trp-, respectively, prefixed by (R)-or (S)-as appropriate. 298.2 & 0.1 K in a water-jacketed 20 an3 titration vessel which was closed to the atmosphere with the exception of a small exit for the nitrogen stream. The 0.100 mol dm- Ni(ClO,), ,Cu(ClO,), and Zn(C10,), stock solutions were standardized by EDTA titration in the presence of Murexide indicator in the first two cases and Eriochrome Black T in the case of Zn(C10,),.l5 Ion exchange of Co2+ on an Amberlite HRC-120 cation-exchange column in the acid form followed by back titration of the liberated acid was used as the standardization method for the 0.100mol dm-3 Co(ClO,), stock solution. In all titrations, standardized 0.100 mol dm-3 NaOH was titrated against the species of interest in solutions 0.010 mol dmP3 in HClO, and 0.090 mol dm-3 in NaClO,. Thus the pK, values of /3CDpnHi+ and TrpH' were determined from titrations of 10.00cm3 aliquots of their 0.001 mol dm-' solu-tions. The stability constants for the formation of the /3CDpn. (R)-Trp- and /3CDpn + (S)-Trp-complexes were J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 determined by titration of 5.00 an3 each of 0.001 mol dm-3 solutions of either (R)-TrpH' or (S)-TrpH + and /3CDpnH;+, and the stability constants for the formation of the +[M(Trp)] +, [M(/3CDpn)12 and related complexes were determined by titration of 10.00 cm3 aliquots of 0.001 mol dm-3 solutions of either TrpH+ or j?CDpnH$+ with either 0.095 cm3 or 0.045 cm3 of M(ClO,), solution added.The sta- bility constants for the formation of [M(BCDpn)(R)-Trp] + and [M(/3CDpn)(S)-Trp] and related complexes were deter- + mined by titration of 5.00 cm3 each of 0.001 mol dm-3 solu- tions of either (R)-TrpH' or (S)-TrpH+ and /3CDpnHi+ with 0.045 cm3 of M(ClO,), solution added. E, and pK, values were determined by titration of 0.010 mol dm-3 HClO, (0.090 mol dm-3 in NaClO,) against 0.100 mol dmP3 NaOH.Derivations of the stability constants were carried out using the program SUPERQUAD.16 At least three runs were performed for each system, and at least two of these runs were averaged ;the criterion for selection for this averag- ing being that X2 for each run was e12.6 at the 95% con- fidence level. ' Results and Discussion General Aspects Several complexes formed in aqueous solutions of BCDpn, M2+ and tryptophan in the 2.5-11.5 pH range of this study. Their stabilities were determined from the differences between the pH profiles arising from titration against NaOH of solu- tions containing different combinations of the complexing species using the program SUPERQUAD. The sequence of these titrations was : (i) pK, determinations of /3CDpnHi+ and TrpH+, followed by determination of the stability con- stants of complexes in solutions of (ii) M2+and TrpHf, (iii) M2+ and BCDpnH$+, (iv) /3CDpnHq+ and either (R)-TrpH+ or (S)-TrpH+, and (v) M2+, PCDpnHi'and either (R)-TrpH' or (S)-TrpH+.The pK,s determined in (i) were employed as known values in the determination of stability constants in (ii)-(v), and the stability constants determined in (ii)-(iv) were employed as known values in the determination of stability constants in (v). The titration data were fitted to equilibria containing the minimum number of species required for a good fit, and any newly determined species found to be <5% of the total BCDpn or tryptophan concen- trations were considered to be insignificant.Two such pH titration profiles are shown in Fig. 1. Plots of the major species present in the Cu2 +-PCDpn-(S)-tryptophan system are shown in Fig. 2 and 3. The stability constants of the major M2+ complexes appear in Table 1, and those for other 12 2 1.30 1 , 1.40 , , 1 , , , , , , , , .50 1.60 1.70 1.80 , , 1.90 2 10 volume of NaOH added/cm3 Fig. 1 Titration profiles for (a) BCDpnH:+ (5.04x lo-, mol dmP3) and (R)-TrpH+ (5.08 x mol dm-3), and (b) BCDpnH:+ (5.02x mol dm-3), (R)-TrpH+ (5.05 x mol dm-j) and Cu(ClO,), (4.50x lo-, mol dm-3), each in aqueous 0.010 mol dm-3 HClO, and 0.090 mol dm-3 NaClO,, against 0.101 mol dm-3 NaOH. h 40 v m .-0 n 20 0 4.5 5.5 6.5 7.5 8.5 PH Fig. 2 Plot of Cu2+ species in a solution 0.00095, 0.001 and 0.001 rnol dm-' in total Cu2+, BCDpn and (S)-tryptophan concentrations, respectively, plotted relative to total [BCDpn] = total [(S)-Tryp-tophan] = 100%.(a) [Cu((S)-Trp)]+,(b) CU", (c) [Cu((S)-Trp),], + '. ,(4 CCNBCDP~XS)-T~PI (4 CCU(BCDP~XS)-T~PI (f[Cu(BCDpn)I2+,(9)[Cu((S)-?rp)OH], (h) [Cu(/3CDpn)OH]+ and (i) [Cu(/3CDpn)((S)-Trp)OH]. Table 1 Stability constants" characterizing metallo-6A-(3-aminopropylamino)-6A-deoxy-~-cyclodextrinsand related complexes in aqueous solution at 298.2K and I 0.10(NaCIO,)= M2+ coz+ Ni2+ b cu2+ Zn2+ log(K2/dm rnol -') 4.22f0.02 5.2 f0.1 7.35 _+ 0.04 4.96 _+ 0.08 10g(K3/dm3 mol-') 2.5 & 0.2 3.1 f0.1 3.09 f0.04 3.0f 0.1 log(KJdm3 mol-') 4.41 f0.05 5.42f0.03 8.11 f0.03 4.90f0.04 log(KJdm mol -') iog(~,./dm~ mol-') iog(~',,/dm~ mol-') log(K7,/drn3 mol- ') log(K7,/dm3 mol-') 4.01 0.08 4.04f0.03 (0.1) 4.32 0.05 (0.09) 4.67 f0.03 5.1 f 0.2(0.2) 4.1 f 0.2 (0.2) 7.20f 0.07 8.09 f 0.05(0.06) 7.85 f0.07 (0.07) 5.4 * 0.1 (0.2) 5.29 f 0.05 (0.1) 5.3 f0.1 (0.1) 5.3 f0.1(0.1) " Errors quoted for K (the mean of N runs) represent the standard deviations, CT = J{[Z(K,-K)']/(N -1)) where Kiis a value from a single run for the best fit of the variation of pH with added volume of NaOH titrant obtained through SUPERQUAD, and i = 1,2,...,N.When a K derived in this way was employed as a constant in the subsequent derivation of another K, the error associated with the first K was propagated in the derivation of the second K.For the diastereomers, the first and second errors quoted are calculated assuming 100 and 99% enantiomeric purity of tryptophan, respectively. 'Ref. 14. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 741-100, I Similar deprotonations were not reliably detected for the Co2+ and Zn2 + analogues, because the precipitation of a metal hydroxide species above pH 8.5 and 7.5, respectively, h2 60 \\ I In .-0 !? 40 20 0. 4.5 5:5 6.5 7.5 8.5 9.5 PH Fig. 3 Plot of non-Cu2+ species in a solution 0.00095, 0.001 and 0.001 mol dm-3 in total Cu2+, j?CDpn and (S)-tryptophan concen- trations, respectively, plotted relative to total [j?CDpn] = total [(S)-tryptophan] = 100%. (a) BCDpnH;+, (b) (S)-Trp, (c) BCDpnH+ and (4BCDpn.species appear below. A detailed discussion of the various equilibria now follows. Cyclodextrin Equilibria The acid dissociations of the diprotonated amino-propylamino substituent of BCDpnH;+ : Ka 1 BCDpnH;' =BCDpnH+ + H+ (1) K.2 j?CDpnH+ BCDpn + H+ (2) derived from data in the pH range 6.0-11.5 are characterized by pK,, = 7.39 0.04 and pK,, = 9.9 f 0.1. The acid disso- ciations of TrpH' characterized by pK,s of 2.40 f 0.02 and 9.28 0.01, and derived from data obtained in the pH ranges 2.0-3.0 and 8.0-10.5, respectively, are similar to those in the literature.' For complexations of either (R)-Trp- or (S)-Trp-by PCDpn : KIR BCDpn + (R)-Trp-BCDpn -(R)-Trp-(3) Kis BCDpn + (S)-Trp-BCDpn.(S)-Trp- (4) 1og(KlR/dm3 mol-') = 3.41 f. 0.02 (0.05) and log(Kls/dm3 mol-') = 3.40 f 0.07 (0.1) were derived from data in the pH range 8.5-11.5, where the first and second errors are calcu- lated on the basis of tryptophan being 100 and 99% pure, respectively. Complexation of 6"-(3-Aminopropylamino)-6*-deoxy-/?-cyclodextrin and the Tryptophan Anion by Divalent Metal Ions The stability of the metallocyclodextrin formed by BCDpn : K2 M2+ + BCDpn [M(BCDpn)12+ (5) varies with the nature of M2+ as shown by the variation of the magnitude of K, in the sequence Co2+ < Ni2+ < Cu2+ > Zn2+ (Table 1) as anticipated from the Irving-Williams series.' In the cases of [Ni(BCDpn)I2+ and [Cu(/?CDpn)]'+, pK,s of 9.20 f 0.04 and 7.84 f 0.03, respec- tively, were determined and are thought to correspond to the deprotonation of aqua ligands bound to the metal centres.obscured the formation of [Co(BCDpn)OH] and+ [Zn(fiCDpn)OH] and rendered titrations above these pHs + impractical. The formation of [M(PCDpnH)13+ : M2+ + BCDpnH+ K3 [M(BCDpnH)I3+ (6) is less favoured (Table 1) as anticipated from the charge repulsion between M2 + and BCDpnH and the monodentate + nature of BCDpnH'. The pK, of [M(BCDpnH)l3+ is 8.3 & 0.1, 7.83 f. 0.02, 5.74 & 0.05 and 8.1 & 0.1 when M2+ = Co2+, Ni2+, Cu2+ and Zn2+, respectively. These values probably characterize the deprotonation of the mono- protonated aminopropylamino substituents of BCDpnH+ in the metallocyclodextrins. The stability constants K, and K, and the corresponding pK,s were derived from data obtained in the pH ranges 6.0-8.5, 5.5-8.5, 5.5-9.0 and 5.5-7.5, when M2+ = Co2+, Ni2+, Cu2+ and Zn2+, respectively. The formation of [M(Trp)] and [M(Trp),] also occurs: + K4-M2+ + Trp-4[M(Trp)]+ (7) K5 [M(Trp)l+ + TrP-=CM(Trp),l (8) The stability constants, K, and K, ,determined in this study (Table 1) are in reasonable agreement with those in the liter- ature,17 and also exhibit variations anticipated from the Irving-Williams series." For [Ni(Trp)] and [Cu(Trp)] +,+ pK,s of 9.1 & 0.1 and 7.28 & 0.07, respectively, were deter- mined, which probably correspond to the deprotonation of aqua ligands bound to the metal centres.Similar deprotona- tions were not reliably detected for the Co2+and Zn2+ ana- logues, because the precipitation of a metal hydroxide species above pH 8.5 and 7.5, respectively, interfered with the titra- tions. The stability constants K, and K, and the correspond- ing pK,s were derived from data obtained in the pH ranges 6.5-8.5, 5.0-9.0, 3.0-6.5 and 5.5-7.0, when M2+ = Co2+, Ni2+, Cu2 + and Zn2+, respectively.Enantioselectivity in the Complexation of (R)-and (8-Tryptophan Anion by Divalent Metal Complexes of 6"-(3-Aminopropylamino)-6"deoxy-/?~yclodextrin The stability of the complexes formed by [M(j?CDpn)]'+ with (R)-Trp- and (S)-Trp- : K6R [M(PCDpn)12 + (R)-Trp-[M(BCDpn)(R)-Trp]++ (9) Kss [M(BCDpn)12++ (S)-Trp-[M(BCDpn)(S)-Trp]+ (10) also varies with the nature of M2+ as shown by the variation of the magnitude of K6, and K6s in the sequence Co2+d Ni2+ < Cu2+> Zn2+ (Table 1).In addition, there is a ten-fold enantioselectivity for (S)-Trp- when M2+ = Ni2+, as a comparison of K6, with K6, shows. When M2+ = Co2+ and Cu2+, there is a moderate enantioselectivity for (S)-Trp-, but when M2+ = Zn2+, no enantioselectivity is observed. The effect of enantioselectivity on the concentrations of several species in the Ni2+ system is shown in Fig. 4. The lower stabilities of /?CDpn -(R)-Trp- and BCDpn (S)-Trp- by comparison with those of [M(BCDpn)(R)-Trp] and+ [M(BCDpn)(S)-Trp]+,demonstrate that M2 + strengthens the complexation of Trp- . However, as [M(PCDpn)(R)-Trp]+ and [M(BCDpn)(S)-Trp]+ (KbR and K6s) are less stable than J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 60 I h $ 40 v 20 0 7.0 7.5 8.0 8.5 9.0 PH Fig. 4 Plot of selected species in a solution 0.00095,0.001 and 0.001 mol dmP3 in total Ni2+, BCDpn and either (R)-tryptophan or (S)-tryptophan (indicated by primed letters) concentrations, respectively, plotted relative to total [BCDpn] = either total [(R)-tryptophan] or total [(S)-tryptophan] = 100%. (a) flCDpnH+, (af) BCDpnH+, (b) “i((R)-Trp)I +7 (4 “i((S)-Trp)l+ (4CNi((R)-Trp),l, (4“i((S)-9 Trp),l, (4CNi(BCDpn)Y+r (4“i(BCDPn)lZ +r (4 CNi(BCDPd (R)- Trp] and (e’)[Ni(flCDpn)(S)-Trp] +.+ [M(Trp)]+ (K4)when M2+ = Co2+, Ni2+ and Cu2+, it appears that the factors stabilizing complexation of (R)-Trp- and (S)-Trp- by BCDpn and M2+ in [M(BCDpn)(R)-Trp]+ and [M(BCDpn)(S)-Trp] do not reinforce each other.In + +contrast, [Zn(BCDpn)(R)-Trp] and [Zn(BCDpn)(S)-Trp]+ are more stable than [Zn(Trp)]+, consistent with mutual reinforcement of the complexation of (R)-Trp- and (S)-Trp- by BCDpn and Zn2+, but there is no enantioselection between (R)-Trp- and (S)-Trp-. The structure envisaged for [M(BCDpn)(R)-Trp]+ and [M(BCDpn)(S)-Trp]+ has the indole moiety of Trp-inside the cyclodextrin annulus with the Trp- chiral centre in the vicinity of the primary hydroxy groups of the cyclodextrin, and the Trp- amine and carboxylate groups coordinated to M2+,as shown in Fig. 5. It has been argued that the indole moiety is only inside the cyclodextrin annulus for the dia- stereomer with the higher stability in the Cu2+ complexes of 6A-[4-(2-aminoethyl)imidazoyl]-6A-deoxy-/.?-cyclodextrin and 6A-deoxy-6A-[2-(4-imidazoyl)ethylamino]-/3-cyclo-dextrin, which preferentially complex (S)-Trp- and (R)-Trp-, re~pectively.’~.’~We have no evidence for such major struc- tural differences in the complexes studied here.The influence of the nature of M2+ on the stabilities of [M(jICDpn)(R)-Trp]+ and [M(BCDpn)(S)-Trp] + reflects the Fig. 5 A possible structure for [M(pCDpn)(S)-Trp]+, where the cyclodextrin annulus is shown as a truncated cone with the narrow and wide ends representing the circles delineated by primary and sec- ondary hydroxy groups, respectively. variation in the ionic radii of six-coordinate Co2+, NiZ+, Cu2+ and Zn2+, which are 0.745, 0.69, 0.73 and 0.74 A,” respectively, and the geometric constraints arising from ligand-field effects in Co2+, Ni2+ and CU”.~O It is particu- larly interesting that [Zn(BCDpn)(R)-Trp]+ and [Zn(BCDpn)(S)-Trp]+ are of the same stability, whereas the analogous diastereomers for the other three metals are of dif- ferent stability.This suggests that the absence of ligand-field- generated geometric constraints on d” Zn2+ allows more flexibility in the structures of [Zn(BCDpnXR)-Trp]+ and [Zn(BCDpn)(S)-Trp]+ and as a result enantioselectivity is decreased. In contrast, the d9 electronic configuration for the similar sized Cu2 + imposes a tetragonally distorted octa- hedral geometry which may place greater constraints on the interaction of the chiral centres of (R)-Trp- and (S)-Trp- with the BCDpn moiety and decrease the stability of [Cu(BCDpn)(R)-Trp]+ by comparison with that of [Cu(BCDpn)(S)-Trp]+.Similar arguments may be applied in the cases of d7 Co2+ and d8 Ni2+ whose six-coordinate geometries more closely approach regular octahedrons. The greater enantioselectivity caused by Ni2 + indicates that the size of the metal centre is important, and that a difference of 0.04 A can result in a substantial change in the degree of enantioselectivity.The stabilities of [Cu(BCDpn)(R)-Trp] and [Cu(BCDpn) + (S)-Trp]’ + : K~R [Cu(BCDpn)12+ + (R)-Trp [Cu(/?CDpn)(R)-Trpl2+ (11) [Cu(PCDpn)12+ + (S)-Trp Kis [Cu(BCDpnXS)-Trp] + (12) are lower than those of [Cu(flCDpn)(R)-Trp] and+ [Cu(BCDpn)(S)-Trp]+ (Table 1) and there is no significant enantioselectivity, probably because (R)-Trp and (S)-Trp act as monodentate ligands and are less sterically constrained than bidentate (R)-Trp- and (S)-Trp-.The deprotonations +of [Cu(/3CDpn)(R)-Trp] +, [Cu(BCDpn)(S)-Trp12 , + +[C@CDpn)(R)-Trp] and [Cu(pCDpn)(S)-Trp] are char- acterized by pK,s of 6.72 f0.08 (O,l), 6.6 & 0.1 (0.2), 9.48 & 0.07 (0.09) and 9.37 & 0.04 (0.05), respectively. The pK,s for the first pair may characterize the deprotonation of either the TrpH+ or the BCDpnH’ ligand, but an unam- biguous assignment is not possible. The pK,s for the second pair probably characterize the deprotonation of an aqua ligand.These reactions were -not detected when M2+ = Co2+, Ni2+ and Zn2+. The stability constants for the complexations shown in eqn. (9)-(12) were derived from data obtained in the pH ranges 7.5-8.7, 7.0-9.2, 4.5-9.5 and 6.5-8.0, when M2+ = Co2+, Ni2+, Cu2+ and Zn2+, respectively. We gratefully acknowledge funding for this research from the University of Adelaide and the Australian Research Council, and the award of an Australian Postgraduate Priority Research Award to S.E.B. References 1 N. J. Smith, T. M. Spotswood and S. F. Lincoln, Carbohydrate Res., 1989, 192,9. 2 S. E. Brown, J. H. Coates, S. F. Lincoln, D. R. Coghlan and C. J. Easton, J. Chem. SOC.,Faruday Trans., 1991,87,2699.3 S. E. Brown, J. H. Coates, P. A. Duckworth, S. F. Lincoln, C. J. Easton and B. L. May, J. Chem. Soc., Faraday Trans., 1993, W, 1035. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 743 4 I. Tabushi, Y. Kuroda and T. Mizutani, J. Am. Chem. SOC., 1986, 14 S. E. Brown, J. H. Coates, C. J. Easton, S. J. van Eyk, S. F. 5 108,4514. I. Tabushi, Y. Kuroda, M. Yamada and T. Sera, J. Zncl. Lincoln, B. L. May, M. A. Stile, C. B. Whalland and M. L. Wil- liams, J. Chem. SOC., Chem. Commun., 1994,47. Phenom., 1988,6,599. 15 A. I. Vogel, Quantitative Inorganic Analysis, Longmans, London, 6 K. Harata, Bull. Chem. SOC. Jpn., 1987,60,2763. 3rd edn., 1961. 7 D. Greatbanks and R. Pickford, Mag. Reson. Chem., 1987, 25, 16 P. Gans, A. Sabatini and A. Vacca, J. Chern. SOC., Dalton Trans., 208. 1985,1195. 8 R. J. Clarke, J. H. Coates and S. F. Lincoln, Adv. Carbohyd. 17 Critical Stability Constants, ed., R. M. Smith and A. E. Martell, 9 Chem. Biochem., 1989,46,205. J. Szejtli, Cyclodextrin Technology, Kluwer, Dordrecht, 1988. 18 Plenum Press, New York, 1975, vol. 1. H. Irving and R.J. P. Willliams, J. Chem. SOC., 1953, 3192. 10 J. F. Stoddard, Cyclodextrins, Royal Society of Chemistry, Cam- 19 R. D. Shannon, Acta Crystallogr., Sect. A, 1976,32, 751. bridge, 1990. 20 A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 11 W. Saenger, Incl. Comp., 1984, 2, 231. New York, 4th edn., 1980. 12 G. Impellizzeri, G. Maccarrone, E. Rizzarelli, G. Vecchio, R. Corradini and R. Marchelli, Angew. Chem., Int. Ed. Engl., 1991, 30,1348. 13 V. Cucinotta, F. DAlessandro, G. Impellizzeri, G. Maccarrone and G. Vecchio, J. Chem. SOC., Chem. Commun., 1992, 1743. Paper 3/060201; Received 8th October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000739

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 745-750 Hydrogen Evolution Reaction on Electrodes coated with Conducting-polymer Films Krzysztof Maksymiukt and Karl Doblhofer Fritz-Haber-lnstitut der Max-Planck-Gessellscha ft, Faradayweg 4-6,0-14195 Berlin, Germany The cathodic hydrogen evolution reaction has been studied on gold electrodes coated with poly(N-methyl- pyrrole) (PMPy), and with a polymer mixture of PMPy and poly(4-styrenesulfonate) (PMPy-PSS). In all cases, Hf ions were found to permeate the film and react at the metal-electrode surface. In particular, for low concentra- tions of HCI, the PMPy film hinders significantly the movement of the depolarizer to the electrode. On the other hand, in the case of PMPy-PSS the ti+ transport across the film proceeds readily, because the H+ ions consti- tute counter-ions to the immobilized sulfonate groups, present at a concentration of ca.1 mol dm-3. The effect of the partitioning equilibria and the Donnan potential on the observed potential dependence of the charge-transfer rate is discussed in detail. It is shown that the presence of the cation-exchanger coating (PMPy-PSS) enhances the rate of electrochemical H+ reduction (relative to the uncoated electrode) when the concentration of H+ in the electrolyte is small. The diffusion coefficient of H+ ions in the PMPy-PSS matrix was determined: D = 1 x cm2 s-'. Oxidation/reduction reactions of redox species from electro- lytes on electrodes coated with electron-conducting polymer films are a subject of considerable interest from the stand- point of electrocatalysis.'-'o Such reactions may proceed by two fundamentally different mechanisms.First, the polymer may act as a mediator for electron transfer between the elec- trode and the depolarizer in the electrolyte; in this case, the reduction of Ox from solution may be formulated: krcd Ox + Poly -Red + Poly+ (1) where kred is the (bimolecular) rate constant, Poly and Poly+ are the reduced and oxidized polymer sites, and Red is the reduced form of the depolarizer in the electrolyte. This mechanism has been found to be operative in many cases, whereby the electron transfer usually proceeds at the polymer surface;' '-I6 nevertheless, a penetration of the redox species into the matrix is also possible.",'* In this case, the electron- transfer reaction proceeds in a three-dimensional reaction zone; the cathodic current may be described by:'5*19 i-= -nFAk,,, ~cply(x)cox(x)dx (1) where d is the polymer film thickness, A the electrode surface area, cpoly(x)and c,,(x) are the concentrations of polymeric sites and of Ox in the film.In our recent paper,I5 we have studied reduction reactions of selected inorganic redox oncouples: Fe(CN)2-'4-, Ru(NH3)2+I2+ and Eu~+/~+ a rotating-disk electrode (RDE) proceeding on the polymer surface. The limiting reduction currents were found to be lower than in the absence of the polymer, which was caused by a slow rate of the mediated reaction [eqn. (l)], as obtained from Levich-Koutecky plots.Much less attention has been given to the second mecha- nism, in which the depolarizer penetrates the polymer film without reacting with it. The electron-transfer reaction may proceed on the metallic substrate, i.e. at the electrode/ polymer interface. An example is the reduction of H+ ions, as found by Bard and co-~orkers'~and Schultze and co-workers20.21 for typical conducting polymers, such as poly- pyrrole and polyaniline. Clearly, in this case the depolarizer is partitioned from the electrolyte solution into the polymer phase, and transported across the polymer phase. Therefore, t On leave from the Department of Chemistry, Warsaw Uni-versity, Pasteura 1, PL-02-093 Warszawa, Poland. the membrane properties of the polymer will exert a decisive influence on the rate of the charge-transfer reaction.It is the aim of this work to analyse the kinetics of such a charge-transfer reaction as a function of the membrane properties of conducting-polymer coatings. Thus, we present and discuss results concerning the H+ reduction on elec-trodes coated with poly(N-methylpyrrole) (PMPy), and with poly(N-methylpyrrole) containing immobilized poly(4-styre- nesulfonate) (PMPy-PSS). PMPy is an anion exchanger in the oxidized state. PMPy-PSS constitutes a cation-exchanger matri~,~~,~~where cations from the electrolyte along with the produced PMPy sites compensate the negative charge of + the -SO, groups in the matrix. Experimental Electrochemical measurements have been performed using a two-compartment cell with working gold or platinum elec- trodes of surface area 0.28 cm2, a saturated calomel reference electrode (SCE) and a counter electrode prepared from the same metal as the working electrode.The working electrodes were polished using diamond paste. The platinum electrode surface was additionally activated by cyclic voltammetry in 0.5 mol dmP3 H2S04. The polymer films were deposited on the metal electrodes potentiostatically, at 0.7 V, from aqueous solutions containing 0.05 mol dm-N-methylpyrrole (distilled and kept cool in an argon atmosphere) and 0.1 mol dm-3 NaClO, or 0.1 mol dm-3 sodium poly(4-styrenesul- fonate). The chemicals (p.a. products) were used as received. Water was triply distilled.For other details see previous paper^.^ 3924 The following experiment was conducted to estimate the concentration of sulfonate groups in the films. First, the PMPy-PSS-coated electrode was polarized in HCl electro- lyte for 10 min at -0.45 0s. SCE [step (l)]. At this potential the polymer is completely reduced and the concentration of H+ in the polymer phase is approximately equal to the con- centration of sulfonate groups (see below). Then, the coated electrode was rinsed with distilled water, transferred into a 0.1 mol dm-3 KCl electrolyte, and polarized to -0.8 V us. SCE [step (2)]. In step (2), the H+ ions are reduced to H,, and their place is occupied by K+ ions from the electrolyte. The charge measured in this step, Q(H+@'IY)) was assumed to correspond to hydrogen ion discharge. This value was quite reproducible (f10%)when it was measured not later than 30 s after immersion into the KCl solution.For longer times 746 I I I -6-0 E--.h---0:4-3 u-2-0 50 100 150 200 polymerization charge/mC Fig. 1 Charge required for complete cathodic reduction of the H+ present in polymer coatings, Q(H+(vlY)).Polymer films : PMPy-PSS of thickness defined by the polymerization charge (73 mC correspond to ca. 1 pmZ2). The coating electrodes were equilibrated in 0.1 mol dmV3 HCI; the reduction charge was determined in 0.1 mol dm-3 KCl. a gradual decrease in measured charge was observed, appar- ently because of exchange of H+ ions by K+from the solu- tion.As expected, Q(H+(PlY)) was found to change little when the HCl concentration in solution was varied (in the range 0.02-1 mol dm-3), and it was proportional to the film thick- ness (Fig. 1). From this charge the concentration of bound sulfonate groups in the film was evaluated to be about 1 mol dm-3, assuming that a polymerization charge of 26.2 mC em-’corresponds to the film thickness, 0.1 pm. This value .~~was found by Zhou et ~1 for PMPy produced under the same conditions. Results and Discussion Electron-transfer Reaction proceeding at the Metal/Polymer Interface The cyclic voltammograms obtained with the electrodes coat- ed with the polymers PMPy and PMPy-PSS in 0.1 mol dm-3 KCl are represented in Fig. 2.Note that the anodic oxidation of the polymer (PMPy -+ PMPy’) commences at about 0.0 V us. SCE in the case of PMPy and at about -0.2 V us. SCE with PMPy-PSS. Both these oxidation potentials are more positive than the standard potential of the H+/H2 reaction (-0.242 V us. SCE). Since the maximum concentra- tion of H+ used in this work was 1 mol drnp3, one concludes that even thermodynamically the mediated reaction H + + PMPy +*H2 + PMPy+ is not favoured, i.e. it cannot proceed at a significant rate. To support this conclusion, the cathodic H+ reduction was conducted on a platinum and a gold electrode. Both elec- trodes were (Q) coated with identical PMPy-PSS films, and (b) uncoated. The results are presented in Fig. 3. Clearly, if the H+ reduction were mediated by PMPy sites, the reaction would proceed on both coated electrodes at the same elec- trode potential, but this is not the case.In fact, the larger overvoltage of the coated gold electrode is an experimental proof that the charge-transfer reaction, at least in the case of gold, proceeds indeed at the metal surface. Hydrogen Reaction on PMPy-coated Electrodes In Fig. 4, H+ reduction currents on Au/PMPy electrodes are compared with the corresponding results on uncoated Au J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 T 10.5 mA IIIIIIIII -0.2 0 0.2 0.4 E/Vvs. (SCE) Fig. 2 Cyclic voltammetric curves of gold electrodes (area, 0.28 cm2)covered with the ‘conducting polymers’ PMPy (a) and PMPy- PSS (b), in 0.1 mol dm-3 KCI.Scan rate, 0.1 V s-’; charge used for polymerization, 90 mC. electrodes. The measured currents do not depend on the rota- tion speed, both at uncoated and coated electrodes, indicat- ing that diffusion/convection in solution is not the rate- determining step. The reduction of H+ proceeds in the poten- tial range where the polymer is completely reduced and constitutes a neutral organic phase. Electroneutrality coup- ling requires that both H+ and C1-, i.e. HC1, enter into the polymer. The partitioning equilibrium may be described by : where cr$ and c&, are the‘ equilibrium concentrations of HCl in the polymer and in the electrolyte, respectively, and I I0-NI5 -0.2 -aE2 >r C.--$ -0.4 -0 c. 2 3 t J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 NI5 -0.2 I-1.0 I -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 electrode potential, €/V vs. SCE Fig. 4 H+ reduction current observed with Au electrodes coated with PMPy (---) in (a)1, (b) 0.1 and (c) 0.02 mol dm-j HCl. Rota- tion speed, 900 rpm; scan rate, 1 mV s-’; charge used for poly- merization, 90 mC for the electrode of area 0.28 cm2. For comparison, the corresponding currents obtained with two HCl con- centrations on the uncoated electrodes are included (-). is the difference in standard chemical potential in the two phases, i.e. the partial molar Gibbs energy of tran~fer.’~ In aqueous electrolytes the polymer (in the reduced state) is unswollen and constitutes a phase of low relative permit- tivity.Considering that the HCl molecule is rather polar, one would expect Apicl to have a large positive value. Further- more, the degree of dissociation of the HCl partitioned in the polymer phase should be small. Although it is difficult to make a quantitative prediction, one expects the concentration of free H+ in the polymer to be significantly smaller than in the electrolyte. The situation is represented schematically in Fig. 5. Comparing the theoretical considerations with the results of Fig. 4, it appears that the result obtained with 0.02 mol dmP3 HCl is plausible. The stationary flux of H+ across the polymer film is defined by the diffusion coefficient and the concentration gradient of H (cpjY/d),which are both small + and lead to a reduction current which is not measurable on the current scale of Fig.4. C 4‘ I 0 d Fig. 5 Schematic representation of the partitioning equilibrium of H+ (HCl) between the electrolyte and polymer (PMPy) 747 Unexpectedly, at the larger HCl concentrations the H+ reduction currents rise by factors much larger than the rise in HCl concentration in the electrolyte (x 5, x 10). The current plateau at 0.1 mol dmP3 HCl indicates that the current is still limited by the rate of H+ transport across the film. One con- cludes that the equilibrium concentration of H+ in the polymer rises more than proportional to the HCl concentra- tion in the electrolyte, i.e. the value of A&,-, decreases with increasing HCl concentration.The reason for this may be the beginning of protonation of the N-methylpyrrole units (the pK, value of N-methylpyrrole is -2.926). This would have an autocatalytic effect on the H transport, because of swelling + of the polymer associated with the solvation of the produced ions. Hydrogen Reaction on PMPy-PSS-coated electrodes The membrane state of a polymer containing a significant concentration of fixed charges is readily discussed in a quan- titative way. The -SO, groups in the considered polymer PMPy-PSS require cations as counter-ions. As a conse-quence of the large ion concentration, the polymer is highly swollen, and it constitutes a polyelectrolyte gel. Across the interface between this solvated polymer and a liquid electro- lyte, the well known ‘Donnan equilibrium’ defines the par- titioning of the exchangeable ions, in particular of H + : where A#D = -#’ is the ‘Donnan potential’, i.e.the interfacial potential difference between the polymer and the electrolyte. Consider the situation in which the electrolyte contains only HCl. The electroneutrality condition for the polymer phase may then be formulated: ct;.,Iy = [x-]+ cg!y (4) where c are the concentrations of the species indicated as subscripts, and [X-] is the fixed-anion concentration. Com- bining eqn. (3) and (4) under the assumption that the concen- trations equal the activities, u, and [X-] = 1 mol dm-3, one obtains the cp?, czly and A&, results summarized in Fig. 6. Note that, over a considerable electrolyte concentration range, the concentration of the considered depolarizer H + in > E..aU -0 --50 --100 -2 -1 0 1 log[cS/moldw3] Fig. 6 Equilibrium concentrations of mobile cations (cEjy)(a) and anions (CE!~)(b) in an anion exchanger of fixed-charge density, [X-]= I rnol dm-3, as a function of the electrolyte (HCl) concentra- tion, cs. (c) A&,, the Donnan potential, as a function of the electro- lyte concentration. the polymer does not depend in a significant way on its con- centration in the electrolyte, i.e. crjyx constant x [X-]. This has a remarkable consequence for the dependence of the CT rate on the H+ concentration in the electrolyte. Consider the distribution of the electric potential across the uncoated and coated electrode at a certain value of -4’.This corresponds to a particular value of the elec- trode potential, E, at which cathodic H reduction proceeds + (Fig. 7).Two concentrations of H+ in the electrolyte are con- sidered, ‘1’ and ‘2’. In the case of the uncoated electrode [Fig. 7(a)] the dependence of the rate of H, production on 4, is a consequence of the linear dependence of the rate of the electron-transfer process on the depolarizer concentra- tion. It may be formulated, disregarding double-layer effects : i-= -nFAk-cL+ exp( -(54g) i-= -nFAk-c;, exp[ -RT where i-is the reduction current (at the considered over- voltages, the back reaction may be disregarded); k-is the cathodic CT rate constant; a is the cathodic CT coefficient; and 4 is the electric potential of the phase defined by the superscript.The logarithm of the current depends linearly on E, with a slope of (59/a) mV decade-’. The situation is rep- resented schematically in Fig. 8, where c1 is a reference con- centration and a = *. In the presence of the film [Fig. 7(b)]the CT rate is defined by the concentration of H+ in the polymer and the potential drop across the metal/polymer interface. Assuming the same electrochemical rate constant as in the case of the uncoated electrode (see Section 4.3.2 of ref. 25 for a discussion of the validity of this assumption), one may write: J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3 Fig. 8 Illustration of the difference between a fixed-charge polymer coated and an uncoated electrode, with respect to the charge-transfer current density, I, as a function of the electrode potential, E (relative scale), and the depolarizer concentration in the electrolyte (relative to a reference concentration cl): (a) cl, (b) 10cl, (c) lOOc,.The arrows point to the change in I corresponding to the reduction of the depo- larizer concentration by a factor of 10 (lOOcl -+lOc,). In Fig. 9 experimental results are summarized, which support the above analysis. At a given current density, the cathodic overvoltage increases by ca. 120 mV decade-’ reduction of H+ concentration in the case of the uncoated gold electrode. On the other hand, in the presence of the PMPy-PSS coating the overvoltage increases only by about 59 mV decade-l, as expected (Fig.8). It was found that the experimental results obtained with the coated electrodes are much more reproducible than the ‘clean’-gold results. The strong dependence of the rate of hydrogen evolution on the slightest variations of the surface state of gold electrodes is well known, see for instance ref. 27. The above result has a remarkable consequence. In the case of dilute ionic depolarizers the rate of the electrode reac- i-= -nFAk-cg!Yexp (6b) tion may be enhanced, at constant overvoltage, by coating [-aF(ERiA4D)]the electrode with an ion exchanger for which the depolarizer Since the effective depolarizer concentration, ~fi’!~, ions constitute counter-ions. remains largely constant, a change of the electrolyte concentration will affect the charge-transfer rate via the Donnan potential, eqn.qb). For example, when at constant applied E the elec- trolyte concentration is reduced from lOOc, to lOc, (see Fig. 8), the CT rate will be reduced only by a factor of about three, compared with a factor of 10 in absence of the fixed- charge polymer film. (a1 (b 1 Me ;I electrolyte Me ’/I poly [electrolyte Fig. 7 Distribution of the electric potential, 4, across the interfaces between electrolyte (S) and an uncoated (a) and polymer-coated (b) electrode of the same metal, Me, at a certain value of -4’. Two concentrations of H+ in the electrolyte are considered, ‘1’ and ‘2’. Ad,, is the (negative) Donnan potential. 0-NI E” -0.2 -a E.a -.-5 4.4In Q)U -E -0.6 g3 -0.8 --1.0 -I I I I I -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 electrode potential, €/V vs.SCE Fig. 9 H+ reduction currents on rotated uncoated (-) and PMPy-PSS-coated (---) gold electrodes, as a function of electrolyte (HCl) concentration: (a)0.02, (b) 0.1 and (c) 1 mol dm-’ HCl. Rota- tion speed, 900 rpm; scan rate, 1 mV s-’; charge used for poly- merization, 90 mC for an electrode of area 0.28 an2. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Note that the above consideration relates to the kinetics of the CT reaction. The equilibrium potential of the redox system is the same in the presence and absence of the polymer film:25 When equilibrium prevails across the inter- face between metal and solvated polymer, one may formulate Using the definition of the electrochemical potential, ii, one obtains : where K is a constant.The equilibrium condition across the polymer/electrolyte interface may be formulated as : Combining eqn. (7) and (9), one sees immediately that in the presence and absence of the coating the same electrochemical equilibrium prevails : Thus, the same equilibrium potential is expected in the pres- ence and absence of the coating. Diffusion Coefficient of H+ Ions in the PMPy-PSS Film At larger overpotentials, the rate-determining step in the H+-reduction reaction is eventually the transport of hydrogen ions towards the electrode surface. Fig. 10 presents two typical current-potential curves on rotated disk electrodes obtained for reduction of 4 mmol dmA3 H+ (HCl).The sup- porting electrolyte (1 mol dm-3 KCl) should minimize the effect of migration. The limiting current obtained for the Au/ PMPy-PSS electrode is distinctly lower than the current on the bare gold electrode. The Levich-Koutecky analysis of I I ‘I 0.6 1 cy 0.4 r I a E.-. t I I 1 0 0.02 0.04 0.06 1/w’/2 Fig. 11 Levich-Koutecky plots of limiting currents, obtained from experiments as represented in Fig. 10. w is the electrode rotation rate in rpm. (0)Au/PMb-PSS, (V) Au. such results is given in Fig. 11. It demonstrates that the process at the bare metal electrode is controlled by diffusion in the solution, as opposed to the situation in the presence of the polymer film.The results of such analyses show that i, (currents from Levich-Koutecky plots, extrapolated to 1/ 01/2-,0) is inversely proportional to the film thickness (Fig. 12; the film thickness is expressed in terms of the charge used for polymerization). The results indicate that in the case of the coated elec- trodes i, defines the rate of diffusion of H+ ions across the film. This process may be described1g2 by the formula: FADi, = -cP$?d where A is the electrode area and D is the diffusion coefficient of H in the polymer. Using eqn. (1l), the diffusion coefficient + of H+ ions in the polymer film may be determined. To do so, the polymerization charge, 26.2 mC cm-2, is again taken to correspond to a film thickness d = 0.1 pm,” and the ion- 0.15 I -10 :0 -1.2 -1.0 -0.8 -0.6 50 100 150 200 electrode potential, €/V vs.SCE polymerisation charge/mC Fig. 10 Current-potential curves obtained on (a) an uncoated and Fig. 12 Dependence of i, (obtained from Levich-Koutecky plots) (b) a PMPy-PSS-coated rotating-disk (gold) electrode, in 0.02 mol on PMPy-PSS-film thickness (in mC used for polymerization; 73 dm-3 HCI. Polymerization charge, 90 mC for an electrode of area mC correspond to ca. 1 pm2’).Electrolyte: aqueous solution of 20 0.28 an2;rotation speed, 900 rpm; scan rate, 1 mV s-’. mmol dm-3 HCl-1 mol dm-3 KCI. 750 exchange equilibrium constant between H+ and K+ is assumed to be l.28*29The value of the diffusion coefficient determined from the slope of the line in Fig.12 is D = 1 x cm2 s-’,i.e. lower than in aqueous solution by a factor of about 102.30 Conclusions The cathodic hydrogen evolution reaction was studied on gold electrodes coated with poly(N-methylpyrrole) (PMPy), and with a polymer mixture of PMPy and poly(Cstyrenesu1- fonate) (PMPy-PSS). In all cases, the H+ ions were found to permeate the film and react at the metal-electrode surface. In particular, for low concentrations of HCl, the PMPy film hinders significantly the movement of the depolarizer to the electrode. As the HCl concentration in the electrolyte is increased, the permeability of the PMPy film rises consider- ably, probably because of the commencement of protonation and swelling of the PMPy matrix.In the case of PMPy-PSS, H+ transport across the film proceeds readily, because the H+ ions constitute counter-ions to the immobilized sulfonate groups, present at a concentration of ca. 1 mol dm-3. The diffusion coefficient of H+ ions in the PMPy-PSS matrix was found to be D = 1 x cm2 s-’. The effect of the partitioning equilibria and the Donnan potential on the observed potential dependence of the charge- transfer rate is discussed in detail. It is shown that the pres- ence of the cation-exchanger coating (PMPy-PSS) enhances the rate of electrochemical H+ reduction (relative to the uncoated electrode) when the concentration of H+ in the electrolyte is small compared with the fixed-charge concen- tration.The basic reason for this is the fact that the H+ con- centration in this polymer, which is effective in the electron-transfer reaction, is almost unaffected by changes of the H+ concentration in the electrolyte (see Fig. 6). Thus, a change of the electrolyte concentration (at constant E) will affect the charge-transfer rate only uia the Donnan potential. For example, when at constant applied E the electrolyte con- centration is reduced from lOOc, to lOc, (see Fig. 8), the CT rate will be reduced by a factor of about three, compared with a factor of 10 in the absence of the fixed-charge polymer film. K.M. is grateful to Alexander von Humboldt-Stiftung for financial support of his research stay in Germany. References (a) W.J. Albery and A. R. Hillman, Annu. Rep. Prog. Chem., Sect. C, 1981, 78, 377; (b) A. R. Hillman, in Electrochemical Science and Technology of Polymers, Elsevier, London, 1987,vol. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1, pp. 103-292;(c) W. J. Albery and A. R. Hillman, J. Electro-anal. Chem., 1984,170,27. 2 C. P.Andrieux, J-M. Dumas-Bouchiat and J-M. Savant, J. Elec-troanal. Chem., 1982,131,l; 1982,142,l; 1984,169,9. 3 D.L. Miller and J. O’M. Bockris, J. Electrochem. SOC., 1992, 139, 967. 4 A. Deronzier and J-C. Moutet, Acc. Chem. Res., 1989,22,249. 5 G. Bidan, E. M. Genies and M. Lapkowski, J. Chem. SOC.,Chem. Commun., 1988,533. 6 M. Fabrizio, G.Mengoli, M. M. Musiani and F. Paolucci, J. Electroanal. Chem., 1991,300, 23.7 J. Desilvestro and 0.Haas, Electrochim. Acta, 1991,36, 361. 8 F. Mitzutani, S. Iijima, Y. Tanabe and K. Tsuda, J. Chem. SOC., Chem. Commun., 1983,148,299. 9 (a) T. Ikeda, C. R. Leidner and R. W. Murray, J. Electroanal. Chem., 1982, 138, 343; (b) C.R. Leidner and R. W. Murray, J. Am. Chem. SOC., 1984, 106, 1606; (c) B. J. Feldman, A. G. Ewing and R. W. Murray, J. Electroanal. Chem., 1985, 194,63. 10 Y.Xie and F. C. Anson, J. Electroanal. Chem., 1993,344,405. 11 N. Oyama, Y. Ohnuki, K. Chiba and T. Ohsaka, Chem. Lett., 1983,1759. 12 V. E. Kazarinov, M. D. Levi, A. M. Skundin and M. A. Vorotyntsev, J. Electroanal. Chem., 1989,271, 193. 13 M. M. Lohrengel, J. W. Schultze and A. Thyssen, in Elektrisch leitende Kunststoffe, ed.H. J. Mair and S. Roth, Hanser Verlag, Munchen, 1989,p. 377. 14 M.D.Levi and E. Yu. Pisarevskaya, Synth. Met., 1991,45,309. 15 K. Maksymiuk and K. Doblhofer, Electrochim. Acta, 1994, 39, 217. 16 K. Maksymiuk and K. Doblhofer, Synth. Met., 1993,55, 1382. 17 R. A. Bull, F-R. F. Fan and A. J. Bard, J. Electrochem. SOC., 1982,129,1009. 18 R. C. M. Jakobs, L. J. J. Janssen and E. Barendrecht, Electro-chim. Acta, 1985,30, 1085. 19 K. Doblhofer, J. Electroanal. Chem., 1992,331, 1015. 20 B. Pfeiffer, A. Thyssen and J. W. Schultze, J. Electroanal. Chem., 1989,260,393. 21 D.Stockert, M. M. Lohrengel and J. W. Schultze, Synth. Met., 1993,55,1323. 22 Q-X. Zhou, L. L. Miller and J. R. Valentine, J. Electroanal. Chem., 1989,261,147. 23 C. Zhong, K. Doblhofer and G. Weinberg, Faraday Discuss. Chem. SOC., 1989,88,307. 24 C. Zhong and K. Doblhofer, Electrochim. Acta, 1990,35, 1971. 25 K. Doblhofer, in Electrochemistry of Novel Materials, ed. J. Lip- kowski and P. N. Ross, VCH, New York, in the press. 26 Y. Chiang and E.B. Whipple, J. Am. Chem. Soc., 1963,852763; H.Beyer and W. Walter, Lehrbuch der Organischen Chemie, S. Hirzel Verlag, Stuttgart, 21st edn., 1988,p. 708. 27 K.Gossner and C. Lomer, 2.Phys. Chem., Neue Folge, 1963,37, 115. 28 T. R. E. Kressman and J. A. Kitchener, J. Chem. SOC., 1949, 1190; 1201; 1208. 29 S.Peterson, Ann. N. Y. Acad. Sci., 1953,57, 144. 30 G.J. Brug, H. Sluyters-Rehbach, J. H. Sluyters and A. Hamelin, J. Electroanal. Chem., 1984,181,245. Paper 3/05961H; Received 5th October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000745

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 751-754 751 Response Kinetics of Polymer-coated Bulk Acoustic Wave Devices on Exposure to Gases and Vapours Neville J. Freeman,* lain P. May and Donald J. Weir GEC-Marconi Hirst Research Centre, Elstree Way, Borehamwood, Hefts, UK WD6 IRX The response of bulk acoustic wave (BAW) devices coated with various polymeric materials to step changes in vapour concentration is shown to be modelled accurately by the sum of two exponentials. It is apparent that the parameters of the exponentials are dependent upon both the coating material used and the gas or vapour present. An automated method of fitting the response kinetics is described. This is shown to allow character- isation of the sensor’s response before steady state is reached and to provide additional information for classi-fication of the analyte.Chemical sensors which use piezoelectric devices such as surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices (e.g. crystal resonators) to transduce changes in a chemically selective coating material are well known.’-4 These piezoelectric devices can form part of oscillators whose frequency of operation is dependent upon a number of factors of which mass loading is one (hence the term mass balance). A number of equations relating mass and frequency change have been derived of which the Sauerbrey equation’ is most commonly employed for BAW devices: Sf= -2.3 x 1O6f;(6m/A) (1) where 6m is the change in mass (g), A is the piezoelectrically active surface area (cm2) andf, (MHz) is the uncoated fre- quency of the BAW device. The use of piezoelectric devices as chemical sensors was proposed as long ago as 19642 for the measurement of humidity.The piezoelectric transducer’s response to vapours is modified by coating the surface with a suitable material such as a gas chromatograph (GC) stationary phase. As vapours concentrate in the surface coating, the change in the coating’s acoustic properties alters (generally reduces) the fre- quency of oscillation. Although much empirical work on the identification of suitable coatings for given applications has been reported in the literat~re~-~ less attention has been paid to the physical changes occurring in polymeric coatings on absorbing vapours and their consequences for the frequency of operation.Recent studies on the response of piezoelectric sensors (both SAW and BAW device^)^,^ have investigated the importance of viscoelastic changes in coatings applied to such devices on absorption of vapours. The term ‘mass balance’ should therefore be used with some caution when referring to coated piezoelectric devices. In the current paper, the response kinetics of piezoelectric crystals with various coatings are modelled and an automatic method of fitting the model is presented. The response is accurately modelled by the sum of two exponentials implying that two concomitant physical processes are occurring. It is shown that this model allows the characterisation of the response before steady state is reached and provides additional information for the classi- fication of the analyte.Experimental Apparatus ated vapours were passed to a mixer module where flow dilution took place as required. Finally the mixed vapour stream was passed into the sensor chamber which was placed in a second water bath (Grant Instruments) held at 25°C. The vapour generation system and sensor chamber were designed to minimise the dead-space in the sensor chamber which was estimated to be ca. 15 cm3. Frequency changes were measured with respect to a reference source (Marconi Instruments 2022D) whose frequency was set above that of the coated crystal. Thus on exposure to the vapour the fun- damental frequency of the crystal falls and the difference between the two increases.A personal computer (Opus PCV) was used to control the experiments and log the data. The experiments consisted of repeats of four distinct phases: initially the sensor chamber was flushed with nitro- gen (60 min), after which the carrier stream was bubbled through the sample liquid for 30 min in order to equilibrate the generated vapour, while the nitrogen purge of the sensor chamber continued. Following the completion of these two phases the sensors were alternately exposed to the sample vapour and purged with laboratory nitrogen for periods shown in the results. The sequence of events was computer controlled by switching a set of solenoid valves (Precision Dynamics). Chemicals Toluene, methanol, chloroform and dichloromethane AnalaR Grade (BDH Ltd.) were used as supplied.Coating materials AT1000, OW01 and Carbowax were obtained from Alltech Ltd. and ethyl cellulose from Aldrich Ltd. Samples of linalol, nitrogen carrier gas control I-I -flow . control sensormixer I chamberI liquid sample IVapours were generated using an automated system (outlined exhaust gas in Fig. 1) in which gas wash bottles were placed in a water Fig. 1 A schematic diagram of the vapour generation system used. bath (Grant Instruments) at 20°C in order to saturate a dry- The sensor chamber is switched between diluted sample vapour and nitrogen carrier stream flowing at 200 ml min-’. The satur- laboratory nitrogen. 752 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 dibutyl sulfide and isoamyl acetate were provided by Firme- 6.0 nich SA. 5g 5.0 Preparation of Sensors 4.0 10 MHz (fundamental frequency) AT cut quartz crystals with u gold-plated electrodes (McKnight) were spray coated using an air brush charged with dilute solutions of the required Y-~~~~coating material in an appropriate solvent. The coatings were w 1.0 typically applied until the fundamental frequency was 0 100 200 300 400 500 600 depressed by ca. 10 kHz (in laboratory air environment) time/s which may be estimated to be equivalent to a coating thick- ness of a few hundred nanometres using eqn. (1) and the density of the coatings (assuming the coatings to be elastic). 2950 T Results and Discussion Fig.2 shows an example response from an ethyl cellulose coating piezoelectric crystal sensor when exposed to a toluene loaded vapour and then ‘purged’ with laboratory nitrogen. The difference frequency is defined throughout as a reference frequency (arbitrarily set at 10 MHz for convenience) minus the frequency of oscillation of the coated crystal. The frequency response is defined as the absolute fre- quency of the coated crystal, which decreases during the sample phase as the mass loading increases and conversely increases during the purge phase as the mass loading decreases. Initially a number of functions were fitted by hand to the ‘expose’ and ‘purge’ phases separately including single I and multiple exponentials and polynomials.The natural L logarithm of the response is shown in Fig. 3(a) in order to illustrate the poor fit obtained with a single exponential. Of the functions tried, the sum of two exponentials appears to give the best fit: 0 200 400 600 800 1000 1200 time/sR(t) = a, -a2 exp(-a,t) Fig. 3 (a) Natural logarithm of the purge phase of the response shown in Fig. 2 after removal of the dc offset. If the response consist- -a4 exp(-a,[); sample phase (2) ed of a single exponential, this plot should be a straight line. (b) The R(t)= a, -a2[1 -exp(-a,t)] example response shown in Fig. 2 (-) and the associated dual- exponential fit (offset by 10 Hz for clarity) (---). -a4[ 1 -exp(-a5t)] ; purge phase (3) An example of the fit obtained is shown in Fig.3(b). In order to ensure that the observed kinetics related to the are significantly altered by the coating used. Further tests vapour and coating properties rather those those of the showed that while addition of more coating increased the apparatus, a number of differently coated crystals were amplitude of the response the time constants were not signifi- exposed to the same vapour in the same apparatus. The cantly affected. results (see Fig. 4) clearly indicate that the response kinetics 2950 T 2650 f 2600 2550 L 2450 0 40 80 120 160 200 240 280 320 360 400 4402500t 0 200 400 600 800 1000 1200 percentage from final estimate ti me/s Fig. 4 Response of four differently coated crystal resonators to the Fig. 2 An example of the response of an ethyl cellulose coated same vapour in the experimental apparatus.The responses have been crystal on exposure to toluene vapour (1-600 s) and subsequently to expressed as a percentage of the steady-state amplitude in order to laboratory nitrogen (600-1200 s). The response is measured in terms illustrate the different rate constants observed with each coating of the difference in frequency between a fixed reference source and material. (a) AT1000, (b) OV101,(c) Carbowax 20 m, (d)ethyl cellu- the coated crystal. lose. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 In order to evaluate the ability of the dual-exponential function to fit the kinetics of the response of piezoelectric devices across a number of coatings and analytes, an auto- mated method of fitting is required.Unfortunately, the fitted function is non-linear with respect to the two rate constant parameters and no linearising transform (e.g. logarithms) is obvious. An iterative fitting process was therefore adopted based on the Levenburg-Marquadht (LM) alg~rithm.~ Initial work with synthetic data demonstrated that the fitting error was a unimodal function of the parameters (al-,), but that the gradient of the error surface could be shallow leading to slow convergence (see Fig. 5). The LM algorithm is well suited to such surfaces as it transforms from a simple gra- dient descent scheme into a quadratic root finding method as it approaches the minimum, thus reducing the convergence time.It was found that the iterative fitting process could fail if the sign of the estimate of one of the two rate constant parameters (a2,u4) went negative as this led the value of R(t) to increase exponentially with time and in some cases caused the calculation program to overflow. The algorithm was modified to reset the value of the rate constant parameters to a small positive value if they went negative. This modification removed the overflow problem and did not appear to prevent convergence. In order to generate statistics concerning the estimated parameters, it is necessary to know the noise associated with each measurement. The noise model used was based on known inaccuracies in the measurement process, i.e. the sam- pling (k0.5s) and gain time (k1 Hz).If only the first expo- nential is considered and the distribution of errors is assumed to be rectangular, the standard deviation associated with each measurement can be computed as : dt)= (C2.O + a2a3 exp(-a3 t)]2/12.0)1'2 (4) A plot of the estimate of a(t) for the example data shown in Fig. 2 is given in Fig. 6. If this noise model is taken as a good estimate then the fit shown in Fig. 3 is found to be significant at the 99.9% confidence level. This noise model is likely to be conservative as it does not include the second exponential or other noise sources (e.g. temperature variation). Inclusion of the noise model into the fitting process, rather than assuming a uniform value of a(t),weighted the different regions of mea- sured response and was found to speed up convergence con- siderably.Further work showed that the rate of convergence was also highly dependent on the quality of the initial estimate of the parameters. This initial estimate was calculated according to the heuristic process in Fig. 7. This makes a number of assumptions based on fitting several responses by hand. For example, the larger of the two exponentials is assumed to account for 90% of the steady-state response and to have the larger rate constant. The uniqueness of fit was tested on a number of experimental results in which the rate constant 45000 TI 40000 i! 35000 5 2 30000 25000 20000 15000 -50-40-30-20-10 0 10 20 30 40 50 percentage from final estimate Fig.5 Variation in 2' as a function of the value of one of the parameters of the dual-exponential equation. While the function is unimodal it is also shallow and gradient-based techniques may there- fore take some time to converge. =. 753 3000 T T 25 I 2900 20 128005 27001 2600 'c 2500 2400 , \z -o 2300 2200 ~ =m m....a.mm..m..."m==~==m==,a~e=ee.ee=n,~e ~ '~~'~0 ~'' 0 7.5 15.0 30.0 45.0 60.0 75.0 ti me/s Fig. 6 Estimate of ~(t)for the example data shown in Fig. 2: (0) difference frequency, (u)estimated standard deviation estimates (a3 and a,) were deliberately swapped. The result- ant fits yielded results for al-as which were within two stan- dard deviations of those obtained with the 'normal' method of estimation demonstrating the robustness of the final solu- tion.The incorporation of this initial heuristic estimate was found to improve the rate of convergence still further. In order to assess the reproducibility of the algorithm it was tested on six successive exposures to one vapour. The results of this are given in Table 1 and show that the stan- dard deviation of the parameter estimate was never more than 5% of the mean. The algorithm was also tested for tirne/s Diagram illustrating the heuristic method used to determine aninitial estimate of the parameter values of the fitting equation. The rate constant of the fast exponential (a3)is calculated from the time taken for the response to fall by half the steady-state amplitude, while the slow rate constant (as)is estimated from the time taken to com-plete the last 10%of the response's amplitude. Table 1 Estimated response parameters of six consecutive 'remove' phases following exposure to toluene vapour repeat number a1 a, a3 a4 a5 probability 248 7 430 0.110 36.2 0.018 1 .Ooo 2483 439 0.110 33.6 0.016 0.999 248 1 433 0.110 35.0 0.016 0.999 2480 429 0.111 35.3 0.016 1.Ooo 2480 447 0.113 38.1 0.017 1.Ooo 2479 444 0.110 35.8 0.016 1.Ooo mean 2482 431 0.111 35.7 0.017 1.Ooo standard 3 8 0.001 1.5 0.001 0.001 deviation 754 Table 2 Estimated response parameters from decreasing data lengths” ~ n um ber of samples a1 a2 a4 a5 42 1 2395 495 0.213 15.3 0.027 300 2385 503 0.218 17.1 0.031 200 2380 507 0.220 18.4 0.033 100 2381 505 0.219 18.0 0.032 50 2302 514 0.283 86.9’ 0.098’ 25 2303 511 0.283 88.6’ 0.100’ 10 2320 440’ 0.236’ 130.3’ 0.260’ ” Sampling interval, 1.5 s.’The parameters were insignificantly dif- ferent from zero at the 95% confidence level. robustness to short data lengths. The results of this are shown in Table 2. While the parameter estimates do change with data length, this is not entirely surprising as in the case with 50 samples an exponential with a half-time of ca. 30 s is being estimated in combination with an exponential whose amplitude is 25 times larger over a period of some 75 s. Further, note that the cases of significant parameter variation may be detected from the statistics generated by the pro- cedure.These results demonstrate that the parameters of the sensor response may be estimated from an initial portion of the response before steady state is reached, thereby poten- tially speeding up sensor operation. BAW devices are frequently referred to as ‘mass balance’ devices. A simple mass absorption model is likely to lead to responses to step changes in vapour concentrations which are well fitted by a single exponential. This model is supported by the general observation that increasing the amount of coating material deposited on the surface of the device (and hence the volume of coating material) leads to an increase in the sensitivity of piezoelectric devices. The results here suggest that a secondary process is occurring which typically accounts for 10% of the overall response.The nature of the second process has not yet been positively identified but can- didates include a separate surface adsorption phenomenon and changes in the viscoelastic properties of the coating as the concentration of vapour in the material builds up. Our lack of knowledge of the identity of the mechanisms underlying the response kinetics does not prevent us from exploiting them to provide additional information from the sensor to classify the atmosphere it is sensing. Fig. 8 shows the response kinetics of an ethyl cellulose coated crystal to three odorous compounds, while Table 3 gives the estimated value of the fast exponential’s rate constant (parameter a3) and the standard deviation of the estimate.These results demonstrate that it is possible to discriminate between these three compounds using a single sensor. This discrimination could be made irrespective of compound concentration or the amount of coating on the device as these factors do not Table 3 Estimated rate constant of the fast exponential part of the ‘remove’ phase of the response of an ethyl cellulose coated crystal resonator to three compounds, and the standard deviation of the estimate standard deviation compound rate constant, a, of estimate dibutyl sulfide 0.050 0.002 linalol 0.030 0.001 isoamyl acetate 0.09 1 0.009 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1001 , i ” 0 50 100 150 200 250 300 350 400 450 500 time/s Fig.8 Response of an ethyl cellulose coated sensor to three differ- ent compounds. The responses have been expressed as a percentage of the steady-state amplitude in order to make the variation in the rate constants clear. (a)Dibutyl sulfide, (b)linalol, (c)isoamyl acetate. change the rate constant of the response, merely its ampli- tude, The characterisation of the response kinetics therefore offers more information for classification of the analyte than the steady-state response amplitude alone. Conclusions The response kinetics of coated piezoelectric crystal reson- ators to a variety of vapours appears to be well modelled by the sum of two exponentials. The parameters of these expo- nentials depend on the chemical nature of both the coating and the vapour.An automated method of estimating the parameters of the non-linear kinetics equation is presented which is relatively robust to short data lengths. This allows the response of piezoelectric sensors to be characterised before steady state is reached, potentially reducing sampling time. The accuracy of the dual-exponential fit suggests that two concurrent processes are occurring such as diffusion into the coating and changes in the coating’s viscoelastic proper- ties. Characterisation of the sensor’s response in this way has been shown to offer more information for the classification of the analyte than use of the steady-state response amplitude alone. The rs gratefully acknowledge support from Firmenich SA ould like to thank Dr. L. Wunsche and Dr. M. Kearney Ior useful discussions. References G. Sauerbrey, Z. Phys., 1959,155,206. W. H. King, Anal. Chem., 1964,36,1735. J. J. McCallum, Analyst (London), 1989, 144, 1173. C. G. Fox and J. F. Alder, Analyst (London), 1989,114,997. J. W. Grate and M. H. Abraham, Sensors Actuators B, 1991,3,85. J. B. Cooper, J. H. Edmondson, D. M. Joseph and R. S. Newbo-wer, IEEE Trans. Biomed. Eng., 1981,243,459. J. W. Grate, M. Klusty, R. A. McGill, M. H. Abraham, G. Whiting and J. Andonian-Haftvan, Anal. Chem., 1992,64,610. S. J. Martin and G. C. Frye, Appl. Phys. Lett., 1990,57, 1867. W. H. Press, B. P. Flannery, S. A. Teukolsky and W. T. Vetter-ling, Numerical Recipes in C, Cambridge University Press, Cam- bridge, 1988. Paper 3/05198F; Received 31st August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000751

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 755-762 Influence of Structure on the Optical Spectra of Eu3+ in Pb(PO,), Glass :Molecular Dynamics Simulation and Crystal-field Theory G. Cormier and J. A. Capobianco" Department of Chemistry and Biochemistry, Concordia University, 1455 de Maisonneuve Blvd. W., Montreal, Canada H3G IM8 C. A. Morrison Microphotonics Division, U.S.Army Research Laboratory, Adelphi, MD, 20783,USA An investigation of the structural factors which lead to the marked differences between various spectral features of rare-earth-metal ions doped in metal metaphosphate and silicate glasses is reported. The investigation was based on a simulated structural/spectral model of an Eu3+-doped lead metaphosphate glass [Eu3+ : Pb(PO,),] that was compared to a previously reported Eu3+-doped sodium disilicate glass [Eu3+ : Na,Si,O,].The models were generated with a computational method that couples molecular dynamics simulation and point-charge crystal-field calculations. It is proposed that the marked differences in several spectroscopic features of Eu3+ ions doped in a lead metaphosphate glass are essentially due to a reduction in the width of the energetic distribution of local fields experienced by the Eu3+ ions. This distribution is shown to be influenced considerably by the presence of medium-range order in the local environment of the Eu3+ ions due to the lack of rigidity of the phosphate backbone. It is well known that optical properties of rare-earth-metal ions doped in glasses are closely related to the glass structure and composition.For the past 30 years, much emphasis has been placed on relating the chemical bonding, symmetry, and coordination within the glass network to the observed optical properties. Since the advent of the tuneable laser, energy- selective spectroscopic techniques, such as laser-induced fluo-rescence line narrowing (FLN), have been used extensively in attempts to elucidate the structural dependence of the lumi- nescent properties of rare-earth-metal doped glasses. Some insight into the rare-earth-metal local environment has been gained from such studies. Nevertheless, FLN experiments still correspond to the investigation of a macroscopic behaviour of a doped glass. Thus, any attempts to infer structural infor- mation from these experiments is complicated by the pres- ence of an overwhelming amount of accidental degeneracy.For this reason, it follows that the search for uniqueness in an atomic structural model for doped glasses, several of which have been proposed through the years,'-' might well be futile. Recently we have demonstrated"-' a new computational technique that is capable of linking experimentally deter- mined optical spectra of a doped glass and a simulated atomic structural model of the glass. This computational technique is related to ab initio crystal-field calculations, exemplified by the lattice summation technique,'3*'4 where crystal-field parameters are derived from the interaction between the impurity ion and the electrostatic potential of the surrounding lattice.In previous papers, we have used molecular dynamics (MD), an atomic-level structural simula- tion technique, to simulate a structural model of an Eu3+-doped sodium disilicate glass. Knowing the position and charge of each atom, we can calculate the electrostatic poten- tial at every individual rare-earth-metal site by summing each individual atomic contribution. Solutions to the electronic crystal-field Hamiltonian can then be calculated from the crystal-field parameters generated from the simulated glass matrix for each of the individual doped rare-earth-metal ions. Thus, we can calculate the splitting of each J manifold and the transition probabilities between all individual com-ponents of each J manifold.Once convoluted, this informa- tion permits the simulation of the optical absorption and emission spectra of rare-earth-metal ions doped in an amorphous matrix. Previously, we presented (i) an analysis of the local structure of Eu3+ ions doped into simulated amorphous silica (SO, : Eu3+) and sodium disilicate glass (Na,O -2Si0, :Eu3+)and (ii) a validation of the simulated structure of the doped sodium disilicate glass via the simula- tion of the optical spectroscopy (absorption and emission) of the Eu3+ ions doped in this glass. The aims of this research are (i) a detailed study of the local environment of Eu3+ ions doped into a lead meta- phosphate glass [Pb(PO,),], (ii) the simulation of the optical spectrum of the Eu3+ ions with the computational method outlined in the previous paragraph and (iii) a comparison between the structural/spectral models of Eu3+-doped lead metaphosphate and Eu3+-doped sodium disilicate glasses, in order to elucidate the structural factors that influence various spectral features observed in these glasses.Lead metaphosphate was chosen as the base glass because of the extensive studies of the optical and spectroscopic properties of rare-earth-metal doped metal phosphates that have been performed in the past 20 year^."-'^ The interest in such glasses lies in their ease of fabrication and the extremely wide range of compositional possibilities with which to tailor physical (optical and mechanical) properties of interest for specific technological applications.In a recent paper, we have simulated the glass structure of lead meta- phosphate using the molecular dynamics method." We pre- sented a model of the short- and medium-range structure, together with an independent confirmation of the presence of previously observed two-dimensional chain structures,20 rather than the conventional three-dimensional frame-work2'-22 postulated in other oxide (e.g. silicate, borate) glasses. Experimental The laboratory sample of Eu3+-doped lead metaphosphate glass, which is referred to in the remainder of this text, has been studied previously by absorption, emission and FLN spectros~opies.~~.~~The experimental details concerning the fabrication of this sample are found in ref.23 and 24. The room-temperature absorption spectrum of the experimental sample was redone with greater spectral resolution. The spec- trum was recorded in the region 380-600 nm, using a Cary 5E spectrophotometer with a 0.3 nm spectral bandwidth, a signal-averaging time of 1 s, and a step size of 0.1 nm. The fluorescence spectrum of the laboratory glass presented in this study was taken from the original spectral data gra- The reader ciously supplied by Dr. P-P. Pro~lx.~~~~~is directed to these references for experimental details. Simulation Procedures Molecular Dynamics Simulation The force law used in the molecular dynamics calculations is derived from a pairwise (two-body) ionic potential, which includes the Pauling repulsive term.It is of the same form as .~that described by Mitra et ~2 and was used by ~ us in~ previously reported The associated force 2719 law is found to be where qi and q, are the ionic charges, uiand ajare the ionic radii of the atoms i andj, n is a measure of the hardness of the repulsion, and rij is the distance between atoms i andj. The various parameters found in the force law were pre-viously published with the MD simulation of (i) undoped lead metaphosphate” (parameters for phosphorus, lead and oxygen) and (ii) Eu3+-doped in sodium disilicate10.12 (parameters for europium). Table 1 presents the ionic param- eters used with the force law. The instantaneous force, for solving the Newtonian equa- tions of motion, was determined for each ion over the set of atomic neighbours within 5.5 A using a screened Coulomb force.The length of 5.5 A is large enough to include the neighbours of importance (ca. 100 ions) and small enough that any increase will not have any effect on the structural characteristics of the glass. Once the instantaneous force on each atom was computed, at each time step (At = 1.0 x s), there was an update of the atomic coor- dinates and velocities using Verlet’s alg~rithm.~’ The composition of the simulated glass is given in Table 2, with other relevant parameters. A cubic cell was used in these calculations with periodic boundary conditions to eliminate the possibility of surface effects. The simulations were carried out at constant volume for each temperature step.The size of the cell, for the lowest-temperature steps, was adjusted to give the correct room-temperature density of undoped Pb(PO& glass, which was determined to be p = 4.74 g ern-,. For the Table 1 Force-law parameters used in the simulations element ionic radius, a/%i ionic charge, q/e 0 1.20 -1.136 P 0.15 2.840 Pb 0.99 1.136 Eu 1.oo 1.704 Hardness parameter, n = 10. Table 2 Simulation parameters for Eu3+-doped Pb(PO,), glass no. of 0 ions 420 no. of P ions 140 no. of Pb ions 67 no. of Eu ions 2 simulated density/g cmP3 4.74 oxygen molar volume/cm (mol O2-) -12.894 length of box side/%i 20.795 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 highest-temperature steps, the size of the box was increased slightly in order to simulate the effect of expansion due to temperature.The initial set of coordinates for the atomic ensemble, which contained 70 molecules of Pb(PO,), ,was derived from the unit cell of crystalline lead metaphosphate.28 This initial atomic ensemble was heated from 300 to 15000 K in a total of 25000 time steps of 1 fs. At 15000 K, we substituted three arbitrarily chosen lead atoms with two europium atoms, generating an atomic ensemble with the composition 2Eu(PO,), -67Pb(PO,), . The quenching procedure for the doped glass was as follows. The doped melt was thermalised at 15OOO K for an additional 200 0oO time steps (20 ps). The temperature of the melt was then lowered to 7500 K and then to 5000 K, each in 2000000 time steps.The melt was then allowed to thermalise at 2500 K for a duration of 18750 ~~ (75 x 250) time steps. After each 250th time-step, a configu- ration was stored for further processing. This ensured a total of 75 configurations, for a total of 150 europium ions. Each of these 75 configurations then underwent quenching from this initial temperature of 2500 to 300K in four steps at 2500, 1250, 600 and 300 K, each of 30000 time steps. Finally, each configuration was thermalised at 300 K for an additional 30000 time steps. Therefore, each of the 75 configurations of a doped glass was simulated between 750250 and 738 750 time steps for a total of 750-739 ps. The quenching rate was calculated to be ca.2 x lo1, K s-’. The simulated structure was verified throughout the run by monitoring various parameters, including average atomic dis- placements, to dispel the possibility of diffusion at the final temperatures of the quenching procedure. Pair, cumulative and bond-angle distribution functions were calculated and averaged throughout the quenching procedure for each con- figuration. However, the structural parameters presented in this paper are given for a temperature of 300 K. An averaging of the pair distribution function (PDF), cumulative distribu- tion function (CDF) and bond-angle distribution was per- formed for the last loo00 time steps of each temperature run, with a sample taken after every 10 time steps. The distribu- tion functions reported had step increments of 0.1 A for the PDF and CDF and of 1”for the bond-angle distribution and were an average of all of the 75 configurations. Optical Spectroscopy Simulation The crystal-field Hamiltonian that describes the interaction of the Eu3+ion with the host lattice can be written asz9 ZCEF = 1A,*, 1CCnm(ii) (2) nrn I where the first sum covers those values of n and m allowed by the symmetry of the site of the rare-earth metal. With n even, eqn.(2) was used to calculate the crystal-field splittings; for n odd, it was used to calculate transition probabilities. The second sum is over i = 6 electrons of the 4f6 configuration of the Eu3+ ion. The simplest description of the crystal field uses the point-charge model, in which the atoms of the lattice surrounding the rare-earth-metal are described by point charges.This model neglects both the finite spatial extent of the ligand charge density and the wavefunction overlap of the optically active 4f electrons with the ligands.” For point charges, eqj located at R,, the crystal-field components, A,, , of eqn. (2) are given by (3) In eqn. (2) and (3), the irreducible spherical tensors, C,,(r), are related to the spherical harmonics Y,,(8, 4).,O J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Using the atomic positions obtained from the molecular dynamics simulation and choosing appropriate ionic charges for each atom type, we can calculate the crystal-field com- ponents, A,, .Using the three-parameter theory of crystal fields proposed by Leavitt et we can calculate the crystal-field parameters, B,, .In order to calculate the intensity of line-to-line transitions in the simulated emission and absorption spectra, we used (i) the oscillator strength between the individual components a and b,hb, for the absorption process32 and (ii) the transition probability between the individual components a and b, Azr, for the emission process.32 The individual components a and b belong to the initial and final electronic manifolds of the electronic transition studied. Both the oscillator strengths and the transition probabilities require the calculation of the radiative line strength, sob, which, according to Condon and Sh~rtely,~~is given by the square of the following matrix element: sab =I<bI Ia> l2 (4) where P is the appropriate operator (electric or magnetic dipole).Electric-dipole intensity calculations were performed using the 'full' Judd-Ofelt theory of induced electric dipole transition^.^^.^^ Within the electronic configuration 4f ",mag-netic dipole transitions are parity allowed. Thus, the calcu- lation of the line strength is more straighforward than for the electric dipole case. With the proper dipole moment operator, eqn. (4) can be directly used to calculate the magnetic dipole line strength. Finally, one must note that J-mixing of eigen- states is included in the calculation of the magnetic-dipole and electric-dipole line strengths. In order to generate a graphic representation of the simu- lated emission and absorption spectra, the calculated energies are collated and sorted.A Gaussian bandshape is assigned to each of the energies. The spectral envelope, Z(E),is given by" where the first sum is over the No =150 Eu3+ configurations and the second sum is over the 49 possible line-to-line 'Do +7F, (J =0-6) transitions for the emission spectrum or is over the (4 x 29) possible line-to-line 'L,, 5D3, 5D2, 'D1, 5D,t7F,,, transitions for the absorption spectrum, The Ek,abare the he-to-line transition energies for each of the Eu3+ ions, such that Ek,ab =IE, -Eb Ik. The width, W, Of each individual Gaussian has been chosen to be ca. 5 cm-' for the emission spectrum and ca.10 cm-I for the absorption spectrum. These widths were chosen so that the 150 Eu3+ ions of the simulated glass effectively represent the macro- scopic ensemble of doped ions found in the experimental glass. The difference in these widths stems from the fact that the two experimental spectra (absorption and emission) were taken at different resolutions. The intensity parameters, ILTrb, found in eqn. (9,where type represents either absorption or emission, are derived from the calculated oscillator strengths or transition probabilities, respectively. The intensity param- eter calculated for the emission process also includes the radi- ative branching ratio pertaining to the calculated radiative transition. Results The average short-range environment of the europium ion has been verified by (i) the calculated Eu-0 pair and cumula- tive distribution functions and (ii) the Eu-M (M =Pb and P) pair distribution functions (PDF).These distribution func- tions, obtained from the room-temperature simulated struc- ture, are shown in Fig. 1 and 2. The average Eu-0 200 225 250 575 3ffi 225 350 375 402I distance/A 20 25 30 35 40 45 50 55 60 65 70 75 80 distance/A Fig. 1 Pair distribution function of 0-Eu atomic pair for simulated Eu3+ :Pb(PO,), glass. Inset shows cumulative distribution function of the 0-Eu pair. interatomic distance, in its first coordination shell, was calcu- lated to be 2.45 A, with a full width at half maximum (FWHM) of 0.28 A. The distribution function does not return to a null value after the first peak, indicating that no clear distinction exists between the first and second coordination shells. For this reason, the average coordination number of the Eu-0 first-shell polyhedra is difficult to assess.The first minimum between the first and second coordination shells occurs at ca. 3.2 A. At this cut-off distance, the average coor- dination of the Eu polyhedra is found to be 6.2 oxygen atoms, as determined from the cumulative distribution func- tion of the Eu-0 pair, shown in the inset to Fig. 1. Fig. 2 show the P-Eu, PbEu pair distribution functions for the simulated Eu3+ :Pb(PO,), glass, together with the Si-Eu PDF for the simulated Eu3+ :Na20* 2Si0, glass that was studied previously." The P-Eu and PbEu PDFs for the simulated metaphosphate glass have maxima at 3.75 8, 3 2.0 .-v)'I y 10 a 20 25 30 35 40 45 50 55 60 65 70 75 80 distance/A Fig.2 Eu-M [M =Pb (. ..)and P (-)I pair distribution func- tions for simulated Eu3+ :Pb(PO,), glass. Also shown is Eu-Si PDF for simulated Eu3+ :NaSi,O, glass [(---) taken from ref. lo]. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 (FWHM = 0.41 A) and 4.55 8, (FWHM = 2.32 A), respec-tively. We notice that the P-Eu PDF has sharp, surprisingly well defined, first and second coordination shell peaks, whilst the Pb-Eu PDF is quite the contrary, presenting broad, undefined peaks. This is not unexpected owing to the differ- ent function of each cation in the glass matrix, the former being a glass-former, the latter a network modifier.Fig. 3 pre-sents a comparison of the probability distribution of oxygen T 0 1 2 3 4 5 6 7 8 9 10 coordination Fig. 3 Probability distribution of oxygen neighbours about an Eu3+ ion within a coordination sphere of 3.2 A. Comparison between simulated Eu3+ :Pb(PO,), (a)and Eu3+ :NaSi,O, (0) glasses (taken from ref. 10) 0 25 scale x 10----f E 380 400 420 440 460 480 500 520 540 560 580 600 I/nm Fig. 4 Comparison between room-temperature 5L,, 'D,, ,,,,,t 7F0,absorption spectra of experimental (-) and simulated (. . .) Eu3'+:Pb(PO,), glasses Table 4 Absorption barycentres and linewidths of the experimental and simulated Eu3 + : Pb(PO,), glasses' Eu" : Pb(PO,), Eu3+ : Pb(PO,), experimental glassb simulated glass barycentre FWHM assignment /cm-' /cm -' barycentre FWHM /cm-' /cm-' 'Do t7F1 17004 253 16 957 256 'Do t7F0 17 280 42 17 279 60 'D, t7F, 18 695 206 18 730 164 'D, t7F0 19 026 47 19 024 72 'D, +'F, 'D, + 7F, 'D, t7F, 21 519 24 137 -130 208 - 21 265 21 546 24 064 z 250 72 260 'D, t'F, -- 24 377 88 'L, t7F, -- -- 'L, t7F, 25 413 84 25 388 144 a Only the electronic transitions which have been simulated are reported.Results taken from ref. 23 and 24. +neighbours about an Eu3 ion between the simulated Eu3+ : Pb(PO,), and Eu3+ : NaSi,O, glasses. The coordi- nation number represents the number of oxygen ions found surrounding a Eu3+ ion within a coordination sphere of 3.2 A.This figure shows that although average coordination numbers are similar between the doped phosphate and sili- cate glass, the overall distributions of local environments are drastically different. Table 3 summarises the structural parameters derived from the pair and cumulative distribution functions for the first and second coordination shells of the various atomic pairs present in the simulated Eu3+: Pb(PO,), glass. In this table, the average interatomic distances and their associated widths (FWHM) are shown as well as average coordination numbers. The number in brackets, found in the last column of this table, refers to the distance at which the average coor- dination number has been calculated. These distances were obtained from the position of the first minima situated between the first and second coordination shell peaks. We have given the details, in a previous paper," of the calculational steps required to obtain the simulated optical spectra.The first and most important of these, is the proper choice of partial ionic charges for all the atomic species present in the simulated glass. The charges have a direct influence on the position and the width of each of the simu- lated electronic transitions. This influence stems from the simulation of an overall covalency effect between locally bound ions. The partial charges that were used in the calcu- lation of the crystal-field parameters [eqn. (3)] have been determined empirically (under a certain set of specific conditions) and produce a proper simulation of the various spectral features found in both the absorption and emission spectra.The first of the above-mentioned conditions was to maintain strict electroneutrality of the atomic ensemble. Sec- ondly, the charge of the europium ion was kept at full value, representing a complete electrostatic interaction with its sur- rounding ligands. Thirdly, in order to simulate partial cova- lency of the Pb2+ ion interaction^,^^ the lead ion's charge was fixed at a slightly lower value than its full value. These conditions lead to the following set of partial charges: oxygen = -0.94e phosphorus = +2.4593e lead = + 1.50e europium = +3.00e which were found to yield simulated spectra, in excellent agreement with their laboratory counterparts. Table 3 Structural parameters derived from the pair and cumulative distribution functions for first and second coordination shells of the various atomic pairs present in the Eu3+ :Pb(PO,), glass atomic pair 1st peak/A FWHMIA 2nd peak/A FWHM/A 0-0 2.45 0.2 1 3.2 & 5.1 0.67 & 1.7 0-P 1.51 0.18 3.85 0.65 0-Pb 2.43 0.40 4.5 1.5 0-EU 2.45 0.28 4.5 1.22 P-P 3.04 0.25 4.95 1.2 P-Pb 3.71 0.60 5.0 1.3 P-EU 3.75 0.41 5.1 1.19 Pb-Pb ca.3.9 ca. 2.4 ca. 7.0 - Pb-Eu 4.55 2.32 ca. 7 - coordination no. (distance/A) 5.82 (3.2) 4.00 (2.1) 6.94 (3.4) 6.20 (3.2) 2.00 (3.5) 5.50 (4.3) 5.78 (4.3) 5.00 (5.5) 5.03 (5.8) J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Discussion .-c 20"i 3 > c.-gc .-580 600 620 640 660 680 700 720 A/nmFig. 5 Comparison between room-temperature 5D0+ 7F0,,.2, ,, emission spectra of experimental (-) and simulated (. . -) Eu3+ : Pb(PO,), glasses Fig. 4 shows a comparison between the absorption spec- trum of the laboratory glass to the spectrum of the computer- generated Pb(PO,), : Eu3+ glass, in the region between 380 and 600 nm. The relative population of the 7F0and 7F1man-ifolds was taken into account in the simulated absorption spectrum by calculating the Boltzmann distribution at a tem- perature of 300 K for each of the Eu3+ ions and incorpor- ating the results in eqn. (5). Assignments, band positions and widths of the measured and simulated absorption spectra for the Pb(PO,), : Eu3+ glass are presented in Table 4.We observe, in Fig. 4, that an excellent agreement exists between the barycentre positions, widths, and overall shapes of the optical transitions, for the two spectra (simulated and labor- atory glass). The 'Do -+ 7F, (J = 0-4) simulated and experimental emission spectra are shown in Fig. 5. The spectral region investigated is between 570 and 720 nm. Although the simu- lated 'Do -+ 7FS,6 transitions were calculated, they are not shown because no emission was detected in these regions for the laboratory glass. Table 5 presents a comparison of the emission barycentres and their associated linewidths for the simulated and laboratory sample of the Eu3+ : Pb(PO,), glass.Although the comparison between the experimental and the simulated absorption and emission spectra is more than qualitatively adequate, certain discrepancies can be found in both simulated spectra, especially regarding the simulations of the hypersensitive transitions 5Do-,7F2 and 'D, t7F0. These discrepancies have been discussed previously,' ',3 and since no attempt has been made to alleviate the problems presented by the absence of dynamic coupling in the crystal- field calculations, the problem and its solution remain essen- tially the same. Table 5 Emission barycentres and linewidths of the experimental and simulated Eu3+ : Pb(PO,), glasses Eu3+ : Pb(PO,), experimental glass Eu3+ : Pb(PO,), simulated glass assignment barycentre/cm- FW HM /cm - barycen tre /cm - FWHM /cm - 5D0+ 7F0 17 283 48 17 282 68 'Do + 7F,5DO+7F2 16 900 16244 300 368 16 901 16 283 316 268 5D0+ 'F,5D0+ 7F, 15295 14292 188 112 15 335 14 316 122 218 Extensive investigations of the luminescent properties of Eu3+ ions (or any other trivalent rare-earth-metal ion for that matter) doped into oxide glasses have shown that absorption and emission spectra can be affected by varying the glass composition.This phenomenon is exemplified by the wide range of physically different local environments experienced by Nd3+ ions in various oxide, fluoride and mixed anion glasses, which leads to extensive variations in barycentre positions and widths as shown by their respective absorption3* and emission39 spectra.A more specific example is the difference in position and width exhibited by the 5D0--+ 7F0transition of the Eu3+ ion doped into a sili- cate glass and a phosphate glass. In a europium-doped mag- nesium aluminosilicate glass of the cordierite stoichiometry (MgO . AI2O3* 2.5Si02),40 the previously mentioned tran-sition is situated at 17 314 cm- ' and has a room-temperature FWHM of 119 cm-'. In the case of a europium-doped lead metaphosphate glass,23 the same room-temperature tran-sition is positioned at 17283 cm-' and has an FWHM of 48 cm-l. This leads to a difference of 31 cm-' (ca. 1 nm) between the positions of the barycentres and to a factor of nearly 2.5 between linewidths.The difference in the position of the barycentres has tradi- tionally been attributed to a difference in the overall crystal- field strength of the average local rare-earth-metal en~ironment.~'As for the widths, it is normally assumed that the smaller the value of width of a given transition, the less disturbed are the local environment^.^^ According to these premises, there would be an increase in the order of the local structure surrounding the rare-earth-metal ions present in the doped metaphosphate glass when compared with the doped silicate glass. Thus, it would be worthwhile to examine in greater detail the differences in the chemical surroundings of the doped europium ions as presented by both structural models, i.e.the doped sodium disilicate" and lead meta- phosphate glasses simulated by the MD technique. Our first concern was to verify the possibility of an increase in the overall symmetry of the individual configu- rations while comparing a silicate to a metaphosphate glass. Although for a glass, an increase of the symmetry might not be significant since there still remains a distribution of differ-ent local environments ; the increase could still be construed as an indication of ordering in the glass. The following two analogies, although divergent in views, examine the possible significance of the results. First, one can imagine a rare-earth- metal doped crystal having the dopant ions situated in a site of C1 point symmetry.Although the site itself does not exhibit any order, translational symmetry from one unit cell to another still remains, and an optical spectrum will neces- sarily show crystalline characteristics. The importance of this analogy resides in the presence (or rather the absence, within the limits of inhomogeneous broadening, in the case of the crystal) of a distribution of local environments. Secondly, a comparison could be made with the microscopic processes occurring during ceramitisation or during nucleation. These processes necessarily start with ordering in the short-range domain (<8 A), where a reorganisation of the local environ- ments leads to the formation of the proper unit cell for a doped crystal. This reorganisation would then proceed through the medium- and long-range domains.The distribu- tion of possible local environments exists at the onset of the process, but this distribution diminishes with time. In order to quantify the spatial arrangement of the oxygen ligands about the Eu3 + ions, we calculated the quadrupole moment for each of the individual configurations. This pro- cedure has been delineated in ref. 10, where we have found 760 that none of the 150 individual Eu3+ configurations, of the simulated Eu3+-doped sodium disilicate glass, had a point- group symmetry higher than C,. The same analysis of the quadrupolar moments on the simulated Eu3+ : Pb(P03), also shows that none of the simulated local environments show any elements of symmetry.That is to say that the point-group symmetry of the Eu3+ ions doped in a lead metaphosphate glass is rigorously C,. It is important to note that this determines only the presence of symmetry elements in the spatial distribution of the oxygen ligands that were identified to be directly connected to the europium ions at a maximum distance of 3.2 A. Obviously, this does not give any indication of ordering in the surrounding matrix; yet it still dispels the postulate that because of the nature of the rare- earth-metal dopant, its ligand shell would have a regular arrangement very similar to its crystalline ~ounterpart.'-~ Returning to the two analogies that we have previously dis- cussed, it is then necessary to propose at this point that the possible ordering experienced by the Eu3+ ions in the lead metaphosphate glass is not due to an increase of symmetry (ordering in the short-range domain, leading to an eventual- ity of ceramitisation) but, rather, would be due to a substan- tial decrease of the distribution of possible local fields experienced by the Eu3+ions. Our second concern was to identify if the Eu3+ ions were situated in specific regions of the Pb(P03), glass matrix.Before discussing this point, let us briefly review the salient structural features of the lead metaphosphate base glass. Pre- vious have postulated the presence of phosphate chains of varying lengths that were connected to each other uia the metal cations bonded with the non-bridging oxygen atoms of the phosphate tetrahedra.This structural organis- ation differs markedly with the typical three-dimensional backbone of network-forming cations with a random dis- tribution of network-modifying cations as postulated by Zachariasen, in oxide glasses, and exemplified by modified silicate glasses. A connectivity study of the phosphate back- bone of a molecular dynamics simulated lead metaphosphate glasslg essentially showed the presence of primary and sec- ondary phosphate chains together with smaller amounts of phosphate ring structures. Furthermore, this study showed the presence of a secondary network made up of the modifier lead cations linked by non-bridging oxygen atoms, as postu- lated by the modified random network theory.,, Table 6 presents the results of an analysis of the connec- tivity of the oxygen atoms found in the first coordination shell of the Eu3+ ions (Eu-0 interatomic distance G3.2 A).An identification of the number of metal cations bonded to these oxygen atoms shows the following results. First, nearly all (ca. 98%) of the first coordination shell oxygen atoms are non-bridging oxygen atoms (NBO) of the phosphate tetra- hedra. The amount of non-bridging oxygen atoms, in metal metaphosphate glasses, is substantial since the presence of long chains ensures two NBO per phosphate tetrahedra, three NBO for terminal phosphate groups, and four for ortho- phosphate groups. Although there is no empirical way of cal- culating precisely the amount of NBO, as is the case for silicate glasses,43 the HPLC experiments of Sales et aL2' and Table 6 Percentage of first coordination shell oxygen ions having x bonded M cations (M = Pb2+ or P5+) X Pb2+ P5+ 23 0.84 0.00 2 7.79 0.42 1 52.42 98.21 0 38.95 1.37 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the MD simulation of Cormier et ~1.'~indicate the amount of various species present in the glass. The value of the NBO : BO ratio, determined from the simulated undoped lead metaphosphate glass," was found to be 2. The very high percentage of NBO connected to the Eu3+ ions clearly indi- cates that the rare-earth-metal ions are essentially connected to the phosphate backbone but are not an intrinsic part of it. Secondly, we observe that ca.60% of the oxygen atoms sur- rounding the Eu3+ are also shared by one or more Pb2+ ions. This prompted us to calculate the number of Eu3+ ions that are directly bonded (through one non-bridging oxygen) with a lead ion. The result obtained showed that all of the Eu3+ ions have at least one lead ion in their second coordi- nation shell. These points confirm the fact that the Eu3+ ions are situated amongst the secondary Pb2 + ionic network, a network which has been clearly identified in undoped lead metaphosphate g1a~s.l~ This secondary network acts as a 'buffer' zone between adjacent phosphate chains, and should therefore be substantially amorphous. Any degree of organis- ation present in the metaphosphate glass will arise from an ordering of the phosphate chains (primary and secondary). This ordering should occur much more readily than in the case of the three-dimensional framework of the modified sili- cate glasses, because of the greater degree of freedom of the chain structure.Under the premise of the modified random network theory,22 this secondary ionic network exists in counterpart to the crystalline structure. In crystalline lead metaphosphate, individual phosphate chains are infinite in length and parallel to each other, separated by columns of lead oxide. The bonding of the Eu3+ polyhedra to the glass matrix was shown to be essentially through non-bridging oxygen atoms (Table 6). As such, the degree of organisation of the local environment of the europium-oxygen polyhedra will be necessarily dependent on the degree of organisation of the phosphate backbone.This degree of organisation can be investigated by analysing the metalkeuropium pair distribu- tion function, where the metal corresponds to either lead or phosphorus. These pair distribution functions have been presented in Fig. 2. What is remarkable about this figure is the surprisingly well defined first and second coordination shell peaks of the Eu-P PDF. A comparison with the Eu-Si PDF of the simulated Na20 * SiO, :Eu3+ glass" imme-diately shows the difference in ordering of the local frame- work surrounding the Eu-0 polyhedra. Several factors provide evidence for this. First, the values of the associated widths for the first and second coordination peaks are sub- stantially smaller in the case of the phosphate glass.Specifi- cally, for the phosphate glass, they are A, = 0.41 A and A, = 1.19 A, whereas we find for the silicate glass, Al = 0.72 A, whilst the width of the second coordination peak is not dis- tinct enough to be accurately measured. Secondly, the ratio between the maxima of the first and second coordination peak is significantly greater in the lead metaphosphate glass in comparison to the corresponding ratio of the sodium dis- ilicate glass. These ratios are 3.3 and 1.9, respectively. This indicates that the first coordination shell of phosphorous atoms, as seen by the europium atoms, is clearly more dis- tinct than the second coordination shell.In the case of the doped sodium disilicate glass, the corresponding shells tend to be more indistinguishable. This is also observed for the height of the minima between the first and second coordi- nation peaks. In the case of a network-former-ligand pair, the minimum between the first and second peak is necessarily a null value, indicating a high degree of short-range order. A network-modifier-ligand pair will show a distinct increase in the height of this minimum, yet will not attain the height of a network-modifier-network-modifier or anionic ligand- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 anionic ligand pair. Fig. 2 shows that, in the case of the europium-phosphorus pair, this height is comparable with that of a network-modifier-ligand pair.All these factors are directly linked to the substantial degree of short-range order in the local environment of the europium-oxygen polyhedra as a result of the presence of medium-range order of the phosphate backbone. In Table 6, we notice in the simulated glass the presence of a small quantity of oxygen atoms that are determined to be 'free'. These oxygen atoms, previously observed in other simulated doped are not connected to the phos: phate backbone and are shared between modifying lead cations and doped europium ions. The formation of free oxygen atoms has been ascribed to the presence of neigh- bouring cations with large field strengths, such as Pb2+ and Eu3+. As mentioned at the start of this section, rare-earth-metal doped lead metaphosphate glasses show exceptional behav- iour in their spectral features.We have argued that this is due to the presence of substantial ordering in the local environ- ment of the rare-earth-metal dopant compared to that found in a more conventional type of glass, such as a doped sodium silicate glass. This should lead, as suggested previously, to a noticeable decrease in the energetic distribution of possible local fields experienced by the doped rare-earth-metal ions. This ordering of the phosphate backbone and its influence on the local environment of the europium-oxygen polyhedra should be observed in the spectroscopic features of the simu- lated europium-doped lead metaphosphate glass. The last section of the Discussion will deal with such spectroscopic evidence.Since we are not restricted by the direct study of an experi- mental spectrum, it is possible to establish an indirect link between the MD-generated structural model of europium- doped lead metaphosphate glass and the experimental spec- trum of the corresponding laboratory glass. This link is established through a proper duplication of the experimental spectrum of the laboratory doped glass, by the calculated spectrum generated from the structural information (atomic positions, partial charges) obtained from the MD simulated model. As such, the knowledge of the exact local atomic con- figurations surrounding each of the doped ions, together with the possibility to identify and isolate individual structural contributions to the simulated spectra, allows for the pos- siblity of spectra/structure correlations.In order to investigate the postulated reduction in the dis- tribution of local field in doped lead metaphosphate glass, we calculated the crystal-field ~trength,~' S,, , using the follow- ing equation : (6) where the B,, are crystal-field parameters3' and the sum over n covers the values of 2, 4, 6. The crystal-field strength was plotted vs. the excitation wavelength (the position of the 'Do manifold representing the 'Do + 'F, transition) for each of the 150 Eu3+ configurations of (i) the simulated Eu3+ : +Pb(P03), glass and (ii) simulated Eu3 :Na,O. 2Si02 glass" (Fig. 6). Since the crystal-field parameters are essen- tially geometric in nature, directly influenced by the nature and the position of each atom in the surrounding lattice, SCF is seen to be a quantitative measure of the strength of the electrostatic interaction between the rare-earth-metal ion and the surrounding lattice.In Fig. 6 we observe the same general trends for both simu- lated glasses. First, a somewhat linear decrease of the crystal- 00 --.5 574 575 576 577 578 579 580 581 A/nm Fig. 6 Crystal-field strength, S,,, as a function of simulated 5D,t 'F, transition wavelength. Comparison between simulated Eu3+ : Pb(PO,), (0)and Eu3+ : NaSi,O, (0)glasses (taken from ref. 10) fielA ctr-nnth ic ceon hmtxirmen Z7A Z cant4 Z70 Z nm ffir tho LlllJ (1bLCI IJCU c ..oy a sumranrial vertical spreaa or tne possiue values OI tne crystal-field strength for a given excitation energy. Secondly, this decrease is followed by a sharp fall to an asymptotic value which is known as the zero-field energy.Thirdly, the overall spread in possible excitation energies reflects the broadness of the 'Do +7F0transition in both glasses, i.e. it is much smaller in the case of the phosphate glass. Lastly, we observe that for a given value of excitation, the vertical spread in possible crystal-field strengths is much greater in the silicate glass than in the phosphate glass. The average values with their standard deviations of the crystal-field strengths were found to be 282.7 cm-' (a = 69.0 cm-') and 350.2 cm-' (a = 98.0 cm-') for the phosphate and silicate glasses, respectively.This vertical spread is a consequence of the widespread presence of accidental degeneracies in the doped glasses. Accidental degeneracy represents cases where a substantial difference in the local environment leads to a difference in crystal-field strength, yet gives the same value of the difference in position between the 'Do excited state and 'F, ground state of the Eu3+ ion. All these points are clear spectroscopic indications of the considerable influence that the local environment, provided by the glass, has on the electronic levels of the rare-earth- metal dopant. As postulated from the MD-generated struc- tural model, the phosphate glass shows greater ordering in the local environment of the rare-earth-metal dopant.Conclusions We have presented an investigation of the structural factors which lead to the marked differences between various spectral features of rare-earth-metal ions doped into metal meta-phosphate and silicate glasses. The investigation was based on a simulated structural/spectral model of an Eu3+-doped lead metaphosphate glass [Ed + : Pb(PO,),] which was compared to a (previously reported) Eu3+-doped sodium dis- ilicate glass (Eu3+ : Na,Si,O,). The models were generated 762 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 with a computational method that couples molecular dynamics simulation and point-charge crystal-field calcu- lations. The structural model of the Eu3+ : Pb(P03) glass, shows that the doped Eu3+ ions are bonded in their first coordi- 13 14 N.Karayianis and C. A. Morrison, Rare Earth lon-Host Crystal Interactions 1. Point charge lattice sum in Scheelites, Harry Diamond Laboratories, Adelphi MD, 1973, HDL-TR-1648. N. Karayianis and C. A. Morrison, Rare Earth Ion-Host Crystal Interactions 2. Local distortion and other efects in reconciling lattice sums and phenomenological B,, , Harry Diamond Labor- nation shell to approximately six non-bridging oxygen atoms. The non-bridging oxygen atoms are themselves an intrinsic part of the phosphate backbone. The point group for the local environment of the Eu3+ ions was found to be C,.Fur-thermore, the Eu3+ ions are situated in the secondary network, which is made up of the modifier lead cations linked 15 16 17 atories, Adelphi MD, 1975, HDL-TR-1682. 0.K.Deutschbein, C. Pautrat and I. M. Svirchevsky, Rev. Phys. Appl., 1967, 1,29. V. B. Kravchenko and Yu P. Rudnitskii, Sov. J. Quantum Elec- tron., 1979,9, 399. N. E. Alekseev, V. P. Gapontev, M. E. Zhabotinskii, V. B. Krav- chenko and Yu P. Rudnitskii, Laser Phosphate Glasses, Nauka, by non-bridging oxygen atoms. Finally, substantial medium- range order was observed in the local environment of Eu3+ ions. We attribute this to the ordering of the phosphate back- bone. It was previously shown that europium ions doped in a sodium disilicate glass are influenced to a greater degree by their bonding and energetic requirements than by the topol- ogy of the silicate framework. Although the bonding and ene- getic requirements have a substantial influence on the local rare-earth-metal environments in the phosphate glass, we have shown that the topology of the phosphate backbone has a much greater influence.We propose that the marked differ- ences in several spectroscopic features of Eu3+ ions doped into a lead metaphosphate glass are essentially due to a reduction in the width of the energetic distribution of local fields experienced by the Eu3+ ions. This is due to the pres- ence of medium-range order in the Eu3+ environments, which is a direct result of the lack of rigidity of the phosphate backbone (comprised essentially of chains and large ring structures). 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Moscow, 1980. M. J. Weber, D. C. Ziegler and C.A. Angell, J. Appl. Phys., 1982, 53,4344. G. Cormier, J. A. Capobianco and A. Monteil, J. Non-Cryst. Solids, 1993, in the press. B. C. Sales, R. S. Ramsey, J. B. Bates and L. A. Boatner, J. Non-Cryst. Solids, 1986,87, 137. W. H. Zachariasen, J. Am. Chem. SOC., 1932,54,3841. G. N. Greaves, A. Fontaine, P. Lagarde, D. Raoux and S. J. Gurman, Nature (London), 1981,293,611. J. A. Capobianco, P. P. Proulx, M. Bettinelli and F. Negrisolo, Phys. Rev. B, 1990,42,5936. P. P. Proulx, Ph.D. Thesis, Concordia University, 1992. S. K. Mitra, Philos. Mag. B, 1982,45, 529. S. K. Mitra and R. W. Hockney, Philos. Mag. B, 1983,48,151. L. Verlet, Phys. Rev., 1967, 159, 98. K. H. Jost, Acta Crystallogr., 1964, 17, 1539. C. A. Morrison and R. P. Leavitt, in Handbook on the Physics and Chemistry of Rare-Earths, ed.K. A. Gschneider and L. Eyring, North-Holland Amsterdam, 1982, ch. 46. C. A. Morrison, Crystal-fieldsfor Transition-Metal Ions in Laser Host Materials, Springer-Verlag, Berlin, 1992. R. P. Leavitt, C. A. Morrison and D. E. Wortman, Rare Earth lon-Host Crystal Insteractions 3. Three Parameter Theory of We gratefully acknowledge the Natural Science and Engin- eering Research Council of Canada for financial support. We thank Prof. Marco Bettinelli for graciously supplying the 32 Crystal-felds, Harry Diamond Laboratories, HDL-TR-1673. W. T. Carnall, in Handbook on the Physics and Chemistry of Rare-earths, ed. K. A. Gschneider and L. Eyring, North-Holland, Amsterdam, 1979, ch. 24. europium-doped lead metaphosphate glass and its absorption spectrum.33 34 E. U. Condon and G. H. Shortley, The Theory of Atomic Spectra, Cambridge University Press, Cambridge, 1957, ch. 9. B. R. Judd, Phys. Rev., 1962,127,750. References 35 36 G. S. Ofelt, J. Chem. Phys., 1962,37, 511. M. J. Weber, L. A. Boatner and B. C. Sales, J. Non-Cryst. Solids, 1 C. R. Kurkjian, P. K. Gallagher, W. R. Sinclair and E. A. Sigety, Phys. Chem. Glasses, 1963,4,239. 2 S. Mockovciak, J. Pantoflicek and K. Patek, Phys. Status Solidi, 1965, 11,401. 3 S. K. Barber, Interaction of Radiation with Solids, ed. A. Bishay, Plenum Press, New York, 1967, pp. 593-601. 4 D. K. Rice and L. G. DeShazer, Phys. Rev., 1969,186,387. 5 M. M. Mann and L. G. DeShazer, J. Appl. Phys., 1970,41,2951. 6 J. T. Fournier, and R. H. Bartram, J. Phys. Chem. Solids, 1970, 31, 2615. 7 C. C. Robinson, J. Non-Cryst. Solids, 1974,15, 1; 11. 8 R. Reisfeld and Y. Eckstein, J. Solid State Chem., 1972,5, 174. 9 R. Reisfeld and N. Lieblich, J. Phys. Chem. Solids, 1973, 34, 1467. 10 G. Cormier, J. A. Capobianco and A. Monteil, J. Non-Cryst. Solids, 1993, 142,225. 11 G. Cormier, J. A. Capobianco, A. Monteil and C. A. Morrison, 37 38 39 40 41 42 43 44 45 1985, 74, 167. J. P. Morley, J. D. Saxe and F. S. Richardson, Mol. Phys. 1982, 47, 379. C. Brecher, L. A. Riseberg and M. J. Weber, Phys. Rev. B, 1978, 18, 5799. C. Brecher, L. A. Riseberg and M. J. Weber, J. Lumin., 1979, 18/19, 651. J. A. Capobianco, P. P. Proulx and N. Raspa, Chem. Phys. Lett., 1989,160,591. M. J. Weber, in Laser Spectroscopy of Solids, ed. W. M. Yen and P. M. Selzer, Springer-Verlag, Berlin, 2nd edn., 1986. D. E. C. Corbridge, The Structural Chemistry of Phosphorus, Elsevier, Amsterdam, 1974. J. S. Jen and M. R. Kalinowski, J. Non-Cryst. Solids, 1980, 38-39, 21. S. A. Brawer and M. J. Weber, J. Chem. Phys., 1981,75,3522. R. P. Leavitt, J. Chem. Phys., 1982,77, 1661. Phys. Rev. B, 1993,48, 16290. 12 G. Cormier, Ph.D. Thesis, Concordia University, 1993. Paper 3/05383K; Received 8th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000755

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 763-771 Synthesis, Structures and Electrical Properties of the Charge-transfer Salts of 4,5=Ethylenedithio-4’,5’-(2-oxatrimethylenedithio)diselenadithiafulvalene (E0ST)t with Linear Anions [I;, IBr;, ICI;, 12Br-, AuBr,, Au(CN);] Toshio Naito,” Akiko Tateno, Takashi Udagawa and Hayao Kobayashi Department of Chemistry, Faculty of Science, Toho University, Miyama 2-2-1,Funabashi, Chiba 274, Japan Reizo Kato Institute for Solid State Physics, The University of Tokyo, Roppongi, Minatoku, Tokyo 106,Japan Akiko Kobayashi Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Takashi Nogami Department of Applied Physics and Chemistry, The University of Electrocommunications , Chofu, Tokyo 182,Japan An unsymmetrical donor 4,5-ethylenedithio-4‘,5’-(2-oxatrimethylenedithio)-diselenadithiafulvalene (EOST)? has been synthesized.Single crystals of some charge-transfer salts of EOST have been prepared electrochemically. The crystal structures of EOST and its charge-transfer salts were analysed by X-ray crystallography. The dc resistivities were measured on the single crystals. Eight of the salts with linear anions [(EOST),I,, a-(EOST),IBr,, /3-type IBr, salt, (EOST),ICI,, (EOST),I,Br, a-and p-type AuBr, salts and Au(CN), salt] retained their metallic conductivity down to low temperatures (G35K) and some were found to be isostructural. The overlap integrals of some salts and the tight-binding band was examined for (EOST),I,.The result suggested that it has a quasi-one-dimensional electronic structure and that EOST molecules interact most strongly in a side-by-side direction. The crystal and electronic structures of (EOST),I, are discussed by comparison with the isostructural compound (EOTT),IBr, [EOTT 1 4,5-ethylenedithio-4’,5’-(2-oxatrimethylenedithio)tetrathiafulvalene]. A few years ago Kato et al.reported the improved synthesis of BEDT-TSeF [bis(ethylenedithio)tetraselenafulvalene or BETS].’ BETS is an analogous donor to BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene] but was expected to form a more isotropic and thus more stable molecular metal owing to its four selenium atoms incorporated in the TTF skeleton.’ In fact most of the charge-transfer salts of BETS turned out to be stable metals down to low temperatures.lt2 In a series of BETS salts, the characteristic molecular struc- ture of the donor [(i) rather planar, (ii) five-membered and six-membered rings with even protrusions of the molecular n orbital at the eight chalcogen atoms]’ often results in iso- tropic crystal structures and intermolecular interactions.However, although BETS is such a good donor and produces the largest fraction of metallic salts of all the donors, few become superc~nducting.~Nakano et a2. reported that (EOTT),IBr, and (EOTT),AuI, retained metallic properties down to ca. 15 K and 4.2 K, re~pectively.~The latter salt in particular sometimes reduces its electrical resistivity more rapidly at low temperatures rather than around room tem- perature and the resistivity ratio between 300 K and 4 K is over We have come to think that such a new donor for superconductors could lie between BETS and EOTT.Thus we synthesized the title molecule (EOST), which is analogous to both donors. The salts of EOST with linear anions can be expected to be good molecular conducting systems for the following reasons : first (EOTT),IBr, has a columnar struc- ture of EOTT molecules with some intercolumnar S-S con-t IUPAC recommended nomenclature: 2-(5,6-dihydro- 1,3- diseleno [4,5-4[1,4] dithiin-2-ylidene)-l,3-dithiolo[4,5-d] [1,3,6]oxa-dithiepine. tacts shorter than the van der Waals di~tance.~The substitution of Se for S in the TTF skeleton would enable more close interaction in a side-by-side direction, which, in turn, would offer a more stable metallic state as found in BETS salts.’*2 Secondly, improved solubility could be achieved by unsymmetrical combination of these donors, which might lead to improved quality of the crystal of the radical cation salts.We report here the synthesis of the unsymmetrical donor based on the unit of BETS and OTT [bis( 2-oxa trimethylenedi thio)tetrat hiafulvalene] .4-6 Electrical properties and crystal structures of several radical cation salts with linear anions are described. Experimental Materials All chemicals were reagent grade from Wako Chemical Co. and used as received unless noted otherwise. Triethyl phos- phite was vacuum distilled, sealed under nitrogen and stored in the refrigerator until use.All solvents were degassed with high-purity dry nitrogen for at least a few minutes before use. 4,5-(2-0xatrimethylenedithio)- 1,3-dithiole-2-t hione, 21 There are some reports of synthetic routes to 2 and 3.4-6 In a 1 1 three-necked flask equipped with 200 and 500 ml drop- ping funnels with pressure-equalizers, 4,5-bis(benzoylthio)- 1, 3-dithiole-2-thione, 1,’ (20.28 g, 50 mmol) was treated with a solution of sodium (6.90 g, 0.3 mol) in 150 ml of methanol $ IUPAC recommended nomenclature: 2, 1,3-dithiolo[4,5-dl[ 1,3, 6loxadithiepine-2-thione; 3, 1,3-dithiolo[4,5-d-J[ 1,3,6]oxadithiepin-2- one; 4, 5,6-dihydro-l,3-diselenolo[4,5-4[1,4]dithiin-2-one. under a nitrogen atmosphere.To this dark-red solution was added 500 ml of methanol containing ammonium acetate (25 g) followed by bis(chloromethy1) ether* (1 1.50 g, 100 mmol) in 125 ml of methanol with stirring. The solution immediately turned crimson-red with a sticky orange precipitate. The mixture then became dard red again within a few minutes and was stirred overnight at room temperature. The precipi- tates were filtered off, washed with methanol followed by hexane and then dried in uacuo. The light-yellow sponges obtained were redissolved in CH,Cl (0.1 g 25 ml-') and evaporated to a fifteenth of its volume under reduced pres- sure. The resulting pale-yellow needles were collected, washed successively with acetone, methanol, ethanol, hexane and finally with ether and dried in uacuo; yield: 8.52 g (71%).The crude product could be recrystallized also from CHCl, or CHCl,-ether (1 : 5) or CH,Cl, , but the appearance of the purified product depended upon the solvent and the tem- perature used. 4,5-(2-0xatrimethylenedithio)-1,3-dithiole-2-one,37 Hg(OAc), (3.077 g, 9.657 mmol) was added, in one portion with stirring, to a solution of 2 (1.123 g, 5.013 mmol), 225 ml chloroform and 225 ml glacial acetic acid in a 500 ml Erlen- meyer flask. The stirring was continued at room temperature for 30 min. The milky suspension was filtered and the filtrate was washed successively with water, saturated aqueous sodium hydrogencarbonate and water again. The organic layer was dried with anhydrous sodium sulfate, then decanted off, evaporated to dryness in uucuo and then the residue was chromatographed on silica gel using the mixed solvent of chloroform and hexane (1 :1) as an eluent.The first colour- less portion gave a crude product of 3; yield: 0.779 g (74%) The off-white needles obtained turned colourless after re-crystallization from ethanol; mp 163-164 "C. Elemental analysis: C,H,S,O, calculated (%) H: 1.80, C: 26.77, S: 57.17, 0: 14.26; found (%) H: 1.77, C: 26.82, S: 57.13. m/e = 224(M+).'H NMR 6(CS,)4.82(4 H, s, CH,). 4,5-Bis(ethylenedithio)-1,3-diselenole-2-one,47 This compound was synthesized by following the procedure of ref. 1. EOST Method A. Compounds 2 (0.54 g, 2.23 mmol) and 4 (0.67 g, 2.23 mmol) were suspended in triethyl phosphite (50 ml) under a dry nitrogen atmosphere and slowly warmed to 110- 120"Cwith stirring and held in that temperature range for 30 min.The resultant orange mixture was allowed to cool to room temperature and reddish-brown precipitates were fil- tered off, washed successively with acetone, methanol, ethanol t See footnote 1on previous page. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 and hexane and dried in uucuo. The reaction was highly selec- tive and only a trace amount of self-coupling products formed. Recrystallization from boiling CS, (1 1) followed by column chromatography (silica gel, eluent CS, : CH,Cl, = 1 :1) gave analytically (HPLC) pure red shiny blocks; yield: 550 mg (50%). Elemental analysis : Cl,H8S,Se,0 calculated (Yo)H: 1.63, C: 24.29, S: 38.91, Se: 31.94, 0:3.24; found (Yo) H: 1.63, C: 24.20, S: 39.15, Se: 30.69.m/e = 496 (M+). IR (KBr): 1418(w), 1303(w), l224(vw), 1055(m), 920(m), 719(vw), 681(vw), 490 cm-'(vw). For the crystal data, see Table 2 (later). Method B. A similar procedure to method A was followed with 3, (0.500 g, 2.23 mmol) instead of 2. This allowed more formation of self-coupling products than method A and thus required more tedious work-up for isolation of the desired compound; crude coupling products were filtered off from tri- ethyl phosphite, washed with methanol, dried in uucuo and purified by column chromatography (silica gel, eluent CS,), then HPLC (Kusano Kagaku-kikai Co., Si-lo), and finally recrystallized first from o-C,H,Cl, , then from CS, : CH,Cl, = 4 : 3; yield: 287 mg (26%) Charge-transfer Salts of EOST Single crystals of the charge-transfer salts of EOST were pre- pared by use of standard electrocrystallization techniques. All chemicals were purified prior to use' and handled inside a drybox. The crystal growth was carried out in a standard H-cell (without glass frit) using platinum electrodes of 1 mm in diameter under a nitrogen atmosphere. A typical pro- cedure began with 7-10 mg of EOST and 50-100 mg of the tetrabutylammonium salt of the corresponding anion as the supporting electrolyte in 20 ml of tetrahydrofuran (THF) or chlorobenzene or sometimes a mixture of the two at a con- stant current of 1.5-3.4 FA or a constant voltage of 6.5-7.5 V at room temperature (20°C) for several days.Some of the successful conditions are tabulated in Table 1. After many attempts single crystals of (EOST),I were obtained using 1,1,2-trichloroethane as the solvent instead of those men-,tioned above. After one or two days thin fibrous crystals were observed to have grown on the tip of the electrode and the anode was thickly covered with many thin needles after a further two days. X-Ray Structural Analyses X-Ray crystal structure analyses were made on EOST and its several charge-transfer salts. Details of the crystal data, inten- sity measurement and data processing for the structures are summarized in Table 2. The intensities were measured by the w28 scan on a Rigaku automated four-circle diffractometer with graphite-monochromated Mo-Ka (2 = 0.7107 A) radi-ation. Three standard reflections were measured every 100 or Table 1 Electrolytic conditions for preparation of EOST salts ~- counter ion (X-1 crystal habit" current /PA voltage' P (C,H,)*NX Img EOST /mg solventd time /days 13 AuBr, plates (a-)plates @-)needles 0.8 1.5 - 66 62 83 10 12 8 TCE CB THF 1,Br ICl, plates plates 1.o 1.5 58 63 10 19 CB CB WCW, blocks' needled 0.8 1.5 80 61 6 14 THF CB IBr, (a-)plates @-)needles 1.o 1.5 45 63 16 7 CB CB All crystals are black.Plates are often elongated and appear needles at first sight. Galvanostatic condition. 'Potentiostatic condition, 20 ml. TCE = 1,1,2-trichloroethane, CB = chlorobenzene, THF = tetrahydrofuran.Insulating phase. Highly conducting phase (T--,< 40 K). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Structure determination summary ~~ EOST,I, z-EOST,IBr, EOST,I Br EOST,ICI, EOSTAu(CN), EOST crystal data empincal formula formula weight. M Se4S1.7C2002H1613 1369.71 W l ,C,OOZHI tiIBr* 1275.71 Se4S12C2~02H 1322.69 16*ZBr Se4s I ZCZ002H 1186.81 16rc12 Se2StiC,2N,0H8A~ 743.54 Se2S6C100H8 494.45 crystal colour, habit crystal size/mm3 black plate 0.30 x 0.10 x 0.01 black plate 0.20 x 0.45 x 0.01 black plate 0.21 x 0.45 x 0.01 black plate 0.26 x 0.22 x 0.012 black plate 0.30 x 0.10 x 0.01 orange plate 0.15 x 0.30 x 0.10 cry\tal system space group aIA biA CIA uldegrees pidegrees 7 /degreesviA3 Z triclinic 11.748 (3) 16.441 (5) 4.789 (1) pi 93.93 (2) 96.37 (2) 72.86 (2) 877.7 (4) 1 triclinic 11.634 (3) 16.283 (4) 4.758 (1) 93.18 (2) pi 95.93 (2) 73.55 (2) 859.5 (4) 1 triclinic 11.736 (2) 16.429 (2) 4.793 (3) 93.83 (1) 96.30 (1) 72.89 (1) pi 877.3 (2) 1 triclinic 11.604 (4) 31.105 (7) 4.747 (3) 89.96 (3) 95.80 (4) 83.47 (2) pi 1693 (1) 2 monoclinic 14.116 (9) 10.338 (5) 14.380 (9) p2, lc 116.64 (5) 1876 (2) 4 monoclinic 6.745 (1) 14.151 (2) 17.109 (2) P2Jc 110.02 (9) 1534.2 (6) 4 Dsalc'mg m picm 2.591 74.603 2.464 81.444 2.504 77.201 2.328 62.502 2.659 123.198 2.141 55.43 F(000) 639 603 62 1 1134 1380 960 data collection scan width, A 1.09" 1.12" 0.89b l.OSb 1.93" 1.05b 20mJdegrees 55.0 55.0 55.0 55.0 50.0 55.0 index ranges -15 < h Q 15 -21 < k < 21 0 ~ 1 ~ 6 -15~hh15 -21 c k G 21 OGlG6 Oghdl5 -2lGk421 -6,<1<6 OQhQl5 -40Gk640 -6QlG6 -16 Q h Q 16 O~k~12 0~1~17 -8QhQ0 -18 Q k Q 0 -21 d I < 21 reflections collected 4554 4460 4349 8352 3672 4001 independent reflections 4064 4076 2845 3118 3525 3700 observed reflections 3604 3661 2721 2791 2638 2198 C I PoI > 3W0)I solution and refinement weighting scheme, w7-I no.of parameters refined 191 191 191 280 d 198 4 IF, I~~'~V,J 196 final R. R_ 0.051, 0.067 0.052, 0.067 0.055, 0.065 0.070, 0.079 0.070, 0.093 0.044. 0.053 goodness of fit 0.30 0.23 0.37 0.49 1.07 1.72 Details in common: Rigaku automated four-circle diffractometer AFC-6 [I3, IBr, and Au(CN), salts] or AFC-5R (I,Br, ICI, salts and neutral molecule); Mo-Ku radiation (i.= 0.7107 A); graphite monochromator; 296 k 1 K; -20 scans; o scan speed 8" min-' (except for neutral: 4" min-I); three standard reflections every 100 [I,, IBr, and Au(CN), salts] or every 150 (I,Br, ICI, salts and neutral molecule); refinement by block-diagonal least-squares (except for neutral: full-matrix least-squares) minimizing cw(I F, I -I F, I )'; Aw = A + 0.50 tan 8.'Aw=A +0.30tan0.' IFol~30.0,w~'=20.0+0.01(F,~2;~F,~~ IF,12;lFo1320.0,w-'30.0,w~'=~~(F~)+0.01~F,~~.~~F~J~20.0,~~'=15.0+0.005=u~(F,,)+O.~~~IF,~~. 150 reflections. Backgrounds were counted for 2.0 s at both were made with 15 pm gold wires attached to the crystal with ends of the scan.The data were corrected for Lorentz and gold conducting paste. The typical dimension of the sample polarization effects. Corrections for absorption were made on was ca. 0.4 mm along the needle axis. ICl,, IBr, salts and the neutral EOST. No significant inten- sity variation was observed for the other samples and no cor- Results and Discussion rection was made for absorption. The unit cell dimensions were determined from 20 reflections with 20 < @/degrees< 35 Syntheses [I,, IBr, and Au(CN), salts], from 25 reflections with The synthesis of EOST, as depicted in Fig. 1, was achieved in 30 d Oldegrees <40 (1,Br and IC1, salts) or from 23 reflec- two ways; cross-coupling of 2 and 4 or of 3 and 4. EOST and tions with 39.3 < B/degrees <40 (neutral EOST) by least- two self-coupling compounds formed as by-products are squares refinement.The structures were solved by the quite different from each other in properties such as colour, heavy-atom (Patterson) method (I, salt and the neutral crystal habit and solubility, which permits easy separation. EOST) or the direct method [MULTAN" for Au(CN), salt, However, method A is more convenient for obtaining EOST SHELXS for IC1, salt] and subsequent Fourier syntheses. in high yield. The solubility in the usual polar solvents was For a-EOST,IBr, and the 1,Br salt, which were found by improved relative to the original symmetrical donors. After X-ray to be isostructural with the I, salt, the atomic param- various conditions of electrocrystallization were examined eters of the I, salt were used for the refinement.All were with various counter-anions, very thin needles were obtained refined by the block-diagonal least-squares method using in some cases: the voltage between the cathode and the unique reflections of IFo[ > 3o(F,) except for the neutral anode was found to be a more important factor than the EOST, which was refined by the full-matrix least-squares current or the solubility. Black, thick needles of ca. 0.4 mm method. Atomic scattering factors were taken from ref. 11. All length of the I, salt were obtained by the galvanostatic (0.8non-hydrogen atoms were refined with anisotropic thermal PA) electrolysis of EOST (10 mg) with (C4H,),N13 (66 mg) in parameters except for the light atoms of the Au(CN), ion trichloroethane (20 ml) at 20°C for 1 week.The Au(CN), salt which were refined with isotropic thermal parameters. Some was collected as curved thin needles by the electrolysis of hydrogen atoms were found on D-maps, other hydrogen EOST in THF at 20°C. When THF-C,H,Cl (ca. 1 : 1-2 : 1)atoms were located at the calculated positions with Biso= 4.0 or THF-CS, (3 : 1) was used as solvent instead of THF alone A'. The charge-transfer salts computation was carried out by a mixture of curved thin needles and thick needles was pro- using the UNICS I11 program package', and HITACHI duced regardless of the potential or the current. Two kinds of M-680H computer at The Computer Centre in The Uni- AuBr, salts crystallized depending on the electrolytic condi- versity of Tokyo, while all calculations were performed using tions.One (a-phase) was obtained by electrolysis in chloro- the TEXSAN', on neutral EOST. benzene with a constant current, 1.5 pA. It consisted of elongated plates with dimensions ca. 1 x 0.1 mm'. The other Resistivity Measurements @-phase) was obtained by electrolysis in THF and consisted The electrical resistivities were measured by a conventional of thin needles. The voltage between anode and cathode was four-probe method. The electrical contacts on the sample kept constant at 6.9 V. IC1, salt was obtained after galvano- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 4 method B 3 EOST Fig. 1 Schematic diagram of the synthesis of EOST static electrolysis of the donor with a current of 1.5 pA in chlorobenzene for nine days.IBr, salt was collected in two different phases. One [a-(EOST),IBr,; galvanostatic (1.0 PA) electrolysis in C6H,C1] consisted of shiny square plates, with typical dimensions 4 x 2 mm2 and the other [P-phase; gal- vanostatic (1.5 PA) electrolysis in C6H,C1] consisted of elon- gated plates. The I,Br salt was prepared as black shiny plates by galvanostatic (1.0 PA) electrolysis in C6H,Cl. Electrical Properties of EOST Salts Fig. 2 shows the temperature dependence of the electrical res- istivity of (EOST),I, . The room-temperature conductivity is ca. 40 S cm-', which is comparable to the isostructural salt (EOST),IBr,.4 However, the behaviour of the EOST salt presents an important contrast to the EOTT salt below ca.150 K; the resistivity of the former decreases monotonically down to 4.2 K while the latter has two anomalies. (EOTT),IBr, slowly reduces in resistivity (with a small hump around 100 K) down to 15 K where the resistivity saturates and makes an upturn. A similar anomaly was found for some other EOTT salts with linear anion^^.'^ but the origin of the anomaly remains to be clarified. However, the metal insta- bility was actually suppressed in (EOST),I,. As shown in Fig. 3, the Au(CN), salt exhibited metallic properties down to ca. 40 K. The room temperature conductivity (oRT)lies between 15 and 40 S cm-'. Although the metal-insulator (M-I) transitions of the salt were very clear, the transition ztemperature (TM-, 35-40 K) could not be determined clearly since the crystal was especially fragile below 70-80 K and had some sample-dependence. The Au(CN), salt was found to have another morphology that has a 1 :1 ratio of the component.This salt was an insulator. The a-AuBr, salt retained metallic conductivity down to 4 K, while the P--1.5--5 -2.0-c Qv 0 2 -2.5-I I I I 0 100 200 300 T/K Fig. 2 Temperature dependence of electrical resistivity (p) of (a) (EOST),I, and (b)(EOTT),IBr; AuBr, salt exhibited sharp M-I transition at 28 K (see Fig. 4). Both oRTlie around 60 S cm-'. The oRTof the ICl, salt is ca. 60-290 S cm-' and the salt retains its metallic property down to 4 K as shown in Fig.5. a-(EOST),IBr, maintained its metallic property down to 4 K, while the P-salt manifested its metallic property only down to 27 K, where a clear M-I transition occurred (see Fig. 6). Each oRTis ca. 80 and 100- 140 S cm-', respectively. 1,Br salt had oRTof ca. 60 S cm-'. In this salt, the unsymmetrical anion (I-I-Br-) introduces disorder at the anion site in the crystal. As for the metallic conductivity, many organic salts including the asymmetric anions have been reported to date" and most of them are insulators at low temperatures. However, interestingly, as shown in Fig. 7, the resistivity of this salt decreased mono- tonically down to 4 K. Thus, whether the anion is unsymmet- rical or not does not appear to affect the electronic structure so much as does the packing mode of the donor.Therefore it might be true that whether the charge-transfer salt exhibits metallic properties or not depends mostly upon the character of the donor, the inclination of how to aggregate themselves. These experimental facts indicate that EOST is certainly a good donor for yielding metallic charge-transfer salts with linear anions. The salts with other counter anions remain to be studied; measurements of the conductivity of the salts described above, at high pressures and/or at lower tem-peratures, are in progress. Molecular, Crystal and Electronic Structure of EOST Salts The crystal and molecular structures of neutral EOST are displayed in Fig. 8. EOST takes the dimerized structure in I I I I -1.2-h Eo -1.4-W m --1.6--1 .a-I I I I 0 100 200 300 TIK Fig.3 Temperature dependence of electrical resistivity (p) of the Au(CN), salt (metallic phase) of EOST. Arrows indicate the insignifi- cant jump in the original data probably due to the micro-cracks in the crystal or some trouble in the contacts, which have been con- nected by translation. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 h h5 5 -3.5 I0 100 200 300 TIK II I I I I I 5- 4- 3- 2- 1- 0- -1 - Fig. 4 Temperature dependence of electrical resistivity (p) of the AuBr, salt of EOST; (a) a-phase; (b) 8-phase. Arrows in (a) indicate the insignificant jump in the original data probably owing to the micro-cracks in the crystal or some trouble in the contacts, which have been connected by translation.the neutral state. The dimers are arranged orthogonal to neighbouring dimers and form layers in the bc plane. The unit cell contains one crystallographically independent mol- ecule. This is isostructural to EOTT,4 BETS,l BEDT-TTF16 and other similar donors." In the dimer the EOST molecule overlaps the other in a head-to-tail manner with gliding along its long axis. The overlapping mode within the dimer is a ring-over-bond type. The donor molecule is fairly warped; the mean deviation of the central diselenadithiaethylene moiety from the least-squares plane is 0.0270 8, and the maximum deviation is 1.21 8, observed at the carbon atom of the ethylene group.These characteristics of the molecular -2.0r I 1 1 -3.5'0' " ' '' " " " I'100 200 300 TIK 0 100 200 300 T/K 3' I I I I I I (6) Fig. 6 Temperature dependence of electrical resistivity (p) of IBr, salt of EOST; (a) a-phase; (b) fl-phase and crystal structure are commonly observed among the neutral donors. 1*4*16-1 * The diameters of the hetero-rings defined as the distance between the sulfur or selenium atoms are 3.477 A (six-membered ring), 3.175 A (five-membered ring with Se), 2.977 (five-membered ring with S), 3.395 A (seven-membered ring). Therefore the ratios between the diameters of the outer and inner rings are 3.477/3.175 = 1.10 and 3.395/ 2.977 = 1.14, respectively. These values are so close to 1.00 that many intermolecular chalcogen-chalcogen interactions can be expected along the side-by-side directions.' Actually, in spite of the short intradimer distance (3.45 A), the short contacts between chalcogen atoms (S.--S < 3.70 A, S.* .Se 6 3.85 A, Sea -.Se < 4.00 8,)are observed more often between dimers than within a dimer. There is no disorder at II I I -2.0-h5 F QW 0,--2.5-I I I I 0 100 200 300 TIK Fig. 5 Temperature dependence of electrical resistivity (p) of Fig. 7 Temperature dependence of electrical resistivity (p) of (EOST),ICl, (EOST),I,Br J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Fig. 8 (a)Molecular structure and overlapping mode and (b)crystal structure of EOST the ethylene nor oxatrimethylene groups.Fig. 9(b)shows the crystal structure of (EOST),I, . The crystal data together with those of other salts are summarized in Table 2. The tables of anisotropic thermal parameters, mean-square displacement tensor of atoms and final atomic positional parameters with thermal parameters for the salts described herein have been deposited.? The unit cell contains two EOST molecules at the t Deposited at the Cambridge Crystallographic Data Centre. Fig. 9 (a)Molecular structure and overlapping mode and (b)crystal structure of (EOST),I, . Broken lines indicate the short contacts between chalcogen atoms. (c) Crystal structure in (EOST),ICI, . J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 general positions and the I, molecule on the inversion centre.No disorder was observed in the anion. The donor molecules stack along the c axis to make a columnar structure, which is isostructural to (EOTT),IBr, .4 The interplanar separation is 3.68 A, which is almost comparable to those of the iso- structural EOTT salts; 3.63 8, for (EOTT),I3,l4 3.62 A for (EOTT),IBr, ,4 (EOTT),AuI; and (EOTT),AuBr, ,I4 3.61 8, for (EOTT),TCl, .I4 The EOST molecule is almost planar except for the ethylene and oxatrimethylene groups. The carbon atoms of the ethylene group have large thermal parameters but no conformational disorder was found. The oxatrimethylene group stands nearly upright on the molecu- lar plane, which increases steric repulsion and hinders the maximal overlap of the n-conjugated system between the neighbouring molecules.In fact they stack in such a way that one molecule glides not only along its long axis but also along the short axis above the neighbouring molecule. Such an overlapping mode is also found in the series of EOTT salts and contrasts with the neutral EOST. This characteristic molecular structure results in an unusual network of intermo- lecular interaction where the short chalcogen-chalcogen con-tacts are observed more often between the neighbouring two donors in different columns rather than in the same column. Besides these interactions there was also found a short contact between donor and anion (C-H.. -I), which some researchers find important in the case of BEDT-TTF salts.” Another characteristic of the stacking mode is that the unsymmetrical donors are all oriented in the same direction in the column as shown in Fig.9(b). In most of the unsym- metrical donor salts reported to date, the donor molecules stack alternately in a head-to-tail manner within the column.21 A stacking mode similar to that of (EOST),I, was also found in (EOTT),I, ,14 (EOTT),IBr, ,4 (EOTT),ICl, ,I4 (EOTT),AuIi and (EOTT),AuBr, .I4 (EOST),I, ,however, is expected to have smaller anisotropy than these isostructural EOTT salts, judging from the chalcogen-chalcogen contacts (see below) and the fact that the former turned out to be a more stable metal than the latter. Thus we calculated the overlap integrals and tight-binding band structure in order to estimate the differences between (EOST),I, and the iso-structural EOTT salts.The calculated overlap integrals of I, salt are tabulated together with the arrangement of the donor molecules with the corresponding suffixes in Fig. lqa). Similar calculations on the other salts showed that all these salts have a similar band structure, which is consistent with their similar electrical behaviour. As mentioned before, adding to the sulfur atoms, the selenium atoms in EOST take part in the short contacts including those not present in the EOTT salts. With respect to the value of the overlap inte- grals, the general trend, for example those with suffixes a, and a, are dominant, is much the same as for the correspond- ing EOTT salts. However, the donor molecules have larger overlaps in every direction in EOST salts than in EOTT ~a1ts.l~The previous band calculation on (EOTT),IBr; had revealed that the salt has a quasi-one-dimensional open Fermi surface and that the donors interact strongly nearly perpendicular to the stacking axis, i.e.in a side-by-side direc- tion. As for the Fermi surface of (EOST),I, shown in Fig. 11, the curvature is in substantial agreement with that of (EOTT),IBr,.4,14 We also tried to calculate the band struc- ture where the d orbitals of the sulfur and selenium atoms were taken into consideration. The resultant overlap integrals are variable depending on the parameters of the d orbitals. Using the latter overlap integrals we again obtained similar Fermi surfaces in regard to (EOTT),IBr:4 and (EOST),J, .Therefore the parameters of the d orbitals of chalcogen atoms remain to be settled; however, at this stage we could safely conclude that all the isostructural salts of EOST in question Fig. 10 Donor arrangement (a) in (EOST),I, , a-(EOST),IBr, , (EOST),I,Br, and (b)in (EOST),ICl,. c = -4.21, p1 = -4.27, pz = -3.83, a, = 15.54 and a2 = 13.15. Fig. 11 Energy band structures of (EOST),I,, Present band struc- ture results from the transfer integrals where d orbitals of the sulfur and selenium atoms not accounted for in the calculation. See text. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 i Fig. 12 Crystal structure of (EOST)Au(CN), and (EOTT),IBri4 have similar electronic structure, at least qualitatively, and that small additional intracolumnar inter- actions would effect the electronic structure and thus could lead to such a quantitative difference in the stability of the metallic properties as mentioned above.With respect to the crystal structure of the Au(CN), salts only the insulating phase has been clarified and is depicted in Fig. 12. The unit cell contains four EOST and four Au(CN), molecules, all of them on the general positions. In this salt a pair of EOST cations are arranged face to face to make the dimeric cation sheet in the bc plane and the dimer is separated by a pair of Au(CN), ions located in the same sheet to reduce the strong Coulombic repulsion between EOST cations. There is no column or sheet which makes the conduction path, so such a structure leads to insulating behaviour.In contrast to the case of the I, salt, two donors of Au(CN), salt directly overlap each other in a head-to-tail manner within the dimer. Another difference from the I, salt is that both ethylene and oxatrimethylene groups extend toward the outside so that the two molecules appear to bend away from each other. The IClalt is isostructural with the IQalt as well as a-(EOST) ,1Br2 and (EOST),I,Br (see Table 2 and Fig. 9 and 10). Accordingly, all the features of the crystal and molecular structure found for the I, salt are also found for the latter two salts. However, the length of the b axis of the IC1, salt, whose direction is along the long axis of the donor, is double the others.Because there appears to be little interaction between the donor molecules through the anion sheet along this direction, it can be explained why the doubling of the b axis does not destroy the metallic electronic structure of the salt. The interplanar distances are 3.690 8, [(EOST),I,Br], 3.623 A, 3.531 8, [EOST),ICl,], 3.684 8, [a-(ESOT),IBr,]. The ICl, salt has two crystallographically independent columns of regularly stacking donor molecules and both of these have smaller interplanar distances than those of the other salts. This close stacking, however, again hardly influ- ences the electronic structure of the salt since the band struc- ture depends mainly upon the interactions a, and a,, as suggested by the calculation mentioned above.The ICl,, cc-IBr, and 1,Br salts essentially share the molecular arrange- ment, intermolecular interactions and thus electronic struc- tures of the I, salt. Conclusion In order to develop an organic superconductor based on a new donor, EOST and its charge-transfer salts were synthe- sized. EOST is more soluble in the usual polar solvents than its original symmetrical donors, BETS’ and OTT,5 and is thus convenient for purification and electrochemical oxida- tion. The electrical resistivity of (EOST),I, , a-(EOST),IBr, , (EOST),ICl, , (EOST),I,Br and the a-type AuBr, salt decrease monotonically down to 4 K. All but the last were found to be isostructural with (EOTT),I, ,14 (EOTT),IBr, ,4 (EOTT),ICl, ,14 (EOTT),AuI; and (EOTT),AuBr, .14 (EOST),ICl, has double the length of the b axis of the others, the arrangement of the molecules and anions is the same.The tight-binding band calculation suggested that (EOST),I, has an open quasi-one-dimensional Fermi surface, i.e. has a similar electronic structure to the EOTT salts. The calcu- lation of their overlap integrals showed that the other EOST salts also have similar electronic structures. The Au(CN), salt has two different morphologies ; one is (EOST)Au(CN), which is an insulating phase and has been examined by X-ray crystal structure analysis and the other is metallic at least down to 40 K, its structure has yet to be clarified. Another phase of both the IBr, and the AuBr, salt exhibited a clear metal-insulator transition at CQ.27-28 K. All these salts could be said to be sufficiently stable metals to be prospective superconductors at lower temperatures, under some pressure if necessary. The tight-binding band calculation suggested that the trihalides of EOST have a quasi-one-dimensional electronic structure. On the other hand the metallic property of the EOST salts in question is more stable than EOTT salt^^,'^ but less stable than BETS salts’*2 as evidenced by J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 771 the conductivity measurements. All but two of the EOTT salts are known to be semiconductors, at least in low tem- peratures. 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Cryst., 1979,50,43; K. Kikuchi, M. Kikuchi, T. Namiki, K. Saito, I. Ikemoto, K. Murata, T. Ishiguro and K. Kobayashi, Chem. Lett., 1987, 931; G. C. Papavassiliou, G. A. Mousdis, J. S. Zambounis, A. Terzis, A. Hountas, B. Hilti, C. W. Mayer and J. Pfeiffer, Synth. Met., 1988, 27, B379; A. Terzis, A. Hountas and G. C. Papavassiliou, Solid State Commun., 1988,66, 1161; R. Kato, H. Kobayashi and A. Kobayashi, Chem. Lett., 1989, 781; L. Ducasse, A. Fritsch, D. Chasseau and J. Gaultier, Synth. Met., 1990, 38, 13; J. S. Zam-bounis, C. W. Mayer, K. Hauenstein, B. Hilti, W. Hofherr, J. Pfeiffer, M. Burkle and G. Rihs, Adv. Mater., 1992, 4, 33; R. Kato, S. Aonuma, Y. Okano, H. Sawa, A. Kobayashi, K. Bun and H. Kobayashi, Chem. Lett., to be published. Paper 3/04962K; Received 16th August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000763

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J CHEM. SOC'. FARADAY TRANS., 1994. Wj). 773--7XI Reduction of Cerias with Different Textures by Hydrogen and their Reoxidation by Oxygen Vincent Perrichon,* Ahmidou Laachir, Gerard Bergeret, Roger Frety and Louise Tournayan lnstitut de Recherches sur la Catalyse, CNRS,2 avenue Einstein, 69626 Villeurbanne Cedex, France 01ivier Touret Rhdne Poulenc, Centre de Recherches d'Aubervilliers, 52 rue de la Haie Coq,93308 Aubervilliers, France Successive reduction steps of CeO, particles by hydrogen between 300 and 1070 K have been followed by temperature-programmed reduction (TPR) and in situ magnetic measurements on several samples with different BET surface areas. The nature of the phases present in cerias reduced between 670 and 1270 K was determined by X-ray analysis.Finally, reoxidation by oxygen or air was studied at room temperature for all the reduced samples. Magnetic and TPR results show a direct relationship between the degree of reduction and the BET surface area. Indeed, for most of the samples, the degree of reduction at 620-670 K determined by magnetism corre- sponded to the creation of one layer of Ce3+ ions at the surface of the ceria. A similar relationship between the BET surface area and the extent of reduction was established using the area of the low-temperature TPR com- posite peak, the maximum of which was found to be constant at 810 K. When the reduction progresses further into the bulk, two main phases were evidenced: first, an expanded cubic CeO,-, phase derived from the initial ceria by a dilatation of the whole structure and, for deeply reduced samples, the hexagonal Ce,O, phase.A new intermediate phase, cubic Ce,O,, was also observed on samples reduced at 1070-1 170 K. Complete reoxidation by oxygen occurs at room temperature, for all reduction percentages below ca. 60°/,,, i.e. as long as the reduced phase remained in the cubic form. When the hexagonal Ce,O, phase has been formed, the reoxidation cannot be completed at 294 K. The structure of ceria is face-centred cubic (fluorite type). It can be described as a pile of oxygen ions. linked together by the edges, with the cerium atom at the centre of the oxygen cube in octahedral coordination. 'q2 Some oxygen entities in this structure are very mobile and can easily be removed tc give non-stoichiometric compositions, stable in a reducing atmosphere with the general formula CeOz-x, where 0 < x d 0.5.3However, in the presence of oxygen.these com- pounds are easily reoxidized to the stable CeO, . This oxygen lability and the possibility of large deviations from stoichiorn- etry explain the prevalent use of ceria in catalysts for auto- motive post-combustion. In such systems, cerium oxide behaves as an oxygen partial pressure reg~lator.'.~ For this reason, several studies have appeared which have focussed on elucidating the conditions of these redox processes in order to obtain a better understanding of their mechanisms.6 '_' From these studies, it appears that the texture of the initial cerium oxide has a great influence on the interaction between H,and CeO,.However, any quantitative determination of z in the Ce02-, reduced phase cannot be accurate because most of the results are obtained from TPR studies which do not take into account the reduction of impurities (nitrates. carbonates) or hydrogen fixation, all quantities which can be important in dispersed cerias. l6 The control of the reduction extent by oxygen reoxidation is also rarely given. Moreover. although the Ce-0 phase diagram is well established for samples prepared at high temperat~re,~ there is a lack ol' phase determination for the solids obtained by reduction of dispersed cerias, which are those used in catalysis. All these data are necessary to establish a macroscopic scheme of the oxygen transfer in CeO, during reduction-oxidation pro-cesses.In a recent paper," we have studied, by several comple- mentary techniques and particularly by in situ magnetic SUS-ceptibility measurements, the hydrogen reduction processes of two ceria samples having surface areas of, respectively, 5 and I15 m' g-' . The reduction began at 470 K, irrespective of the initial surface. With the low-surface-area ceria, a small reduction plateau at 620 K attributed to the reduction of the ceri$i surface was observed as well as an intermediate state at a higher temperature of reduction (900 K), having the formal composition CeO, 83, r.e. close to that of Ce,O, ,.In the case of the 115 m2 g-' sample. the observation of a plateau corre- sponding to the reduction of the surface depended on the operating conditions, but again a stabilized composition close to CeO, g4 could be obtained at 670 K.The reoxidation by tixygen of the reduced oxides was almost complete at room temperature. Ir the present work, we have generalized these studies to othcr samples of intermediate surface area and to higher reduction extents, in order to see if the same reduction steps can be observed for various ceria types and if the reversibility of the redox process can be effective whatever the solid and redLCtion percentage. In parallel with the quantitative deter- mindion of the reduction percentage by TPR and magne- tism, the evolution of the ceria structure was followed by X-rz y diffraction on samples sealed under hydrogen after redLCtion.The results are presented and discussed in terms of the topology of the reduction and reoxidation processes. together with the structure of the reduced phase. Experimental Materials Twc different HSA cerias were obtained from RhBne-Poulenc I.0.M. with references C1 and C2. They differed by the exis- tence of microporosity in the former (equivalent to almost (5o(J, of the BET surface area), whereas micropores were absent for the latter. Their purity is close to 99.5'%, with lanthanum as the main impurity. By treatment of the initial cerias in various conditions, five new samples were prepared. All the samples studied are pre- sented in Table 1, which also gives the BET surface area and the equivalent microporous surface calculated according to the 't method'.'' Among the samples, C1-400 and C1-850 have already been characterized by means of several tech- niques.In order to compare the samples under the same condi- tions, they were standardized with a pretreatment in situ under oxygen flow (4 1 h-') at 673 K for 1 h (heating rate, 5 K min-I). After that, they were evacuated or placed in an inert gas flow for 1 h at 673 K before cooling to room temperature. Temperature-programmed Reduction (TPR) The hydrogen consumption (1% H, in argon) was followed with a thermal conductivity detector in an apparatus described previously." The catalyst was in a U-shaped reactor. The flow rate was 1.1 1 h-' and the heating rate 8 K min-'.Magnetic Measurements The Faraday microbalance and procedure for calculation of the magnetic susceptibility have been described else-where."." The method allows determination of the para- magnetic Ce3+ ion content (note that Ce4+ in CeO, is diamagnetic). The experimental conditions corresponded to those used previ0us1y.l~ The ceria sample (0.10-0.13 g) was first stan- dardized under oxygen at 673 K, and then evacuated down to 0.1-0.2 mPa at the same temperature. After cooling to 294 K, the sample was placed under a hydrogen flow (4 1 h-I) and heated (4 K min-') with incremental steps of 50 K. For each step, the sample was kept for 2 h at the temperature before cooling to 294 K, always in H,, in order to measure the susceptibility.The latter was found to be nearly the same if the sample was cooled under vacuum instead of hydro- gen." X-Ray Diffkaction (XRD) The X-ray diagrams were obtained using the Debye-Scherrer method with Mo-Ka radiation or Cu-Kcr in some cases. Since the reduced cerias are extremely sensitive to air reoxidation, a special procedure was followed which consisted of sealing the reduced sample in a quartz tube under hydrogen partial pres- sure. Then, in a glove box flushed with oxygen-free dry argon, the sample was transferred into a controlled-atmosphere cell J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 equipped with a beryllium window enabling the X-ray dif- fraction pattern to be recorded. Results Influence of the Initial BET Surface Area on the Reduction Process Study by TPR Three samples were examined for this study and compared with C1-400: C2-600, C2-800 and C1-850.After the stan- dardization treatment, TPR was performed up to around 1100 K. The curves are shown on Fig. 1. Each curve presents a first composite peak beginning at about 500 K with a maximum close to 810 K. The shapes of this peak are not identical for the different samples. Shoulders and slope changes are visible as indicated by the arrows on Fig. 1. Fig. l(b) and (c), relative to the same precursor, present a similar initial profile, whereas for Fig. l(a), the hydrogen consumption is initially rather slow, and then increases quickly between 630 and 660 K. Another minor slope change is also observed at 760-740 K for Fig.l(a) and (b). All these differences have to be connected with a specific chemistry of the oxide surface anions depending on the sample surface. An attempt has been made with FTIR spectroscopy to correlate them with differ- ent types of surface hydroxy groups.20 Nevertheless, although the first peak results from several steps, it can be related to a global process corresponding to the consumption of the surface oxygen species as evidenced bef~re.~~'~' Then, after this qst peak, when the temperature is increased above 900 K, the beginning of a new peak is observed on the curves and must be due to the bulk reduction. For C1-400 [Fig. l(a)] the nature and origin of the nega- tive peak at 965 K were already characteri~ed.'~*~~ This peak arises from the reduction of bulk carbonates" which are present in the solid and are desorbed under vacuum at around 810 K.It can also be due to the desorption of hydro- gen fixed by the solid during the TPR.16 For C2-600 [Fig. l(b)], the negative peak is not clearly observed or, more prob- ably, it occurs at 880 K with a lower intensity, giving rise to the small positive peak centred at 965 K. It must be remarked that this sample was prereduced at 873 K before being reoxidized at room temperature. Such a sample exhibited an almost complete absence of carbonate species, as shown by FTIR spectroscopy.21 This absence of carbonates must therefore explain the absence of the negative peak at 890 K.Assuming that the first peak is due only to the reduction of the various surface oxygen species, it is interesting to note that no major shift is observed at the maximum temperature of this peak. For every sample, the maximum is observed at Table 1 Origin and texture of the different CeO, samples reference c1-400 C1-700 Cl-800 c1-850 C2-600 c2-640 C2-800 origin and treatment RP -C1 C1-400 + air calcination at 973 K C1-400 + vacuum treatment at 1073 K C1-400 + air calcination at 1123 K SBETaImZg-' 115 55 10 5 RP -C2 + H, reduction at 873 K + air at 300 K 78 C2-600 + H, reduction at 915 K + air at 300 K 21 C2-600 + air calcination 1073 K 54 equivalent microporous area/m2 g-55 10 0 0 0 0 -Determined from N, adsorption at 77 K.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1.1.1 300 500 700 900 TIK Fig. 1 Hydrogen TPR curves corresponding to the various CeO, samples after standardization at 673 K: (a) C1-400, (b) C2-600, (c) C2-800,(d)C1-850. (H2 : Ar = 1: 99; 8 K min-’) around 810 K. This observation is in disagreement with the results of Johnson and MOO^.^ It indicates that whatever the initial texture, the mean binding energies of the surface oxygen are the same and distinct from that of the bulk. It can also be an indication that the pretreatment procedure at 673 K was efficient enough to leave the surface in a clean reproducible state. In order to make a correlation with the BET surface area, we calculated the hydrogen uptake during this first peak.Two methods were tried. The first one used the experimental curve [Table 2 (exp)]. The second one was undertaken in order to minimize the uncertainty due to the recovering of Table 2 Low-temperature peak hydrogen consumption hydrogen consumptionlpmol g -sample exp. sym. predicted c1-400 423 544 617 c1-400 367 512 617 c2-600 239 354 429 C2-800 155 204 302 C1-850 29 25 29 C1-850 47 56 29 exp., experimental result ;sym., from the symmetrical procedure. the surface and bulk reduction processes at high temperature and also to limit the error caused by the negative peak which might be present in each sample. Since it is dificult to decon-volute the first peak, we have supposed that the surface reduction peak was symmetrical and centred at the maximum temperature (see Fig.1). Thus the hydrogen uptake was cal- culated by doubling the H, consumed between 470 and 810 K, the temperature of the maximum of the low-temperature peak. The corresponding hydrogen quantities are certainly overestimated compared with the real phenomenon. They are presented in Table 2 (sym.). The values of this second series are all higher than those of the first ones. Also included in this table for comparison are the values predicted according to the method proposed by Johnson and MOO^.' These authors assumed a simple model in which the ceria surface is constituted by adjacent oxygen ions (ionic radius, 0.14 nm) the number of which can be easily calculated for spherical or cubic particles.By supposing that the reduction of the surface cerium species corresponds to the elimination of one fourth of the surface oxygen ions (capping oxide ions), it is easy to obtain the corresponding values for hydrogen. The relationships between BET surface area and the differ- ent hydrogen consumptions are shown graphically in Fig. 2. The variations are roughly linear in both cases, which indi- cates that the first overall TPR peak may be related directly to the reduction of the ceria surface. However, the slopes (respectively, 3.1 and 4.2 pmol H, mP2)differ by about 35%, and are well below the predicted one (almost 5.4 pmol H, mP2). This difference can be due to a too simplified model.In particular, it does not take into account the repartition of the crystallographic planes of ceria which may vary when the cal- cination temperature increases. Moreover, the value for the ionic radius of the oxygen ion could be different. One can also think of a specific reason connected with the physico- chemistry of the ceria surface or the experimental conditions. For example, a too low pressure of hydrogen during the TPR could induce a low reduction rate and be in competition with simple oxygen depletion (due to both sintering of the ceria and the lability of some oxygen species) or a decrease of ‘mobile oxygen’ content due to sintering. Consequently, we A / 7 500 Is, 0 1 4005 0.-w n5 300 v) 0 $ 200 Is,2 -0>.= 100 0 25 50 75 100 SBET/m2 9-’ Fig. 2 Variation of the hydrogen consumption in the low-temperature TPR peak as a function of the BET surface area of CeO, : (a) experimental curve, (b) symmetrical procedure, (c) pre-dicted values. See text for more details about the calculation methods. applied the magnetic method to measure the reduction extent of the ceria surface with pure hydrogen. Magnetic Study The reduction percentages estimated from the magnetic sus-ceptibilities at 294 K, are shown in Fig. 3 as a function of the reduction temperature for six samples of Table 1. The pre- vious results for C1-400 and C1-850 are presented again for a complete comparison, Further, the run for C-400 was re-peated and extended to temperatures higher than 673 K, the preparation temperature and the limit of the previous study. In every case, the beginning of the reduction is observed at 470 K.Then, as the temperature increases, the reduction extent increases, and the more so for higher BET surface areas. For all the samples, the curves exhibit an inflexion point or even a plateau in the 620-720 K temperature domain. It must be recalled that on the curves each experi- mental point was obtained after a 2 h isothermal reduction. Thus, the different equilibrium points before obtaining the reduction plateau value correspond to different germination and surface diffusion steps which, in fact, reveal a specific surface chemistry depending upon the sample.For higher temperatures and in the case of the non-microporous solids [Fig. 3(b) and (c)], an important increase in reduction extent is obtained at about 800 K. At 900 K, an 70 I 1 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 intermediate state seems to be reached, corresponding to a 36-42% reduction percentage, i.e. to cerium oxides having the composition CeO,.,,-CeO,.,, . For the microporous cerias (Cl-400 and C1-700), no real intermediate state was evidenced at about 900 K. The curves show a progressive but irregular increase, with several slope changes. This behaviour can be explained by two main phenomena which probably occur simultaneously : (i) the elimination of bulk carbonates as evidenced by TPR which induces a transient local dis- order and (ii) the decrease of the BET surface area which is effective on this sample for reduction temperatures higher than 720 K.22 Our previous data on C1-400 and C1-850 have evidenced a rough relationship between the reduction extent at 670 K and the BET surface area.The values calculated here at the 620-720 K plateau for all the samples lead to the almost linear relationship shown in Fig. 4, although the C1-400 point is clearly above the line. The slope is slightly inferior (15%) to the one calculated in the model of Johnson and Mooi. This relatively good agreement supports the model which predicts at the plateau the elimination of one fourth of the surface adjacent oxygen ions. It confirms the idea that for tem-peratures G620-670 K, the reduction process is mainly limited to the uppermost layer of the ceria.The dispersed ceria C1-400 is an exception with a reduction greater than one layer, which is probably due to the more dispersed char- acter of this oxide for which the cubic model of Johnson and Mooi may be very crude. Indeed, the numerous lattice defects must be favourable for a faster diffusion of the oxygen species in the bulk. It could also be connected with the higher germi- nation process rate related to specific surface sites.” By comparison, the reduction values obtained by TPR are smaller than those deduced from the magnetic study. This can be explained by the differences in experimental pro- cedures. During magnetic measurements, longer reduction times and higher hydrogen pressures favour the equilibrium states and a fraction of the bulk oxygen can reach the surface and be reduced. In TPR experiments, although the higher reduction temperature must favour oxygen diffusion, the decrease of the BET area would limit the participation of the bulk oxygen.Thus, it seems normal to obtain lower values for TPR. Another reason can contribute to the lower TPR 0 100 200 300 400 500 600 700 800 900 1000 ternperature/K Reduction percentages of cerias of different specific areas obtained from the magnetic susceptibility as a function of the reduction temperature (2 h at each temperature): (a) C1-400, (b) C1-700, (c) C2-600,(d) C2-640, (e) C1-800, cf)C1-850. (*)C1-400, (0)C1-700, (+) C2-640, (*) C2-600, (x) C1-800, (0)(21-850.200% reduction corresponds to the total reduction of CeO, to Ce,O, . SBET/m2 9-’ Fig. 4 Variation of the reduction percentage calculated at the plateau of the magnetic curve (Fig. 3) as a function of the BET surface area of CeO, .Comparison between the experimental (a) and predicted (b) values. 100% reduction corresponds to the total reduction of CeO, to Ce,O, . J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 values: The TPR data are calculated on the basis of the initial sample weight, i.e. without taking into account the weight loss occurring during the standardization treatment. This is not the case for magnetism. The difference can be esti- mated to about 7% for C1-400 but is much smaller for C1-850 with a specific surface area of 5 m2 8-l.Therefore, even if it is not possible to obtain a complete agreement between the TPR and magnetic results, both sets of data are directly related to the BET surface area of the initial ceria. Accordingly, these methods can be very useful for determining the surface area of peculiar ceria samples, as, for example, the supported cerias. The TPR technique is easy to handle and commonly utilized in research laboratories, in contrast to the magnetic technique. However, the latter has the great advantage of selectively measuring the formation of Ce3+ ions and gives reliable data, irrespective of the content of metallic salts or impurities in the sample, with the excep- tion of strongly paramagnetic or ferromagnetic species.X-Ray Study of the Reduced Phase The composition of the oxide obtained after reduction at 900 K, CeO1.8S1.79, is close to that of the monoclinic phase Ce6011 (CeOla8J or eventually Ce7O12 (CeO,.,,) which have been proposed to explain some diffraction pattern^.,^.,^ The reduced solid could also correspond to a mixture of CeO, and Ce203. Another important point is the nature of the cerium oxide when the reduction is performed at tem- peratures >900 K. In order to characterise these points, we have studied the evolution of the X-ray diagrams of CeO, reduced under pure hydrogen at temperatures up to 1270 K. The starting material C1-400, which has the highest BET surface area, was chosen to obtain the highest reduction per- centage.Additionally, a run was performed with the C1-850 sample. These samples were heated for 15 h under 3.6 1 h-' of hydrogen at fixed temperature, cooled under hydrogen to room temperature and analysed as noted above. Table 3 summarizes the results obtained after reduction at 673 and 873 K. From the position and the intensity of the diffraction peaks, it can be seen that the initial cubic structure of CeO, is maintained but with a slight shift of all the lines. The cell parameters determined from the set of interplanar spacings are 5.433 and 5.509 A, for the Cl-400 samples reduced at 673 and 873 K, respectively. This corresponds to a lattice expansion of the ceria of 0.4% for the sample reduced at 673 K and of 1.8% for the sample reduced at 873 K.No evidence for the Ce6011phase was found. In particular, the (043) Bragg line with spacing d = 2.447 A, characteristic of the Ce6011phase, is absent on all the patterns. The results are similar after reduction at 873 K of the low-surface-area sample C1-850. In this case, the interplanar spacing is 5.502 A, with an expansion of 1.7%. The X-ray diffraction pattern corresponding to the reduction temperature of 1073 K is shown on Fig. 5(4. In addition to the peaks of the expanded CeO, phase (cell parameter 5.564 A, lattice expansion 2.8%), new well defined peaks appear with low intensity (Table 4). No referenced JCPDS-ICDD phase could be attributed to these peaks which could correspond to a cubic structure.The diffractograms of the solids reduced at 1173 [Fig. qa)]and 1273 K [Fig. qb)] are more complex. The expanded 050,phase is always present (cell parameter 5.555 and 5.520 A, lattice expansion 2.7 and 2.0% for 1173 and 1273 K, respectively) and all the peaks of the Ce,03 phase are observed. After reduction at 1273 K, the concentration of the CezOJphase increases at the expense of the expanded ceria. It must be stressed that the unknown cubic phase evidenced Table 3 Interplanar spacings, d/& calculated from the XRD pat- terns of =ria samples reduced at 673 and 873 K: comparison with the JCPDS-ICDD data for CeO,,Ce,O, ,and &,03a CeO, Ce@, ,' Ce,03f JCPDS JCPDS JCPDS 34-394 32-196 23-1048 c1-4006 c1-400" cubic monoclinic hexagonal 3.38 (20) 3.37 (30) 3.32 (20) 3.152 3.184 3.1234 (100) 3.23 (100) 3.22 (100) 3.20 (100) 3.03 (20) 3.03 (30) 3.01 (20) 2.964 (40) 2.945 (100) 2.7147 2.7556 2.7056 (30) 2.796 (100) 2.773 (100) 2.447 (80) 2.391 (20) 2.274 (20) 2.254 (30) 1.9235 1.9452 1.9134 (52) 1.973 (100) 1.968 (100) 1.960 (loo) 1.945 (35) 1.737 (60) 1.733 (30) 1.730 (60) 1.708 (40) 1.6392 1.6611 1.6318 (42) 1.675 (60) 1.685 (4) 1.637 (30) 1.623 (18) 1.5684 1.5897 1.5622 (8) 1.588 (40) 1.550 (40) 1.516 (2) 1.545 (40) 1.4727 (5) 1.3558 1.3779 1.3531 (8) 1.3823 (2) 1.2444 1.2645 1.2415 (14) 1.2941 (8)1.2113 1.2314 1.2101 (8) 1.2464 (11) 1.1957 (7) 1.1742 (3) 1.1410 (4) 1.1084 1.1244 1.1048 (14) 1.0775 (7) Intensities are given in parentheses.Reduction temperature 673 K;0 :Ce = 1.84 :1. Reduction temperature 873 K;0 :Ce = 1.70:l. dO:Ce=2.00:1. 'O:Ce= 1.83:l. /O:Ce= 1.50:l. The compositions were extrapolated from magnetic measurements performed on samples treated in similar conditions. for the sample reduced at 1073 K is also observed for the sample reduced at 1173 K, but is no longer detected after reduction at 1273 K. In addition, for both samples, small extra peaks appear as shoulders on the expanded ceria peaks, 10 15 20 25 30 35 40 2Bfdegrees Fig. 5 XRD pattern of Ceo, reduced at 1073 K by hydrogen (a) and further reoxidized room temperature (b). Note the Ka,-a, split-ting for the 28 angles >20°. The asterisk denotes the new cubic phase.778 Table 4 Interplanar spacings, d, and relative intensities of the new phase appearing after reduction at 1073 and 1173 K: comparison with the JCPDS-ICDD data (22-0369)for cubic La,O, cerium oxide (our data) La,O, (JCPDS) peaks,"2Oldegrees d/A III, dlA III, (hkl) 8.96 4.539 45 4.62 10 (211) 3.27 100 (222) 2.832 35 (400)15.54 2.623 50 2.668 10 (411) 17.19 2.374 50 2.413 5 (332)18.69 2.184 100 2.220 10 (431) 2.003 40 (440)22.65 1.806 85 1.836 10 (611) 23.84 1.717 95 1.747 5 (541) a Mo-Ka = 0.71 A. on the high-angle side. They can be attributed to the normal cerium dioxide. This beginning of reoxidation was due to a tiny air leak in the measurement cell, as could be deduced from the evolution of the spectrum with time.The increase of the lattice parameter was previously noted by Bauer and Gingerich,' and also by Ray et This lattice expansion is due to the reduction of some Ce4+ ions to Ce3+,the radius of the Ce3+ ion being larger than that of Ce4+ (128.3 pm instead of 111 pm26).However, this increase was associated with the splitting of some peaks suggesting a rhombohedra1 symmetry. Here, no such splitting was observed. The structure remains unchanged and the unit cell varies with the reduction temperature, as shown on Fig. 7. A slight expansion is already detected after reduction at 673 K, but cannot be considered as certain. Then, the increase is evident and reaches a maximum for 1100 K. For higher reduction temperatures, the lattice expansion decreases, prob-ably because Ce3+ ions begin to leave the expanded CeO, phase to form the Ce203phase.The new cubic phase can be tentatively assigned. As shown in Fig. 8 and Table 4, it seems to present some links with the 10 15 20 25 20/deg rees Fig. 6 XRD pattern of CeO, reduced at 1173 K (a) and 1273 K (b). The letters a and b show the expanded CeO, phase and the normal hexagonal Ce,O, phase, respectively. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ______-0 5.4 I I I 500 750 1000 1250 1500 T/K Fig. 7 Variation of the unit-cell parameter of the CeO, cubic phase us. reduction temperature cubic La203 phase (JCPDS-ICDD 22-0369). The two struc-tures most commonly found for rare-earth-metal sesquiox-ides, M203, are the so-called hexagonal A and cubic C structures.,' A monoclinic B form of some of these oxides has also been recorded.In cases where M,O, crystallizes with more than one structure, A appears to be stable at the highest temperature, B at lower and C at the lowest temperatures. The hexagonal A and cubic C forms are known for lantha-num (JCPDS-ICDD 05-0602 and 22-0369, respectively), but only the A hexagonal form is reported for cerium to our knowledge (JCPDS-ICDD 23-1048). The cubic C structure is closely related to that of CaF, from which it may be derived by removing one quarter of the anions and then rearranging the atoms slightly.26It is interesting to recall that the cerium dioxide CeO, crystallizes with the fluorite structure. If the cubic structure is assumed for our unknown cerium phase, a value of 1113 pm for the unit-cell parameter can be deter-mined from the interplanar spacings of Table 4.This value can be compared to the value of 1121 pm interpolated from the curve of the unit-cell parameter of the cubic C structure for different known lanthanides vs. the ionic radius of 220) I (311)I I I I I I 11 i-..z cQ) c, .-(mi I I I I I (b ) I (SSO?'400? I i (sn?I I I I 10 15 20 25 2t?/degrees Fig. 8 XRD pattern of CeO, reduced at 1073 K compared with reference data extracted from the JCPDS-ICDD file. (a)CeO, ceria-nite (34-0394);(b)cubic La,03 (22-0369). J. CHEM SOC FARADAY TRANS..1994. VOI.. 90 Shannon and Prewitt" for the valency 111 and a coordi nation number of 6 (Fig.9). The agreement is relatively good considering that the main peaks of the cubic C structure of Ce,O, [peaks (222). (400). (440) and 162211 are not distin- guishable from those of expanded CeOz [peaks (11 I). (200). (220) and (31l)] because these two phases are cubic and thc unit-cell parameter of Ce20, is twice that of CeOz (1 1.13 A for Ce,O, and 5.564-5.555 A for CeO, reduced at 1073 dnc! 1173 K, respectively). Reoxidation by Oxygen of the Reduced Cerium Oxides The reoxidation by oxygen of the reduced samples at rwm temperature was followed by magnetic measurements and !)! X-ray diffraction. The objective here was to ascertain in N hat conditions the reoxidation can be complete and ho\\ the reoxidation process varies with reduction extent..Mugnrric I sotherni jbr O.Yj*yen.A dsorptio 11 Two samples were prepared with a reduction extent hisher than that expected for the reduction of the surface alone. Accordingly, the reduction of C1-400 and C1-850 at 971 K was to 54 and 35"z11, corresponding to CeO, ,73 and CeO ,hZ5. respectively. Their reoxidation was compared with that of CeO,,,, (from C1-400) the reduction of which was close to that of the surface.lS Small doses of oxygen were introduced at room tem-perature. and the variations of the magnetic susceptibilit! were followed as a function of the weight increase expressed in the number of adsorbed oxygen molecules.The results are shown in Fig. 10. The experimental points follow straight lines with slopes equal to -6.8 and -6.5 emu (cgsi-F mol-0,,respectively. They are close to the previous results and correspond to the chemisorption of one oxygen atom on two Ce3+ ions (predicted value. -7.3 x lo-')). The smaller value observed for the slope was shown to be due to a slight excess of adsorbed oxygen. Effectively. after 15 h, the totdl quantity of adsorbed oxygen on CeO,,-, was 820 pmol g- '. a value higher than the 785 pmol necessary for total reoxida- tion. After evacuation, the final magnetic susceptibilit), w;ts 0.02 lo-' emu (cgs) g-'. which confirms the almost complete reoxidation. It indicates that for limited reduction percent- 11.5 11.21 "? 11.0 I I I I I I I 10.5 I 100 110 120 r'pm Fig.9 Unit-cell parameter. ti. of cubic Me20, extracted from the JCPDS-ICDD file rs. the metal ionic radius for diffttrznt lanthanides" (Lalency 111. six oxygen nearest neighbours) 0 200 40C 63'1 800 1000 No2.pmol 9 ' Fig. 10 Magnetic susceptibilitj changes 15 the number of oxqgen nacllrcule\ [,V(O,)] adsorbed at 294 K on reduced CeO, ((11 (e0, ,,.lhlCrO, HZ5.10Ce0,-7 ages. loiver than about 60",o, the reoxidation of the reduced cma to CeO, occurs in the same manner. independent of the rt.duction extent and of the texture of the initial ceria. 3rirdj. 01 the Reosiduriorr Procwv hj. X-Ruj, Dlfrucrion 7 he reoxidation of reduced ceria was also followed by X-ray diflraction.As mentioned above. the sample was reduced by hvdrogen in a separate device. cooled at room temperature under hydrogen and transferred under argon into the X-ray cell. Then. several successive patterns were recorded after it~troductionof increasing pressures of air into the cell. For the cerias reduced at 673 or 873 K, i.e. with a rt,duction degree lower than 60°;1.the introduction of air at room temperature progressively reoxidizes the solids. For p,irtial reoxidation, the patterns of both the expanded and the normal ceria were observed. For the sample reduced at 8'3 K, no modification of the lattice of the expanded ceria ~a?;evidenced. After 24 h under air. only the initial CeOz phse was observed.These results indicate that the migration 0,' oxygen species into the bulk is effective at room tem-p1:rature. We can suppose that the reoxidation occurs pro-gressii.ely into the bulk. with. inside the same particle. bl)undaries between the two phases, CeOz and expanded CeO,. There would not have been a progressive modification oi (he lattice of the expanded ceria during the adsorption of o\!gen. However, our results can also be explained simply by mass-transfer limitation. the inner particles of the bed not being reoxidized owing to a lack of oxygen. The reoxidation of samples reduced at 1073. 1173 and 1.'73 K led to the following observations: (i) The introduction ot air on the sample modifies quick]) and deeply the spectra ol the reduced phases.and after one day under air, the e) panded ceria and the cubic Ce,O, have completely disap- pt.ared. (ii) There is no evidence of a progressive modification ot the lattice of the expanded ceria. (iii) Even after two days utider air at room temperature. the normal hexagonal Ce,O, pliase is always detected. as shown in Fig. 11. N'e can conclude that. when the hexagonal Ce,O, phase is formed during the reduction. it is impossible to obtain com- plete reoxidation at room temperature. On the other hand. the dilatation of the lattice to generate the expanded ceria wrlich maintains the initial cubic structure is very favourable for reoxidation of the solid b) oxJgen. These experimental a b L I I 1 I I 1 I 10 15 20 25 28/degrees Fig.11 XRD pattern of CeO, reduced at 1173 K and then left under air for 48 h at room temperature. The letters a and b corre-spond to the normal CeO, phase and to the hexagonal Ce,O, phase, respectively. facts could be important for the redox processes occurring in the three-way catalytic converters during the rich and lean- burn steps. Discussion The influence of the surface area on the reduction of ceria by hydrogen has been observed previ~usly.~~’*’ ’ The present results give more quantitative information and allow charac- terisation of several points during the reduction processes. When the reduction is performed by isothermal steps, we have evidenced a plateau for the Ce3+ content between 620 and 720 K, as seen in the magnetic susceptibility data.An almost linear relationship was found between the degree of reduction calculated at this plateau and the initial BET surface area. The corresponding slope is very close to that which can be predicted assuming a compact model of capping oxygen ions, with one fourth of these ions eliminated upon reduction. From this result, we have concluded that the first reduction step corresponds to that of the surface alone. The TPR results confirm this conclusion. Two main reduction processes by hydrogen were observed. They corre- spond, respectively, to the surface and bulk oxygen reduction. The corresponding binding energies of these oxygen species in the solid are different enough to lead to well separated phenomena during the reduction.This model is supported by the fact that whatever the initial surface area of the ceria sample, the temperature of the maximum of the first peak remains constant, at about 810 K. An examination of the small shoulders in the first composite TPR peak has shown that the surface reduction is, in fact, more complex and involves the reduction of different surface species, the popu- lation of which may vary from one oxide to another. Thus, the initial reduction process results from germination and surface diffusion steps, which progressively involves the whole surface. This model of reduction corresponds to a cherry-like attack scheme where after the completion of the surface reduction, the following step is the bulk reduction at higher temperatures.One can deduce from our results that the oxygen ion mobility in the bulk is low as long as the initial lattice of CeO, remains non-expanded. In this general scheme, the changes, with texture, of the surface oxygen binding energy distributions are probably limited and do not affect the TPR results. This conclusion is important in the study of modified ceria samples. As noted above, any change observed in the position and the shape of the first TPR peak will be directly attributed to the modification of the chemistry of the ceria surface. This is the case in the presence of anionic species J. CHEM. SOC. FARADAY TRANS., 1994, VOL 90 (after impregnation),28 residual or doping ele- ments in the bulk.Such modifications are also well known when a metal is present on the ceria surface, for which much lower temperatures are observed for the reduction of the surface.Q. 18.29.30 A calculation by two methods of the number of surface oxygen ions using the first TPR peak was presented above. The values are lower than that obtained by magnetic mea- surements or with the m0de1.~ The numerous parameters which influence the validity of the TPR results have been already discussed in detail.16 Another question must also be considered. Indeed, we have observed in the case of highly dispersed cerias that an important sintering occurred after a partial reduction, as soon as the treatment temperature is higher than 720 K2’ As the maximum of the TPR curve is located at 810 K, it is quite probable that, under hydrogen and at this temperature, the actual surface of the catalyst is lower than the initial one.Under these conditions, the quan- tity of hydrogen calculated from TPR for highly dispersed samples may be systematically lower. For low surface areas, this phenomenon is probably limited, but the precision of the measurement is poor because the deconvolution of the peaks is difficult and no clear conclusion can be established. However, in spite of these limitations, it remains that both the TPR and magnetic results of Fig. 2 and 4 can be usefully applied to estimate the initial surface area of an unknown ceria sample. The case of the highly dispersed ceria C1-400 was found by magnetic analysis to be unusual.The degree of reduction observed at the plateau was higher than expected from Fig. 4. As already noted, this may be caused by a higher germination rate and diffusion processes. It could also indicate that the energy required for bulk diffusion is lower in the case of high- surface-area ceria; this seems plausible in view of the defects in dispersed solids. This effect could also be due to the reduction and the elimination of the inner polycarbonate species which may be already effective at this temperature and would deeply modify the top surface layers.” Although the magnetic method shows some stable inter- mediate compositions during the reduction (Fig. 3), no evi- dence of a definite suboxide phase was obtained.For moderate reduction percentages, there is only one phase observed by X-ray diffraction which has the expanded cubic structure of CeO, . No evidence for an initial ceria phase was found, as could have been the case if the bulk reduction had progressed by zones. This suggests that as soon as the expanded ceria is obtained, the mobility of the oxygen ions becomes high in the lattice. This is very well illustrated by the facile reoxidation of such samples upon introduction of oxygen at room temperature. When the reduction extent is increased, the hexagonal Ce,O, phase is nucleated, with the intermediary formation of a cubic Ce203 phase. The hexagonal lattice of Ce203 is more stable. It presents many fewer anionic vacancies than the expanded cubic lattice of CeO, -x.Consequently, the diffu- sion of oxygen ions will need a much higher activation energy and the reoxidation process will not be completed at room temperature as soon as the hexagonal Ce,O, phase is formed. This fact could be of some importance in the three- way catalysts in which the gas composition changes with high frequency (0.5-5 Hz), as soon as the redox processes are not limited to the surface. Conclusions The interaction of hydrogen with cerias of different surface areas was followed by TPR, in situ magnetic susceptibility measurements and X-ray diffraction. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 781 By the magnetic method, a stable reduced state is found between 620 and 720 K.The degree of reduction obtained at this plateau varies linearly with the BET surface area. The corresponding slope is very close to that which can be calcu- lated assuming a compact model of capping oxygen ions with one fourth of the surface ions eliminated upon reduction. It is concluded that the formation of one Ce3+ ion layer on the surface corresponds to a stable intermediate state. The TPR data confirm this conclusion. Although the agreement with the predicted value is poor, a linear relationship could be obtained between the BET surface area and the hydrogen consumption during the low-temperature reduction compos- ite peak. These results open the possibility of determining the contribution of the ceria surface in some complex systems containing cerium dioxide.For temperatures between 720 K and about 900 K, which correspond to reduction extents probably lower than 50-60%, the bulk reduction preserves the initial structure of the 5 6 7 8 9 10 11 12 13 14 15 P. Loof, B. Kasemo and I(.E. Keck, J. Catal., 1989,118, 339. H. C. Yao and Y. F. Yao, J. Catal., 1984,86,254. M. F. L. Johnson and J. Mooi, J. Catal., 1987, 103, 502; 1993, 140,612. S. H. Oh and C. C. Eickel, J. Catal., 1988, 112, 543. J. Barrault, A. Allouche, V. Paul-Boncour, L. Hilaire and A. Percheron-Guegan, Appl. Catal., 1989,46,269. K. Otsuka, M. Hatano and A. Morikawa, J. Catal., 1983, 79, 493. K. Otsuka, M. Hatano and A. Morikawa, Znorg. Chim. Acta, 1985,109, 193. J. G. L. Fierro, J. Soria, J. Sanz and J. M. Rojo, J. Solid Stare Chem., 1987,66, 154. S.Bernal, F. J. Botana, R. Garcia, Z.Kang, M.L. Lopez, M. Pan, F. Ramirez and J. M. Rodriguez-Izquierdo, Catal. Toda,y, 1988,2, 653. C. Li, Y.Sakata, T. Arai, K. Domen, K. Maruya and T. Onishi, J. Chem. SOC., Faruday Trans. I, 1989,85,929; 1451. A. Laachir, V. Perrichon, A. Badri, J. Lamotte, E. Catherine, J. C. Lavalley, J. El Fallah, L. Hilaire, F. Le Normand, 13. Quemkre, G. N. Sauvion and 0.Touret, J. Chem. SOC.,Faraday cubic ceria but an appreciable expansion of the lattice is observed. Under these conditions, complete reoxidation can be effective at room temperature. For higher reduction per- centages, the hexagonal Ce203 phase is formed and its pres- ence decreases the reversibility of the redox process since most of the Ce,O, phase remains stable under air at room 16 17 18 19 Trans., 1991,87, 1601.F. M. Z. Zotin, L. Tournayan, J. Varloud, V. Perrichon and R. Frtty, Appl. Catal. A: Gen., 1993,98,99. B. C. Lippens and J. H. de Boer, J.Catal., 1965,4,319. L. Tournayan, N. R. Marcilio and R. Frety, Appl. Catal., 1991, 78, 31. J. P. Candy and V. Perrichon, J. Catal., 1984,89,93. temperature. Finally, a new intermediate phase, cubic Ce203, was observed on the ceria samples reduced at 1070-1 170 K. 20 21 J. C. Lavalley and M. Waquif, personal communication. J. C. Lavalley, A. Badri, E. Catherine and J. Lamotte, personal communication. The authors are indebted to F. Luck and E. Quemere for helpful discussions. Prof. S. Bernal is acknowledged for numerous discussions, as well as Dr. G. A. Martin and Mrs. F. M. Z. Zotin. Part of this research was done with the finan- 22 23 ,24 A. Laachir, Doctoral Thesis No 241-91, University of Lyon I, 1991. 0.T. Sorensen, J. Solid State Chem., 1976,18,217. S. P. Ray, A. S. Nowick and D. E. Cox, J. Solid State Chem., 1975,15,344. cial support of the M.R.E.S. The authors also thank P. Moral for his technical assistance. 25 26 G. Brauer and K. A. Gingerich, J. Znorg. Nucl. Chem., 1960, 16, 87. R. D. Shannon, C. T. Prewitt, Acta Crystallogr., Sect. B, 1969, 25, 295; 1970, 26, 1046; R. D. Shannon, Acta Crystallogr., Sect. References D. A. Johnson, in Advances in Inorganic Chemistry and Radio- chemistry, ed. H. J. Emeleus and A. G. Sharpe, Academic Press, London, 1977, vol. 20, p. 1. M. P. Rosynek, Catal. Rev. Sci. Eng., 1977, 16, 111. R. Korner, M. Riken, J. Nolting and I. Riess, J. Solid State Chem., 1989,16,136. B. J. Cooper, W. D. J. Evans and B. Harrison, Catalysis and Automotive Pollution Control, ed. A. Crucq and A. Frenet, Else- vier, Amsterdam, 1987, vol. 1, p. 117. 27 28 29 30 A, 1976,32,751. A. F. Wells, Structural Inorganic Chemistry, Oxford University Press, London, 3rd edn., 1962, p. 464. J. Barbier, Jr, F. Marsollier and D. Duprez, Appl. Catal., 1992, !m,11. S. Bernal, J. J. Calvino, G. A. Cifredo, J. M. Rodriguez-Izquierdo, V. Perrichon and A. Laachir, J. Catal., 1992,137, 1. A. Trovarelli, G. Dolcetti, C. Leitenburg, J. Kaspar, P. Finetti and A. Santoni, J. Chem. SOC., Faruday Trans., 1992,88,1311. Paper 3/05314H;Received 6th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000773

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

3. CHEM. SOC. FARADAY TRANS..1994. 90(1. 783-786 Water Adsorption in Active Carbons described by the Du binin-Astakhov Equation Fritz Stoeckli* and Timur Jakubovt Chemistry Department, University of Neuchatel, Av. de Bellevaux 51, CH-2000Neuchatel, Switzerland Andre Lavanchy GRD Laboratorium , CH-3700 Spiez,Switzerland It is shown that the adsorption of water by a variety clf active carbons can be described within the framework of Dubinin’s theory. Owing to the low values of the characteristic energy, E (0.8-2.5 kJ mol-’), the Dubinin- Astakhov equation becomes S-shaped in the range 0 3 < pipo < 0.7 and provides a good basis for the fit of the adsorption branch of type V isotherms near room temperature. The parameters of the equation are almost temperature invariant and consequently a good agreement IS also found in many cases for the enthalpies of immersion into water, as predicted by the extension of Dubinin’s theory.The adsorption of organic and simple inorganic molecules b?, active carbons corresponds to type I or type I1 isotherms and is well described by Dubinin’s theory.’ -3 In this approach. the micropore volume is gradually filled by the adsorbate. whose density varies with the strength of the adsorption potential. It is also assumed that at the end of the process. the average density is close to that of the free liquid. The adsorption of water, on the other hand. corresponds to a type V i~otherm,~owing to the low affinity of thic adsorptive for carbonaceous surfaces.A specific model has therefore been proposed by Dubinin and Serpinski‘ to describe the adsorption branch of the isotherm. (No quanti-tative description exists, so far, for the desorption branch.i It takes into account the fact that the adsorption of uater depends essentially on the presence of hydrophilic or so-called primary centres. where the formation of clusters begins. This type of adsorption and its thermodynamic consequence< have been investigated by different authors.6 lo In the present paper, we show that in the case of micro- porous carbons the adsorption branch of the water isotherm can also be described by the classical equation of Dubinin and Astakhov. .Vd = N,, exp[-(APE)”] In this expression. N, represents the amount adsorbed in mmol-at temperature T and relative pressure p/po ; .V.3c,is the limiting amount adsorbed in the micropores, A = RT In ( pojp); and n and E are temperature-invariant parameters.which depend on the system under investigation. Typicall), ti varies between 1.5 and 6--7. It is convenient to use the actual volume of the micropore system W, = Na, V,. where LA rep-resents the molar volume of the adsorbate. Since W, is an invariant. one often uses the volumes I+’ and W, and their ratio. instead of N, and N,, . It has been shown that the influence of the adsorbate could be expressed by a scaling factor p. called the affinity c(1effi- cient, such that E = PE,. By convention, the referznce vapour is benzene and /?(C,H,) = 1. For typical organic and inorganic vapours adsorbed by active carbons.E, varies from 15 to 30 kJ mol-I and. as shown recently,” it is an inierse function of the average micropore width L/nm = 10.8 (E, -1 1.4). t Permanent address Institute of Physical Chemistry, Ruman Academy of Sciences, Leninsktt Prospek t 3 1. 1 17915 mom)^. Russia 4 mathematical analysis shows that eqn. (1) has an inflex- .on point, but in the case of high values of E it is found at my low relative pressures. Consequently, the isotherm is -xactically of type I. On the other hand, for small values of E rt>pically E < 2-3 kJ mol-I near room temperature), the iso- *herm becomes markedly S-shaped and it can be used to tiescribe the adsorption branch of a type V isotherm.It .tppears that the water adsorption isotherm can be fitted to cqn. (1) practically over the entire range of relative pressures, .darting near or below p/po = 0.1. Moreover, it is also found $hat in the case of water, E and n do not vary appreciably xith temperature, which explains the satisfactory agreement Kith the known thermodynamic consequences of eqn. , 1 ),2,12.I3 such as the differential heat of adsorption and the >:nthalpy of immersion into water. These observations suggest that Dubinin’s theory provides ,i satisfactory background for the description of water idsorption by microporous carbons. In a later study, we shall <:ompare this approach with the generalized equation pro- 2osed by Sircar.I4 The latter also describes adsorption iso- ‘herms of types I.IV and V, but the underlying model is .Merent. Theoretical Following the model proposed by Dubinin and Serpinski, the idsorption of water occurs around primary centres, where ,:lusters are forrned.l5 In the region of 0.4 < pip, < 0.8, the sotherm is given by PIP, = u/[c(u, + U)(l -ka)] (2) In this equation, a represents the amount of water, usually n mmol g-adsorbed at p’p, ; a, is the number of primary :entres characterized by the number of molecules directly ittached to them, implicitly a 1 : 1 ratio; k is a constant related to the total amount of water, as,adsorbed at pip, = 1 ind c is the ratio between the rate constants of adsorption ind desorption. The relevance of c has been described else- lvhere and it appears that it is related to the molar enthalpy ,f immersion into water.The range of validity of eqn. (2) has recently been extended qy Barton er by adding a fourth parameter. It has also been shown that for typical active carbons -reated in uacuo at 400-5oO‘C and containing a uniform type .,f primary site (probably of the carbonyl type), the enthalpy Jf immersion into water is given by6.’ Ahi,J g--‘ = -25a, -0.6(a,-ao) (3) 784 For carbons with important external surface areas, S,, one should add the term -(0.035 J mV2)S,, but in most cases this is a relatively small correction. A mathematical analysis of eqn. (1) shows that the Dubinin-Astakhov isotherm is always S-shaped, but this feature becomes significant only for small values of E/RT.This is illustrated by the model isotherms of Fig. 1, where T= 293 K, n = 2 and E varies from 25 to 1. Under these conditions, E/RT decreases from 10.2 to 0.41 and the inflex- ion point is gradually shifted towards higher values of p/po. As a consequence, the isotherm changes from type I to type V. Fig. 2, on the other hand, shows the influence of n on the steepness of the isotherm for a typical case (E = 2 kJ mol-' 1.o $ 0.5 0.0 0.0 0.5 1.o PIP0 Fig. 1 Model isotherms corresponding to eqn. (1) with T = 293 K, n = 2 and E = 1 (a),2 (b), 4 (c), 9 (d) and 25 (e) 1.o $ 0.5 0.0 0.0 0.5 1.o PIP, Fig. 2 Model isotherms corresponding to eqn. (1) with T = 293 K, E = 2 and n = 1 (a),2 (b), 3 (c), 5 (d)and 9 (e) J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 and T = 293 K). It appears that for n = 1, the isotherm is almost linear. From a mathematical point of view, the features illustrated by Fig. 1 and 2 reveal the usefulness of eqn. (1) for the description of water adsorption by active carbons, as well as other systems, in general. Moreover, provided that E and n are temperature invariant, the thermodynamic consequences of eqn. (1) should also be valid. As shown elsewhere, the fol- lowing expression can be derived for the net differential heat of adsorption associated with the Dubinin-Astakhov eq~ation'*'~-~~ qnet= E[(ln i/e)lln + (aT/nXln 1/8)'/"-'] (4) where 8 = NJN,, and a is the thermal expansion coefficient of the pure adsorptive.(However, it may be slightly different in the adsorbed state.) Since qne' = qSt-AHvap,the integration of eqn. (4) from 8 = 0 to 1, leads to the molar enthalpy of immersion into the corresponding liquid. It is a negative quantity, given by AhJJ mol-I = -E(1 + df)T(l + l/n) (5) where r is the classical 'Gamma' function. If one introduces the micropore volume, W0/cm3 g-', and the molar volume of the adsorbate, VJcm3 mol-l, eqn. (5) becomes AhJJ g-' = -EWo(l + olT)r(l + l/n)/Vm (6) For the classical case of the Dubinin-Radushkevich equation, where n = 2, one obtains an expression which has been used extensively for the characterization of active carbon~~*~~' ',16 AhJJ g-' = -EW,(l + ~T)Z'/~/~V~(7) From the tabulated values of the 'Gamma' function," it appears that if n varies from 2 to 6, the numerical value of eqn.(6) increases by only 5%. This suggests that the enthalpy of immersion depends mainly on the parameters W,and E (or BE,) of the Dubinin-Astakhov equation. Experimental A number of well characterized active carbons were used and their main properties are given in Table 1. The techniques applied in the present study (water adsorption between 263 and 293 K and immersion calorimetry at 293 K) have been described in detail earlier.2-'6 The carbons present a wide range of structural properties, with average micropore widths between 0.7 and 2 nm. On the other hand, the number of hydrophilic centres, a,, varies between 1.3 and 8% of the total amount of water adsorbed by the different solids.This means that the inflexion point of Table 1 Main properties of the microporous carbons, derived from the adsorption of organic vapours and of water, by using eqn. (1) organic vapour water adsorption carbon E0BJ mol-' w0/cm3g -1 L/nm TJK EJkJ mol-' W0/cm3 g-' n 1 CMS 26.2 0.25 0.8 293 1.86 0.24 4.20 275 2.01 0.27 4.12 2 U-03B 21.1 0.43 1.1 293 1.37 0.43 4.68 3 U-03N 16.9 0.52 2.0 293 0.87 0.52 2.67 263 0.98 0.50 2.94 4 u-02 20.0 0.43 1.2 293 1.19 0.45 2.32 5 N-125 16.8 0.64 2.0 293 1.17 0.57 4.22 6 DCG-5 21.2 0.54 1.1 293 1.79 0.49 1.87 7 PLW 23.9 0.46 0.9 293 2.18 0.45 6.46 8 PLWK 22.9 0.41 0.9 293 2.27 0.45 7.55 9 ALCA 19.4 0.50 1.3 293 2.23 0.45 7.67 10 MSC-V 27.1 0.17 0.7 293 2.39 0.17 3.28 11 MSC-VR 27.1 0.19 0.7 293 1.89 0.17 4.61 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the water adsorption isotherm is shifted from p/po =0.7 to 0.3, as a,/a, increase^.^.^ Calculations were based on the thermal expansion coeffi- cient and the molar volume of the pure water, assuming that these properties were not too different in the adsorbed state. Results and Discussion The water adsorption isotherms were fitted to the Dubinin- Astakhov equation [eqn. (l)] and the corresponding results for n, E and W,(H,O) are given in Table 1. As illustrated by the typical examples shown in Fig. 3, eqn. (1) leads to a good fit for the various samples.In the case of samples CMS and U-03N the adsorption of water was investigated between 263 and 293 K. It appears that for both solids, the parameters n and E do not vary appreciably with temperature. Although the range of tem-perature is still limited to 30 K, this suggests that the require- ment for temperature invariance is fulfilled to a good approximation, at least near room temperature. As illustrated in Fig. 4,a plot of W/W, vs. RT In (po/p) leads to a unique characteristic curve1,* for each carbon, as found in the case of organic vapours. This also corresponds to an earlier observa- tion by Hassan et al.," for the adsorption of water by carbon BPL between 288 and 308 K. /* a 0 0.4 0.8 P/Po Fig.3 Fit of the water adsorption isotherms to eqn. (1) for carbons N-125 at 293 K (m), U-02 at 293 K (0)and CMS at 275 K (A).See also Table 1. 1 2 RT In(p,/p)/kJ mol-' Fig. 4 Characteristic curves for the adsorption of water by carbons CMS at 275 and 293 K (0,m) (a) and U-03N at 263 and 293 K (0,rn) (4 Table 2 Calculated and experimental enthalpies (in J g-') of immersion of the active carbons into water at 293 K Ahi 1 27 27 2 28 32 3 21 35 5 31 32 6 44 41 7 47 45 8 50 45 9 49 44 10 19 23 11 15 14 It is, therefore, not surprising to find a relatively good agreement between the enthalpies of immersion into water calculated by eqn. (6) and the corresponding experimental values obtained at T = 293 K (Table 2).In some cases, however, a poor agreement was observed. One reason may be the ageing of the material (some isotherms were determined several years ago), and/or the presence of water-soluble materials in the active carbon, two factors which affect Ahi. From a practical point of view, the description of water adsorption by means of the Dubinin-Astakhov equation [eqn. (l)] presents a number of advantages. First, it means that a relatively simple relation can be used to predict the adsorption isotherm at different temperatures and relative pressures, as in the case of organic vapours. In practice, only one isotherm is needed at a convenient temperature, for example 293 K, and the results can even be cross-checked with the corresponding enthalpy of immersion.However, the important property of temperature invariance must still be checked over a wider range of temperature and an important test will be provided by the comparison of Ah, measured at different temperatures (ca. 278 to 308 K) with the values pre- dicted by eqn. (6). Secondly, since n and E are almost temperature invariant, eqn. (4) can be used to calculate the corresponding heat transfer in the system. This possibility is of great importance in the case of dynamic adsorption of vapours by active carbons in the presence of water, and in the computer simula- tion of such proces~es.'~ From a theoretical point of view, on the other hand, the present study shows that nearly 50 years after its formulation, Dubinin's theory for the filling of micropores can be extended to the adsorption of water by active carbons.It is likely that this description also applies to other vapours with type I11 or V isotherms on carbons and possibly on other microporous solids. This aspect will be presented later. It is obvious that the present description can be transposed to the case where the water adsorption isotherm begins as type I at low pressures. The overall isotherm may simply be considered as a superposition of Langmuir and Dubinin- Astakhov contributions, including the thermodynamic conse- quences of the two models. At the present time, it is not possible to define exactly the meaning of parameters n and E in the case of water adsorp- tion.As shown above, mathematical modelling suggests that the 'steepness' of the isotherm depends on n,whereas E has a direct influence on the position of its inflexion point. As shown it depends on the state of oxidation of the surface. The data presented in Table 1 suggest that no direct corre- lation exists between E and the classical characteristic energy, E,, associated with type I or I1 isotherms. This is not too Table 3 Correlation between the characteristic energies, E/kJ mol-', obtained from eqn. (1) and (8),for T = 293 K E 1 5.6 1.86 2.04 2 2.5 1.37 1.25 3 2.1 0.87 1.18 4 1.3 1.19 0.98 5 1.6 1.17 1.03 6 3.6 1.79 1.57 10 7.9 2.39 2.65 11 3.8 1.89 1.58 surprising, since the adsorption of water depends essentially on specific sites, as opposed to the case of organic vapours where the adsorption potential within the micropores domi- nates the process of volume filling. One may therefore expect that E depends more on the hydrophilic/hydrophobic charac- ter of the solid expressed by a,/a,, than on the average micropore size.A possible correlation is suggested by comparison of eqn. (3) and (6) for the enthalpies of immersion into water, which are not mutually exclusive, and the following empirical rela- tion can be obtained, As shown in Table 3, for a number of systems the values of E obtained from eqn. (1) and (8), and T = 293 K, agree within 15-20%, which indicates some degree of self-consistency. Eqn.(8) also suggests that at room temperature E should be close to 0.6 kJ mol-l, when a, vanishes. The corresponding isotherm, with an average value of n near 3-4, should therefore describe the adsorption of water in the micropores of pure carbon. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 At the present time, further work is needed to explore the consequences of eqn. (1) and results will be presented in due course. References 1 M. M. Dubinin, Carbon, 1989,21,457; 1989,29,481. 2 D. C. Bansal, J. B. Donnet and H. F. Stoeckli, Active Carbon, Marcel Dekker, New York, 1988. 3 H. F. Stoeckli, Carbon, 1990,28, 1. 4 S. J. Gregg and K. S. W. Sing, in Adsorption, Surface Area and Porosity, Academic Press, London, 1982. 5 M. M. Dubinin and V.V. Serpinski, Carbon, 1981,19,402. 6 H. F. Stoeckli, K. Kraehenbuehl and D. Morel, Carbon, 1983, 21, 589. 7 F. Kraehenbuehl, C. Quellet, B. Schmitter and H. F. Stoeckli, J. Chem. SOC.,Faraday Trans. 1, 1986,82,3439. 8 M. J. B. Evans, Carbon, 1987,25,81. 9 S. S. Barton, M. J. B. Barton and J. MacDonald, Carbon, 1991, 29, 1105; 1992,31, 123. 10 H. F. Stoeckli and D. Huguenin, J. Chern. Soc., Faraday Trans., 1992,88,737. 11 H. F. Stoeckli, P. Rebstein and L. Ballerini, Carbon, 1990, 28, 907. 12 H. F. Stoeckli,Izv. Akad. Nauk SSSR (Ser. Khim.), 1981,63. 13 H. F. Stoeckli and F. Kraehenbuehl, Carbon, 1981,19,353. 14 S. Sircar, Carbon, 1987, 25, 39. 15 G. G. Malenkov and M. M. Dubinin, Izv. Akad. Nauk SSSR (Ser. Khim.), 1984, 1217. 16 H. F. Stoeckli, D. Huguenin and P. Rebstein, J. Chem. Soc., Faraday Trans., 1991,87, 1233. 17 W. H. Beyer, CRC Standard Mathematical Tables, CRC Press, Boca Raton, FL, 1981. 18 N. M. Hassan, T. K. Ghosh, A. L. Hines and S. K. Loyalka, Carbon, 199 1,29,68 1. 19 F. Meunier, F. M. Sun, F. Kraehenbuehl and F. Stoeckli, J. Chem. SOC.,Faraday Trans. 1,1988,84,1973. Paper 3/06455G;Received 28th October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000783

出版商:RSC    

年代:1994

数据来源: RSC