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Modification of the Kaolinite Hydroxyl Surfaces through the Application of Pressure and Temperature, Part III
Journal of Colloid and Interface Science 214, 380 –388 (1999) Article ID jcis.1999.6209, available online at http://www.idealibrary.com on
Modification of the Kaolinite Hydroxyl Surfaces through the Application of Pressure and Temperature, Part III Ray L. Frost,* ,1 Janos Kristof,† Elisabeth Horvath,‡ and J. Theo Kloprogge* *Centre for Instrumental and Developmental Chemistry, Queensland University of Technology, 2 George Street, G.P.O. Box 2434, Brisbane, Queensland 4001, Australia; †Department of Analytical Chemistry, University of Veszprem, H8201 Veszprem, P.O. Box 158, Hungary; and ‡Research Group for Analytical Chemistry, Hungarian Academy of Sciences, H 8201 Veszprem, P.O. Box 158, Hungary Received December 7, 1998; accepted March 11, 1999
Kaolinite hydroxyl surfaces have been modified by the combined application of heat and pressure in the presence of water at 120°C and 2 bars and at 220°C and 20 bars. X-ray diffraction shows that some of the layers are expanded. It is hypothesized that this expansion occurs at the edges of the crystals due to the intercalation of water. The X-ray diffraction data is supported by diffuse reflectance infrared spectroscopy, with additional hydroxyl stretching bands observed around 3550 and 3590 cm 21. These bands are attributed to adsorbed water and to edge-intercalated water. Additional bands are observed in the hydroxyl deformation region around 895 and 877 cm 21. The position of these bands depends on the defect structure of the kaolinite and the conditions under which the kaolinite was thermally treated. Additional water bending vibrations were observed at 1651 and 1623 cm 21 for the thermally treated high-defect kaolinite and at 1682 and 1610 cm 21 for the low-defect kaolinite. The bands at 1651 and 1682 cm 21 are attributed to the bending modes of water coordinated to the kaolinite surface. The role of water in the edge intercalation of water in the high- and low-defect kaolinites is apparently different. © 1999 Academic Press Key Words: kaolinite surfaces; hydroxyls; water structure; defect structures; infrared spectroscopy; effect of pressure; effect of temperature increase.
INTRODUCTION
The role of water in clay mineral structures and the control of clay mineral surfaces is significant (1–3). For example, the loss of water by halloysite may cause the contraction from a 10 Å to a 7 Å structure. Further, the intercalation of kaolinites with organic molecules followed by deintercalation with water often results in the kaolinite forming an 8.4 Å hydrated kaolinite (3). Water is often found in clay minerals, particularly smectites and disordered kaolinites, such that extreme experimental methods must be used to achieve a dry clay (2). Then of course, the structure of the clay may be altered or even destroyed. On exposure to water vapor, many clays absorb water to return to their more stable structure. 1
To whom correspondence should be addressed.
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
Infrared spectroscopy has often been used to determine changes in kaolinite surface structures (3–5). Water in clay minerals has three main vibrational spectroscopic regions, (a) the hydroxyl stretching region, 3300 to 3600 cm 21, (b) the water H–O–H bending region centered on 1630 cm 21, and (c) the water librational region in the 400 to 700 cm 21 range. Each of these regions reflects the hydrogen bonding of water that may be effected by the presence of salts and structure making and breaking chemicals. The difficulty with using the first region (a) is that the hydroxyl stretching vibrations of water overlap with the hydroxyl stretching region of the clays. The water librational region (c) overlaps with the low-frequency region of the clays and is also difficult to determine with precision. The window in infrared spectroscopy for the study of water in clay minerals is the water bending mode region (b) centered on 1630 cm 21. The vibrational spectroscopy of water in kaolinites can be measured either with infrared or with Raman spectroscopy. The infrared spectra of water are intense because of the large change in dipole moment for the water vibrations. Consequently, low concentrations of water in clay minerals may be determined using infrared spectroscopy. Raman spectroscopy, on the other hand, is not as useful for the measurement of water. X-ray diffraction provides information about the spacing between the layers, but nothing about the molecular interactions. X-ray diffraction may measure the presence of water but only through the expansion of the layers in the absence of other molecules. To explore these molecular interactions, infrared or Raman spectroscopy may be employed. While Raman spectroscopy is useful for the measurement of kaolinite hydroxyl stretching frequencies, it is not useful for studying water bands (6). Thus infrared spectroscopy is more likely to show the role of water in the molecular interactions with the kaolinite surfaces. The infrared spectra of kaolinites are characterized by four bands centered around 3692, 3670, 3650, and 3619 cm 21. The first three bands are attributed to the inner surface hydroxyls and the fourth band to the inner hydroxyl (6 –9). An additional band may be resolved at around 3685 cm 21 in very highly
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ordered kaolinites. This band is normally Raman active and infrared inactive. In halloysites (a less ordered kaolin polytype) an additional Raman and infrared band may be observed at 3627 cm 21. This band is attributed to the inner hydroxyl, which has a different frequency because of the rolling of the halloysite layers. Water bands are observed at frequencies lower than those of the hydroxyl stretching frequencies of kaolinites. Water bands are found in the 3300 to 3600 cm 21 region. Any additionally observed bands in the DRIFT spectra of kaolinites may be attributed to the molecular interactions of water with the kaolinite surfaces. Kaolinite by its very nature has five different surfaces. These are the hydroxyl surface of the inner hydroxyls, the siloxane hydrophobic surface, the two outer surfaces of the kaolinite, and the surfaces at the end of the crystals. This last surface, the edge surface, is important, as it is proposed that it is this surface which is opening on thermal treatment. It is this surface which is interacting with the water molecules. Kaolinite structure can be modified by intercalation with salts such as potassium acetate (6, 7). This intercalation has been shown to be effected by the application of both pressure and temperature (8). Such intercalation inherently involves the incorporation of water into the structures. This water may be as interstitial water or adsorbed water. However, in order to distinguish between the effect of intercalation and the effect of pressure and temperature, it is necessary to attempt to modify kaolinite surfaces by thermal treatment without the intercalating molecule. In this paper we attempt to ascertain the effect of elevated pressure and temperature on the surfaces of kaolinite. EXPERIMENTAL METHODS
Intercalation under Pressure Intercalation under high pressure and temperature was carried out in a High Pressure Asher (Anton-Paar, Austria). Two hundred milligrams of Kiralyhegy or Szeg kaolinite and 20 cm 3 of water were placed in the quartz bomb of the HPA equipment that is closed by a quartz lid. A teflon gasket between the bomb and the lid ensured the gas-tight sealing of the bomb. Then the bomb with the lid was placed in the metal heating block of the equipment and a nitrogen pressure of 80 bars was applied over the lid to prevent escape of vapors from the bomb. In 30 min the temperature was increased to 220°C and kept constant for 8 h. After the bomb was cooled to room temperature, the pressure was released and the clay was separated from the solution by centrifugation. During this treatment the temperature and the outside pressure were recorded by the computer controlling the process. At 220°C the nitrogen pressure increases to 120 bars. It is not possible to measure the pressure inside the bomb, but it does not exceed 23 bars. The pressure inside the bomb is taken to be nominally 20 bars. In addition, a parallel experiment was conducted in which the
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same kaolinites were heated only to 120°C. In this case the inside pressure did not exceed 2 bars. X-Ray Diffraction XRD analyseswere 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 an in-house 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 CuK a from a long finefocus Cu tube, operating at 40 kV and 40 mA. The samples were measured at 50% relative humidity in stepscan mode with steps of 0.02° 2u 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 Diffuse reflectance infrared Fourier transform spectroscopy (commonly known as DRIFT) analyses were undertaken using a Bio-Rad 60A spectrometer. Five hundred and twelve scans were obtained at a resolution of 2 cm 21. Spectral manipulation such as baseline adjustment, smoothing, and normalization was performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH, USA). Band component analysis was undertaken using the Jandel “Peakfit” software package, which enabled the type of fitting function to be selected and allowed specific parameters to be fixed or varied accordingly. Band fitting was undertaken 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 r 2 correlations greater than 0.995. RESULTS AND DISCUSSION
X-ray Diffraction The normal method for the phase analysis of kaolinites is the use of X-ray diffraction. The 001 d-spacing reflects the distance between adjacent layers in the c axis direction. Kaolinites normally show a symmetric or slightly asymmetric XRD peak with a d-spacing around 7.2 Å (9). This spacing depends on the defect structures of the kaolinites and the presence of impurities. Figure 1 shows the 001 spacings for both high- and low-defect kaolinites and these kaolinites treated at elevated temperatures and pressures in the presence of water. The two kaolinites do show some asymmetry on the low-angle side. Clearly, however, on hydrothermal treatment of the kaolinites, expansion to some of the layers occurs along the c direction. This is observed from the asymmetry on the low-angle side of
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areas of these peaks are 65, 25, and 10% for the treatment of the low-defect kaolinite at 120°C and 2 bars, and are 45, 30, and 25% for the treatment at 220°C and 20 bars. It is apparent that the hydrothermal treatment has a greater effect on the low-defect kaolinite when this kaolinite is treated at the higher temperature. The high-defect kaolinite more readily expands on hydrothermal treatment compared with the low-defect kaolinite. It is proposed that the hydrothermal treatment of the kaolinites forces the edges of the kaolinite layers to separate. The layers are slowly peeling apart at the edges with the center of the layers remaining at the normal d-spacings. DRIFT Spectroscopy
FIG. 1. X-ray diffraction patterns of (a) high-defect kaolinite, (b) treated at 120°C and 2 bars and (c) at 220°C and 20 bars, and of (d) low-defect kaolinite, (e) treated at 120°C and 2 bars and (f) 220°C and 20 bars.
the 001 peak. An analysis of the peak profiles may be carried out using a curve fitting procedure. The results of this type of analysis are reported in Table 1. While it is envisaged that this may not be a strictly valid technique to study the layer expansion, it is a convenient means of studying the kaolinite expansion. The nonexpanded high- and low-defect kaolinites have a 001 d-spacing of 7.21 and 7.17 Å (9). The low-defect kaolinite shows some asymmetry, and the 001 d spacing has been fitted with two peaks at 7.17 and 7.23 Å. The band areas of these two bands are 89 and 11%. The high-defect kaolinite is strongly asymmetric before any treatment. The 001 peak profile was fitted with two bands at 7.21 and 7.4 Å. The band areas of these peaks were determined as 62 and 38%. The high-defect kaolinite shows two additional 001-peaks at 7.35 and 7.6 Å after treatment at 120°C and 2 bars. The peak areas after thermal treatment, for the three peaks at 7.21, 7.34, and 7.59 Å, are 19, 44, and 35%. Some slight differences in the areas are observed, but essentially the hydrothermal treatments at 120 and 220°C are the same. The low-defect kaolinite shows three peaks at 7.17, 7.25, and 7.42 Å after treatment at 120°C and 2 bars. The
Figure 2 shows the DRIFT spectra of the high- and lowdefect kaolinites untreated, hydrothermally treated at 120° and 2 bars, and also at 220°C and 20 bars. Table 2 reports the band component analyses of these spectra. The infrared reflectance spectrum of the high-defect kaolinite shows bands at 3692, 3671, 3651, and 3619 cm 21 with minor components at 3687 and 3627 cm 21. The high-defect kaolinite is disordered and is an halloysite with broken tubes, as determined by electron microscopy. Such kaolin polytypes show an additional band at 3627 cm 21 which is attributed to an inner hydroxyl. Disordered kaolin polytypes such as the one used in this study show infrared peaks at ;3600 and 3564 cm 21. These bands are attributed to intercalated or interlayer water and adsorbed water. Indeed the comment can be made that the only means of distinguishing between kaolinites and halloysites is by the presence of interlayer water (11). The low-defect kaolinite is a highly ordered kaolinite and the infrared spectrum shows bands at 3692, 3683, 3669, 3650, and 3619 cm 21. Although it has often been stated that the Raman active/infrared inactive vibrational mode at 3683 cm 21 is not observed in the infrared spectra, band component analysis always shows a low-intensity band at this frequency in the infrared spectra of highly
TABLE 1 Component Analysis of the X-Ray Diffraction Patterns
Kaolinite Low-defect Low-defect, 120°C and 2 bars Low-defect, 220°C and 20 bars High-defect High-defect, 120°C and 2 bars High-defect, 220°C and 20 bars
Peak 1 (Å) 6 0.01 Å/% area
Peak 2 (Å) 6 0.01 Å/% area
7.17 89% 7.17 65% 7.17 45% 7.21 62% 7.21 19% 7.21 19%
7.23 11% 7.25 25% 7.25 30% 7.40 38% 7.35 44% 7.34 47%
Peak 3 (Å) 6 0.01 Å/% area
7.42 10% 7.39 25%
7.60 37% 7.59 35%
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FIG. 2. DRIFT spectra of the hydroxyl stretching region of (a) high-defect kaolinite, (b) treated at 120°C and 2 bars and (c) at 220°C and 20 bars, and of (d) low-defect kaolinite, (e) treated at 120°C and 2 bars and (f ) at 220°C and 20 bars.
ordered kaolinites. A band is observed at 3590 cm 21. Such a band is representative of interlayer water or intercalated water. Bands are often observed at this frequency in disordered kaolinites and halloysites. Upon thermal treatment of the high defect kaolinite at 120°C and 2 bars, the infrared reflectance spectrum shows bands at 3692, 3668, 3648, and 3620 cm 21. No band was observed at 3627 and 3687 cm 21. Additional bands were observed at 3585 and 3555 cm 21. The determination of the exact position of these water bands is difficult and so some variation in the exact position occurs. Importantly, the intensity of the 3585 cm 21 band has increased from ;2.2% to 23.7% of the overall band intensity. The intensity of the bands of adsorbed water changed from 29.4 to 15.7%. The total water intensity of the two water bands is 31.6% for the untreated high-defect kaolinite and 39.4% for the thermally treated kaolinite. The X-ray diffraction of this thermally treated high-defect kaolinite showed additional expansion of the original kaolinite to 7.35 and 7.6 Å. Therefore it is proposed that this expansion occurs at the edges of the kaolinite crystals and that the water fills these voids in the kaolinite edge surfaces. When the high-defect kaolinite is treated at 220°C and 20 bars, infrared reflectance bands are observed at 3694, 3669, 3650, and 3620 cm 21. It is observed that some changes in the peak positions of bands 1 to 5 occur, and this is attributed to (a) experimental error and (b) the band component analysis of overlapping bands. The intensity of the 3585 and 3550 cm 21 bands are 17.7 and 7.3%. These values are less than those for the high-defect kaolinite thermally treated at 120°C and 2 bars. The intensity of the 3692 cm 21 band decreases upon thermal
TABLE 2 Band Component Analysis of the DRIFT Spectra of the Hydroxyl Stretching Region of (a) High-Defect Kaolinite, (b) Treated at 120°C and 2 Bars and (c) at 220°C and 20 Bars, and for (d) Low-Defect Kaolinite, (e) Treated at 120°C and 2 Bars and (f ) at 220°C and 20 Bars
High-defect kaolinite
High-defect, 120°C and 2 bars High-defect, 220°C and 20 bars Low-defect kaolinite
Low-defect, 120°C and 2 bars Low-defect, 220°C and 20 bars
Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area
Band 1
Band 2
Band 3
Band 4
Band 5
Band 6
Band 7
Band 8
3692 21.5 25% 3692 29.0 22.0% 3694 23.1 19.9% 3692 20.3 28.0% 3691 28.9 27.2% 3690 28.7 27.0%
3687 5.0 0.5%
3671 18.8 5.6% 3668 33.6 15.9% 3669 46.0 27.3% 3669 15.2 12.2% 3668 15.5 8.5% 3668 15.6 8.0%
3651 29.7 12.9% 3648 20.8 3.5% 3650 29.7 7.3% 3650 17.4 26.2% 3650 18.9 17.9% 3650 19.9 18.8%
3627 14.6 4.7%
3619 13.0 19.7% 3620 30.6 19.4% 3620 19.8 20.5% 3619 9.2 20.3% 3619 10.8 13.3% 3619 11.3 14.2%
3600 34.0 2.2% 3585 104 23.7% 3585 78.0 17.7% 3590 179 10.8% 3610 135 18.5% 3593 179 18.9%
3564 149 29.4% 3555 171 15.7% 3550 160.0 7.3%
3683 9.9 2.5%
3440 215 13.7% 3450 183 10.8%
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treatment from 25 to 22% (for the 120°C, 2 bars experiment) and to 19.9% (for the 220°C and 20 bars experiment). This decrease in intensity is attributed to the bonding of the inner surface hydroxyls to water molecules. At the same time, an increase in intensity of the 3668 cm 21 band is observed. The intensity increases from 5.6 to 15.9% and to 27.3%. When the low-defect kaolinite is thermally treated at 120°C and 2 bars, the infrared reflectance spectrum shows bands at 3691, 3668, 3650, and 3619 cm 21. An additional band is observed at 3610 cm 21 which makes up 18.5% of the total band intensity. When the low-defect kaolinite is thermally treated at 220°C and 20 bars, infrared reflectance bands are observed at 3690, 3668, 3650, and 3619 cm 21. The water bands are observed at 3593 and 3450 cm 21. The water bands have 18.9 and 10.8% of the total band intensity. These values are comparable to those obtained for the low-defect kaolinite thermally treated at 120°C and 2 bars. X-ray diffraction of the low-defect kaolinite showed that upon thermal treatment some expansion occurred. Again it is proposed that this expansion occurs at the edge of the crystals and that the space between the kaolinite layers is filled with non-hydrogen-bonded water molecules and these water molecules result in the infrared reflectance band at 3610 cm 21. The reason the band occurs at this frequency is the non-hydrogen bonding of the water molecules (10, 12). The water molecules are fitting into the expanded interlayers of the kaolinite. Normally adsorbed water displays bands at ;3550 cm 21. In the case of the thermally treated low-defect kaolinite (120°C and 2 bars) the water band is found at 3440 cm 21. The frequency of the hydroxyl stretch of water is extremely sensitive to the strength of the bond between the water molecule and the cation or surface to which the water molecule is chemically bonded (10). The lower the wavenumber, then the stronger the water bonding is. Thus the observation of the band at 3440 cm 21 suggest that these water molecules are strongly hydrogen bonded. This would suggest that the water bonded at the edges of the kaolinite is more strongly bound than adsorbed water. Two models may be proposed: first, the water is hydrogen bonded to the kaolinite surface through the “outer” hydroxyls or the inner surface hydroxyls at the edges of the kaolinite crystals; second, these are water molecules, which are bonded to the edge water molecules. The more likely situation is that the water molecules are strongly bonded to the edge inner surface hydroxyls, which by their very nature are exposed at the surface. Water bonded to water is likely to be in the 3500 to 3550 cm 21 region. Information on kaolinite surfaces may also be obtained by studying the hydroxyl deformation modes. The spectra of these vibrations are shown in Fig. 3, and the results of the band component analyses are reported in Table 3. The high-defect kaolinite infrared reflectance spectrum shows bands at 942, 913, and 877 cm 21 with band intensities of 21.5, 72.7, and 5.8% of the total intensity of the band profile. Previously, the 877 cm 21 band was attributed to hydroxyl deformation of non-hydrogen-bonded inner surface hydroxyls (13). The band
FIG. 3. DRIFT spectra of the hydroxyl deformation region of (a) highdefect kaolinite, (b) treated at 120°C and 2 bars and (c) at 220°C and 20 bars, and of (d) low-defect kaolinite, (e) treated at 120°C and 2 bars and (f ) at 220°C and 20 bars.
at 913 cm 21 is assigned to the inner hydroxyl deformation mode and the band at 942 cm 21 to the hydroxyl deformation of the inner hydroxyls hydrogen-bonded to the adjacent siloxane layers. When the kaolinite is thermally treated at 120°C and 2 bars, infrared reflectance bands are observed at 938, 913, 891, and 875 cm 21. The intensities of these bands are 20.6, 66.2, 6.9, and 6.3%. The additional band at 891 cm 21 is assigned to inner surface hydroxyls weakly hydrogen-bonded to water molecules. When the high-defect kaolinite is thermally treated at 220°C and 20 bars, infrared reflectance bands are observed at 937, 913, 894, and 877 cm 21. The intensities of these bands are 25.3, 60.3, 6.0, and 8.4% respectively. Several observations may be made: first, the intensity of band 1 increases upon thermal treatment, second, the intensity of band 3 decreases, and third, the intensity of band 4 is constant with the two thermal treatments. The increase in intensity of band 1 attributed to the inner surface hydroxyls strongly hydrogen bonded may be attributed to the hydrogen bonding of the water molecules to the edge hydroxyls.
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TABLE 3 Band Component Analysis of the DRIFT Spectra of the Hydroxyl Deformation Region of (a) High-Defect Kaolinite, (b) Treated at 120°C and 2 Bars and (c) at 220°C and 20 Bars, and for (d) Low-Defect Kaolinite, (e) Treated at 120°C and 2 Bars and (f ) at 220°C and 20 Bars Band 1 High-defect kaolinite
High-defect, 120°C and 2 bars
High-defect, 220°C and 20 bars
Low-defect kaolinite
Low-defect, 120°C and 2 bars
Low-defect, 220°C and 20 bars
Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area
942 22.3 21.5% 938 25.0 20.6% 937 24.5 25.3% 940 18.3 30.8% 940 18.3 29.8% 940 19.3 32.6
The low-defect kaolinite infrared reflectance spectrum shows four bands at 940, 923, 913, and 901 cm 21. The intensities of these band are 30.8, 22.5, 40.9, and 5.7%. Compared to the spectrum of the high-defect kaolinite, the intensity of band 1 is significantly higher. An additional band at 923 cm 21 is observed and the intensity of band 3 is reduced. The band at 923 cm 21 is attributed to the inner surface hydroxyls which are weakly hydrogen bonded to the next adjacent siloxane layer. When the low-defect kaolinite is thermally treated at 120° and 2 bars, bands are observed at the same frequencies. The intensities of these bands are now 29.8, 17.8, 48.8, and 4.1%. When the low-defect kaolinite is thermally treated at 220°C and 20 bars, bands are observed at 940, 924, 914, 900, and 879 cm 21. The intensities of these bands are 32.6, 17.7, 41.9, 7.0, and 0.7%. The observation of a band at 879 cm 21 suggests that the kaolinite is becoming disordered upon the thermal treatment. Band 4 increases in intensity upon thermal treatment and this increase is attributed to the interaction of water molecules with the inner surface hydroxyl groups. Band 5 at 879 cm 21 is also observed. Such a band is observed for high-defect kaolinites, and this observation supports the concept that the kaolinite defect structures are increasing upon thermal treatment at 220°C and 20 bars based on the X-ray diffraction results. While it is valid to compare intensity changes from one experiment to the next for the same band, it may not be valid to compare intensity changes between bands at different positions because the bands may have different molar absorptivities. Band 1 is constant in intensity, as is seen by comparing the values of the untreated kaolinite to the values for the two thermally treated kaolinites. Band 2 shows a decrease in inten-
Band 2
Band 3
923 19.0 22.5% 923 19.0 17.8% 924 18.3 17.7%
913 23.9 72.7% 913 25.2 66.2% 913 23.3 60.3% 913 17.9 40.9% 914 20.3 48.2% 914 19.1 41.9%
Band 4
891 33.5 6.9% 894 13.9 6.0% 901 30.3 5.7% 896 29.4 4.1% 900 24.0 7.0
Band 5 877 17.7 5.8% 875 16.9 6.3% 877 19.0 8.4%
879 14.0 0.7%
sity. Band 3 increases then decreases. Band 4 displays an increase in intensity. The role of water in kaolinites may be also followed by the study of the H–O–H bending vibration at ;1630 cm 21. The position of the bending vibration reflects the bonding of the water to the kaolinite surfaces (11). The DRIFT spectrum of the low-defect kaolinite shows the absence of water. Water bands, however, are observed for the high-defect kaolinite (Table 4). A broad band centered at 1630 cm 21 is observed. When the high-defect kaolinite is thermally treated, two bands are observed at 1651 and 1623 cm 21. Figure 4 shows the band component analysis of the high-defect kaolinites thermally treated at 120°C and 2 bars. The relative intensities of these two bands for the high-defect kaolinite are 39.0 and 61.0 for the 120°C/2 bars experiment and 34.0 and 66.0 for the 220°C/20 bars experiment. The band at 1651 cm 21 is attributed to water hydrogen-bonded to the kaolinite surface. Adsorbed water has characteristic infrared frequencies similar to those of liquid water, so the band at 1623 cm 21 is assigned to adsorbed water. This type of water is the water fitting into the spaces formed between the slightly expanded edges of the kaolinites. When the kaolinites are treated at 220°C and 20 bars, changes in the defect structures may be introduced into the kaolinites as is indicated by the X-ray diffraction results. This may be the reason the relative intensities of the two bands are different in the two experiments. Figure 5 represents the band component analysis of the low-defect kaolinite thermally treated at 120°C and 2 bars. When the low-defect kaolinite is thermally treated, two bands are observed at 1682 and 1605 cm 21. The intensities of these
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TABLE 4 Band Component Analysis of the DRIFT Spectra of the Water Bending Region of (a) High-Defect Kaolinite, (b) Treated at 120°C and 2 Bars and (c) at 220°C and 20 Bars, and of (d) Low-Defect Kaolinite, (e) Treated at 120°C and 2 Bars and (f ) at 220°C and 20 Bars Band 1 High-defect kaolinite
High-defect, 120°C and 2 bars
High-defect, 220°C and 20 bars
Low-defect kaolinite
Low-defect, 120°C and 2 bars
Low-defect, 220°C and 20 bars
Band center/cm 21 Bandwidth/cm 21 Band area/% Band center/cm 21 Bandwidth/cm 21 Band area/% Band center/cm 21 Bandwidth/cm 21 Band area/% Band center/cm 21 Bandwidth/cm 21 Band area/% Band center/cm 21 Bandwidth/cm 21 Band area/% Band center/cm 21 Bandwidth/cm 21 Band area/%
two bands are 45 and 55% for the 120°C/2 bars experiment and 58 and 42% for the 220°C/20 bars experiment. The shift in frequency of the water bending modes from the normal posi-
Band 2
Band 3
1630 53.0 100 1651 13.7 61.0 1650 29.0 34.0
1623 41.5 39.0 1621 46.2 66.0
Band 4
nil
1682 42.0 45.0 1682 41.3 58.0
1604 40.3 55.0 1605 41.5 42.0
tion of around 1630 cm 21 is significant. The shift to a higher frequency is indicative of a strongly hydrogen-bonded water, and the shift to a lower frequency is representative of more
FIG. 4. Band component analysis of the DRIFT spectrum of the water bending region for the high-defect kaolinite thermally treated at 120°C and 2 bars.
MODIFICATION OF KAOLINITE HYDROXYL SURFACES, III
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FIG. 5. Band component analysis of the DRIFT spectrum of the water bending region for the low-defect kaolinite thermally treated at 120°C and 2 bars.
weakly hydrogen-bonded water molecules. The 1682 cm 21 band is assigned to a water molecule coordinated to the kaolinite surface while the 1605 cm 21 band is attributed to the edge intercalated water molecules. In the hydroxyl stretching region two bands attributed to water were observed at around 3450 and 3595 cm 21. The hydroxyl stretching band at 3450 is associated with the water bending band at 1682 cm 21 while the hydroxyl stretching vibration of water at 3595 cm 21 is associated with the water bending mode at 1605 cm 21. It is possible to envisage models for the edge expansion of the kaolinites. Figure 6 shows such a model of the expansion of the edge surface of the high- and low-defect kaolinites. The low-defect kaolinite will have a regular stack of layers. The
FIG. 6. Model of the defect structures of kaolinites, their expansion under thermal treatment, and the possible location of the water molecules.
high-defect kaolinite has layers with disordered stacking. When the low-defect kaolinite is expanded at the edges, the two layers expand congruently, and the water molecule may fit in between these two curved edges. The circles represent the water molecules. When the high-defect kaolinite expands at the edges, then more space is available for the water molecules to fit into or at least adsorb on these surfaces. Thus this model would predict that the water molecules are behaving differently in the low- and high-defect kaolinites. This then results in a difference in the infrared spectra of the water hydroxyl stretching vibrations and the water bending modes. CONCLUSIONS
Both low- and high-defect kaolinites were subjected to thermal treatments at 120°C and 2 bars and 220°C and 20 bars. This thermal treatment caused some expansion in the layers as determined by X-ray diffraction. It is proposed that this expansion occurs at the edges of the kaolinite crystals. The layers of the kaolinites are peeling apart upon thermal treatment. The thermal treatment with water at the elevated temperatures and pressures caused the water molecules to be incorporated into the space between the expanded edges of the kaolinite crystals. Such a conclusion may have important consequences for the thermal treatment of other clay minerals, for example micas, in acids at elevated temperatures.
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Subtle changes in the infrared reflectance spectra of the thermally treated kaolinites were observed. Additional reflectance bands were observed around 3600 and 3550 cm 21, which were attributed to edge intercalated water and to adsorbed water. Importantly, it is proposed that this edge surface is the fifth surface of the kaolinite crystals where the interaction of the clay and water occurs. A model is proposed of the interaction between the water molecules and the kaolinite surfaces, showing the edge intercalation. ACKNOWLEDGMENTS The financial and infrastructure support of the Queensland University of Technology Centre for Instrumental and Developmental Chemistry is gratefully acknowledged. Commercial Minerals (Aust.) Pty Ltd is thanked for financial support through Mr. Lew Barnes, Chief Geologist. Financial support from the Hungarian Scientific Research Fund under grant OTKA T25171 is also acknowledged.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Prost, R., Spectrochim. Acta 31A, 1497 (1975). Sposito, G., and Prost, R., Chem. Rev. 82, 553 (1982). Constanzo, P. M., and Giese, R. F., Clays Clay Miner. 38, 160 (1990). Barrios, J., Planc¸on, A., Cruz, M. I., and Tchoubar, C., Clays Clay Miner. 25, 422 (1977). Brindley, G. W., Chih-Chun, K., Harrison, J. L., Lipsiscas, M., and Raythatha, R., Clays Clay Miner. 34, 233 (1986). Frost, R. L., Kristof, J., Paroz, G., and Kloprogge, J. T., J. Colloid Interface Sci. 198, 478 (1998). Frost, R. L., Tran, T. H., and Kristof, J., Clay Miner. 32, 587 (1997). Frost, R. L., and Kristof, J., Clays Clay Miner. 45, 68 (1997). Frost, R. L., and Van Der Gaast, S. J., Clay Miner. 32, 293 (1997). Frost, R. L., Kristof, J., Paroz, G., and Kloprogge, J. T., J. Colloid Interface Sci. 198, 227 (1998). Churchman, G. J., and Carr, R. M., Clays Clay Miner. 23, 382 (1975). Frost, R. L., Kristof, J., Paroz, G., and Kloprogge, J. T., J. Colloid Interface Sci. 198, 216 (1998). Frost, R. L., Clays Clay Miner. 46, 280 (1998).
Modification of the Kaolinite Hydroxyl Surfaces through the Application of Pressure and Temperature, Part III Ray L. Frost,* ,1 Janos Kristof,† Elisabeth Horvath,‡ and J. Theo Kloprogge* *Centre for Instrumental and Developmental Chemistry, Queensland University of Technology, 2 George Street, G.P.O. Box 2434, Brisbane, Queensland 4001, Australia; †Department of Analytical Chemistry, University of Veszprem, H8201 Veszprem, P.O. Box 158, Hungary; and ‡Research Group for Analytical Chemistry, Hungarian Academy of Sciences, H 8201 Veszprem, P.O. Box 158, Hungary Received December 7, 1998; accepted March 11, 1999
Kaolinite hydroxyl surfaces have been modified by the combined application of heat and pressure in the presence of water at 120°C and 2 bars and at 220°C and 20 bars. X-ray diffraction shows that some of the layers are expanded. It is hypothesized that this expansion occurs at the edges of the crystals due to the intercalation of water. The X-ray diffraction data is supported by diffuse reflectance infrared spectroscopy, with additional hydroxyl stretching bands observed around 3550 and 3590 cm 21. These bands are attributed to adsorbed water and to edge-intercalated water. Additional bands are observed in the hydroxyl deformation region around 895 and 877 cm 21. The position of these bands depends on the defect structure of the kaolinite and the conditions under which the kaolinite was thermally treated. Additional water bending vibrations were observed at 1651 and 1623 cm 21 for the thermally treated high-defect kaolinite and at 1682 and 1610 cm 21 for the low-defect kaolinite. The bands at 1651 and 1682 cm 21 are attributed to the bending modes of water coordinated to the kaolinite surface. The role of water in the edge intercalation of water in the high- and low-defect kaolinites is apparently different. © 1999 Academic Press Key Words: kaolinite surfaces; hydroxyls; water structure; defect structures; infrared spectroscopy; effect of pressure; effect of temperature increase.
INTRODUCTION
The role of water in clay mineral structures and the control of clay mineral surfaces is significant (1–3). For example, the loss of water by halloysite may cause the contraction from a 10 Å to a 7 Å structure. Further, the intercalation of kaolinites with organic molecules followed by deintercalation with water often results in the kaolinite forming an 8.4 Å hydrated kaolinite (3). Water is often found in clay minerals, particularly smectites and disordered kaolinites, such that extreme experimental methods must be used to achieve a dry clay (2). Then of course, the structure of the clay may be altered or even destroyed. On exposure to water vapor, many clays absorb water to return to their more stable structure. 1
To whom correspondence should be addressed.
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
Infrared spectroscopy has often been used to determine changes in kaolinite surface structures (3–5). Water in clay minerals has three main vibrational spectroscopic regions, (a) the hydroxyl stretching region, 3300 to 3600 cm 21, (b) the water H–O–H bending region centered on 1630 cm 21, and (c) the water librational region in the 400 to 700 cm 21 range. Each of these regions reflects the hydrogen bonding of water that may be effected by the presence of salts and structure making and breaking chemicals. The difficulty with using the first region (a) is that the hydroxyl stretching vibrations of water overlap with the hydroxyl stretching region of the clays. The water librational region (c) overlaps with the low-frequency region of the clays and is also difficult to determine with precision. The window in infrared spectroscopy for the study of water in clay minerals is the water bending mode region (b) centered on 1630 cm 21. The vibrational spectroscopy of water in kaolinites can be measured either with infrared or with Raman spectroscopy. The infrared spectra of water are intense because of the large change in dipole moment for the water vibrations. Consequently, low concentrations of water in clay minerals may be determined using infrared spectroscopy. Raman spectroscopy, on the other hand, is not as useful for the measurement of water. X-ray diffraction provides information about the spacing between the layers, but nothing about the molecular interactions. X-ray diffraction may measure the presence of water but only through the expansion of the layers in the absence of other molecules. To explore these molecular interactions, infrared or Raman spectroscopy may be employed. While Raman spectroscopy is useful for the measurement of kaolinite hydroxyl stretching frequencies, it is not useful for studying water bands (6). Thus infrared spectroscopy is more likely to show the role of water in the molecular interactions with the kaolinite surfaces. The infrared spectra of kaolinites are characterized by four bands centered around 3692, 3670, 3650, and 3619 cm 21. The first three bands are attributed to the inner surface hydroxyls and the fourth band to the inner hydroxyl (6 –9). An additional band may be resolved at around 3685 cm 21 in very highly
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ordered kaolinites. This band is normally Raman active and infrared inactive. In halloysites (a less ordered kaolin polytype) an additional Raman and infrared band may be observed at 3627 cm 21. This band is attributed to the inner hydroxyl, which has a different frequency because of the rolling of the halloysite layers. Water bands are observed at frequencies lower than those of the hydroxyl stretching frequencies of kaolinites. Water bands are found in the 3300 to 3600 cm 21 region. Any additionally observed bands in the DRIFT spectra of kaolinites may be attributed to the molecular interactions of water with the kaolinite surfaces. Kaolinite by its very nature has five different surfaces. These are the hydroxyl surface of the inner hydroxyls, the siloxane hydrophobic surface, the two outer surfaces of the kaolinite, and the surfaces at the end of the crystals. This last surface, the edge surface, is important, as it is proposed that it is this surface which is opening on thermal treatment. It is this surface which is interacting with the water molecules. Kaolinite structure can be modified by intercalation with salts such as potassium acetate (6, 7). This intercalation has been shown to be effected by the application of both pressure and temperature (8). Such intercalation inherently involves the incorporation of water into the structures. This water may be as interstitial water or adsorbed water. However, in order to distinguish between the effect of intercalation and the effect of pressure and temperature, it is necessary to attempt to modify kaolinite surfaces by thermal treatment without the intercalating molecule. In this paper we attempt to ascertain the effect of elevated pressure and temperature on the surfaces of kaolinite. EXPERIMENTAL METHODS
Intercalation under Pressure Intercalation under high pressure and temperature was carried out in a High Pressure Asher (Anton-Paar, Austria). Two hundred milligrams of Kiralyhegy or Szeg kaolinite and 20 cm 3 of water were placed in the quartz bomb of the HPA equipment that is closed by a quartz lid. A teflon gasket between the bomb and the lid ensured the gas-tight sealing of the bomb. Then the bomb with the lid was placed in the metal heating block of the equipment and a nitrogen pressure of 80 bars was applied over the lid to prevent escape of vapors from the bomb. In 30 min the temperature was increased to 220°C and kept constant for 8 h. After the bomb was cooled to room temperature, the pressure was released and the clay was separated from the solution by centrifugation. During this treatment the temperature and the outside pressure were recorded by the computer controlling the process. At 220°C the nitrogen pressure increases to 120 bars. It is not possible to measure the pressure inside the bomb, but it does not exceed 23 bars. The pressure inside the bomb is taken to be nominally 20 bars. In addition, a parallel experiment was conducted in which the
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same kaolinites were heated only to 120°C. In this case the inside pressure did not exceed 2 bars. X-Ray Diffraction XRD analyseswere 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 an in-house 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 CuK a from a long finefocus Cu tube, operating at 40 kV and 40 mA. The samples were measured at 50% relative humidity in stepscan mode with steps of 0.02° 2u 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 Diffuse reflectance infrared Fourier transform spectroscopy (commonly known as DRIFT) analyses were undertaken using a Bio-Rad 60A spectrometer. Five hundred and twelve scans were obtained at a resolution of 2 cm 21. Spectral manipulation such as baseline adjustment, smoothing, and normalization was performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH, USA). Band component analysis was undertaken using the Jandel “Peakfit” software package, which enabled the type of fitting function to be selected and allowed specific parameters to be fixed or varied accordingly. Band fitting was undertaken 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 r 2 correlations greater than 0.995. RESULTS AND DISCUSSION
X-ray Diffraction The normal method for the phase analysis of kaolinites is the use of X-ray diffraction. The 001 d-spacing reflects the distance between adjacent layers in the c axis direction. Kaolinites normally show a symmetric or slightly asymmetric XRD peak with a d-spacing around 7.2 Å (9). This spacing depends on the defect structures of the kaolinites and the presence of impurities. Figure 1 shows the 001 spacings for both high- and low-defect kaolinites and these kaolinites treated at elevated temperatures and pressures in the presence of water. The two kaolinites do show some asymmetry on the low-angle side. Clearly, however, on hydrothermal treatment of the kaolinites, expansion to some of the layers occurs along the c direction. This is observed from the asymmetry on the low-angle side of
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areas of these peaks are 65, 25, and 10% for the treatment of the low-defect kaolinite at 120°C and 2 bars, and are 45, 30, and 25% for the treatment at 220°C and 20 bars. It is apparent that the hydrothermal treatment has a greater effect on the low-defect kaolinite when this kaolinite is treated at the higher temperature. The high-defect kaolinite more readily expands on hydrothermal treatment compared with the low-defect kaolinite. It is proposed that the hydrothermal treatment of the kaolinites forces the edges of the kaolinite layers to separate. The layers are slowly peeling apart at the edges with the center of the layers remaining at the normal d-spacings. DRIFT Spectroscopy
FIG. 1. X-ray diffraction patterns of (a) high-defect kaolinite, (b) treated at 120°C and 2 bars and (c) at 220°C and 20 bars, and of (d) low-defect kaolinite, (e) treated at 120°C and 2 bars and (f) 220°C and 20 bars.
the 001 peak. An analysis of the peak profiles may be carried out using a curve fitting procedure. The results of this type of analysis are reported in Table 1. While it is envisaged that this may not be a strictly valid technique to study the layer expansion, it is a convenient means of studying the kaolinite expansion. The nonexpanded high- and low-defect kaolinites have a 001 d-spacing of 7.21 and 7.17 Å (9). The low-defect kaolinite shows some asymmetry, and the 001 d spacing has been fitted with two peaks at 7.17 and 7.23 Å. The band areas of these two bands are 89 and 11%. The high-defect kaolinite is strongly asymmetric before any treatment. The 001 peak profile was fitted with two bands at 7.21 and 7.4 Å. The band areas of these peaks were determined as 62 and 38%. The high-defect kaolinite shows two additional 001-peaks at 7.35 and 7.6 Å after treatment at 120°C and 2 bars. The peak areas after thermal treatment, for the three peaks at 7.21, 7.34, and 7.59 Å, are 19, 44, and 35%. Some slight differences in the areas are observed, but essentially the hydrothermal treatments at 120 and 220°C are the same. The low-defect kaolinite shows three peaks at 7.17, 7.25, and 7.42 Å after treatment at 120°C and 2 bars. The
Figure 2 shows the DRIFT spectra of the high- and lowdefect kaolinites untreated, hydrothermally treated at 120° and 2 bars, and also at 220°C and 20 bars. Table 2 reports the band component analyses of these spectra. The infrared reflectance spectrum of the high-defect kaolinite shows bands at 3692, 3671, 3651, and 3619 cm 21 with minor components at 3687 and 3627 cm 21. The high-defect kaolinite is disordered and is an halloysite with broken tubes, as determined by electron microscopy. Such kaolin polytypes show an additional band at 3627 cm 21 which is attributed to an inner hydroxyl. Disordered kaolin polytypes such as the one used in this study show infrared peaks at ;3600 and 3564 cm 21. These bands are attributed to intercalated or interlayer water and adsorbed water. Indeed the comment can be made that the only means of distinguishing between kaolinites and halloysites is by the presence of interlayer water (11). The low-defect kaolinite is a highly ordered kaolinite and the infrared spectrum shows bands at 3692, 3683, 3669, 3650, and 3619 cm 21. Although it has often been stated that the Raman active/infrared inactive vibrational mode at 3683 cm 21 is not observed in the infrared spectra, band component analysis always shows a low-intensity band at this frequency in the infrared spectra of highly
TABLE 1 Component Analysis of the X-Ray Diffraction Patterns
Kaolinite Low-defect Low-defect, 120°C and 2 bars Low-defect, 220°C and 20 bars High-defect High-defect, 120°C and 2 bars High-defect, 220°C and 20 bars
Peak 1 (Å) 6 0.01 Å/% area
Peak 2 (Å) 6 0.01 Å/% area
7.17 89% 7.17 65% 7.17 45% 7.21 62% 7.21 19% 7.21 19%
7.23 11% 7.25 25% 7.25 30% 7.40 38% 7.35 44% 7.34 47%
Peak 3 (Å) 6 0.01 Å/% area
7.42 10% 7.39 25%
7.60 37% 7.59 35%
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FIG. 2. DRIFT spectra of the hydroxyl stretching region of (a) high-defect kaolinite, (b) treated at 120°C and 2 bars and (c) at 220°C and 20 bars, and of (d) low-defect kaolinite, (e) treated at 120°C and 2 bars and (f ) at 220°C and 20 bars.
ordered kaolinites. A band is observed at 3590 cm 21. Such a band is representative of interlayer water or intercalated water. Bands are often observed at this frequency in disordered kaolinites and halloysites. Upon thermal treatment of the high defect kaolinite at 120°C and 2 bars, the infrared reflectance spectrum shows bands at 3692, 3668, 3648, and 3620 cm 21. No band was observed at 3627 and 3687 cm 21. Additional bands were observed at 3585 and 3555 cm 21. The determination of the exact position of these water bands is difficult and so some variation in the exact position occurs. Importantly, the intensity of the 3585 cm 21 band has increased from ;2.2% to 23.7% of the overall band intensity. The intensity of the bands of adsorbed water changed from 29.4 to 15.7%. The total water intensity of the two water bands is 31.6% for the untreated high-defect kaolinite and 39.4% for the thermally treated kaolinite. The X-ray diffraction of this thermally treated high-defect kaolinite showed additional expansion of the original kaolinite to 7.35 and 7.6 Å. Therefore it is proposed that this expansion occurs at the edges of the kaolinite crystals and that the water fills these voids in the kaolinite edge surfaces. When the high-defect kaolinite is treated at 220°C and 20 bars, infrared reflectance bands are observed at 3694, 3669, 3650, and 3620 cm 21. It is observed that some changes in the peak positions of bands 1 to 5 occur, and this is attributed to (a) experimental error and (b) the band component analysis of overlapping bands. The intensity of the 3585 and 3550 cm 21 bands are 17.7 and 7.3%. These values are less than those for the high-defect kaolinite thermally treated at 120°C and 2 bars. The intensity of the 3692 cm 21 band decreases upon thermal
TABLE 2 Band Component Analysis of the DRIFT Spectra of the Hydroxyl Stretching Region of (a) High-Defect Kaolinite, (b) Treated at 120°C and 2 Bars and (c) at 220°C and 20 Bars, and for (d) Low-Defect Kaolinite, (e) Treated at 120°C and 2 Bars and (f ) at 220°C and 20 Bars
High-defect kaolinite
High-defect, 120°C and 2 bars High-defect, 220°C and 20 bars Low-defect kaolinite
Low-defect, 120°C and 2 bars Low-defect, 220°C and 20 bars
Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area
Band 1
Band 2
Band 3
Band 4
Band 5
Band 6
Band 7
Band 8
3692 21.5 25% 3692 29.0 22.0% 3694 23.1 19.9% 3692 20.3 28.0% 3691 28.9 27.2% 3690 28.7 27.0%
3687 5.0 0.5%
3671 18.8 5.6% 3668 33.6 15.9% 3669 46.0 27.3% 3669 15.2 12.2% 3668 15.5 8.5% 3668 15.6 8.0%
3651 29.7 12.9% 3648 20.8 3.5% 3650 29.7 7.3% 3650 17.4 26.2% 3650 18.9 17.9% 3650 19.9 18.8%
3627 14.6 4.7%
3619 13.0 19.7% 3620 30.6 19.4% 3620 19.8 20.5% 3619 9.2 20.3% 3619 10.8 13.3% 3619 11.3 14.2%
3600 34.0 2.2% 3585 104 23.7% 3585 78.0 17.7% 3590 179 10.8% 3610 135 18.5% 3593 179 18.9%
3564 149 29.4% 3555 171 15.7% 3550 160.0 7.3%
3683 9.9 2.5%
3440 215 13.7% 3450 183 10.8%
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treatment from 25 to 22% (for the 120°C, 2 bars experiment) and to 19.9% (for the 220°C and 20 bars experiment). This decrease in intensity is attributed to the bonding of the inner surface hydroxyls to water molecules. At the same time, an increase in intensity of the 3668 cm 21 band is observed. The intensity increases from 5.6 to 15.9% and to 27.3%. When the low-defect kaolinite is thermally treated at 120°C and 2 bars, the infrared reflectance spectrum shows bands at 3691, 3668, 3650, and 3619 cm 21. An additional band is observed at 3610 cm 21 which makes up 18.5% of the total band intensity. When the low-defect kaolinite is thermally treated at 220°C and 20 bars, infrared reflectance bands are observed at 3690, 3668, 3650, and 3619 cm 21. The water bands are observed at 3593 and 3450 cm 21. The water bands have 18.9 and 10.8% of the total band intensity. These values are comparable to those obtained for the low-defect kaolinite thermally treated at 120°C and 2 bars. X-ray diffraction of the low-defect kaolinite showed that upon thermal treatment some expansion occurred. Again it is proposed that this expansion occurs at the edge of the crystals and that the space between the kaolinite layers is filled with non-hydrogen-bonded water molecules and these water molecules result in the infrared reflectance band at 3610 cm 21. The reason the band occurs at this frequency is the non-hydrogen bonding of the water molecules (10, 12). The water molecules are fitting into the expanded interlayers of the kaolinite. Normally adsorbed water displays bands at ;3550 cm 21. In the case of the thermally treated low-defect kaolinite (120°C and 2 bars) the water band is found at 3440 cm 21. The frequency of the hydroxyl stretch of water is extremely sensitive to the strength of the bond between the water molecule and the cation or surface to which the water molecule is chemically bonded (10). The lower the wavenumber, then the stronger the water bonding is. Thus the observation of the band at 3440 cm 21 suggest that these water molecules are strongly hydrogen bonded. This would suggest that the water bonded at the edges of the kaolinite is more strongly bound than adsorbed water. Two models may be proposed: first, the water is hydrogen bonded to the kaolinite surface through the “outer” hydroxyls or the inner surface hydroxyls at the edges of the kaolinite crystals; second, these are water molecules, which are bonded to the edge water molecules. The more likely situation is that the water molecules are strongly bonded to the edge inner surface hydroxyls, which by their very nature are exposed at the surface. Water bonded to water is likely to be in the 3500 to 3550 cm 21 region. Information on kaolinite surfaces may also be obtained by studying the hydroxyl deformation modes. The spectra of these vibrations are shown in Fig. 3, and the results of the band component analyses are reported in Table 3. The high-defect kaolinite infrared reflectance spectrum shows bands at 942, 913, and 877 cm 21 with band intensities of 21.5, 72.7, and 5.8% of the total intensity of the band profile. Previously, the 877 cm 21 band was attributed to hydroxyl deformation of non-hydrogen-bonded inner surface hydroxyls (13). The band
FIG. 3. DRIFT spectra of the hydroxyl deformation region of (a) highdefect kaolinite, (b) treated at 120°C and 2 bars and (c) at 220°C and 20 bars, and of (d) low-defect kaolinite, (e) treated at 120°C and 2 bars and (f ) at 220°C and 20 bars.
at 913 cm 21 is assigned to the inner hydroxyl deformation mode and the band at 942 cm 21 to the hydroxyl deformation of the inner hydroxyls hydrogen-bonded to the adjacent siloxane layers. When the kaolinite is thermally treated at 120°C and 2 bars, infrared reflectance bands are observed at 938, 913, 891, and 875 cm 21. The intensities of these bands are 20.6, 66.2, 6.9, and 6.3%. The additional band at 891 cm 21 is assigned to inner surface hydroxyls weakly hydrogen-bonded to water molecules. When the high-defect kaolinite is thermally treated at 220°C and 20 bars, infrared reflectance bands are observed at 937, 913, 894, and 877 cm 21. The intensities of these bands are 25.3, 60.3, 6.0, and 8.4% respectively. Several observations may be made: first, the intensity of band 1 increases upon thermal treatment, second, the intensity of band 3 decreases, and third, the intensity of band 4 is constant with the two thermal treatments. The increase in intensity of band 1 attributed to the inner surface hydroxyls strongly hydrogen bonded may be attributed to the hydrogen bonding of the water molecules to the edge hydroxyls.
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TABLE 3 Band Component Analysis of the DRIFT Spectra of the Hydroxyl Deformation Region of (a) High-Defect Kaolinite, (b) Treated at 120°C and 2 Bars and (c) at 220°C and 20 Bars, and for (d) Low-Defect Kaolinite, (e) Treated at 120°C and 2 Bars and (f ) at 220°C and 20 Bars Band 1 High-defect kaolinite
High-defect, 120°C and 2 bars
High-defect, 220°C and 20 bars
Low-defect kaolinite
Low-defect, 120°C and 2 bars
Low-defect, 220°C and 20 bars
Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area Band center/cm 21 Bandwidth/cm 21 Band area
942 22.3 21.5% 938 25.0 20.6% 937 24.5 25.3% 940 18.3 30.8% 940 18.3 29.8% 940 19.3 32.6
The low-defect kaolinite infrared reflectance spectrum shows four bands at 940, 923, 913, and 901 cm 21. The intensities of these band are 30.8, 22.5, 40.9, and 5.7%. Compared to the spectrum of the high-defect kaolinite, the intensity of band 1 is significantly higher. An additional band at 923 cm 21 is observed and the intensity of band 3 is reduced. The band at 923 cm 21 is attributed to the inner surface hydroxyls which are weakly hydrogen bonded to the next adjacent siloxane layer. When the low-defect kaolinite is thermally treated at 120° and 2 bars, bands are observed at the same frequencies. The intensities of these bands are now 29.8, 17.8, 48.8, and 4.1%. When the low-defect kaolinite is thermally treated at 220°C and 20 bars, bands are observed at 940, 924, 914, 900, and 879 cm 21. The intensities of these bands are 32.6, 17.7, 41.9, 7.0, and 0.7%. The observation of a band at 879 cm 21 suggests that the kaolinite is becoming disordered upon the thermal treatment. Band 4 increases in intensity upon thermal treatment and this increase is attributed to the interaction of water molecules with the inner surface hydroxyl groups. Band 5 at 879 cm 21 is also observed. Such a band is observed for high-defect kaolinites, and this observation supports the concept that the kaolinite defect structures are increasing upon thermal treatment at 220°C and 20 bars based on the X-ray diffraction results. While it is valid to compare intensity changes from one experiment to the next for the same band, it may not be valid to compare intensity changes between bands at different positions because the bands may have different molar absorptivities. Band 1 is constant in intensity, as is seen by comparing the values of the untreated kaolinite to the values for the two thermally treated kaolinites. Band 2 shows a decrease in inten-
Band 2
Band 3
923 19.0 22.5% 923 19.0 17.8% 924 18.3 17.7%
913 23.9 72.7% 913 25.2 66.2% 913 23.3 60.3% 913 17.9 40.9% 914 20.3 48.2% 914 19.1 41.9%
Band 4
891 33.5 6.9% 894 13.9 6.0% 901 30.3 5.7% 896 29.4 4.1% 900 24.0 7.0
Band 5 877 17.7 5.8% 875 16.9 6.3% 877 19.0 8.4%
879 14.0 0.7%
sity. Band 3 increases then decreases. Band 4 displays an increase in intensity. The role of water in kaolinites may be also followed by the study of the H–O–H bending vibration at ;1630 cm 21. The position of the bending vibration reflects the bonding of the water to the kaolinite surfaces (11). The DRIFT spectrum of the low-defect kaolinite shows the absence of water. Water bands, however, are observed for the high-defect kaolinite (Table 4). A broad band centered at 1630 cm 21 is observed. When the high-defect kaolinite is thermally treated, two bands are observed at 1651 and 1623 cm 21. Figure 4 shows the band component analysis of the high-defect kaolinites thermally treated at 120°C and 2 bars. The relative intensities of these two bands for the high-defect kaolinite are 39.0 and 61.0 for the 120°C/2 bars experiment and 34.0 and 66.0 for the 220°C/20 bars experiment. The band at 1651 cm 21 is attributed to water hydrogen-bonded to the kaolinite surface. Adsorbed water has characteristic infrared frequencies similar to those of liquid water, so the band at 1623 cm 21 is assigned to adsorbed water. This type of water is the water fitting into the spaces formed between the slightly expanded edges of the kaolinites. When the kaolinites are treated at 220°C and 20 bars, changes in the defect structures may be introduced into the kaolinites as is indicated by the X-ray diffraction results. This may be the reason the relative intensities of the two bands are different in the two experiments. Figure 5 represents the band component analysis of the low-defect kaolinite thermally treated at 120°C and 2 bars. When the low-defect kaolinite is thermally treated, two bands are observed at 1682 and 1605 cm 21. The intensities of these
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TABLE 4 Band Component Analysis of the DRIFT Spectra of the Water Bending Region of (a) High-Defect Kaolinite, (b) Treated at 120°C and 2 Bars and (c) at 220°C and 20 Bars, and of (d) Low-Defect Kaolinite, (e) Treated at 120°C and 2 Bars and (f ) at 220°C and 20 Bars Band 1 High-defect kaolinite
High-defect, 120°C and 2 bars
High-defect, 220°C and 20 bars
Low-defect kaolinite
Low-defect, 120°C and 2 bars
Low-defect, 220°C and 20 bars
Band center/cm 21 Bandwidth/cm 21 Band area/% Band center/cm 21 Bandwidth/cm 21 Band area/% Band center/cm 21 Bandwidth/cm 21 Band area/% Band center/cm 21 Bandwidth/cm 21 Band area/% Band center/cm 21 Bandwidth/cm 21 Band area/% Band center/cm 21 Bandwidth/cm 21 Band area/%
two bands are 45 and 55% for the 120°C/2 bars experiment and 58 and 42% for the 220°C/20 bars experiment. The shift in frequency of the water bending modes from the normal posi-
Band 2
Band 3
1630 53.0 100 1651 13.7 61.0 1650 29.0 34.0
1623 41.5 39.0 1621 46.2 66.0
Band 4
nil
1682 42.0 45.0 1682 41.3 58.0
1604 40.3 55.0 1605 41.5 42.0
tion of around 1630 cm 21 is significant. The shift to a higher frequency is indicative of a strongly hydrogen-bonded water, and the shift to a lower frequency is representative of more
FIG. 4. Band component analysis of the DRIFT spectrum of the water bending region for the high-defect kaolinite thermally treated at 120°C and 2 bars.
MODIFICATION OF KAOLINITE HYDROXYL SURFACES, III
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FIG. 5. Band component analysis of the DRIFT spectrum of the water bending region for the low-defect kaolinite thermally treated at 120°C and 2 bars.
weakly hydrogen-bonded water molecules. The 1682 cm 21 band is assigned to a water molecule coordinated to the kaolinite surface while the 1605 cm 21 band is attributed to the edge intercalated water molecules. In the hydroxyl stretching region two bands attributed to water were observed at around 3450 and 3595 cm 21. The hydroxyl stretching band at 3450 is associated with the water bending band at 1682 cm 21 while the hydroxyl stretching vibration of water at 3595 cm 21 is associated with the water bending mode at 1605 cm 21. It is possible to envisage models for the edge expansion of the kaolinites. Figure 6 shows such a model of the expansion of the edge surface of the high- and low-defect kaolinites. The low-defect kaolinite will have a regular stack of layers. The
FIG. 6. Model of the defect structures of kaolinites, their expansion under thermal treatment, and the possible location of the water molecules.
high-defect kaolinite has layers with disordered stacking. When the low-defect kaolinite is expanded at the edges, the two layers expand congruently, and the water molecule may fit in between these two curved edges. The circles represent the water molecules. When the high-defect kaolinite expands at the edges, then more space is available for the water molecules to fit into or at least adsorb on these surfaces. Thus this model would predict that the water molecules are behaving differently in the low- and high-defect kaolinites. This then results in a difference in the infrared spectra of the water hydroxyl stretching vibrations and the water bending modes. CONCLUSIONS
Both low- and high-defect kaolinites were subjected to thermal treatments at 120°C and 2 bars and 220°C and 20 bars. This thermal treatment caused some expansion in the layers as determined by X-ray diffraction. It is proposed that this expansion occurs at the edges of the kaolinite crystals. The layers of the kaolinites are peeling apart upon thermal treatment. The thermal treatment with water at the elevated temperatures and pressures caused the water molecules to be incorporated into the space between the expanded edges of the kaolinite crystals. Such a conclusion may have important consequences for the thermal treatment of other clay minerals, for example micas, in acids at elevated temperatures.
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Subtle changes in the infrared reflectance spectra of the thermally treated kaolinites were observed. Additional reflectance bands were observed around 3600 and 3550 cm 21, which were attributed to edge intercalated water and to adsorbed water. Importantly, it is proposed that this edge surface is the fifth surface of the kaolinite crystals where the interaction of the clay and water occurs. A model is proposed of the interaction between the water molecules and the kaolinite surfaces, showing the edge intercalation. ACKNOWLEDGMENTS The financial and infrastructure support of the Queensland University of Technology Centre for Instrumental and Developmental Chemistry is gratefully acknowledged. Commercial Minerals (Aust.) Pty Ltd is thanked for financial support through Mr. Lew Barnes, Chief Geologist. Financial support from the Hungarian Scientific Research Fund under grant OTKA T25171 is also acknowledged.
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