J . Phys. Chem. 1985,89, 5557-5565
5557
A Vibrational Spectroscopic Characterization of the Solid-Phase Behavior of n -C,,H,,NH3CI and ( n-C,,H2,NH3),CdCI,' H. L. CasaI,* D. G . Cameron,2 and H. H. Mantsch Division of Chemistry, National Research Council of Canada, Ottawa, Canada K I A OR6 (Received: May 10, 1985)
The solid-phase behavior of n-CloH21NH3C1(CloCI) and (n-C10H21NH3)2CdC14 (CloCd) was investigated by infrared and Raman spectroscopy. The ability of vibrational spectroscopy to study the structural and dynamic aspects of the solid-phase behavior of layered systems has been exploited to compare CloCl and CloCd. The nature of the four solid phases of CloC1 (phases I, 11, 111, and IV) is discussed and compared with that of the three solid phases of CloCd (phases A, B, and C). The virgin crystals of CloCl form a well-ordered rigid phase which on heating converts to a liquid-crystalline phase. Cooling induces formation of a disordered solid phase which on annealing re-forms the virgin crystals. In the case of CloCd the data demonstrate the presence of intramolecular as well as intermolecular disorder in the intermediate phase. A correspondence is found between the solid-phase behavior of CloCd and that of n-alkanes and aqueous lipid bilayers.
Introduction Perovskite-type layer compounds of general formula (nCnH2n+lNH3)2MX4are of much current interest from both magnetic and structural points of view.3 They undergo solidsolid phase transitions and, because of their pronounced two-dimensional structure, present interesting magnetic properties. In the case of (n-CloH21NH3)2CdC14 (hereinafter abbreviated as CloCd), parallel sheets of corner-sharing CdC16 octahedra are held together by the n-decylammonium groups. The NH3+groups of the chains occupy the cavities of the CdC14 layers and are bonded to them by hydrogen bonds to the chlorine atoms (see Figure 8 of ref 5 ) . Thus, the general arrangement of the alkyl chains is comparable to the bilayer structure of biological membranes. The solid-phase behavior of CloCd has been extensively studied by various p!ysical t e c h n i q ~ e s . ~In* ~the temperature range 10-60 "C it undergoes two phase transitions. The lower temperature transition at 34 OC involves the onset of orientational disorder in the alkyl chains; the higher temperature transition a t 39 "C involves the "conformational melting" of the alkyl chains. These transitions resemble those of saturated diac,yl- or dialkylphosphatidylcholine bilayers' and therefore also resemble the transitions observed in solid, odd-numbered n-paraffins.* In the low-temperature phase of CloCd the alkyl chains are tilted by 40" with respect to the bilayer normal. There are two types of chains labeled A and B. A chains contain a gauche bond between carbon atoms 1 and 2 and B chains contain a gauche bond between positions 2 and 3. Thus, all chains in the low-temperature phase contain conformational defect^.^ The chemical precursor of CloCd, n-CloH21NH3C1 (hereinafter abbreviated as CloCl), forms, at room temperature, interdigitated bilayer^.^ In this phase the alkyl chains are in the extended all-trans conformation; Le., there are no conformational defects. At 48 OC it converts to a noninterdigitated liquid-crystalline-like (1) Issued as NRCC No. 24741. (2) Present address: The Standard Oil Co. (Ohio), 4440 Warrensville Center Rd; Cleveland, OH 44128. (3) For a review of recent literature see ref 4. (4) G. F. Needham, R. D. Willett, and H. F. Franzen, J. Phys. Chem., 88, 674 (1984). ( 5 ) R. Kind, S. Plesko, H.Arend, R. Blinc, B. Zeks,, J. Seliger, B. Lozar,
J. Slak, A. Levstik, C. Filipic, V. Zagar, G. Lahajnar, F. Milia, and G. Chapuis, J. Chem. Phys., 71, 2118 (1979). (6) R. Blinc, M. I. Burgar, V. Rutar, B. Zeks, R. Kind, H. Arend, and G. Chapuis, Phys. Rev.Lett. 43, 1679 (1979). (7) J. F.Nagle, Annu. Rev. Phys. Chem., 31, 157 (1980); H.L. C a d and H.H. Mantsch, Biochim. Biophys. Acta, 779, 381 (1984). (8) M. G. Broadhurst, J. Res. Narl. Bur. Stand., Sect. A . 66, 241 (1962). (9) R. Kind, R. Blinc, H.Arend, P. Muralt, J. Slak, G. Chapuis, K. J. Schenk, and B. Zeks, Phys. Rev.A, 26, 1816 (1982).
0022-3654/85/2089-5557$01.50/0
phase.1° This transition is not reversible and on cooling a third phase is formed which is characterized by the presence of chain orientational disorder and gauche bonds." Only after prolonged annealing does the system revert to an ordered structure similar to that of the virgin crystals. We have investigated the solid-phase behavior of CloCd and CloCl by infrared and Raman spectroscopy. These studies are directed toward a correlation of known structural and dynamic properties of these systems with the vibrational spectra. At the same time, the vibrational spectra yield new information on the presence of intrachain conformational defects in the intermediate phase of CloCd. In the case of CloCl it is shown that the NH3Cl groups do not follow the behavior of the alkyl chains; different hysteresis is shown by the NH3Cl groups and the alkyl chains. Since the completion of the present work, a detailed study of the phase behavior of CloCd by vibrational spectroscopy has appeared.'* Our results are in accord with that work and serve as a complement.
Experimental Section The sample of n-decylammonium chloride (CloCI) used in this study has been described previously." Bis(n-decylammonium) tetrachlorocadmate (CloCd) was prepared by reaction of ClOCl with stoichiometric amounts of CdC12 in ethanol. The mixture was refluxed for 30 min, the solvent was evaporated, and the solid was recrystallized three times from the same solvent. The purity of CloCl and CloCd was checked by elemental analysis. The parent n-decylamine (Fluka) was determined to be >99% pure by gas-liquid chromatography using a Hewlett-Packard Model 5995 GC-MS system with a lO-m, 0.22-mm i.d. Ultra-1 column (cross-linked methyl silicon). This ensured that there were no artifacts due to impurities and/or the presence of chain homologues of different length? It is reflected in very sharp transitions of CloCd. Infrared spectra of CloCl and CloCd were recorded from 12pm-thick films between KBr windows. The films were deposited from ethanol. Raman spectra were recorded from neat crystals sealed in glass capillaries. Infrared spectra at various temperatures were recorded at 2-cm-' resolution with a Digilab FTS-19 spectrometer by procedures described e1sewhere.I' Raman spectra were excited with 200 mW of 5145-A radiation from a CR12 argon ion laser and collected with a computer(10) V. B u s h , P. Cemicchiaro, P. Corradini, and M. Vacatello, J . Phys. Chem., 87, 1631 (1983). (1 1) H. L. Casal, H. H. Mantsch, and D. G. Cameron, Solid State Commun. 49, 571 (1984).
(12) L. Ricard, M. Rey-Lafon, and C. Biran, J. Phys. Chem., 88, 5614 (1984).
Published 1985 American Chemical Society
5558
The Journal of Physical Chemistry, Vof. 89, No. 25, 1985
Casal et al.
r f: 0 0)
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C
I
I
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I
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I
I
1200
I
I
I400
I
I
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1600 2700
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I
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I
31 1
Wavenumber shift, cm-l
Figure 1. Raman spectra of CloCI: at 25 OC (phase I, virgin crystals); at 60 OC (phase 11); at 25 OC (phase 111) recorded 2 h after recording the phase I1 spectrum; and at 25 OC (phase IV) recorded 28 days after recording the phase 111 spectrum. The bands marked with asterisks are artifacts introduced by the spectrometer
controlled Spex 14018 double monochromator equipped with a cooled RCA C3 1034 photomultiplier. The Raman spectra were collected with a spectral slit width of -2 cm-I, and frequency and sensitivity calibrations were applied to each spectrum. The temperature was controlled to f 0 . 5 OC by using a laminar flow of dry nitrogen.
Results and Discussion A . n-CloH21NH,Cl (CloCI). i. Raman Spectra. Figure 1 shows Raman spectra of the four solid forms of Cl0C1. The bottom trace is the spectrum of phase I (at 25 "C) formed by the CloCl virgin crystals after recrystallization from methanol. Phase I1 CloCl is formed on heating above 48 OC; the spectrum of phase I1 was recorded at 60 OC. Phase I11 forms on cooling phase 11; the corresponding spectrum of phase I11 was obtained at 25 OC, 2 h after recording that of phase 11. The spectrum of phase I V was recorded after equilibrating the sample for 28 days at 25 OC. Phase I. The spectrum of phase I in Figure 1 is that typical of polymethylene chains in the all-trans conformation. The high degree of conformational order is particularly manifested in those bands sensitive to chain conformation, such as the longitudinal acoustical mode (LAM).I3 In the spectrum of phase I the LAM band appears at 218 cm-I. The frequency of this mode in the spectrum of solid n-decane is 230 cm-I, while it is 206 cm-' for solid n-undecaneI4 and 213 cm-' for solid n-decylamine; thus the frequency of 218 cm-' for CloCl is compatible with a conformationally highly ordered chain. The difference in frequency with respect to the n-alkanes is due to end effects known to affect these modes in hydrogen-bonded systems.15 The Raman bands in the C-C stretching region (1050-1 150 cm-') are also typical of systems with rigid all-trans polymethylene chains and the broad bands characteristic of gauche conformers16 are absent in the spectrum of phase I. The CH2 rocking band at 1176 em-' is well resolved and of medium intensity demon(13) R. F. Schaufele and T. Shimanouchi, J . Chem. Phys., 47, 3605 (1967). (14) J. R. Scherer and R. G. Snyder, J . Chem. Phys., 72, 5798 (1980). (15) G. Minoni and G. Zerbi, J . Phys. Chem., 86, 4791 (1982). (16) J. L. Lippert and W. L. Peticolas, Proc. Natl. Acad. Sci. U.S.A.,68, 1572 (1971).
strating that the alkyl chains of phase I CloClare tightly packed and that little motion is present. The CH2 scissoring mode shows correlation field splitting and gives rise to two bands at 1418 and 1440 cm-'. The crystal structure of CloC1 is monoclinic (space group P2,) with two molecules in the unit cell? This type of packing is known to induce splitting of several vibrational m o d e ~ . ' ~ J ~ The C-H stretching region in the spectrum of phase I is similar to that observed in the spectra of n-alkanes, phospholipid bilayers, and other polymethylene compound^.'^ In this case the CH, antisymmetric stretching band at -2880 cm-' is very sharp and markedly more intense than the rest of the C-H stretching bands. The peak height ratio r = h2850/h2880 of the two C H 2 stretching bands (at 2850 and 2880 cm-') is 0.46. This value is comparable to that observed in the spectra of n-alkanes, fatty acids, and their esters when packed in rigid crystals in the all-trans conformation.20 The value of this parameter depends mainly on interchain interactions and has been correlated with known crystal structures of n-alkanes20 However, the value of r = 0.46 in the spectrum of virgin CloCl (Figure 1) is lower than that observed for virgin CloCd ( r = 0.60). These differences in r can, in principle, be attributed to the fact that phase I ClOClis interdigitated whereas CloCd is not. Levin et aL2' found that when the acyl chains of phospholipid bilayers become interdigitated the values of r decrease. However, the values of r also depend on interchain packing which is different for CloCd and CloCl. Phase ZZ. The liquid-crystal-like phase I1 is obtained from phase I by heating above 48 OC. The Raman spectrum of phase I1 in Figure 1 clearly reflects the presence of conformational disorder in this phase. The LAM band, for example, is a broad feature at -226 cm-I with a full-width at half-height of 28 cm-'. Broad LAM bands are also observed in the spectra of liquid n-alkanes (17) F. J. Boerio and J. L. Koenig, J . Chem. Phys., 52, 3425 (1970). (18) R. G. Snyder, J . Chem. Phys., 71, 3229 (1979). (19) D. F. Wallach, S. P. Verma, and J. Fookson, Biochim. Biophys. Ada, 559, 153 (1978). (20) R. G. Snyder, S. L. Hsu, and S. Krimm, Specrrochim. Acta, Par? A, 34, 395 (1978). (21) C . Huang, J. T. Mason, and I. W. Levin, Biochemistry, 22, 2775 (1983).
Spectroscopic Characterization of Solid-Phase Behavior and appear at 250 cm-I for n-decane and at 248 cm-' for n-undecane.22 The C-C stretching bands in the 1050- and 1150-cm-l region are different from those in the spectrum of phase I in Figure 1, e.g., a broad C-C stretching band at 1080 cm-I, characteristic of gauche conformers is present, while the band at 1130 cm-' characteristic of trans bonds is much weaker. The other C-C stretching trans band at 1060 cm-l is masked by a strong band at -1055 cm-I due to N H 3 rocking. The peak height ratio h,,,/h, '30 has been used to monitor chain conformational disorder in other systems. In this case we find this ratio to be 1.8 at 60 "C which is less than the values usually found in the spectra of liquid n-alkanes or lipid bilayers in the liquid-crystalline phase.24 This low value of h(gauche)/h(trans) is an indication that, while the chains are conformationally disordered, the concentration of gauche defects is less than in the other systems mentioned above. This conclusion is supported by a value of -0.75 for the order parameter calculated from 13CN M R measurements on C10Cl.25 In the CH2 scissoring region the band due to correlation field splitting is absent and all bands have broadened considerably. Likewise, the C H 3 rocking band at 890 cm-' has decreased in intensity and new CH, rocking bands at 845 and 870 cm-' are present indicating the presence of chain conformational defects involving gauche bonds next to the chains ends. The C H stretching region of the Raman spectrum also changes drastically on transition from phase I to phase 11. The spectrum of phase I1 in Figure 1 is very similar to that of liquid n-alkanes and lipid bilayers in the liquid-crystalline phase.24 The peak height ratio r = hzsso/hzssois 0.96 which can be compared with values of 1.4 in the spectra of liquid n-alkanes.20 The broadening of the CH2 antisymmetric stretching band (-2880 cm-l) is particularly evident and is a definitive manifestation of the presence of liquidlike chains. A further difference between the phase I and phase I1 Raman spectra of CloCl is found in the bands originating from the NH3Cl group (900-1200 cm-I). In the spectrum of phase I, sharp, strong bands are observed for the different vibrational modes of this group. A tentative assignment of these bands can be given by comparison with the spectra of related compounds.26 However, in the spectrum of phase I1 CloCl in Figure 1 many NH3CI bands are absent or have broadened to the extent that their intensity is negligible relative to that of the alkyl chain bands. The different intensity of the NH3C1 modes in phases I and I1 is due to the different geometries of the NH3CI group in the two phases. In phase I each nitrogen atom is bonded via three hydrogens to three chlorines and each chlorine atom shares hydrogen bonds with three nitrogen atoms. The N-H--CI distances vary from 3.15 to 3.19 A.9 This yields a rigid arrangement for the NH,Cl group which gives rise to site-symmetry splitting of the various vibrational modes of this group. In phase I1 the basal expansion of the crystal induces larger interchain distances, which in turn allow motional (as well as conformational) disorder to occur. This situation induces a local symmetry for the NH3C1 group under which many of the vibrational modes are Raman forbidden.26 Thus, the weak intensities of the NH3Cl Raman bands in the phase I1 spectra are probably due to differences in local symmetry of the NH,Cl group induced by disorder. Phase ZZI. When phase I1 CIoCl is allowed to cool to room temperature, a metastable phase I11 is obtained." This phase I11
-
-
-
-
(22) R. G. Snvder. J . Chem. Phvs.. 76. 3921 (1982). (23j R. G. Syider; J . Chem. Ph;s.; 471 1316 (1967j. (24) P.T. T. Wong and H. H. Mantsch, Biochim. Biophys. Acta, 732,92 (1983); C. Huang and I. W. Levin, J. Phys. Chem., 87, 1509 (1983). (25) B. Lozar, M. I. Burgar, R. Blinc, R. Kind, and H. Arend, Solid State Commun., 44, 737 (1982).(26) J. E. Stewart, J . Chem. Phys., 30, 1259 (1959). J. Bellanato, Spectrochim. Acta, 16, 1344 (1960). T. Uno, K. Machida, and K. Miyajima, Spectrochim. Acta, Parr A 24, 1749 (1968). I. A. Oxton and 0.Knop, J . Mol. Structure, 37, 59 (1977). I. A. Oxton, J . Mol. Structure, 54, 1 1 (1979). C. Sourisseau and G. Lucazean, J . Raman Spectrosc., 8, 311 (1979). C. Si-
guenza, P.Galera, E. Otero-Aenlle, and P. F. Gonzalez-Diaz, Spectrochim. Acta, Part A , 37,459 (1981). 2.Iqbal, H. Arend, and P.Wachter, J. Phys. C , 14, 1497 (1981). C. N. R. Rao, S. Ganguly, and H. R. Swany, J . Chem. SOC.,Faraday Trans. 2, 77, 1825 (1981).
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985 5559 retains the c axis dimension of phase I1 and is therefore not interdigitatede9The arrangement of phase I11 therefore closely resembles that of lipid bilayers. Evidence for both intra- and intermolecular disorder is found in the infrared spectrum of phase I11 CloCl" and similar information is also available from the Raman spectrum of phase I11 in Figure 1. The LAM band is at 222 cm-' (compared to 2 16 cm-l for phase I and 226 cm-' for phase 11). In phase I11 it is broader than in phase I, but narrower than in phase I1 (its half-width is 14 cm-' in phase I, 28 cm-' in phase 11, and 21 cm-I in phase 111). This is evidence for the presence of gauche conformers in phase 111. Further evidence for conformational disorder in the alkyl chains of phase I11 CloClcomes from the CH3 rocking mode. Although this band, at 890 cm-l, has regained some of its intensity on cooling from 60 OC, it is broader than in the phase I spectrum and there are additional weak bands at 875 and 845 cm-l which can be related to the presence of conformational defects.27 The individual bands in the CH2 scissoring region are more clearly resolved in the phase I11 spectrum than in the phase I1 spectrum, the CH, bending mode is evident at 1480 cm-I. We ascribe a very weak shoulder at 1425 cm-' to the component of the CHI scissoring mode, split due to the correlation field. In the spectrum of phase I in Figure 1 this band appears at 1418 cm-l. In phase I11 the interchain interactions are weaker and thus the correlation field induces a smaller separation of the CHI scissoring components. When polymethylene chains are completely decoupled they behave as rigid rotors and no interchain vibrational coupling of CH2 scissoring or rocking modes is observed. The small splitting observed indicates that in phase I11 CloCl the alkyl chains are correlated to a degree comparable to that found in n-alkanes in their rotator or hexagonal phase.28 The relative intensities of the Raman bands in the C H stretching region are practically identical with those observed in the spectra of hexagonally packed n-alkanes (r = 0.79 for CloCl and r = 0.76 for hexagonal n-C36H74at 347.5 K).20 In the spectrum of phase I11 CloClthe NH3C1bands reappear. However, the C N stretching band, which is at 948 cm-I in phase I, occurs at 925 cm-"in phase 111; the NH, rocking mode observed at 1054 cm-' in phase I, completely overlaps the C-C stretching mode at 1060 cm-' in phase 111; and the NH3 rocking band which is at 1162 cm-l in phase I, shifts to 1048 cm-' in phase 111. These differences in frequency of the NH3CI modes suggest that the geometry of this group in phase I11 differs from that in phase I. Since phase I11 is not interdigitated, but more ordered than phase 11, we propose that the reappearance of the NH3Cl bands in the Raman spectrum indicate a reoccurrence of the nonequivalence of N-H-CI distanw, inducing site-symmetry splitting of the NH, modes. Phase IV. The Raman spectrum of phase IV is shown in Figure 1 after annealing phase I11 for 28 days. Apart from minor differences, this spectrum is practically identical with the spectrum of phase I. However, the peak height ratio of the two CH2 scissoring components at 1418 and 1440 cm-' is 0.88 in phase IV and 1.0 in phase I. As this ratio is related to the shape of the unit cell in which the alkyl chains pack,'* the differences between the spectra of phase I and phase IV indicate small differences in the unit cell shape with regard to the angle of tilt of the alkyl chains relative to the layer normal. The same conclusion regarding differences between phases I and IV was reached from the corresponding infrared spectra." ii. Infrared Spectra. Figure 2 shows the infrared spectra of the four phases of CloCl measured under conditions identical with those used to record the Raman spectra for phases I, 11, and 111, except that the Raman spectrum of phase IV was obtained after equilibrating at 25 O C for 28 days, while the infrared spectrum of phase IV was recorded after only 13 days equilibrating at the same temperature.
-
(27) G. Zerbi, R. Magni, M. Gussoni, K. Holland-Moritz, A. Bigotto, and S. Dirlikov, J . Chem. Phys., 75 3175 (1981). (28) H. L. Casal, H. H. Mantsch, D . G . Cameron, and R. G. Snyder, J . Chem. Phys., 77, 2825 (1982).
5560
Casal et al.
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985
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Wavenumber,cm-' Figure 2. Infrared spectra of CloCI: at 25 O C (phase I, virgin crystals); at 65 'C (phase 11); at 25 spectrum; and at 25 O C (phase IV) recorded 13 days after recording the phase I11 spectrum.
Temperature Dependence of the Infrared Spectra. The temperature-induced changes in the alkyl chains and in the ionic NH3Cl moiety can be followed by measuring the frequency of C H 2 and N H 3 modes, respectively. Figure 3A shows the temperature dependence of the C H 2 symmetric stretching mode us(CH,), while Figure 3B and Figure 3C show the data for the N H 3 symmetric bending mode, 6,(NH3), and for the NH, torsional overtone. These temperature profiles were obtained from the same spectra; the rates of heating and cooling were 0.125 K min-' with the cooling cycle starting after the sample was kept at 60 OC for 30 min. The nonreversible nature of the conformational melting transition of CloCl is clearly evident in the plots of Figure 3. In all cases we observe a noncoincidence of the frequency values between the heating and cooling cycles. The transition itself covers a 4 K range on heating and a 6 K range on cooling. The fact that this transition covers such a wide temperature range does not indicate that the compound is impure as sharp transitions are found for CloCd prepared from CloCl (vide infra). In this respect we note that Kind et aL9 reported at least two overlapping first-order transitions for CloCI between 315 and 333 K as measured by DTA.9 The width of the heating transition as observed from the infrared frequencies can thus be explained as being a consequence of these two overlapping thermal events. In fact, the transition from phase I to phase I1 CloCl involves two major structural rearrangements, Le., the removal of the interdigitation between alkyl chains from different layers and the conformational melting brought about by the introduction of rapid "kink" diffusion. We have recorded infrared spectra on a heating cycle at 0.5 OC intervals and did not find clear evidence for two separate phase changes. However, the infrared spectrum may well be unable to distinguish the two events observed by DTA. The phase 1-11 transition involves A H = 6 kcal/mol and A S R per R-C-C-R group.g The shift in frequency of the us(CH2) mode is -2.5 cm-' (Figure 3A). Comparable frequency shifts have been observed in other systems involving similar A H and AS values.29 However, the values of the u,(CH,) frequency in the
-
O C
(phase 111) 2 h after recording the phase I1
spectra of the disordered phase I1 (Le. T > 48 "C) are lower than those found in other systems after the melting of long alkyl, acyl, or alkane chains. This indicates that a certain degree of conformational order is still present, probably as short segments with all-trans groups. This is also clearly evident from the spectra of Figure 2 where the components of the CH, wagging (1 350-1 150 cm-I) and rocking (1 170-700 cm-I) band progressions are still visible in the infrared spectrum of phase 11. This observation reinforces the earlier observations from the Raman spectra (see above) and also correlates with previous studies of the order parameter.25 The three modes investigated in Figure 3 give a different view of the organization and temperature dependence of phase 111, obtained during the cooling cycle. Previous measurements of this transition of CloCl by 14N and 3sCl N Q R found hysteresis of 2 "C and suggested that the nonreversibility is limited to the alkyl chains and that the NH3Cl groups are the same in phases I11 and I.30 Our infrared spectra, on the other hand, indicate that the hysteresis is not limited to the chains; in fact, the largest difference is found in the NH, torsional overtone frequency (Figure 3C). This frequency in phase I11 not only does not coincide with that of phase I but is also very sensitive to temperature changes, while it is almost insensitive in phase I. The presence of kink and gauche defects in the alkyl chains of phase 111 ClOClgives rise to bands3' at 1367, 1340, 1300, 1080, and 820 cm-' in the infrared spectrum of phase 111" and is also manifested in the frequency values of the v,(CH2) mode. The formation of phase IV after prolonged equilibration of phase 111 restores the order to the system and, most likely, the interdigitation. This annealing process was followed by measuring ~~
~~~~
(29) H. L. Casal and H. H. Mantsch, Biochim. Biophys. Acta, 735, 387 (1983). (30) J. Seliger, V. Zagar, R. Blinc, H. Arend, and G. Chapuis, J . Chem. Phys., 78, 2661 (1983). (31) M. Maroncelli, S. P. Qi, H. L. Straws, and R. G. Synder, J . Am. Chem. Soc., 104, 6237 (1982).
The Journal of Physical Chemistry, Vol.89, No. 25, 1985 5561
Spectroscopic Characterization of Solid-Phase Behavior
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as involving the onset of orientational disorder.s The second or main transition at T,, = 39 "C involves chain melting and leads to a disordered phase (Amaa) which parallels the liquid-crystalline phase of lipid bilayers and phase I1 of CloCl. The structure of the intermediate phase between T,, and T,, has been deduced to be Pmnn on the basis of symmetry and dynamic considerations.s It has been modeled by assuming rigid alkyl chains flipping around their long axis between two equivalent orientations separated by 90". We present in this section the Raman and infrared spectra of the three phases of CloCd. For the purpose of simplicity we shall refer to these phases as A, B, and C, A being the room temperature virgin crystals, B the intermediate phase, and C the high-temperature liquid-crystalline phase. i. Raman Spectra. The general characteristics of the Raman spectra of CloCd are shown in Figure 4. Phase A . The Raman spectrum of phase A is that of a well-ordered solid, as expected from the known crystal structure of CloCd at room temperature. The principal differences between the spectrum of phase A CloCd in Figure 4 and that of phase I CloCl in Figure 1 are a consequence of the very different hydrogen-bonding arrangement in which the NH3C1group participates; other differences arise due to the alkyl chains. The LAM band32appears at 232 cm-I in the spectrum of phase A CIoCd. This value is almost identical with that found in the spectrum of solid n-decane (230 cm-I)l4 but higher than that found in the spectra of hydrogen-bonded polymethylene chains (21 3 cm-' for n-decylamine and 218 cm-' for CloCl) and reflects the presence of gauche bonds in the alkyl chains of CloCd. The NH$l groups in CloCd are involved in weaker hydrogen bonds than those in CloCl;this is evident from the weak bands of the NH3C1 group in the Raman spectrum of phase A CloCd compared to that of phase I Cl0CI. As pointed out above, when the NH, group is symmetrically bonded the local symmetry of this group is manifested in low intensity of the Raman bands. Another difference between the CloCl and CloCd Raman spectra due to the different alkyl chain arrangements is manifested in the two components of the CH2 scisssoring mode at 1420 and 1440 cm-'. The frequency separation 6v is 18 cm-' for CloCd and 22 cm-' for CloCl while the intensity ratios are h1420/h1440 = 0.74 for CloCd and hl418/h1440 = 1.0 for CloCI. These different intensity ratios are a consequence of different angles of tilt of the alkyl chain^.^^^ The different frequency separation of the two components of the CH2 scissoring mode indicates a weaker intermolecular interaction for the alkyl chains in CloCd than in CloCI. This correlates with the different packing characteristics of CloCl and CloCd imposed by the underlying constraints of the metal halide lattice of CloCd. The region of the CH stretching bands in the phase A spectrum is very similar to that of phase I CloCl except that, as discussed peak height ratio is greater for CloCd previously, the r = hzsso/hzsso ( r = 0.60 for CloCd and r = 0.46 for CloC1). Phose B. At Tc, = 34 "C CloCd undergoes a transition to an intermediate phase (phase B). This transition has been ascribed to the onset of orientational disorder in the arrangement of the alkyl chains. In the Raman spectrum of phase B CloCd (at 37 "C), shown in Figure 4, the appearance of both intra- and intermolecular disorder is evident. The LAM band has broadened considerably indicating the presence of conformational disorder. This is confirmed by other vibrational features; for example, C-C stretching bands due to gauche bonds at 1090 cm-' as well as CH3 rocking bands due to end-gauche defects (840 and 870 cm-I) appear with considerable intensity in the spectrum of phase B. The presence of conformational disorder in the CloCd alkyl chains in the intermediate phase B has been detected previously by vibrational spectroscopy;I2 however, no evidence was found for disorder near the methyl end groups. Interchain vibrational coupling of the CH2 scissoring mode due to the correlation field has almost disappeared in phase B. The
I
I
,
2847 20
30
40
50
60
TEMPERflTURE, ' C Figure 3. Temperature dependence of the v,(CH,) frequency (A), the 6,(NH3) frequency (B), and the NH3 torsional overtone (C) in the infrared spectra of C&I: (@) heating cycle; (0) cooling cycle.
infrared spectra every few hours for a period of 13 days. During the first 10 days we observed only minimal changes in the spectra; however, after 10 days the spectrum started to change gradually with the intensity of the gauche and kink hands decreasing while the correlation field splitting of CH2scissoring and rocking bands appears. B. (n-Cl&21NH3)2CdC14(CloCd).The thermal phase behavior of CloCd has been studied e~tensively.~ Virgin crystals at room temperature are monoclinic ( P Z l / n with four molecules in the unit cell); the alkyl chains contain one gauche bond and are tilted 40" with respect to the bilayer normal. CloCd undergoes two transitions. The first transition at T,, = 34 OC has been interpreted
-
~~~~~~~
~
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(32) There is an overlapping band at -220 cm-' which can be assigned to Cd-CI stretching.
5562
Casal et al.
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985
r
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m
C c
C
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1
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Figure 4. Raman spectra of CloCdat 25 OC (phase A), at 37 O C (phase B), and at 48 "C (phase C). The band marked with an asterisk is an artifact
introduced by the spectrometer.
-
AI, component has shifted to higher frequencies and is only evident as a weak shoulder at 1426 cm-l compared to the strong band at 1422 cm-l in the phase A spectrum. This is a consequence of the onset of orientational disorder in the alkyl chains. However, the persistence of correlation field splitting (although weak) indicates that the chains are still correlated. This is confirmed in the infrared spectra (vide infra). The C H stretching region of the phase B spectrum is characteristic of the Raman spectra of an intermediate phase. The value of r = 0.69 is comparable to that found in the spectra of oddnumbered n-alkanes when packed in the so-called hexagonal or rotator phaseZoor that of lipid bilayers in the phase between their pretransition and main t r a n ~ i t i o n . ~ ~ The spectrum of phase B, in summary, indicates a phase comparable with hexagonally packed n-alkanes and parallels phase I11 C,oCI. Besides the known orientational disorder we also find evidence for conformational disorder in the alkyl chains and even at the end methyl groups. This could explain why the value of the c axis of phase B of CloCd is the same as that of phase A.' If gauche defects, leading to a shorter effective chain length, are introduced and at the same time the chains become parallel to the bilayer plane, these two events would have opposite effects on the value of the c axis. Phase C. The spectrum of phase C CloCd in Figure 4 is typical of a liquid-crystalline phase similar to that of phase I1 CloCI. The LAM band (overlapped with the Cd-CI stretching band36 at -220cm-I) is now broad and weak and appears at -225 cm-I, Le., the same frequency as that of phase I1 CloC1. The rest of the Raman spectrum is also characteristic of liquidlike chains and is practically the same as that of liquid n-alkanes. We also note that the bands due to the NH3CI groups are very weak or absent. As in the case of phase I1 CloC1we attribute this to local symmetry constraints of the N H 3 group in phase B CloCd. (33) I. W. Levin in 'Advances in Infrared and Raman Spectroscopy", Vol. 11, R. J. H. Clark and R. E. Hester, Ed., Heyden, London, 1984, pp 1-48. (34) D. G. Cameron, H. L. Casal, and H. H. Mantsch, Biochemistry, 19, 3665 (1980). (35) Such changes are observed in odd-numbered n-alkanes at their "rotator" transition and in lipid bilayers at the so-called pretransition. (36) R. Mokhlisse, M. Couzi, N. B. Chanh, Y . Hagel, C. Hauw, and A . Mqresse, J . Phys. Chem. Solids, 46, 187 (1985).
ii. Infrared Spectra. The infrared spectra of phases A, B, and C CloCd are shown in Figure 5. The general characteristics of these spectra follow closely those of CloC1in Figure 2. However, differences are apparent and deserve comment here. The N-H stretching vibrations of CloCd are at higher frequencies than those of CloC1 while the N H 3 bending vibrations are at lower frequencies; thus in CloCd the NH3 symmetric bending overlaps the CH2 scissoring mode (1500-1470 cm-I). These differences in frequency are a direct consequence of the weaker hydrogen bonds of the N H 3 group in CloCd. Temperature Dependence of the Infrared Spectrum of CloCd. In order to cover the two thermal events of Cl0Cd we have measured infrared spectra at various temperatures between 10 and 65 OC. The heating rate was the same as that for CloC1(Le., 0.125 K mi&). After the heating cycle was completed a spectrum was obtained after 10 h at 28 OC. No cooling cycle was performed. Figure 6 shows the temperature dependence of two alkyl chain modes and of one mode from the NH3CI group. Figure 6A presents the behavior of the C H 2 symmetric stretching mode (v,(CH2)); Figure 6B shows the behavior of a NH3 rocking band (p(NH3)), and Figure 6C shows the corresponding plot for one component of the CH2 wagging band progression. In the plot of Figure 6A there are three clearly defined regions: the first one from 12 to 34 "C, the second from 34 to 39 OC, and the third one from 40 to 62 "C. The first inflexion at -34 O C marks the effect of Tc,;the second at -39 OC marks Tc2and is associated with chain melting. The temperature dependence of the frequency of the us(CH2) mode (at -2850 cm-I in Figure 5 ) has been extensively used to monitor order-disorder transitions; in this case we find that there is a continuous change in frequency in all three phases (see Figure 6A), as opposed to other cases where a frequency shift is only observed at phase changes. In this particular example the frequency of this mode could be affected by temperature-induced variations of the overlapping N-H stretching bands (see Figure 5). However, the two transitions are clearly distinguished in Figure 6A and the changes in slope are sharp. The increase in frequency can be correlated with the introduction of conformational disorder. The temperature dependence of the p(NH3) mode in Figure 6B differs from that of v,(CH,) in Figure 6A. In the case of p(NH,) the effect of T,, is small with a minor inflexion point at
Spectroscopic Characterization of Solid-Phase Behavior
The Journal of Physical Chemistry, Vol. 89, No. 25, 1985 5563
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34 OC while the effect of Tc2is much more marked giving rise to a 8-em-' increase in the frequency of this mode. This is a consequence of the environmental change of the NH3Cl groups. In phase C the NH, groups are not deformed while the hydrogen bonding imposes restrictions in phases A and B. The distortion of the NH, group can be followed by examining the bands in the 550-450-cm-' region (Figure 5). In the spectra of phases A and B there are four and three bands, respectively. It has been shown37that these bands, due to overtones of the NH, torsional mode, are present only when the N H 3 group is distorted from its "normal" C,, local symmetry. Thus, the absence of these bands in the phase C spectrum indicates either the removal of distortion in this disordered liquid-crystalline-like phase or that the local symmetry of the NH, is C,,due to motional averaging. Also, the presence of the NH, torsional overtone bands in the phase B spectrum confirms that the strong hydrogen-bonding network is still present in the ionic "head-group" of this molecule. The phase A to phase B transition does not induce large rearrangements of the head group. The nature of the T,, transition a t 34 "C can be examined by monitoring the CH2 rocking, scissoring, and wagging vibrations of the alkyl chains. In the case of CloCd the CH2scissoring mode overlaps with the NH, symmetric bending mode and is therefore difficult to study. The CH2 rocking head band at -720 cm-', however, is not obscured. The temperature dependence of the CH2wagging band shown in Figure 6C confirms the nature of the transition at T,,. There is a -0.6-cm-I increase in frequency a t T,, (-34 "C); no data are plotted for temperatures higher than T,, since the wagging bands are not present in the spectrum of the high-temperature phase C (Figure 5). The frequency of the CH, wagging modes increases by 1-2 cm-' when n-paraffins or lipid bilayers undergo a transition of the orthorhombiehexagonal type.34,38The present
(37) R. D.Waldron, J . Chem. Phys., 21, 734, (1953).
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case can thus be correlated with these observations and may be assigned to the effect of increased chain mobility. The intensity of the CHI wagging bands decreases markedly at T,, which is also observed in the other examples of orthorhombic to hexagonal transition^.^^*^^ In the infrared spectrum of phase A the CHI rocking mode is split in two bands at 720 and 728 cm-I, in a pattern typical for orthorhombic and/or monoclinic lattices. This is in accord with the known crystal structure of CloCd in phase A. On transition to phase B at 34 OC the splitting is drastically reduced. The trend of changes can be followed by measuring difference spectra between consecutive temperatures. Such difference spectra obtained by subtracting the low-temperature spectrum from the hightemperature consecutive one are displayed in Figure 7. Each difference spectrum covers the region 750-700 cm-'. Two temperature intervals dominate the plot. The first one between 34.1 and 34.6 OC shows the effect of T,, on this band while the second between 38.7 and 39.3 OC corresponds to T,,. The change at T,, is typical of that observed when such a two-band system collapses to one band. There is a positive lobe at -722 cm-' which corresponds to the growth of the rocking band for phase B while the bands for phase A (at 720 and 728 cm-') are seen to decrease in intensity. However, the difference spectrum for the 35.4-33.6 OC temperature interval still shows the presence of two minima. These two minima are also seen in the following difference spectra but are much weaker. This indicates that even in the spectrum of phase B there are two bands for the CH2 rocking mode. The two bands are separated by less than 3 cm-' and their presence can also be inferred from the flat-top shape of the CHI rocking band in Figure 5 at 37 OC. The presence of a two-band system for the CH2 rocking mode in phase B parallels such a situation in the spectra of n-alkanes in their rotator (or hexagonal) phase28 and indicates that there is correlation between adjacent chains. In the case of n-alkanes the frequency separation of the CH2 rocking mode was associated (38) H.L.Casal, D. G. Cameron, and H. H. Mantsch, Can. J . Chem., 61, 1736 (1983).
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5564 The Journal of Physical Chemistry, Vol. 89, No. 25, 1985
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30
TEMPERflTURE,
'C
Figure 6. Temperature dependence of the v,(CH,) frequency (A), the
p(NH,) frequency (B), and the frequency of the W2 component of the CH2 wagging band progression (C) in the infrared spectra of C,,Cd.
with a relaxation time of the order of 3 ps as calculated from inelastic neutron scattering measurement^.^^ The difference spectrum induced by T,, (39.3-38.7 "C) is that expected for a chain melting event and similar effects have been observed in related systems.34 The main effect is a reduction of the band intensity accompanied by band broadening as seen by the positive going lobes at either side of the negative peak in the difference spectrum in Figure 7. Band broadening is also evident in the absorbance spectrum in Figure 5. Another observation from the plots of Figure 7 relates to the width of both transitions at (39) J . D. Barnes, J . Chem. Phys., 58, 5193 (1973).
Temperature, "C
Figure 7. Infrared difference spectra of Cl0Cd in the region of the CH,
rocking mode (750-700 cm-I) as a function of temperature. Spectra have been normalized with respect to the temperature used to generate the spectra. Temperatures used in the subtraction are indicated on the bottom axis; e.g. the left-most difference spectrum was generated by subtracting the spectrum recorded at 28.6 OC from that recorded at 30.6 "C.
T,, and T,,. In both cases the maximum rate of change occurs in less than 0.5 OC. The transition at Tcz is confirmed as involving mainly the conformational melting of the alkyl chains. The spectrum of phase C CloCd in Figure 5 shows that the components of the CH2 wagging band progression are not present@ and at the same time bands characteristic of gauche conformers3' appear at 1300 and 1340 cm-*. Concluding Remarks This comparative study of the solid phase behavior of CloCl and CloCd yields information regarding the different packing characteristics of the alkyl chains in these two systems. Since the alkyl chains are chemically identical (Le., n-decylammonium) the differences observed for alkyl chain behavior are a consequence of the diffcrent head groups which impose different constraints on the chains. In the case of CloCl the ionic head group forces a very rigid chain packing; this is manifested in the fact that the chains interdigitate and are in the all-trans extended conformation. Also, no intermediate phase is formed prior to the conformational melting leading to phase 11. In fact, phase I1 CloCI, while disordered, still shows characteristics in its vibrational spectra which indicate the presence of ordered segments resembling n-alkanes in their rotator or hexagonal solid phase. In contrast, the low-temperature phase of CIoCd (phase A) is not interdigitated and the alkyl chains contain gauche defects. All chains contain one gauche bond either between positions 1 and 2 (A chains) or positions 2 and 3 (B chains). At T,,, the intermediate phase B is formed and, besides chain orientational disorder, more gauche bonds are introduced. The disorder present in phase B is therefore not limited to the orientations of the alkyl chains as proposed earlier. We note that the introduction of extra conformational disorder at T,, was demonstrated earlier for CIoCdl2and has also been proposed in the case of the intermediate phases of C&d and related compound^.^' The present Raman data indicate that the gauche bonds in phase B are not restricted to positions close to the NH3C1 groups but some end-gauche defects are present. It can thus be proposed that T,, really marks the onset of a premelting stage which is completed at T,,. The solid-phase behavior of CloCd parallels closely that of odd-numbered n-alkanes which also show an intermediate solid phase prior to melting, known as the rotator or hexagonal phase.* In this intermediate phase of n-alkanes there are gauche bonds present)' and also residual intermolecular vibrational coupling.28 The same is found for CloCd in phase B. The correspondence with the behavior of bilayers of saturated diacylphosphatidylcholines is also These direct correlations serve as confirmation of the proposed structure of phase B CloCd. In terms of the concentration of gauche defects in C&l and C&d the present data do not yield a quantitative comparison. (40) The CH2 wagging band progression is not observed in the spectra of liquidlike chains.23 (41) G.F. Needham and R. D. Willett, J. Phys. Chem.. 85, 3385 (1981).
J. Phys. Chem. 1985,89, 5565-5568 However, the spectra presented in this study clearly indicate that CloCd in phase C is more disordered than CloCl in phase 11, i.e., the two phases which have been described as resembling smectic liquid crystals. Again, we ascribe this difference in gauche concentration to the looser hydrogen bond arrangement of CloCd compared to that in CloCI.
5565
-
While the nonreversibility of the conformational melting transition (phase I phase 11) of CloCl is not limited to the alkyl chains, the rearrangement of the head group seems to lag behind that of the alkyl chains. Registry NO.CI&I, 143-09-9; CloCd, 53188-91-3; CdCl,, 10108-64-2.
Clay-Modified Electrodes. 5. Preparation and Electrochemical Characterization of Pillared Clay-Modified Electrodes and Membranes Kingo Itayat and Allen J. Bard* Department of Chemistry, The University of Texas, Austin, Texas 78712 (Received: April 15, 1985)
Methods of preparing pillared clay layers on electrode surfaces by treatment of montmorillonite with [ Fe3(0COCH3),0H.2H20]N03, zirconyl chloride, or hydroxy-aluminum solutions followed by heating are described. X-ray diffraction of the pillared clay layers shows larger basal plane splittings (ca. 17 A) than unpillared clays. The cyclic voltammetric behavior of ions incorporated in these pillared clay layers, e.g., Fe(bp~),~+, in both aqueous and nonaqueous solutions is described and contrasted to that of unpillared clay layers. Pillaring is also shown to attenuate the penetration of anionic species through the films. Free standing pillared clay films (5-10-pm thick) were shown to behave as cation-exchange membranes by measurements of the membrane potential developed across them when separating aqueous NaCl and KCI solutions (0.01 to 1 M).
Introduction There has been considerable interest recently in modified electrodes using ~ l a y , I zeolite: -~ metal oxide,’ and other inorganic layer^.^^^ Modification of electrode surfaces by coverage with a thin layer of clay has been reported in previous studies from this laboratory.12 Spectroscopy and photochemistry of Ru(bpy),*+ (bpy is 2,2’-bipyridine) in colloidal smectites have also k n studied recently.I0 Clays were used extensively as commercial catalysts before they were replaced by more thermally stable and selective zeolite catalysts. However, there have been several recent efforts to study the formation of stable, high surface area modified clays by developing oxide pillars between the layers of expanded layer silicates.” Pillared clays are formed when a “pillar” (e.g., a metal oxide) is introduced between the silicate layers that maintains the interlayer spacing. Since pillared clays can have fixed pore sizes larger than those of zeolites, they can offer a promising new means of facilitating reactions of various molecules.” Moreover, pillaring fxes the interlamellar spacing in the clay particles and makes them less susceptible to large changes via solvation. The above considerations encouraged us to explore the effect of pillaring on the behavior of clay-modified electrode surfaces. We also found in this study that free-standing pillared clay membranes with reasonable mechanical strengths could be prepared easily. Preheated (but unpillared) montmorillonite membranes were described over 40 years ago by Marshall et a1.I2J3 and were suggested for the potentiometric determination of potassium and ammonium ions. However, the properties of these membranes were reported to be very dependent upon the preheating temperatures. Membranes heated below 250 O C reached equilibrium quickly but were extremely fragile and dispersed or cracked readily.12 Marshall et al. mainly used the membranes preheated at 490 O C for montmorillonite. However, it is known that sodium montmorillonite heated at 500 OC for 1 h regains interlayer water upon immersion and expands over long intervals of rehydration.l4*I5 Such membranes did not appear to be useful in electrochemical studies of long duration, and investigations of them were discontinued. ‘Present address: Department of Applied Chemistry, Faculty of Engineering, Tohoku University, Sendai 980, Japan.
0022-3654/85/2089-5565$01.50/0
In this paper, we describe methods of preparation of pillared clay-modified electrodes and pillared-clay membranes and characterize these by electrochemical measurements. Experimental Section Materials. Calcium montmorillonite (STx-1) was purchased from the Source Clay Minerals Repository (University of Missouri, Columbia, MO). The clay, purified by the method described in previous papers,’V2 was used for this study. A colloidal suspension in aqueous solution was prepared by weighing the desired amount of freeze-dried clay and dispersing it in triply distilled water in an ultrasonic bath. O ~ ( b p y ) , ( C l O ~and ) ~ Fe(bpy)3(C106)2were synthesized and purified according to published procedure^.'^^'^
(1) (a) Ghosh, P. K.; Bard, A. J. J. Am. Chem. SOC.1983, 105, 5691. (b) Ghosh, P. K.; Mau, A. W. H.; Bard, A. J. J. Electroanal. Chem. 1984,169, 315. (2) (a) Ege, D.; Ghosh, P. K.; White, J. R.; Equey, J. F.; Bard, A. J. J. Am. Chem. SOC.,1985, 107, 5652. (b) White, J. R.; Bard, A. J. J. Electroanal. Chem., in press. (3) Liu, H.; Anson, F. C. J. Electroanal. Chem. 1985, 184, 41 1. (4) Yamagishi, A,; Aramata, A. J. Chem. SOC.,Chem. Commun. 1984, 119. (5) Kamat, P. V . J. Electroanal. Chem. 1984, 163, 389. (6) Murray, C. G.; Nowak, R. J.; Rolison, D. R. J. Electronanal. Chem. 198j, 164, 389. (7) Zak, J.; Kuwana, T. J. A. Chem. SOC.1982,104,5514. J. Electroanal. Chem. 1983, 150, 645. (8) (a) Ellis, D.; Eckhoff, M.; Neff, V. D. J . Phys. Chem. 1981,85, 1225. (b) Itaya, K.; Ataka, T.; Toshima, S. J. Am. Chem. SOC.1982, 104, 4762. (9) Sinha, S.; Humphrey, B. D.; Fu, E.; Bocarsly, A. B. J. Electronanal. Chem. 1984, 162, 351. (10) Ghosh, P. K.; Bard, A. J. J. Phys. Chem. 1984, 88, 5519. (1 1) Pinnavia, T. J. Science 1983, 220, 365. (12) Marshall, C. E.; Bergman, W. E. J. Am. Chem. SOC.1941,63, 1911. J . Phys. Chem. 1942, 46, 52, 325. (13) Marshall, C. E.; Krinbill, C. A. J. Am. Chem. Soc. 1942, 64, 1814. (14) Van Olphen, H. “An Introduction to Clay Colloid Chemistry”; Wiley: New York, 1977. (15) Grim, R. E. “Clay Mineralogy”; McGraw-Hill: New York, 1953. (16) Gaudiello, J. G.; Bradley, P. G.; Norton, K. A,; Woodruff, W. H.; Bard, A. J. Inorg. Chem. 1984, 23, 3.
0 1985 American Chemical Society
