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J. Phys. Chem. B 2002, 106, 4162-4171
Modification of Kaolinite Surfaces through Intercalation with Deuterated Dimethylsulfoxide Wayde N. Martens,† Ray L. Frost,*,† Ja´ nos Kristof,‡ and Erzse´ bet Horvath‡ Centre for Instrumental and DeVelopmental Chemistry, Queensland UniVersity of Technology, 2 George Street, G.P.O. Box 2434, Brisbane, Queensland 4001, Australia, and Department of Analytical Chemistry, UniVersity of Veszpre´ m, P.O. Box 158, H8201 Veszpre´ m, Hungary ReceiVed: August 3, 2001; In Final Form: January 17, 2002
The surfaces of kaolinite have been modified through intercalation with deuterated dimethylsulfoxide (d-DMSO). X-ray diffraction shows the kaolinite to be expanded from 7.2 to 11.19 Å. Modification of the surface has been explored through (a) changes to the hydroxyl surfaces of the kaolinite and (b) through changes to the d-dimethylsulfoxide inserting molecule. Upon intercalation of the kaolinite with d-DMSO, additional infrared bands at 3660, 3538, and 3502 cm-1 and additional Raman bands at 3660, 3537, 3507, and 3480 cm-1 are observed. The first band at 3660 cm-1 is attributed to the inner-surface hydroxyls hydrogen bonded to the d-DMSO and the other bands to water hydroxyl-stretching modes. Both infrared and Raman spectroscopy shows that significant changes in the molecular structure of the d-DMSO occur upon intercalation. First, the CD stretching modes observed for d-DMSO at 2125 and 2249 cm-1 in the DRIFT spectrum lose the degeneracy and split into 2140 and 2127 cm-1 and 2267, 2250, and 2238 cm-1. The Raman spectrum shows this loss of degeneracy through the bands observed at 2272, 2267, 2263, and 2251 cm-1 for the antisymmetric CD stretching vibration and at 2129 and 2141 cm-1 for the symmetric stretching vibrations. Upon intercalation with d-DMSO, the SdO stretching region shows bands at 1066, 1023, and 1010 cm-1. The 1066 cm-1 band is assigned to the free monomeric SdO group and the 1023 and 1010 cm-1 bands to two different polymeric SdO groups. Bands attributed to the CS stretching vibrations, the in-plane and outof-plane SdO bending and the CSC symmetric bends all move to higher frequencies upon intercalation with d-DMSO. It is proposed that intercalation depends on the presence of water and that the additional bands at 3536 and 3501 cm-1 are due to the presence of water. The precise positions of the hydroxyl stretching modes of water at these positions suggest that water is in a well-defined position within the intercalation structure.
Introduction The intercalation of dimethylsulfoxide (DMSO) into clay minerals has been used to separate the chlorite fractions from the kaolin minerals.1-4 The reason DMSO is so successful at separating the clay minerals is that the kaolins expand from 7.2 to 11.2 Å. This expansion of the kaolin minerals by DMSO followed by deintercalation results in an increase in the stacking disorder of the kaolin.5 Intercalation of DMSO into kaolinite provides a method for the incorporation of other alkali and alkaline metal salts into the kaolin by replacement of the DMSO.6,7 When the kaolinite is expanded with DMSO, a threedimensional ordering incorporating the DMSO into the interlamellar space occurs.8,9 It has been proposed that when this three-dimensional ordering occurs, the DMSO molecule is locked into the kaolinite surfaces first by hydrogen bonding of the SdO to the gibbsite-like hydroxyls and by a coordination of the sulfur to the oxygens of the siloxane surface.8 Such threedimensional ordering then alters the molecular motion of the methyl groups of the DMSO.10-14 It has been proposed that the DMSO molecule is triply H bonded above the octahedral vacancy in the gibbsitic sheet of the kaolinite layer.8 One methyl group is keyed into the ditrigonal hole in the tetrahedral sheet with the other S-C bond parallel to the sheet. The DMSO * To whom correspondence should be addressed. E-mail: r.frost@ qut.edu.au. † Queensland University of Technology. ‡ University of Veszpre ´ m.
molecules are accommodated by significant horizontal displacement of individual kaolinite layers to achieve almost perfect overlap of the octahedral vacancy by the adjacent ditrigonal hole.9 Stable, three-dimensionally ordered complexes were formed from synthetically hydrated, highly ordered kaolinite (d(001) ) 8.40 Å.) with dimethylsulfoxide.15-17 Removal of the intercalated organic compounds by drying or by water washing produced an 8.40 Å hydrate with its ordered layer stacking essentially unchanged. The question may be raised as to why these hydrated kaolinites were formed. Isolated water molecules appear to be keyed into the ditrigonal holes formed by the basal O of the silicate tetrahedra of the 8.4 Å hydrate. These water molecules, referred to as “hole water”, and ions that had replaced approximately 20% of the inner-surface OH groups of the 8.4 Å phase sufficiently altered the interlayer bonding to allow an expansion of the inner-layer spaces by a variety of guest molecules. Further, a 10 Å hydrated kaolin was produced. Hydrated kaolinite with d(001) ) 10.0 Å was synthesized by mild heating of a kaolinite-DMSO suspension, allowing time for the clay to be intercalated, and then dissolving a fluoride salt in the solution. After mild heating of the suspension, the salt and DMSO are removed by repeated water washings. The kaolinite retained interlayer water in the form of a 10 Å kaolinite hydrate. The application of Raman microscopy to the study of modification of kaolinite surfaces through intercalation has
10.1021/jp0130113 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/29/2002
Modification of Kaolinite Surfaces proven most useful.18-20 An additional Raman band, attributed to the inner-surface hydroxyl groups strongly hydrogen bound to the acetate, was observed at 3605 cm-1 for the potassium acetate intercalate with the concomitant loss of intensity in the bands at 3652, 3670, 3684, and 3693 cm-1. Intercalation of halloysite resulted in a Raman pattern similar to that of an intercalated ordered kaolinite. Thus, the intercalated halloysite resembled the intercalated kaolinite at least on a molecular level.20 Here, the conclusion was made that the intercalation process resulted in a decrease in the defect structures of the kaolinite. Remarkable changes in intensity of the Raman spectral bands of the low-frequency region of the kaolinite also occurred upon intercalation. In particular, the 935 cm-1 band increased in intensity upon intercalation. On the basis of the above examples, it can be concluded that Raman spectroscopy has proven most suitable for the spectroscopic analysis of intercalated kaolinites.35-38 One of the difficulties of using DMSO is the overlap or potential overlap of CH and OH bands of kaolinite in both the infrared and Raman spectra.35 In this research, we have replaced DMSO with d-DMSO. A second consideration is that the DMSO bands can overlap with the water OH bands. In this paper, we report the changes in the molecular structure of a low-defect kaolinite intercalated with deuterated dimethylsulfoxide. Experimental Section Intercalation of Kaolinite. The kaolinite used in this study is from Kira´lyhegy in Hungary. The kaolinite is an example of a low-defect kaolinite.19,20 The kaolinite has a Hinckley index of 1.39. The kaolinite was intercalated by mixing 1 g of the kaolinite in anhydrous deuterated dimethylsulfoxide in an ampule, which was sealed under nitrogen and kept at 85 °C for 7 days. Two runs were made, which resulted in 66% and 88% expansion. Both infrared and Raman spectra show the presence of water in the pure, anhydrous d-dimethylsulfoxide. It should be noted that in the preparation of the DMSO-kaolinite intercalate by previous workers water-DMSO mixtures were used.22-24 In fact, Olejnik found the optimum rate of intercalation occurred when the kaolinite was suspended in DMSO containing 9% water.21 X-Ray Diffraction. XRD analyses were carried out on a Philips wide-angle PW 1050/25 vertical goniometer equipped with a graphite-diffracted beam monochromator. The d spacing and intensity measurements were improved by application of a self-developed computer-aided divergence slit system enabling constant sampling area irradiation (20 mm long) at any angle of incidence. The goniometer radius was enlarged from 173 to 204 mm. The radiation applied was Cu KR from a long finefocus Cu tube operating at 40 kV and 40 mA. The samples were measured at 50% relative humidity in step-scan mode with steps of 0.02° 2θ and a counting time of 2 s. Measured data were corrected with the Lorentz polarization factor (for oriented specimens) and for their irradiated volume. Spectroscopy. The DMSO-intercalated kaolinites were prepared in sealed vials. The vials containing the DMSOintercalated kaolinites were placed in a Linkham thermal stage of an Olympus BHSM microscope equipped with 10×, 20×, 50×, and LWD50× objectives. The purpose of placing the samples in the stage was to prevent absorption of moisture from the air. Samples were flushed with dry nitrogen or air depending on the type of experiment. The samples were enclosed to prevent evaporation and contamination of the laboratory atmosphere. The samples were measured by focusing through the sealed vial; thus, no water entered the DMSO-intercalated kaolinite complex
J. Phys. Chem. B, Vol. 106, No. 16, 2002 4163 because the samples were kept under nitrogen. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system, and a chargecoupled device (CCD). Raman spectra were excited by a Spectra-Physics model 127 He-Ne laser (633 nm) at a resolution of 2 cm-1 in the range between 100 and 4000 cm-1. Repeated acquisitions using the highest magnification were accumulated to improve the signal-to-noise ratio in the spectra. Spectra were calibrated using the 520.5 cm-1 line of a silicon wafer. Further details on the spectroscopy have been published elsewhere.24-26 Diffuse reflectance Fourier transform infrared spectroscopic (commonly known as DRIFT) analyses were undertaken using a Bio-Rad 60A spectrometer. A total of 512 scans were obtained at a resolution of 2 cm-1 with a mirror velocity of 0.3 cm/sec. Spectra were coadded to improve the signal-to-noise ratio. Approximately 3 wt % kaolinite or intercalated kaolinite was dispersed in 100 mg of oven-dried spectroscopic grade KBr with a refractive index of 1.559 and a particle size of 5-20 µm. Reflected radiation was collected at ∼50% efficiency. The reason for using this percentage is that any distortion of the infrared absorption bands is avoided. The experimental chamber of the spectrometer was continuously flushed gently with nitrogen. The temperature and humidity in the laboratory were controlled to set limits. Background KBr spectra were obtained, and spectra were ratioed to the background. The diffusereflectance accessory used was designed exclusively for BioRad FTS spectrometers. It is of the so-called “praying monk” design and is mounted on a kinematic baseplate. It includes two four-position sample slides and eight sample cups. The cup (3 mm deep, 6 mm in diameter) accommodates powdery samples mixed with KBr using an agate mortar and pestle in 1-3% concentration. The collection efficiency of this adaptor (part number 099-0931) is approximately 50%. The reflectance spectra expressed as Kubelka-Mink units versus frequency curves are very similar to absorbance spectra and can be evaluated accordingly. The advantage of using DRIFT measurements over the pellet technique is that in this case the likely interference of the mulling agent (intercalation of KBr in a liquid phase under pressure) can be avoided. Spectral manipulation such as baseline adjustment, smoothing, and normalization was performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH). Band component analysis was undertaken using the Jandel “Peakfit” software package, which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a Lorentz-Gauss cross product function with the minimum number of component bands used for the fitting process. The Gauss-Lorentz ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r2 greater than 0.995. Results and Discussion Changes in the Lattice Structure as Determined by X-Ray Diffraction. Figure 1 shows the X-ray diffraction patterns of the low-defect kaolinite and its DMSO and deuterated DMSO intercalates. The X-ray diffraction patterns show that the kaolinites are intercalated to the 98% level (DMSO, XRD trace b), 88% level (d-DMSO, XRD trace c), and 66% level (d-DMSO, XRD trace d). A slight measurable difference is observed between the intercalation with DMSO and d-DMSO. DMSO expands the kaolinite to 11.13 Å, whereas the d-DMSO expands the kaolinite to 11.15 Å. Additionally, an increase in
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Figure 1. X-ray diffraction patterns of (a) kaolinite, (b) DMSO-intercalated kaolinite (98% expansion), (c) d-DMSO-intercalated kaolinite (88% expansion), and (d) d-DMSO-intercalated kaolinite (66% expansion).
the background in the 15°-30° 2θ has occurred with a broad band centered at ∼22° 2θ. The significance of this broad band is that the two intercalated kaolinites are now showing a certain degree of amorphicity. The intensity of this broad band would indicate that ∼10% of the kaolinites are now amorphous. This concept of amorphicity in the intercalation complex fits well with the significant horizontal displacement of individual kaolinite layers to achieve almost perfect overlap of the octahedral vacancy by the adjacent ditrigonal hole.9 An alternative proposal is based on two types of d-DMSOintercalated kaolinites in the intercalate, a single-layer kaolinite and a multidimensional DMSO-intercalated kaolinite. In the case of a single-layer kaolinite, the d-DMSO would simply be adsorbed on the surface. The kaolinite expands from 7.16 to 11.19 Å. This is an increase of 4.03 Å. The size of the SdO group is 3.38 Å, and if the d-DMSO is parallel to the siloxane surface, then there is an imbalance of 0.67 Å. The question arises as to how this gap may be filled. In the model of the dimethylsulfoxide kaolinite intercalate presented by Thompson and Cuff, one of the methyl groups is pointing directly into the ditrigonal cavity with the second methyl group pointing away from this surface.9 The oxygen of the SdO group is pointing toward two or three of the inner-surface hydroxyls. This model however is based on an absence of water in the intercalate and also depends on the presence of nonpolymeric d-DMSO. Changes in the Hydroxyl Surface of Kaolinite upon Intercalation with d-DMSO. The results of X-ray diffraction show that at least three phases are present: (a) d-DMSOintercalated kaolinite; (b) nonintercalated kaolinite; (c) singlelayer kaolinite with no long-distance order. Thus, when obtaining the infrared and Raman spectra of the d-DMSO-intercalated kaolinite, the spectral sum of these three phases will be obtained. If the assumption is made that the amount of the amorphous kaolinite is minimal, then the spectra represent the addition of those of the intercalated and nonintercalated kaolinite. X-ray diffraction shows that the degree of expansion of the d-DMSOintercalated kaolinite is 88%. While the degree of expansion is not necessarily equated to the amount of intercalation, it is a guide to the ratio of the two phases. The degree of intercalation equates to the “coverage” of the inner-surface hydroxyls by the intercalating molecule. DRIFT spectroscopy shows additional bands for low-defect kaolinite intercalated with deuterated dimethylsulfoxide at 3660,
3538, and 3502 cm-1 (Figure 2). This figure displays the DRIFT spectra of kaolinite intercalated with d-DMSO at the 66% and 88% levels. These spectra represent the combination of the infrared spectra of the d-DMSO-intercalated kaolinite and the nonintercalated kaolinite. Kaolinite displays infrared bands at 3620 and 3693 cm-1 with minor components at 3650 and 3668 cm-1. Thus, the additional bands at 3660, 3538, and 3502 cm-1 must be attributed to either the d-DMSO or the bands attributed to the interaction of the d-DMSO and the kaolinite inner-surface hydroxyls. The difference between the DRIFT spectra of the two intercalates appears to rest with the intensity of the bands observed at 3537, 3506, and 3483 cm-1. A decrease in intensity of the band at 3693 cm-1 relative to the intensity of the band of the nonintercalated kaolinite is also observed, and this decrease is attributed to the interaction of the inner-surface hydroxyls and the d-DMSO. Table 1 summarizes the DRIFT and Raman spectra of the low-defect kaolinite intercalated with both DMSO and d-DMSO. Bands in similar positions have been reported using attenuated total reflectance techniques.8 These bands have been previously assigned to the inner-surface hydroxyl groups hydrogen bonded to the SdO of the DMSO. Interestingly, the ν4 mode is observed in the FTIR spectra of the d-DMSO-intercalation complex, as it is often said to be infrared inactive. No bands at ∼3670 and 3650 cm-1 were observed ascribed to the inner-surface hydroxyls of the nonintercalated kaolinite. These bands are no doubt obscured by the intense band at 3655 cm-1 assigned to the inner-surface hydroxyls hydrogen bonded to the d-DMSO. The intensity of the ν1 band was 19% for the 66% d-DMSOintercalated kaolinite and 10% for the 88% example. This low value suggests that the kaolinite was approaching full intercalation. Spectra reported previously show strong intensities in the bands of the 3695 cm-1 region.8,21-22 In these works, the kaolinites were only partially intercalated. Similarly in this research reported here, the kaolinite was intercalated to 66% and 88%. Bands are observed at 3538 and 3502 cm-1 for the d-DMSO-intercalation complex with bandwidths of 25.0 and 29.0 cm-1. The widths of these bands suggest that these bands are attributable to water in the intercalation complex. It is difficult to understand why there should be three different OH frequencies (at 3660, 3538, and 3502 cm-1) with such a wide difference in band positions, as can be described as hydrogenbond formation between the inner-surface hydroxyls and the
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Figure 2. DRIFT spectra of the hydroxyl-stretching region of d-DMSO-intercalated kaolinite with 66% and 88% expansion.
TABLE 1: Table of the Infrared and Raman Band Centers for d-DMSO- and DMSO-Intercalated Kaolinite DMSO-intercalated kaolinite ν1
deuterated DMSO FTIR
deuterated DMSO Raman 298 K
deuterated DMSO Raman 77 K
3693
3696
3695
ν4
DMSO Raman 298 K
DMSO Raman 77 K
3685
ν6
3660
ν7
3655
ν5
3620
3621
3617
3620
3615 3569
νOH1
3537
3537
3533
3536
3530
νOH2
3506
3508
3506
3501
νOH3
3483
3480
3491
3660
3660
3660
3656
νOH4
SdO group. In this paper, we reaffirm that the 3660 cm-1 band is the result of the hydrogen-bond formation of the SdO group and the inner-surface hydroxyls. The model of the intercalation of kaolinite with d-DMSO that we would propose is based upon the cointercalation of d-DMSO and water. The bands observed at 3538 and 3502 cm-1 are ascribed to water hydrogen bonded to the d-DMSO in a three-dimensional structure.8,9 A broad band is also observed in the DRIFT spectra at around 3600 cm-1, and it is likely that this band is attributable to water in the intercalates. Figure 3 displays the water HOH bending region for the d-DMSO-intercalation complex. Two sets of distinct water bands are observed at around 1606 and 1682 cm-1 with relative intensities of 55.4% and 25.8% for the 88% expanded complex and 56.6% and 26.9% for the 66% intercalation complex. The spectral profile at around 1606 cm-1 may be resolved into two bands at 1612 and 1601 cm-1 for the 66% expanded complex and at 1608 and 1585 cm-1 for the 88% expanded complex and the bands are assigned to water in a space-filling role. It is noticeable that the two bands shift to lower wavenumbers with an increased degree of intercalation. In both sets of spectra, a
3475
3663 3658
3489
band assignment unreacted inner-surface hydroxyl stretching band unreacted inner-surface hydroxyl stretching band inner-surface hydroxyl hydrogen bonded to DMSO inner-surface hydroxyl hydrogen bonded to DMSO inner hydroxyl OH stretching band of water in the intercalation complex OH stretching band of water in the intercalation complex OH stretching band of water in the intercalation complex OH stretching band of water in the intercalation complex OH stretching band of water in the intercalation complex
band is observed at 1682 cm-1 and is assigned to water that is strongly coordinated to the d-DMSO. The concept of water strongly hydrogen bonded to the DMSO in the intercalation complex is in harmony with the concept of the DMSO and water forming a polymeric type of structure in the intercalation complex. One possibility is that the water coordinates to the sulfur of the SdO unit. This coordination interaction is strong as evidenced by the high wavenumber position of the waterbending mode. In the DRIFT spectrum of the DMSOintercalated kaolinite, an additional band was observed at 3423 cm-1. This band is attributed to the OH stretching bands of water that is strongly coordinated to the SdO group of the DMSO. The band makes up 1.5% of the total band profile. The other two bands at 3538 and 3502 cm-1are ascribed to water molecules incorporated into the DMSO-intercalate structure. Adsorbed water gives strong infrared bands at 3450 cm-1, the water hydroxyl stretching vibration, and at ∼ 1630 cm-1, the water bending vibrations. For monomeric non-hydrogenbonded water as occurs in the vapor phase, these bands are found at 3755 and 1595 cm-1. For liquid water, the bands occur at 3455 and 1645 cm-1, and for water molecules in ice, the bands
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Figure 3. DRIFT spectra of the water deformation modes for d-DMSO-intercalated kaolinite with 66% and 88% expansion.
are at 3255 and 1655 cm-1.29 The hydroxyl stretching modes of weakly hydrogen-bonded water molecules occur in the 35903500 cm-1 region, and the hydroxyl stretching modes of strong hydrogen bonds occur below 3420 cm-1. When the water is coordinated to the cation in clays as occurs in certain minerals such as hisingerite and palygorskites,29 the water OH stretching frequency occurs at 3220 cm-1. A simple observation can be made that as the water OH stretching frequency decreases the HOH bending frequency increases. The 3220 cm-1 band corresponds to an ice-like structure with O-H‚‚‚O bond distances of 2.77 Å. Thus, the water hydroxyl stretching and the water HOH bending frequencies provide a measure of the strength of the bonding of the water molecules either chemically or physically to the clay minerals. Bands that occur at frequencies above 1650 cm-1 are indicative of coordinated water and chemically bonded water. Bands that occur below 1630 cm-1 are indicative of water molecules that are not as tightly bound. In the case of kaolinite intercalated with d-DMSO, two types of water are present: first weakly hydrogen-bonded water and second coordinated water. The band at 1682 cm-1 is attributed to strongly hydrogen-bonded water. It is proposed that this water is coordinated to the DMSO molecule and may act as the linking molecule in joining several DMSO molecules in a polymer. Upon deintercalation of the DMSO-intercalated kaolinite, this water is then retained to form a hydrated kaolinite.16,17 It is suggested that this water is water that is hydrogen bonded to the DMSO molecules. The band at 3536 cm-1 is associated with the 1610 cm-1 hydroxyl deformation frequency. These bands are attributed to adsorbed water. Thus, these assignments support the concept that the additional bands in the Raman and infrared spectra in the 3500-3600 cm-1 range are associated with the presence of water in the intercalate. Changes in the Kaolinite Surface as Determined by the Raman Spectra of the Hydroxyl Stretching Region of the d-DMSO Complex. The Raman spectrum of the hydroxylstretching region of the deuterated-DMSO complex is illustrated in Figure 4. Bands are observed at 3693, 3685, 3662, and 3620 cm-1. The widths of these bands are 18.5, 10.2, 17.4, and 4.6 cm-1, respectively. The two bands at 3693 and 3685 cm-1 are assigned to the OH stretching modes of the inner-surface hydroxyls not intercalated with the d-DMSO, and the band at 3620 cm-1 is assigned to the inner hydroxyl, while the band at
3660 cm-1, as with the DRIFT spectra, is attributed to the hydroxyl stretching mode of the inner-surface hydroxyls hydrogen bonded to the d-DMSO. In addition, these bands make up 7.4%, 3.6%, 42.8%, and 38.0% of the total band intensity of the 3595-3700 cm-1 region. Three bands are observed at 3539, 3507, and 3480 cm-1 with bandwidths of 30.1, 8.9, and 9.9 cm-1 and relative intensities of 30.4%, 36.4%, and 33.2%. When kaolinite was intercalated with DMSO, additional bands were observed at 3660, 3536, and 3501 cm-1.35-37 The bandwidths of the 3660, 3536, and 3501 cm-1 bands were found to be 13.0, 19.0, and 24.0 cm-1 with relative intensities of 34.0%, 21.5%, and 23.5% of the total area of the band profile. No intensity remained in the ν1-ν4 modes. The absence of intensity in these Raman bands suggested that the kaolinite was fully intercalated upon reaction with DMSO. Thus, bands were observed for both the DMSO and d-DMSO-intercalation complexes at 3660, 3536, and ∼3507 cm-1. These bands were observed whether or not the DMSO was deuterated. The only additional band observed in the d-DMSO-intercalation complex was a band observed at 3480 cm-1. Changes in the Hydroxyl Surface as Studied by DRIFT and Raman Spectra of the Hydroxyl Deformation Modes. Figure 5 displays a comparison of the DRIFT spectra of the hydroxyl deformation modes of d-DMSO-intercalation complex for the 66% and 88% expanded kaolinite. For the 66% expanded complex, three bands are observed at 940, 914, and 905 cm-1 with relative intensities of 21.7%, 71.5%, and 6.75%, and for the 88% expanded complex, bands are observed at 957, 940, 914, and 905 cm-1 with relative intensities of 4.5%, 8.8%, 58.8%, and 27.8%. The first two bands at 940 and 914 cm-1 are ascribed to the hydroxyl deformation modes of the innersurface hydroxyls and the inner hydroxyl of kaolinite. The band at 905 cm-1 is attributed to the hydroxyl deformation of the inner-surface hydroxyl groups that are hydrogen bonded to the -SdO group of the DMSO. The difference between the 66% and 88% intercalation DRIFT spectra is the relative intensities of the bands at 915 and 905 cm-1. The intensity of the 905 cm-1 band increases significantly with the increased degree of expansion. In addition, the band at 957 cm-1 attributed to the in-plane symmetric rocking vibration is not observed for the
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Figure 4. Raman spectra of the hydroxyl stretching region of d-DMSO-intercalated kaolinite with 66% and 88% expansion.
Figure 5. DRIFT spectra of the hydroxyl deformation region of d-DMSO-intercalated kaolinite with 66% and 88% expansion.
66% intercalation complex but is observed for the 88% intercalation complex. Intercalation of the kaolinite with DMSO has caused the rearrangement of the orientation of the kaolinite hydroxyls such that different hydroxyl deformation modes are observed. The DRIFT spectra of the hydroxyl deformation region of kaolinite completely intercalated with DMSO shows two bands at 958 and 905 cm-1 with a minor components at 940 cm-1. The band at 958 cm-1 is assigned to a DMSO symmetric methyl-rocking mode.29 The 940 cm-1 band appears in the spectra of DMSO and d-DMSO at liquid-nitrogen temperatures and may also be ascribed to free, noninteracting HCH in-plane rocking modes. The band occurs at 953 cm-1 for the pure liquid DMSO and moves to the higher frequency of 959 cm-1 upon intercalation. The increase in the frequency of this band upon intercalation is attributed to the interaction between the methyl groups and the siloxane surface. The areas of the 958 and 906 cm-1 bands are 83.7% and 15.2%. The minor components at 940 cm-1 make up 1.0% of the total band area. A single intense band at the
905 cm-1 position with a bandwidth of 16.2 cm-1 dominates the band profile. This band at 905 cm-1 is attributed to the hydroxyl deformation mode of the inner-surface hydroxyls hydrogen bonded to the SdO unit of the DMSO. If the DMSO kaolinite intercalate had three different OH stretching frequencies that were attributed to the hydrogen bonding of the inner-surface hydroxyls to the SdO of the DMSO, then it could be expected that three different hydroxyl deformation modes would be observed. This is not the case. Rather, one single intense band centered at 905 cm-1 is observed. This observation suggests that only one type of hydrogen-bonded inner-surface hydroxyl group is formed. In the DRIFT spectra of the DMSO-intercalated low-defect kaolinite, two water-bending vibrations are observed at 1610 and 1683 cm-1 with areas of 66% and 33%. The bandwidths of these bands were 39.9 and 45.9 cm-1. These two waterbending modes show that two different types of water molecules are present. Figure 6 illustrates the band-component analysis of the Raman
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Figure 6. Raman spectra of the hydroxyl deformation region of d-DMSO-intercalated kaolinite with 66% and 88% expansion.
spectrum of the hydroxyl deformation region of d-DMSOintercalated low-defect kaolinite. For the 66% expanded dDMSO-intercalation complex, hydroxyl deformation modes are observed at 913, 905, and 890 cm-1 with relative intensities of 18.0%, 66.5%, and 15.4%. The bandwidths are 14.1, 13.5, and 13.3 cm-1, respectively. For the 88% expanded complex, bands are observed at 912, 903, and 888 cm-1 with relative intensities of 18.8%, 84.0%, and 3.8%. The bandwidths were 13.1, 12.1, and 15.1 cm-1, respectively. In addition bands are observed at 957 and 939 cm-1. The 957 cm-1 band is assigned to the methyl in-plane rocking vibration and the 939 cm-1 band to the hydroxyl deformation modes of the strongly hydrogen-bonded inner-surface hydroxyls. The difference in the Raman spectra of the hydroxyl deformation modes of the 66% and 88% expanded kaolinite lies with the relative intensities of the 889 and 904 cm-1 bands. The ratio of intensities is around 1:4 for the 66% expanded kaolinite and >1:10 for the 88% expanded complex. These values mean that there is more “free” nonhydrogen-bonded hydroxyls for the 66% expanded complex, and significantly more inner surface hydrogen bonded to the d-DMSO for the 88% d-DMSO-intercalation complex. The Raman spectrum of the hydroxyl deformation region of the DMSO-intercalated low-defect kaolinite is characterized by three major bands at 913, 904, and 899 cm-1. The relative intensities of the three bands are 18.0%, 66.5%, and 15.4% of the total band intensity of this region. The first band is assigned to the inner hydroxyl deformation mode. The band at 904 cm-1 is assigned to the hydroxyl deformation vibration of the innersurface hydroxyls hydrogen bonded to the DMSO. The band at 889 cm-1 may be attributed to the hydroxyl deformation mode of the free or non-hydrogen-bonded inner-surface hydroxyls. The observation of significant intensity at this position harmonizes well with the concept of displacement of the kaolinite layers relative to one another and the observation of some amorphicity in the XRD patterns. Methyl Vibrations. Figure 7 displays a comparison of the DRIFT and Raman spectra of the CD stretching vibrations for the d-DMSO-intercalated kaolinite. The DRIFT spectrum of d-DMSO shows two bands at 2138 and 2250 cm-1 with the 2138 cm-1 band more intense. Upon intercalation of kaolinite with d-DMSO, the 2250 cm-1 band resolves into three components at 2267, 2250, and 2238 cm-1. This set of data is
in good agreement with the Raman data. These bands are attributed to the antisymmetric CH stretching modes. DRIFT spectra show the band at 2140 cm-1 with a shoulder at 2127 cm-1. These two bands are weak in the infrared spectrum but more intense in the Raman spectrum and are assigned to the CD symmetric stretching vibrations. The Raman spectrum of pure d-DMSO shows two bands at 2248 and 2125 cm-1. These bands are attributed to the antisymmetric and symmetric CD stretching modes. Upon cooling to liquid-nitrogen temperature, two CD symmetric stretching modes are observed at 2129 and 2141 cm-1. Such a loss of degeneracy suggests two distinct CD stretching modes, which result from the two CD3 units of d-DMSO being in two very different molecular environments. Upon cooling d-DMSO to liquid-nitrogen temperature, three antisymmetric stretching vibrations are observed at 2253, 2246, and 2243 cm-1. The Raman spectrum of the d-DMSO-intercalated kaolinite displays four bands in the antisymmetric stretching region with bands observed at 2272, 2267, 2263, and 2251 cm-1. The relative intensities of these bands are 27.8%, 22.8%, 28.7%, and 20.6%, respectively. The bands are sharp with bandwidths of 5.5, 5.5, 6.2, and 5.3 cm-1, respectively. Thus, there is an even greater loss of degeneracy for the d-DMSO-intercalated kaolinite than for solid d-DMSO. The 2272 cm-1 band is not observed in the DRIFT spectra, whereas the 2238.5 cm-1 band in the infrared spectra is not observed in the Raman spectrum. The 2272 cm-1 band is infrared-inactive/Raman active, which suggests a symmetric vibration with a center of symmetry. The loss of degeneracy of the CD antisymmetric stretching modes into three bands indicates three different antisymmetric C-D stretching vibrations. Raupach8 showed only two FTIR CH antisymmetric stretching bands at 2936 and 3022 cm-1. Johnston22 reported two Raman CH antisymmetric stretching bands for DMSO at 2913 and 2998 cm-1, which upon intercalation split into bands at 2919 and 2936 for the 2913 cm-1 band and into 3001, 3006, and 3116 cm-1 for the 2998 cm-1 band.22 The loss of degeneracy in the CD symmetric stretching vibrations suggests that two different types of CD3 groups are found in the d-DMSO-intercalation complex. Each of these two CD3 units provides two antisymmetric stretching modes giving a total of four bands. The observation of two sets of bands shows that
Modification of Kaolinite Surfaces
J. Phys. Chem. B, Vol. 106, No. 16, 2002 4169
Figure 7. Comparison of the DRIFT and Raman spectra of the CD stretching region.
there are two different methyl groups in the d-DMSOintercalated kaolinite, one of which is perturbed to high frequency. Such a loss of degeneracy was observed by Johnston, but the spectra of Johnston show a lack of intensity in the 2935 cm-1 band, which may be attributed to the partial intercalation of the kaolinite.22 A comparison may be made between the d-DMSO- and the DMSO-intercalated kaolinite. For the DMSO-intercalated kaolinite, the 2918 and 2935 cm-1 DRIFT bands make up 5.0% and 32.7% of the total normalized band area and have bandwidths of 6.9 and 7.0 cm-1. As with the Raman spectra, these bands show considerable narrowing upon intercalation. Such an observation supports the concept that the DMSO molecules are being held in a “crystalline-like” lattice. A comparison can be made between the spectra of solid d-DMSO and d-DMSO-intercalated kaolinite. The possibility arises that the DMSO is self-coordinated rather than simply hydrogen bonded to the inner-surface hydroxyls of the kaolinite. In other words, the DMSO shows some sort of polymeric behavior within the layers of the kaolinite. The bands at 3016, 3021, and 3029 cm-1 have also narrow bands with bandwidths of 6.5, 7.9, and 6.7 cm-1. These bands make up 8.0%, 35.0%, and 7.0% of the total band area. The 1000-1200 cm-1 Region. The d-DMSO-intercalated system consists of at least two phases (a) the unintercalated kaolinite and (b) the d-DMSO-intercalated kaolinite. There is also the possibility of a single-layer kaolinite phase with some X-ray amorphicity. Thus in the infrared and Raman spectra of the 1000-1200 cm-1 region, the spectra will be a combination of the spectra of all three phases. If the assumption is made that phase 3 presence is minimal, then the spectra of two phases are observed. This region in the DRIFT spectra will be dominated by SiO stretching vibrations, whereas in the Raman spectra the SO vibrations will be observed. Further, the CD3 in-plane methyl bending modes will be observed. Figure 8 displays the DRIFT and Raman spectra of the 10001200 cm-1 region. A strong band at 1024 cm-1 is attributed to the in-plane CD3 bending mode. A small sharp inflection is observed at 1029 cm-1 in the DRIFT spectrum. A comparison may be made between the CD3 and CH3 methyl bending modes. The in-plane methyl bending region of the Raman spectrum of DMSO shows a single band at 1419 cm-1,
which splits into two bands at 1411 and 1430 cm-1 upon intercalation into kaolinite. The 1419 cm-1 band has a bandwidth of 21.9 cm-1. Upon intercalation the 1411 and 1430 cm-1 bands have bandwidths of 10.2 and 9.2 cm-1. The bands have become considerably narrower upon intercalation of the kaolinite. The DRIFT spectrum of this region for the DMSO-intercalated lowdefect kaolinite is complex with bands observed at 1438, 1428, 1408, 1404, 1392, 1374, and 1318 cm-1. The DRIFT spectrum of the DMSO shows two bands at 1414 and 1445 cm-1. A band of low intensity is also observed at 1333 cm-1. This number of bands indicates at least seven different types of H-C-H bending vibrations. Infrared bands are also observed at 1317 and 1302 cm-1. The complexity of the CH bending region supports the concept of different molecular arrangements of the dimethylsulfoxide molecules in the intercalate. Now, these HCH bending vibrations all shift into the 1000-1200 cm-1 region upon deuteration of the DMSO. The Raman spectra of pure d-DMSO displays four bands in the 950-1100 cm-1 region. These are observed at 1007, 1030, 1040, and 1058 cm-1. Raman spectra of pure DMSO show bands at 1056, 1042, 1029, and 1015 cm-1. These bands are attributed to SdO vibrations except for the 1015 cm-1 mode, which is the antisymmetric methyl rocking vibration.30 Thus, the band observed at 1007 cm-1 in the d-DMSO Raman spectrum is assigned to the CD3 rocking vibration. The region centered on 1040 cm-1 is assigned to the symmetric stretching region of SdO. Three Raman bands are identified for pure DMSO at 1056, 1042, and 1029 cm-1. These bands are assigned to the unassociated monomer and the outof-phase and the in-phase vibrations of the dimer.31-34 The bands for d-DMSO occur at 1030, 1040, and 1058 cm-1 and are similarly assigned to the SdO stretching vibrations. For the DMSO-intercalated kaolinite, the 1015 cm-1 band splits into two bands at 1023 and 1010 cm-1 with bandwidths of 12.0 and 9.5 cm-1. For the d-DMSO-intercalated kaolinite, the band observed at 1007 cm-1 splits into two bands at 1009 and 996 cm-1. It is proposed that two types of CD3 groups exist in the DMSO-intercalated kaolinite, one that is interacting with the siloxane surface and a second CD3 unit that is free from interactions. Upon intercalation of the low-defect kaolinite with d-DMSO, new Raman bands are observed at 1079, 1065, 1046, and 1023
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Martens et al.
Figure 8. Comparison of the DRIFT and Raman spectra of the SdO stretching region of d-DMSO-intercalated kaolinites.
cm-1. The band at 1023 cm-1 is assigned to polymeric SdO DMSO units.30-34 The relative intensity of these bands are 58%, 20%, 12%, and 10%, respectively. The relative intensity of the SdO polymeric unit is such that ∼60% of the DMSO molecules are in a polymeric structure within the intercalation complex. The broad bands at 1079, 1065, and 1047 cm-1 are assigned to the symmetric SdO stretching modes of free monomeric DMSO, dimeric DMSO, and trimeric DMSO. Importantly, these results show that more than one type of intercalating DMSO molecule is present. The relative intensity of the 1065 cm-1 band is 12%, and because this band is ascribed to monomeric DMSO, this means that 88% of the DMSO is in a polymeric structure within the intercalate. For pure d-DMSO, two overlapping bands are observed at 613 and 625 cm-1 and are assigned to the symmetric and antisymmetric CS stretching vibrations. The equivalent bands for DMSO were observed at 669 and 699 cm-1. Upon intercalation of the kaolinite with d-DMSO, bands are observed at 631, 640, and 652 cm-1 with relative intensities of 73%, 20%, and 7%, respectively. The first band is assigned to the symmetric CS stretching mode and the second and third bands to the CS antisymmetric stretching vibrations. There is a marked shift to higher wavenumbers for the CS stretching vibrations upon intercalation with d-DMSO. A shift of some 18 cm-1 is observed for the symmetric stretching vibration and 15 and 27 cm-1 for the antisymmetric vibration. For the intercalation of DMSO into kaolinite, an ∼21 cm-1 difference between the unperturbed CS stretching frequencies and the perturbed stretching frequencies was observed. The observation of two CS antisymmetric stretching vibrations is in harmony with the observation of two different CD3 units, one unit that is perturbed to high energy through bonding to the siloxane surface and a second that is not interacting with a kaolinite surface. A second set of bands was observed for the 88% expanded d-DMSO-intercalated kaolinite at 688 and 719 cm-1 with bandwidths of 8.8 and 7.2 cm-1. The significance of the two sets of values for both the antisymmetric and symmetric CS stretching vibrations is that two different types of molecular arrangements of the DMSO molecules are present in the intercalate. The ratio of the two types of molecules is ∼2:5. The bandwidths of the two CS stretching vibrations for d-DMSO at 613 and 625 cm-1 are 14.5 and 11.7 cm-1, respectively. The bandwidths of the CS
stretching vibrations for the d-DMSO-intercalation complex observed at 631, 642, and 652 cm-1 are 7.9, 6.9, and 7.5 cm-1. The bands are substantially narrower for both the asymmetric and symmetric CS stretching vibrations for the d-DMSOintercalated kaolinite. Bands in the Raman spectrum of DMSO at 382 and 333 cm-1 are assigned to the in-plane and out-ofplane C-SdO bends. The equivalent bands in the Raman spectrum of d-DMSO are observed at 340 and 379 cm-1. The band at 304 cm-1 for DMSO is attributed to the C-S-C bend.31-34 A band at 308 cm-1 is observed for d-DMSO. Upon intercalation of the kaolinite with d-DMSO, bands are observed at 319 and 339 cm-1. These bands are attributed to perturbed C-S-C bending modes. The in-plane and out-of-plane C-Sd O bends are observed at 350 and 368 cm-1 for the d-DMSOintercalated kaolinite. For the 88% expanded d-DMSOintercalated kaolinite, two additional bands are observed at 386 and 399 cm-1. Recent results of the simultaneous TG-DTG-DTA experiments show that there are 0.88 mol of dimethylsulfoxide per mole of inner-surface hydroxyl groups present in the intercalation complex.39 Therefore, not every inner-surface hydroxyl group is individually hydrogen bonded to one DMSO molecule. Further such a conclusion is justified because some intensity remains in the bands attributed to the inner-surface hydroxyls even though the kaolinite is completely expanded. This interaction results in the hydroxyl stretching frequencies of 3660 cm-1 for the low-defect kaolinite. These interactions are envisaged to be orientation-dependent and may occur as linear or bent hydrogen bonds. Such orientation dependence was first suggested by Johnston et al.22 The present model is at variance with the model in which one DMSO molecule is hydrogen bonded to three inner-surface hydroxyl groups.9 Importantly, it is proposed that water plays an integral part in the intercalation process: the water molecule acts as the linkage between adjacent DMSO molecules. Hydrogen bonds are formed between the water OH and the lone pairs of the S of the DMSO molecule. Two water hydroxyl groups link two DMSO molecules and form “polymeric” DMSO. This then results in the band formed at ∼1029 cm-1 attributed to polymeric SdO units in DMSO. For the DMSO-intercalated low-defect kaolinite two SdO stretching bands were observed at 1023 and 1010 cm-1. This indicates that two types of
Modification of Kaolinite Surfaces polymeric behavior of the inserting DMSO are involved in the intercalate. A second type of interaction occurs when single monomeric DMSO molecules hydrogen bond to the kaolinite inner-surface hydroxyl layer. Two types of arrangements are recognized: first DMSO molecules that are not hydrogen bonded with water and second DMSO molecules that are hydrogen bonded to water either through one or both of the water OH groups. It is suggested that the water OH units interact with the S of the DMSO. For the low-defect d-DMSOintercalated kaolinite, a band was observed at 1066 cm-1. Such a band was attributed to monomeric DMSO molecules. For the d-DMSO-intercalated kaolinite, two bands were observed at 1058 and 1040 cm-1. Both of these bands are attributed to monomeric DMSO, and it is possible that the two frequencies are due to the DMSO with and without hydrogen-bonded water molecules. The loss of degeneracy of the CH stretching vibrations suggests that the methyl groups are locked into a rigid structure. Such observations have been determined using NMR spectroscopy.10,11 While d-DMSO has two CD stretching bands, upon intercalation of the DMSO into kaolinite, the first band loses degeneracy and splits into two bands and the second into four bands. The blue shift of the CH stretching vibrations suggests that the interactions on the CD have been reduced. In the model, the d-DMSO molecules are shown as parallel to the 001 plane. However, some of the CH may point toward and interact with the siloxane surface. Conclusions A new interpretation of the hydroxyl stretching frequencies based on the presence of water in the d-DMSO intercalate of kaolinite is proposed. Evidence for two types of intercalated water in the d-DMSO-intercalation complex is obtained. Also, evidence for the presence of two types of d-DMSO molecules in the intercalate is obtained as well. These are monomeric and polymeric molecules. A simple interpretation of only one type of intercalating molecule is rejected. Only one new hydroxyl deformation mode was observed upon intercalation, supporting the concept of one SdO hydrogen-bonded inner-surface hydroxyl group. Significantly, the DMSO bands show a decrease in bandwidth upon intercalation; such a decrease is attributed to a “crystal-like” rigid structure of the DMSO in the intercalate. Pronounced changes in the Raman spectra of the inserting molecule (d-DMSO) are noted upon intercalation of the kaolinite with d-dimethylsulfoxide. Splitting of all of the symmetric and antisymmetric modes occurs. The spectra of the d-DMSO resemble the spectra of solid d-DMSO. Such observations confirm the presence of more than one type of DMSO molecule present in the intercalate. The SdO symmetric stretching frequencies confirm the presence of three types of DMSO molecules in the intercalate including monomeric, polymeric, and nonbonded DMSO. The concept of different types of DMSO molecules is supported by the complexity of the 950-1150 cm-1 region where SdO symmetric stretching and the methyl rocking vibrations occur. The CH bending region also shows complexity, thus indicating many types of methyl groups in the intercalating DMSO. Whereas for the ordered kaolinite these bands were very
J. Phys. Chem. B, Vol. 106, No. 16, 2002 4171 sharp, for the disordered kaolinites the bands are broad. Such bandwidths would imply that the intercalation of the disordered kaolinite is different from that of the ordered kaolinite. It is suggested that the ordered kaolinites have a rigid threedimensional structure, whereas the high-defect kaolinites show a less rigid structure. Acknowledgment. The financial and infrastructure support of the Queensland University of Technology Centre for Instrumental and Developmental Chemistry is gratefully acknowledged. References and Notes (1) Gonzalez Garcia, S.; Sanchez Camazano, M. Clay Miner. 1968, 7, 447. (2) Kirkman, J. H. N. Z. J. Sci. 1974, 17, 503. (3) Lim, C. H.; Jackson, M. L.; Higashi, T. Soil Sci. Soc. Am. J. 1981, 45, 433. (4) Churchman, G. J. Clays Clay Miner. 1990, 38, 591. (5) Heller-Kallai, L.; Huard, E.; Prost, R. Clay Miner. 1991, 26, 245. (6) Lapides, I.; Lahav, N.; Michaelian, K. H.; Yariv, S. J. Therm. Anal. 1997, 49, 1423. (7) Lahav, N. Clays Clay Miner. 1990, 38, 219. (8) Raupach, M.; Barron, P. F.; Thompson, J. G. Clays Clay Miner. 1987, 35, 208. (9) Thompson, J. G.; Cuff, C. Clays Clay Miner. 1985, 33, 490. (10) Hayashi, S. Clays Clay Miner. 1997, 45, 724. (11) Hayashi, S. J. Phys. Chem. 1995, 99, 7120. (12) Duer, M. J.; Rocha, J.; Klinowski, J. J. Am. Chem. Soc. 1992, 114, 6867. (13) Lipsicas, M.; Raythatha, R.; Giese, R. F., Jr.; Costanzo, P. M. Clays Clay Miner. 1986, 34, 635. (14) Thompson, J. G. Clays Clay Miner. 1985, 33, 173. (15) Costanzo, P. M.; Giese, R. F., Jr.; Clemency, C. V. Clays Clay Miner. 1984, 32, 29. (16) Costanzo, P. M.; Giese, R. F., Jr. Clays Clay Miner. 1986, 34, 105. (17) Costanzo, P. M.; Giese, R. F., Jr. Clays Clay Miner. 1990, 38, 160. (18) Frost, R. L.; Tran, T. H.; Kristof, J. Vib. Spectrosc. 1997, 13, 175. (19) Frost, R. L.; Tran, T. H.; Kristof, J. Clay Miner. 1997, 32, 587. (20) Frost, R. L.; Kristof, J. Clays Clay Miner. 1997, 45, 68. (21) Olejnik, S.; Aylmore, L. A. G.; Posner, A. M.; Quirk, J. P. J. Phys. Chem. 1968, 72, 241. (22) Johnston, C. T.; Sposito, G.; Bocian, D. F.; Birge, R. R. J. Phys. Chem. 1984, 88, 5959. (23) Anton, O.; Rouxhet, P. G. Clays Clay Miner. 1977, 25, 259. (24) Frost, R. L.; Shurvell, H. F. Clays Clay Miner. 1997, 45, 68. (25) Frost, R. L.; Van Der Gaast, S. J. Clay Miner. 1997, 32, 293-306. (26) Kristof, J.; Frost, R. L.; Felinger, A.; Mink, J. J. Mol. Struct. 1997, 41, 119. (27) Raupach, M. J. Colloid Interface Sci. 1988, 121, 476. (28) Yariv, S. J. Chem. Soc., Faraday Trans. 1 1975, 71, 674. (29) van der Marel, H. W.; Beutelspacher, H. Atlas of infrared spectroscopy of clay minerals and their admixtures; Elsevier: Amsterdam, The Netherlands, 1976. (30) Rintoul, L.; Shurvell, H. F. J. Raman Spectrosc. 1990, 21, 501. (31) Raman, K. V.; Singh, S. J. Mol. Struct. 1989, 194, 73. (32) Raman, K. V.; Singh, S. J. Raman Spectrosc. 1989, 20, 169. (33) Sastry, M. I.; Singh, S. J. Raman Spectrosc. 1984, 15, 80. (34) Singh, S.; Krueger, P. J. J. Raman Spectrosc. 1982, 13, 178. (35) Frost, R. L.; Kristof, J.; Paroz, G. N.; Kloprogge, J. T. J. Phys. Chem. B 1998, 102, 8519. (36) Frost, R. L.; Kristof, J.; Horvath, E.; Kloprogge, J. T. Thermochim. Acta. 1999, 327, 155. (37) Frost, R. L.; Kristof, J.; Horvath, E.; Kloprogge, J. T. J. Phys. Chem. A 1999, 103, 9654. (38) Frost, R. L.; Kristof, J.; Horvath, E.; Kloprogge, J. T. Clay Miner. 2000, 35, 447. (39) Kristof, J.; Horvath, E.; Frost, R. L.; Gabor, M. J. Therm. Anal. Calorim. 1999, 56, 885.
