Hydroxyl deformation in kaolins

The hydroxyl deformation modes of kaolins have been studied by Fourier transform (FT) Raman spectroscopy. Kaolinites showed well-resolved bands at 959, 938 and 915 cm(-1) and an additional band at 923 cm(-1). For dickites, well-resolved bands were observed at 955, 936.5, 915 and 903 cm(-1) Halloysites showed less-resolved Raman bands at 950, 938, 923, 913 and 895 cm(-1). The first 3 bands were assigned to the librational modes of the 3 inner-surface hydroxyl groups, and the 915-cm(-1) band was assigned to the libration of the inner hydroxyl group. The band in the 905 to 895 cm(-1) range was attributed to "free" or non-hydrogen-bonded inner-surface hydroxyl groups. The 915-cm(-1) band contributed similar to 65% of the total spectral profile and was a sharp band with a bandwidth of 11.8 cm(-1) for dickite, 14.0 cm(-1) for kaolinites and 17.6 cm(-1) for halloysites. Such small bandwidths suggest that the rotation of the inner hydroxyl group is severely restricted. For the inner-surface hydroxyl groups, it is proposed that the hydroxyl deformation modes are not coupled and that the 3 inner-surface deformation modes are attributable to the three OH2-4 hydroxyls of the kaolinite structure. For intercalates of kaolinite and halloysite with urea, a new intense band at similar to 903 cm(-1) was observed with concomitant loss in intensity of the bands at 959, 938 and 923 cm(-1) bands. This band was assigned to the non-hydrogen-bonded hydroxyl libration of the kaolinite-urea intercalate. Infrared reflectance (IR) spectroscopy confirms these band assignments.