Modification of Kaolinite Surfaces by Mechanochemical Treatment

Langmuir 2001, 17, 4731-4738

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Modification of Kaolinite Surfaces by Mechanochemical Treatment Ray L. Frost,*,† E Ä va Mako´,‡ Ja´nos Kristo´f,§ Erzse´bet Horva´th,| and J. Theo Kloprogge† Centre for Instrumental and Developmental Chemistry, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane, Queensland 4001, Australia, Department of Silicate and Materials Engineering, University of Veszpre´ m, H-8201 Veszpre´ m, P.O.Box 158, Hungary, Department of Analytical Chemistry, University of Veszpre´ m, H-8201 Veszpre´ m, PO Box 158, Hungary, and Research Group for Analytical Chemistry, Hungarian Academy of Sciences, H8201 Veszpre´ m, PO Box 158, Hungary Received October 16, 2000. In Final Form: May 2, 2001 Kaolinite surfaces were modified by grinding kaolinite/quartz mixtures with mole fractions of 0.25 kaolinite and 0.75 quartz for periods of time up to 4 h. X-ray diffraction shows the loss of intensity of the d(001) spacing with mechanical treatment resulting in the delamination of the kaolinite. Thermogravimetric analyses show the kaolinite surface is significantly modified and surface hydroxyls are replaced with water molecules. Changes in the molecular structure of the surface hydroxyls of the kaolinite/quartz mixtures were followed by infrared spectroscopy. Kaolinite hydroxyls were lost after 2 h of grinding as evidenced by the decrease in intensity of the OH stretching vibrations at 3695 and 3619 cm-1 and the deformation modes at 937 and 915 cm-1. Changes in the surface structure of the OSiO units were reflected in the SiO stretching and OSiO bending vibrations. The decrease in intensity of the 1056 and 1034 cm-1 bands attributed to kaolinite SiO stretching vibrations were concomitantly matched by the increase in intensity of additional bands at 1113 and 520 cm-1 ascribed to the new mechanically synthesized kaolinite surface. Mechanochemical treatment of the kaolinite results in a new surface structure.

Introduction Kaolinite and its polytype, halloysite, are extremely important industrial minerals.1 Many of the bulk properties of kaolinite are determined by the surface properties of the kaolinite. For example, in industry, solid loadings and viscosity are important and the presence of adsorbed surface species can affect these properties. The presence of minor amounts of other clay minerals can also affect the surface properties: for example, the presence of smectite and illite influences the viscosity and solids loadings.2 Very often kaolinites are mechanically ground to delaminate the kaolinite for industrial use. However, the chemical phenomena occurring during this process are not well defined.3-5 Studies of the effect of mechanical treatment of kaolinite have been made over an extensive period of time.6,7 Some studies suggest that the OH group is irrevocably displaced during mechanical treatment.6,7 The importance of knowing not only the mineral science of the kaolinite but also the alteration of the surface structure cannot be underestimated. Mechanochemical †

Queensland University of Technology. Department of Silicate and Materials Engineering, University of Veszpre´m. § Department of Analytical Chemistry, University of Veszpre ´ m. | Hungarian Academy of Sciences. ‡

(1) Murray, H. H.; Keller, W. D. In Kaolins, kaolins and kaolins. In kaolin genesis and utilisation; Murray, H., Ed.; Specialist Publication No. 1, The Clay Minerals Society: Boulder, CO, 1993. (2) Guven, Necip. Molecular aspects of clay-water interactions in clay water interface and its rheological implications; Guven, N., Pollastro, N. M., Eds.; CMS workshop lectures Vol. 4; The Clay Minerals Society: Boulder, CO, 1992; pp 2-69. (3) Kelley, W. P.; Jenny, H. Soil Sci. 1936, 4, 367. (4) Laws, W. D.; Page, J. B. Soil Sci. 1946, 62, 319. (5) Schrader, R. Silikattechnik 1970, 21, 196. (6) Juha´sz, A. Z. Acta Mineralogica-Petrographica 1980, 24, 121. (7) Juha´sz, A. Z.; Opoczky L. Mechanical activation of minerals by grinding: Pulverising and morphology of particles: Academic Press: Budapest, Hungary, 1990.

treatment of the kaolinite may, for example, affect the dehydroxylation temperature. Recently modifications of the kaolinite surfaces have been studied using a combination of intercalation and thermal treatment.8-12 New additional phases of kaolinite were found and modification of the hydroxyl surfaces was extensive even with mild heating. In mechanochemical treatment intense local heating can result. In the kaolinite structure two types of hydroxyl groups can be recognized: hydroxyl groups at the internal surface between adjacent layers, often referred to as inner-surface hydroxyl groups, and hydroxyl groups within a single layer, often referred to as inner hydroxyl groups. The innersurface hydroxyl groups are situated in the outer, unshared plane whereas the inner hydroxyl groups are located in the inner (shared) plane of the octahedral sheet, often termed the gibbsite-like sheet.12-15 The innersurface hydroxyl groups are located on the outside of a single kaolinite layer. However, the kaolinite crystal consists of a stacking of many of these layers, which means that actually the inner-surface hydroxyl groups are located in the space between two adjacent layers; hence the term inner-surface hydroxyl is normally used in the literature. The inner hydroxyl group is within the unit cell and is not (8) Frost, R. L.; Kristof, J.; Horvath, E.; Kloprogge, J. T. Langmuir 1999, 15, 8787. (9) Frost, R. L.; Kristof, J.; Kloprogge, J. T.; Horvath, E. Langmuir 2000, 16, 5403. (10) Frost, R. L.; Kristof, J.; Mako, E.; Kloprogge, J. T. Langmuir 2000, 16, 7421. (11) Frost, R. L.; Kristof, J.; Mako, E.; Kloprogge, J. T. Modification of the hydroxyl surface in cesium acetate intercalated kaolinite between 25 and 300 °C. Langmuir 2000, 16, 7421-7428. (12) Collins, D. R.; Catlow, C. R. A. Acta Crystallogr. 1991, B47, 678. (13) Giese, R. F. Bull. Mineral. 1982, 105, 417. (14) Giese, R. F. Reviews in Mineralogy; Volume 19, Hydrous Phyllosilicates; Bailey, S. W., Ed.; Mineralogical Society of America, BookCrafters Inc.: Chelsea, MI, 1988; Chapter 3. (15) Hess, C. A.; Saunders: V. R. J. Phys. Chem. 1992, 96, 4367.

10.1021/la001453k CCC: $20.00 © 2001 American Chemical Society Published on Web 07/07/2001

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exposed at the surface of the kaolinite layers. Dispersive and Fourier transform (FT) Raman spectroscopy and infrared spectrometry have been used in the study of the kaolinite clay minerals.16-28 Bands for kaolinite have been identified at 3620, 3650, 3667, and 3682 cm-1 with a prominent shoulder at 3692 cm-1. Assignments of these bands were made in terms of the inner and inner-surface hydroxyl groups.19,24 One proposition is that the band assignments were attributable to individual OH oscillators rather than coupled OH units. Other studies suggest that the bands are attributable to coupled in-phase and outof-phase inner-surface hydroxyl groups.20 The current status of the assignment of the kaolinite hydroxyl stretching frequencies and the physics of collection of spectral data have been recently reviewed by Farmer.20 This work defined the effect of crystallite size on the intensity of the hydroxyl stretching bands. As the kaolinite crystal size is reduced, the intensity of the transverse optic vibration is reduced. In a low-defect kaolinite four distinct infrared bands are assigned as follows: the three higher frequency vibrations at 3695 (ν1), 3670 (ν2), and 3658 (ν3) are due to the three inner-surface hydroxyl groups. The band at 3620 cm-1 is due to the inner hydroxyl group.19,24 One commonly accepted view is that the ν1 and ν2 bands are the coupled antisymmetric and symmetric vibrations.19,27,28 The assignment of the ν3 band is open to question, but the suggestion has been made that the band is due to symmetry reduction of the inner-surface hydroxyl.19,27,28 The kaolinite unit cell has four hydroxyl groups, and therefore it would be expected that four infrared and Raman bands would be observed. However, five Raman bands are observed by both dispersive and FT Raman spectroscopic techniques with an additional band centered at 3684 (ν4) cm-1.15 Frost and van der Gaast proposed that this band was the out-of-phase vibration of the coupled 3684 cm-1 mode.19 Recently Shoval et al. have identified this band in the infrared spectra of kaolinites with lowdefect structures.27 In this way all of the inner-surface hydroxyl groups are labeled ν1 to ν4. The 3684 cm-1 band is labeled ν4 in Raman spectroscopy, and the inner hydroxyl frequency is ν5. A previous study reported the rupture of the OH, AlOH, AlOSi, and SiO bonds due to mechanical treatment.29 No determination of what happens to these bonds has been reported. Previous studies also suggest that the layer type structure of the kaolinite is retained without hydroxyls. Comparisons of mechanically treated kaolinite with thermally treated ones often termed metakaolinite have been made. There is an obvious need to determine the changes in the surface structure of kaolinite through (16) Wiewiora, A.; Wieckowski, T.; Sokolowska, A. Arch. Mineral. 1979, 135, 5. (17) Johnston, C. T.; Sposito, G.; Birge, R. R. Clays Clay Miner. 1985, 33, 483. (18) Michaelian, K. H. Can. J. Chem. 1986, 64, 285. (19) Frost, R. L.; van der Gaast, S. J. Clay Miner. 1997, 32, 471. (20) Farmer, V. C. Clay Miner. 1998, 33, 601. (21) Frost, R. L.; Fredericks, P. M.; Bartlett, J. R. Spectrochim. Acta 1993, 20, 667. (22) Frost, R. L. Clays Clay Miner. 1995, 43, 191. (23) Frost, R. L. Clay Miner. 1997, 32, 73. (24) Brindley, G. W.; Chih-Chun, K.; Harrison, J. L.; Lipsiscas, M.; Raythatha, R. Clays Clay Miner. 1986, 34, 233. (25) Farmer, V. C.; Russell, J. D. Spectrochim. Acta 1964, 20, 1149. (26) Farmer, V. C.; Russell, J. D. Proceedings of the 15th Conference of Clays and Clay Minerals; 1967; p 27. (27) Shoval, S.; Yariv, S.; Michaelian, K. H.; Lapides, I.; Boudeuille, M.; Panczer, G. J. Colloid Interface Sci. 1999, 212, 523. (28) Johannson, U.; Frost, R. L.; Forsling, W.; Kloprogge, J. T. Appl. Spectrosc. 1998, 52, 1277. (29) Kristof, E.; Juha´sz, A. Z.; Vassanyi, I. Clays Clay Miner. 1993, 41, 608.

Frost et al.

mechanochemical treatment, particularly at the molecular level. This forms the basic objective of this research. Kaolinite as it often occurs in nature is not a pure mineral and often has other phases present such as quartz and micas. Indeed, in order to undertake this research, it was difficult to find a kaolinite that was quartz free. Because of the uncertainty in the description of the surface structure in the mechanochemical treatment of kaolinite, detailed infrared spectroscopy of kaolinite/quartz mixtures mechanically treated for periods of time is reported. Experimental Section Milling of the Kaolinite/Quartz Mixtures. The kaolinite used in this experiment was natural kaolinite from Sedlec (Zettlitz) in Slovakia. The chemical composition of the kaolinite is the following (% oxide): MgO, 0.26; CaO, 0.54; SiO2, 46.97; Fe2O3, 0.37; K2O, 1.21; Al2O3, 36.32; TiO2, 0.05. This kaolinite has some minor impurities of illite and quartz and was selected for this experiment because of its low quartz content. The kaolinite is a low-defect kaolinite with a Hinckley index of around 0.7. A Fritsch pulverisette 5/2 type stainless steel planetary mill was used to grind the mixture of kaolinite (0.25 mole fraction) and quartz (0.75 mole fraction). Samples were ground for 0, 0.5, 1, 2, 3, and 4 h. Each milling was carried out with a 10 g air-dried sample in an 80 cm3 container using 31.6 g stainless steel balls (10 mm diameter). X-ray diffraction shows that the XRD pattern of the quartz is not altered during the grinding process. X-ray Powder Diffraction. The XRD analyses were carried out on a Philips wide-angle PW 3020 vertical goniometer equipped with curved graphite diffracted beam monochromator at room temperature. The radiation applied was Cu KR1 from a long fine focus Cu tube, operating at 40 kV and 40 mA. The samples were measured in step-scan mode with steps of 0.02° 2θ and a counting time of 1 s. Data collection was performed with PC-APD 3.6 software. Thermogravimetric Analysis. Thermogravimetric analyses were carried out in a Netzsch (Germany) TG 209 type thermobalance under dynamic heating conditions (10 °C/min heating rate) in flowing argon atmosphere of 99.995% purity (Messer Griesheim, Hungary). A ceramic crucible was used for the experiments filled with approximately 20 mg of sample in each case. Diffuse Reflectance Infrared Spectroscopy. Diffuse reflectance Fourier transform infrared spectroscopic (commonly known as DRIFT) analyses were undertaken using a Bio-Rad FTS 60A spectrometer. A total of 512 scans were obtained at a resolution of 2 cm-1 with a mirror velocity of 0.3 cm/s. Spectra were co-added to improve the signal-to-noise ratio. Approximately 3 wt % ground 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. Background KBr spectra were obtained, and spectra were ratioed to the background. The diffuse reflectance accessory used was designed exclusively for Bio-Rad 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 achate mortar and pestle in 1-3% concentration. The reflectance spectra expressed as Kubelka-Munk unit versus wavenumber curves are very similar to absorbance spectra and can be evaluated accordingly. 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 LorentzGauss cross-product function with the minimum number of component bands used for the fitting process. The Lorentz-Gauss 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 0.95.

Modification of Kaolinite Surfaces

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Figure 2. Thermogravimetric analysis of the kaolinite/quartz mixture ground for (a) 0, (b) 0.5, (c) 1, and (d) 2 h. Figure 1. X-ray diffraction patterns of the d(001) spacing of a kaolinite/quartz mixture mechanically ground for (a) 0, (b) 0.5, (c) 1, (d) 2, (e) 3, and (f) 4 h.

Results and Discussion X-ray Diffraction. The XRD patterns of the mechanically ground kaolinite/quartz mixture show the rapid changes in the kaolinite structure during the grinding process (Figure 1). The effect of grinding causes the diminution of the d(001) spacing, and after 2 h of grinding almost no intensity remains in this peak. The significance of the loss of intensity of the d(001) peak means the stacking between the layers is disrupted and lost. The mechanochemical treatment has broken the hydrogen bonding between adjacent kaolinite layers. Thus, the kaolinite has been completely delaminated through the mechanical grinding process. After 4 h of grinding no XRD pattern of the kaolinite is present. This means that the long-range ordering in the layers is disturbed so that there is no regular pattern of atoms, which can cause the diffraction. The XRD pattern only shows a broad peak centered on approximately 25° 2θ. The significance of this pattern rests with the production of a poorly diffracting material through grinding. This material has had its lattice structure destroyed. The modified kaolinite is not necessarily amorphous; rather it is poorly diffracting. Crystallite size is the coherently diffracted domain. The rapid structural degradation of kaolinite is connected with an increase of the mean lattice distortion and is not the consequence of the reduction of the crystallite size. Crystallite size is not equal to particle size. The mechanochemical treatment not only changes the morphology of the kaolinite particles but also causes a reduction in particle size. Thus, other techniques are sought to study the changes in the structure of the synthesized material. Scanning electron microscopy shows only an agglomeration of small spherical particles with no surface morphology. Because the material has little

long-range ordering in its crystalline structure, Raman spectroscopy is not suitable and infrared reflectance spectroscopy is the technique of choice for the study of the changes on the kaolinite surface. This technique enables the changes in surface structure to be followed upon the mechanochemical treatment brought about through dry grinding. Thermogravimetric Analysis. Figure 2 shows the thermogravimetric (TG) and differential thermogravimetric analyses of the ground kaolinite/quartz mixtures. The TG curve for the pure kaolinite shows a single weight loss around 450 °C. Curves b-d show a two-step weight loss. With increased grinding time, the curves shift to lower temperatures. Further, it appears that the weight loss in the first step (>100 and 400 °C). Such observations are confirmed by the differential thermogravimetric curves. Two weight losses are observed. The first weight loss is attributed to the liberation of water formed as a result of mechanochemical dehydroxylation of the kaolinite, while in the second stage (above 450 °C) water is lost in the thermal dehydroxylation process. This dehydroxylation temperature decreases with grinding time. The area of this peak decreases concomitantly as the area of the first weight loss peak centered at approximately 150 °C increases. This weight loss profile ascribed to a water weight loss is rather complex, because of different types of water present in the mechanochemically synthesized material. Changes in the Kaolinite Surface Structure by DRIFT Spectroscopy. The destruction of the molecular structure of the ground kaolinite/quartz mixtures as determined by DRIFT spectroscopy is illustrated in Figure 3. This figure displays the decrease in intensity of the hydroxyl stretching vibrations as a function of grinding time and the concomitant increase in intensity of OH stretching vibrations, attributable to water in the 32003550 cm-1 region. The results of the band component analyses are reported in Table 1. The bands at 3695 (ν1)

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Figure 4. Variation of the relative intensity of the hydroxyl stretching and deformation modes of the kaolinite/quartz mixture as a function of grinding time.

from the out-of-phase vibrations of the inner-surface hydroxyls corresponding to the in-phase vibrations observed at 3695 and 3685 cm-1. This means that the innersurface hydroxyls are no longer behaving in a cooperative vibrational pattern. The mechanochemical treatment causes significant changes in the surface structure at the molecular level. The position of the band attributed to the stretching vibration of the inner hydroxyl at 3619 cm-1 (ν5) is not affected by the grinding process even though the intensity decreases. An increase in bandwidth is noted. This may result from the intense localized heating of the kaolinite surfaces during mechanochemical treatment. Figure 4 shows the decrease in relative intensities of the hydroxyl vibrations as a function of grinding time. The relationship between the length of time of grinding and the relative intensities of the (ν1) and (ν5) vibrations are both linear functions. After 2 h of grinding no significant intensity remains in these bands. Figure 5 shows the hydroxyl deformation modes observed at 937 and 914 cm-1 attributed to the inner-surface and inner hydroxyls, respectively. In harmony with the decrease in intensity of the hydroxyl stretching vibrations, the decrease in intensity of the kaolinite hydroxyl deformation vibrations is linear with grinding time (Table 2). Figure 5 clearly shows a monotonic decrease in intensity of the hydroxyl deformation modes with grinding time. The variation in intensity shown in Figure 4 of these hydroxyl deformation modes is based upon the relative intensities of the 914 and 937 cm-1 bands. This result is significant

Figure 3. Infrared reflectance spectra of the hydroxyl stretching region of kaolinite/quartz mixture mechanically ground for (a) 0, (b) 0.5, (c) 1, (d) 2, (e) 3, and (f) 4 h.

and 3685 (ν4) cm-1 are attributed to the longitudinal and transverse optic vibrations. This latter band is intense in Raman spectra of low-defect kaolinites but is of low intensity in the infrared spectra and is only determined as a component in the overall band profile.27 Interestingly, the (ν4) mode shows a decrease in intensity as the mechanochemical treatment of the kaolinite is taking place. Such a transverse vibration depends on the aspect ratio of the kaolinite crystals, i.e., the thickness of the crystals. Thus, as the size of the kaolinite crystals is reduced, this vibration apparently shifts to lower wavenumbers. The bands observed at 3668 (ν2) and 3652 (ν3) cm-1 also show a decrease in band position with the length of grinding, and after 1 h of mechanochemical treatment no intensity is observed in these bands. These bands result

Table 1. Results of the Band Component Analysis of DRIFT Spectra of the Hydroxyl Stretching Region of the Kaolinites as a Function of Grinding Time kaolinite polytype kaolinite at 25 °C kaolinite ground for 0.5 h kaolinite ground for 1.0 h kaolinite ground for 2.0 h kaolinite ground for 3.0 h kaolinite ground for 4.0 h

band parameters

ν1

ν4

ν2

ν3

ν5

band center/cm-1 %area bandwidth/cm-1 band center/cm-1 %area bandwidth/cm-1 band center/cm-1 %area bandwidth/cm-1 band center/cm-1 %area bandwidth/cm-1 band center/cm-1 %area bandwidth/cm-1 band center/cm-1 %area bandwidth/cm-1

3695 28.5 15.9 3695 19.6 16.4 3695 13.0 16.0 3695 1.2 32.5 3695 0.4 33.0 3695 0.2 33.6

3685 4.8 17.7 3685 1.6 23.2 3676 1.5 21.0

3668 9.5 14.8 3664 7.3 25.3 3661 6.1 29.0

3652 24.6 26.4 3645 6.7 28.0 3640 2.7 25.5

3619 18.5 9.4 3619 12.9 12.3 3620 8.5 13.0 3619 1.9 26.0 3619 0.8 26.0 3619 0.7 26.0

ν6

ν7

ν8

3600 11.4 br 3585 13.5 br 3580 13.9 br

3575 34.8 br 3576 20.4 br 3425 31.6 br 3420 27.9 br 3420 28.9 br

3237 16.9 br 3200 47.8 br 3200 54.0 br 3200 49.4 br 3200 56 br


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Table 2. Results of the Band Component Analysis of DRIFT Spectra of the Hydroxyl and Water Deformation Regions of the Kaolinites as a Function of Grinding Time kaolinite polytype kaolinite at 25 °C kaolinite ground for 0.5 h kaolinite ground for 1.0 h kaolinite ground for 2.0 h kaolinite ground for 3.0 h kaolinite ground for 4.0 h

band parameters

νOH deform 1

νOH deform 2


937 28.6 19.8 937 25.8 19.7 937 19.2 17.6

914 71.4 20.0 914 74.2 19.3 914 80.8 19.0 913 100 16.5 913 100 12.9 not detected not detected not detected

νwater deform 1

νwater deform 2 1630 weak

1677 34.4 40.9 1675 44.6 49.0 1676 40.3 50.6 1676 43.8 51.8 1676 48.5 56.6

1613 65.6 44.7 1616 55.3 45.3 1615 59.7 53.5 1616 56.2 52.0 1615 51.5 49.2

Figure 6. Band component analysis of the water deformation mode for the kaolinite/quartz mixture after grinding for 2 h.

Figure 5. Infrared reflectance spectra of the hydroxyl deformation mode of kaolinite/quartz mixture mechanically ground for (a) 0, (b) 0.5, (c) 1, (d) 2, (e) 3, and (f) 4 h.

as it means that the grinding process results in the loss of the inner-surface hydroxyls before the inner hydroxyls. The grinding process destroys the hydrogen bonding between the adjacent kaolinite layers, as is observed from the loss of the hydroxyl vibrations. At the same time an increase in intensity of bands ascribed to the hydroxyl stretching vibrations of water is observed (Figure 3). The band profile in the 3200-3600 cm-1 range is very broad and as such is difficult to analyze by band component analysis. Nevertheless, in the early stages of grinding two bands are observed at around 3200 and 3576 cm-1 (Table 1). After 2 h of grinding bands are observed at around 3200 and 3420 cm-1. These bands are attributed to the hydroxyl stretching frequencies of water. Also, after 2 h of grinding a third band is observed at around 3585 cm-1.

The band at around 3200 cm-1 is normally associated with water, which is coordinated to a metal ion. Bands at around 3570 cm-1 are associated with bound water and bands around 3585 cm-1 with interlamellar water.31-34 The relative intensity of the interlamellar water is reasonably constant after 2 h of grinding. It is considered that this type of water molecule sits at the edges of the destroyed kaolinite layers. The amount of adsorbed water increases from time zero to 2 h and then remains constant. The grinding process creates a high surface area material, which provides a support for the adsorbed water. The results of the band component analysis for water suggests that the maximum surface area is achieved at or before 2 h of grinding and that further grinding past 2 h makes little difference. It is proposed that the band at around 3200 cm-1 is coordinated water, which is bonded to the aluminum. Such a concept requires further proof. What the proposal means is that the hydroxyls on the gibbsitelike surface of the kaolinite have been replaced with water coordinated to the surface. Mechanochemical treatment causes intense local heating, and sufficient energy is supplied to break the hydroxyl bonds. Three low-intensity bands are observed at 3738, 3742, and 3748 cm-1. Such (30) Frost, R. L.; Kristof, J.; Paroz, G. N.; Tran, T. H.; Kloprogge, J. T. J. Colloid Interface Sci. 1998, 204, 227. (31) Frost, R. L.; Kristof, J.; Paroz, G. N.; Kloprogge, J. T. J. Colloid Interface Sci. 1998, 204, 216. (32) Frost, R. L.; Kristof, J.; Paroz, G. N.; Kloprogge, J. T. J. Colloid Interface Sci. 1998, 204, 478. (33) Frost, R. L.; Kristof, J.; Horvath, E.; Kloprogge, J. T. J. Colloid Interface Sci. 1999, 214, 109. (34) Frost, R. L.; Kristof, J.; Horvath, E.; Kloprogge, J. T. J. Colloid Interface Sci. 1999, 214, 380.

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Table 3. Results of the Band Component Analysis of DRIFT Spectra of the SiO Region of the Kaolinites as a Function of Grinding Time kaolinite/quartz kaolinite at 25 °C kaolinite ground for 0.5 h kaolinite ground for 1.0 h kaolinite ground for 2.0 h kaolinite ground for 3.0 h kaolinite ground for 4.0 h

band parameters

νsilica SiO 1

νsilica SiO 2

νnew surface SiO 3

νsilica SiO 4

νkaolinite SiO 5

νkaolinite SiO 6

νkaolinite SiO 7


1196 9.8 40.2 1194 10.0 38.8 1194 10.0 39.3 1192 14.5 42.7 1192 12.8 44.2 1192 14.1 45.8

1159 20.7 55.4 1161 18.6 48.0 1164 11.0 40.8 1163 9.7 35.2 1164 4.2 24.9 1162 3.0 20.4

1113 2.8 16.5 1119 16.0 68.9 1129 23.8 67.1 1130 22.0 54.7 1131 32.9 75.0 1134 34.0 80.7

1103 29.0 57.1 1100 20.5 60.0 1093 22.6 54.6 1088 24.1 56.7 1089 24.5 61.0 1088 17.5 60.8

1056 10.9 27.0 1056 9.7 27.5 1057 8.2 24.2 1058 3.0 17.7 1059 1.9 15.1 1059 1.5 14.2

1034 15.8 28.0 1035 15.0 29.3 1037 14.3 30.5 1042 9.5 30.9 1045 5.7 31.3 1047 2.4 35.6

1007 11.0 22.0 1008 10.1 23.9 1009 10.0 28.9 1013 16.9 60.3 1016 17.9 79.2 1011 21.3 82.7

Table 4. Results of the Band Component Analysis of DRIFT Spectra of the Low-Frequency Region of Kaolinites as a Function of Grinding Time kaolinite/quartz kaolinite at 25 °C kaolinite ground for 0.5 h kaolinite ground for 1.0 h kaolinite ground for 2.0 h kaolinite ground for 3.0 h kaolinite ground for 4.0 h

band parameters center/cm-1

band %area bandwidth/cm-1 band center/cm-1 %area bandwidth/cm-1 band center/cm-1 %area bandwidth/cm-1 band center/cm-1 %area bandwidth/cm-1 band center/cm-1 %area bandwidth/cm-1 band center/cm-1 %area bandwidth/cm-1

ν1 799 9.7 15.4 799 11.2 15.4 799 10.2 15.0 800 12.5 14.3 800 10.9 13.2 800 13.4 14.1

ν2

ν3

ν4

ν5

ν6 (new surface)

ν7

792 0.95 11.8 794 1.2 12.0 795 2.5 12.7

780 13.6 20.5 779 8.4 16.4 779 8.4 15.4 779 11.0 15.0 780 10.0 14.2 779 13.0 15.4

696 6.5 14.9 695 6.0 15.0 695 5.3 13.7 695 5.6 11.5 695 4.5 10.5 695 6.7 11.2

537 32.8 45.0 537 35.8 50.0 534 32.6 51.4 542 15.6 66.4

520 3.3 24.6 516 2.0 15.7 515 2.3 17.4 518 17.4 28.9 519 20.0 26.0 519 16.9 25.4

470 32.0 34.5 471 30.5 36.5 471 38.9 38.0 483 18.7 25.4 481 23.7 32.3 482 25.0 32.9

bands are attributable to the hydroxyl stretching vibrations of SiOH, confirming the effects of intense local heating on the kaolinite surfaces by the breaking of the SiO bonds and the replacement with SiOH. Confirmation of the water hydroxyl stretching vibrations may be obtained through the analysis of the water deformation modes normally observed at around 1630 cm-1. Figure 6 displays the band component analysis of the water deformation region for the material ground for 2 h. Two bands are observed at 1676 and 1615 cm-1 (Table 2). The first band corresponds with the 3200 cm-1 stretching vibration of coordinated water and the second with the water OH stretching vibration at 3576 cm-1. The ratio of the relative intensities of the two bands remains constant independent of the grinding time. It is possible that the adsorbed water is bonded to the coordinated water and that this adsorbed water is required to maintain the stability of the water coordinated to the aluminum surface. Controlled rate thermal analysis experiments support this concept.35 The observation of the two water deformation vibrations supports the concept of two types of water molecules: (a) adsorbed water and (b) coordinated water. Two regions are identified for silicon-oxygen vibrations: (a) the 980-1300 cm-1 region ascribed to SiO stretching vibrations and (b) the low-wavenumber vibra(35) Kristof, J.; Horvath E.; Frost, R. L.; Kloprogge, J. T. J. Therm. Anal. Calorim. 2001, 63, 279.

ν8

ν9

465 7.4 22.6 467 20.3 28.6 462 13.3 21.4

431 2.0 15.4 431 4.7 19.6 432 1.5 19.1 444 10.8 33.0 445 8.7 33.7 446 9.0 25.9

tions between 400 and 850 cm-1 ascribed to lattice vibrations. Figure 7 displays the SiO stretching region for the kaolinite, the quartz, and the products of the mechanochemical treatment. The results of the band component analysis of the two regions are reported in Tables 3 and 4. The bands at 1056 and 1034 cm-1 are attributed to the SiO stretching vibrations of the siloxane layer of the kaolinite. The bands observed at 1196, 1159, and 1103 cm-1 are associated with the SiO stretching vibrations of quartz. The variation of relative intensity of these vibrations is shown in Figure 8. The relative intensities remain constant upon grinding. Figure 7 clearly shows the decrease in intensity of these bands as a function of grinding time. What this figure also shows is the increase in intensity in the spectral profile at 1113 cm-1, which appears upon the mechanochemical treatment. It is proposed that this band is associated with the new surface phase produced upon grinding. Figure 9 displays the spectra of the low-wavenumber region of the mechanochemically treated kaolinite/quartz mixtures. The bands observed at 799, 780, and 696 cm-1 are vibrations associated with the sheet-lattice structure of the kaolinite. Figure 10 displays the relative intensities of the low-wavenumber vibrations as a function of grinding time. The major changes in the low-wavenumber region are associated with the bending vibrations of the OSiO units. Thus, the band at 431 cm-1 diminishes in intensity


Figure 7. Infrared reflectance spectra of the SiO stretching region of kaolinite/quartz mixture mechanically ground for (a) 0, (b) 0.5, (c) 1, (d) 2, (e) 3, and (f) 4 h.

Figure 8. Variation of the relative intensity of the SiO stretching modes of the kaolinite/quartz mixture as a function of grinding time.

Langmuir, Vol. 17, No. 16, 2001 4737

Figure 9. Infrared reflectance spectra of the low-frequency region of kaolinite/quartz mixture mechanically ground for (a) 0, (b) 0.5, (c) 1, (d) 2, (e) 3, and (f) 4 h.

Figure 10. Variation of the relative intensity of the lowfrequency modes of the kaolinite/quartz mixture as a function of grinding time.

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with grinding time as does the band at 537 cm-1. Significant decrease in intensity occurs in this band. In fact, after 2 h of grinding no intensity remains in the 537 cm-1 band. This band is assigned to the OSiO bending vibration of the hexagonal ring structure of the siloxane layer. Concomitantly a significant increase in relative intensity occurs in the 520 cm-1 band. This band is assigned to the OSiO vibrations of a tetrahedral Si. Thus, this suggests that some opening of the ditrigonal units of the siloxane layer occurs. Conclusions Kaolinite/quartz mixtures were ground for periods of time up to 4 h, and a new poorly diffracting silicate phase was produced. The kaolinite surface hydroxyls were lost after 2 h of grinding and were replaced with water both coordinated and adsorbed on the surface of the aluminumcontaining layer. Upon mechanochemical treatment of the kaolinite with quartz, significant structural alteration occurred rapidly to form a new material with a significantly modified kaolinite surface of reduced crystal size with a somewhat higher surface area. Of the techniques used to characterize the new mechanochemically synthesized material, only infrared spectroscopy and thermal analysis proved worthwhile. Thermal analysis appears to show the loss of hydroxyls from the kaolinite surface and the replacement with water.

Frost et al.

Infrared spectroscopy shows that the hydroxyl stretching vibration intensity is lost after 2 h of grinding. This suggests the optimal time for the production of the delaminated kaolinite. The appearance of water bands upon grinding supports the concept of the replacement of the kaolinite hydroxyl units with water. Such observations are in harmony with the conclusions drawn from thermal analysis. The study of the hydroxyl deformation mode indicates the inner-surface hydroxyls are lost before the inner hydroxyls. The observation of new SiO stretching and bending vibrations at 1113 and 520 cm-1 suggests that the new material has a molecular structure different from that of kaolinite. The concomitant decrease in the SiO stretching and bending modes of kaolinite support the concept of the synthesis of a new material which has a very different surface structure from that of the untreated kaolinite. Acknowledgment. Financial support from the Hungarian Scientific Research Fund under Grants OTKA T25171 and F023531 are also acknowledged. The financial and infrastructural support of the Queensland University of Technology, Centre for Instrumental and Developmental Chemistry, is gratefully acknowledged. Mr. Tama´s Szila´gyi is thanked for carrying out the thermoanalytical measurements. LA001453K

Modification of Kaolinite Surfaces by Mechanochemical Treatment

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