Langmuir 2001, 17, 4067-4073
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Modification of the Hydroxyl Surface in Cesium Acetate Intercalated Kaolinite Ray L. Frost,*,† Ja´nos Kristof,‡ J. Theo Kloprogge,† and Erzse´bet Horvath§ Centre for Instrumental and Developmental Chemistry, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane Queensland 4001, Australia, Department of Analytical Chemistry, University of Veszprem, H8201 Veszprem, PO Box 158, Hungary, and Research Group for Analytical Chemistry, Hungarian Academy of Sciences, H 8201 Veszpre´ m, PO Box 158, Hungary Received August 3, 2000. In Final Form: November 2, 2000 Changes in the hydroxyl surfaces of cesium acetate intercalated kaolinite have been studied over the ambient to predehydroxylation temperature range using a combination of FTIR and Raman spectroscopy, combined with X-ray diffraction. Upon intercalation of a low-defect kaolinite with cesium acetate, the kaolinite layers expanded to 14.0 Å. Upon heating the intercalate to 50 °C, the kaolinite expands to 17.0 Å. Over the temperature range 100-200 °C, a third phase with a d(001) spacing of 12.80 Å was observed. These expansions are reversible, and upon cooling the intercalation complex and upon exposure to air for sufficient lengths of time, the d(001) spacing returned to 14.0 Å. These expansions are in harmony with thermal decomposition measurements. Diffuse reflectance spectroscopy shows that the cesium acetate intercalated kaolinite is almost completely intercalated and that the thermal treatment of the intercalate is reversible. The Raman spectrum of the hydroxyl stretching region of the intercalated kaolinite showed a new band at 3606 cm-1, which was attributed to the inner surface hydroxyl hydrogen bonded to the acetate ion. Mild heating of the intercalated complex to 50 °C caused a rearrangement of the surface structure with a Raman band being observed at 3610 cm-1. It is proposed that the 3610 cm-1 band is associated with the 17.0 Å phase and that the 3606 cm-1 band is associated with the 14.0 Å phase. Further thermal treatment over the 100-200 °C temperature ranges resulted in two hydroxyl bands at 3618 and 3609 cm-1. The 3618 cm-1 band is attributed to the inner hydroxyl. At the predehydroxylation temperature for cesium acetate intercalated kaolinite (∼300 °C), two bands were observed at 3609 and 3619 cm-1. Above this temperature, no hydroxyls are spectroscopically evident. Upon cooling to room temperature, the Raman spectra of the hydroxyl surfaces are identical to that of the initial intercalation complex, showing that the thermal modification of the kaolinite surfaces is reversible. The thermal treatment results in some minor deintercalation.
Introduction Kaolinite contains two types of hydroxyl groups: hydroxyl groups at the internal surface, OHou, often referred to as inner surface hydroxyl groups and hydroxyl groups, OHin, often referred to as inner hydroxyl groups, within the layer. The OHou groups are situated in the outer, unshared plane, whereas the OHin groups are located in the inner shared plane of the octahedral sheet, often termed the gibbsite-like sheet.1-2 The OHou groups are located on the outside of a single kaolinite layer.2 However, the kaolinite crystal consists of a stacking of many of these layers, which means that actually the OHou groups are located in the space between two adjacent layers, and hence the term inner surface hydroxyl is normally used in the literature. The OHin hydroxyl is within the unit cell and not exposed at the surface of the kaolinite; consequently, it is difficult to modify the kaolinite surfaces by interaction with this hydroxyl group. The OHou hydroxyl groups are at the gibbsite-like surface of the kaolinite and are readily modified. When the kaolinite surface is modified through interaction with any number of bonding molecules, changes * To whom correspondence should be addressed. E-mail: r.frost@ qut.edu.au. † Queensland University of Technology. ‡ University of Veszprem. § Hungarian Academy of Sciences. (1) Brindley, G. W.; Chih-Chun, K.; Harrison, J. L.; Lipsiscas, M.; Raythatha, R. Clays Clay Miner. 1986, 34, 233. (2) Frost, R. L.; Kristof, J.; Horvath, E.; Kloprogge, J. T. Langmuir 1999, 15, 8787.
are reflected in the vibrational spectrum, either in the infrared or Raman spectrum or both.2-3 The hydrogens of the inner hydroxyl groups are bound to the oxygen below the aluminum atoms and directed toward the intralayer cavity of the kaolinite. Any modification of kaolinite surfaces is unlikely to alter the inner hydroxyl group, although such a group may be influenced by the interaction of molecules such as hydrazine or cations such as potassium, which may fit into the ditrigonal cavity. The OHin hydroxyls essentially lie parallel to the 001 plane. This hydroxyl points toward the vacant dioctahedral site in the kaolinite structure.3-7 The inner surface hydroxyl groups (OHou) point toward the adjacent siloxane layer. Hydrogen bonds are formed between the gibbsite-like layer and the adjacent siloxane layer. It is these hydrogen bonds which hold many kaolinite layers in a crystal. Upon modification of the surfaces of kaolinite, this interlayer hydrogen bonding is broken and is normally replaced with hydrogen bonding to the reacting molecule.8 Such alter(3) Frost, R. L.; Kristof, J.; Tran, T. H. T.; Kloprogge, J. T. Am. Mineral. 1998, 83, 1182. (4) Frost, R. L.; Kristof, J.; Paroz, G.; Kloprogge, J. T. J. Colloid Interface Sci. 1998, 208, 478. (5) Brindley, G. W. Order-disorder in clay mineral structures. In Crystal Structures of Clay Minerals and their X-ray Identification; Brindley, G. W., Brown, G., Eds.; Mineralogical Society Monograph No. 5; Mineralogical Society: London, 1984; Chapter 2, pp 125-189. (6) Giese, R. F. Bull. Mineral. 1982, 105, 417. (7) Giese, R. F. Kaolin minerals: structures and stabilities. In Reviews in Mineralogy Vol. 19: Hydrous Phyllosilicates; Bailey, S. W., Ed.; Mineralogical Society of America BookCrafters Inc.: Chelsea, MI, 1988; Chapter 3.
10.1021/la001114r CCC: $20.00 © 2001 American Chemical Society Published on Web 05/26/2001
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ation in the hydrogen bonding is reflected in the changes in the vibrational spectrum of the kaolinite. The application of infrared spectroscopy to the study of the changes in the kaolinite structure has proved worthwhile.9 Changes in the intensities of the infrared bands in the low-frequency region for kaolinite also occurred upon intercalation. In particular, the 935 cm-1 band changed in intensity upon intercalation and provided evidence for the acetate ion being hydrogen bonded to the inner surface hydroxyl group. The application of Raman microscopy to the study of intercalated kaolinites has also proved most useful.10-13 An additional Raman band, attributed to inner surface hydroxyl groups strongly hydrogen bound to the acetate, was observed at 3605 cm-1 for the potassium acetate intercalate with the concomitant loss of intensity in the OHou bands at 3652, 3670, 3684, and 3693 cm-1. Intercalation of a high-defect kaolinite resulted in a Raman spectrum similar to that of an intercalated ordered kaolinite. Thus, the intercalated highdefect kaolinite resembled the intercalated low-defect kaolinite, at least on a molecular level.11,12 Here, the conclusion was made that the intercalation process resulted in changes in the kaolinite surfaces such that a decrease in the defect structures of the kaolinite occurred. In a previous study by the authors, the effect of pressure on the intercalation of both an ordered and disordered kaolinite with potassium acetate induced disordering as evidenced by both X-ray diffraction and Raman spectroscopy.13 Intercalation of the ordered kaolinite with potassium acetate (KCH3COO) under a pressure of 20 bar and 220 °C induced new Raman bands at 3590, 3603, and 3609 cm-1 in addition to the normal kaolinite bands. These bands were attributed to the inner surface hydroxyls hydrogen bonded to the acetate. It was proposed that the intercalation under 20 bar pressure at 220 °C caused the differentiation of the inner surface hydroxyl groups, resulting in the appearance of these additional bands. Diffuse reflectance infrared spectra of the potassium acetate intercalated kaolinite pressure formed at 20 bar and at 220 °C showed new bands at 3595 and 3605 cm-1. Upon formation of the intercalate at 2 bar and at 120 °C, additional infrared bands were found at 3592, 3600, and 3606 cm-1. These infrared bands correspond well with the observed Raman spectra. Kaolinite hydroxyl surfaces have been modified upon intercalation with cesium acetate under a range of conditions.14,15 Upon intercalation of both low- and high-defect kaolinites with cesium acetate at 25 °C, additional infrared hydroxyl stretching bands are observed at 3606 and 3603 cm-1.15 Infrared hydroxyl deformation modes were observed at 898 and 910 cm-1 upon intercalation with cesium acetate. These changes are attributed to the hydrogen bonding of the acetate ion to the inner surface hydroxyls. It was shown that the intercalation process involves the incorporation of water into the interlayer structure.4 It was proposed that the effect of intercalation of the highly ordered kaolinite under pressure caused the kaolinite to become disordered and this disordering was dependent on the temperature of intercalation. It was suggested that when pressure is applied to the kaolinite (8) Hess, C. A.; Saunders, V. R. J. Phys. Chem. 1992, 96, 4367. (9) Constanzo, P. M.; Giese, R. F. Clays Clay Miner. 1990, 38, 160. (10) Barrios, J.; Planc¸ on, A.; Cruz, M. I.; Tchoubar, C. Clays Clay Miner. 1977, 25, 422. (11) Frost, R. L.; Tran, T. H. T.; Kristof, J. Clay Miner. 1997, 32, 587. (12) Frost, R. L.; Kristof, J. Clays Clay Miner. 1997, 45, 68. (13) Frost, R. L.; Van Der Gaast, S. J. Clay Miner. 1997, 32, 293. (14) Frost, R. L.; Kristof, J.; Paroz, G.; Kloprogge, J. T. J. Colloid Interface Sci. 1998, 204, 478. (15) Frost, R. L. Clays Clay Miner. 1998, 46, 280.
Frost et al.
crystal in the presence of an intercalating agent, the hydrogen bonds between adjacent layers are broken in order to create space for the intercalating agent between the layers. A direct result is that the order of the kaolinite crystals shows a decrease resulting in more defect structures. The additional bands in both the Raman and infrared spectra evidence this. Changes in both the hydroxyl stretching and deformation regions indicate that water is being incorporated into the kaolinite structure at the elevated temperatures of 120 and 220 °C. Raman spectra of the 25 °C cesium acetate intercalated kaolinites show additional bands at 3606 and 3598 cm-1. These bands are attributed to the inner surface hydroxyls hydrogen bonded to the acetate ions. This paper reports a continuation of our studies of the effect of temperature on the intercalation of cesium acetate into kaolinite. Experimental Methods The Cesium Acetate Intercalated Kaolinite. The kaolinite used in this study is the Kiralyhegy kaolinite from Hungary. Samples were analyzed for phase purity using X-ray diffraction techniques before Raman microprobe spectroscopic analysis. The kaolinite was used purified by sedimentation and size fractioned to
