5402
Langmuir 2000, 16, 5402-5408
Rehydration of Potassium Acetate-Intercalated Kaolinite at 298 K Ray L. Frost,*,† Janos Kristof,‡ J. Theo Kloprogge,† and E. 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, H8201 Veszprem, PO Box 158, Hungary Received November 30, 1999. In Final Form: March 3, 2000 The rehydration of potassium acetate-intercalated kaolinite has been followed using a combination of X-ray diffraction and Raman microscopy. Dehydration of the fully expanded potassium acetate-intercalated kaolinite with initial d(001) spacing of 13.88 Å, in an atmosphere of nitrogen, shows the presence of three expanded kaolinite phases with d(001) spacings of 11.47, 9.6, and 9.2 Å. X-ray diffraction shows the existence of six expanded phases after one minute of rehydration with d spacings of 14.13, 11.56, 11.04, 9.88, 8.90, and 8.55 Å. Rehydration is rapid with the intercalation complex rehydrating in less than 21 min. Raman spectroscopy shows hydroxyl stretching bands at 3632 cm-1 assigned to the inner hydroxyl and at 3601 cm-1 attributed to the inner surface hydroxyl hydrogen bonded to the acetate ion when the intercalation complex is heated to 300 °C under an atmosphere of nitrogen. The position of the inner hydroxyl band is observed at 3630 cm-1 in the 298 K spectra, providing the intercalated kaolinite is not exposed to air. Phase changes of the intercalation complex are determined using the changes in intensity of the inner surface hydroxyl stretching bands. Phase changes are also observed through changes in intensity of the CdO, C-C, and OCO Raman modes.
Introduction Kaolinite (Al2(OH)4[Si2O5]) is a naturally occurring inorganic polymer with a layer structure composed of siloxane and gibbsite-like layers. The siloxane layer is composed of SiO4 tetrahedra linked in a hexagonal array. The bases of the tetrahedra are coplanar, and the apical oxygen atoms are linked to a second layer (the gibbsitelike layer) containing aluminum ions and OH groups. The unit cell of kaolinite shows the existence of four hydroxyl groups. One hydroxyl group (OH1) lies in the ab plane and the other three (OH2-OH4) termed the inner surface hydroxyl group lie at angles between 65 and 73° to the ab plane.1,2 The inner hydroxyl (OH1) points toward the apical oxygen, which is the bridging atom between the siloxane and gibbsite-like surfaces. The inner surface hydroxyls point away from the surface and hydrogen bond to the oxygens of the next adjacent siloxane layer. These hydroxyls in terms of spectroscopy function as in-phase and out-of phase vibrations. Four distinct bands are observed in the infrared spectrum of kaolinite at 3697, 3669, 3652, and 3620 cm-1.3,4 The three higher frequency bands (designated ν1, ν2, and ν3) are assigned to OH stretching modes of the three inner surface hydroxyl groups (OuOH).1 The band at 3620 cm-1 is designated ν5 and is assigned to the stretching mode of an inner hydroxyl group (InOH).5,6 The Raman spectrum in the OH stretching region of kaolinite contains five features.1,6 In addition * To whom correspondence should be addressed. † Queensland University of Technology. ‡ University of Veszprem. § Hungarian Academy of Sciences. (1) Frost R. L.; van der Gaast S. J. Clay Miner. 1997, 32, 471. (2) Johnston, C. T.; Agnew, S. F.; Bish, D. L. Clays Clay Miner. 1990, 38, 573. (3) Farmer, V. C. The Layer Silicates. In The Infrared Spectra of Minerals; Farmer, V. C., Ed.; Mineralogical Society: London, 1974; p 331. (4) Farmer, V. C. Clay Miner. 1998, 33, 601.
to the four frequencies noted above, an infrared inactive feature is observed at 3685 cm-1. This is designated ν4 and is observed as a component of an unresolved transverse/longitudinal optic doublet at 3695/3685 cm-1. The relative intensities of the components of this doublet depend on the orientation of the kaolinite crystal and the aspect ratio, the thickness-to-width ratio of the crystal.1,4 The 3669 and 3652 cm-1 bands (ν2 and ν3) are the out-of phase vibrations and are weak, while the 3620 cm-1 band (ν5) is strong and sharp. The intensity of the OH stretching bands depends on the defect structure of the kaolinite.1,7 In highly ordered kaolinites, ν4 has been detected in the infrared spectrum at 3684 cm-1 when band resolution was carried out.1,6 Intercalation is a process of insertion of molecules between the kaolinite layers and involves the breaking of hydrogen bonds between the kaolinite layers and the formation of new hydrogen bonds with the inserting molecule. These new bonds usually involve the inner surface hydroxyl groups (OuOH), and changes are observed in the intensities of bands assigned to vibrations of these groups. This is observed for intercalated molecules such as potassium acetate, where hydrogen bonding occurs between the carboxylate group of the acetate anion and the inner surface hydroxyls of the gibbsite-like layer. In this case a decrease in the intensity of the corresponding OH stretching bands is observed and additional bands due to new hydrogen-bonded OH groups are observed between 3595 and 3605 cm-1.7-11 To date, no Raman (5) Johansson, U.; Frost, R. L.; Forsling, W.; Kloprogge, J. T. Appl. Spectrosc. 1998, 52, 1277. (6) Shoval, S.; Yariv, S.; Michaelian, K. H.; Lapides, I.; Boudeuille, M.; Panczer, G. J. Colloid Interface Sci. 1999, 212, 523. (7) Frost, R. L.; Kristof, J.; Kloprogge, J. T.; Tran, T. H. Am. Mineral. 1998, 83, 1182. (8) Frost, R. L.; Kristof, J.; Paroz, G. N.; Kloprogge, J. T. J. Colloid Interface Sci. 1998, 208, 478. (9) Frost, R. L.; Kristof, J. Clays Clay Miner. 1997, 45, 551. (10) Frost, R. L.; Tran, T. H.; Kristof, J. Clay Miner. 1997, 32, 587.
10.1021/la9915522 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/19/2000
Potassium Acetate-Intercalated Kaolinite
Langmuir, Vol. 16, No. 12, 2000 5403
experiments have been undertaken to determine the changes in the spectra of the hydroxyl-stretching region of potassium acetate-intercalated kaolinites upon dehydration and rehydration. Further no comprehensive studies of the changes of the inserting acetate ion have been forthcoming. The objective of this research, therefore, is to determine the changes in the Raman spectra of both the hydroxyl-stretching region of potassium acetateintercalated kaolinite and the inserting acetate ion from ambient to pre-dehydroxylation temperatures. Experimental Methods Potassium 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
