Hydrazine is one of the most commonly used entraining agents to penetrate kaolinite, yet the mechanism of intercalation of kaolinite by hydrazine is still in debate. The objectives of this study are to investigate the possible molecular configurations and orientations of hydrazine in the interlayer of kaolinite and the configuration changes induced by water molecules. Water molecules increased the intercalation rate and caused the expansion of the intercalation complex from 0.96 to 1.03 nm. The kinetic effect was likely the result of breaking the self-associations of hydrazine molecules and releasing more "free" hydrazine molecules for the intercalation. H-bonding caused large red shifts of the inner surface OH stretching bands from 3695 to 3626 cm(-1) in the 0.96-nm kaolinite hydrazine intercalation (KHI) complex and to 3570 and 3463 cm-1 in the 1.03-nm KHI complex. The N-H stretching bands of the hydrazine molecules in the KHI complexes became sharper and blue-shifted more than 20 cm-1 compared with the free liquids. The symmetric NH vibrations at 3365 and 3310 cm-1, and the N-N vibration at 1092 cm(-1) became infrared inactive in the 0.96-nm KHI complex. The frequency of the Si-O bands of the kaolinite in the 1.03-nm KHI complex was slightly lower than in the 0.96-nm KHI complex (5 cm(-1) shift). These IR band changes implied that hydrazine molecules have different configurations in the complexes: hydrazine molecules had an eclipsed form in the interlayer of the 0.96-nm KHl complex. The eclipsed configuration has a dipole moment of 3.31 D, which is higher than the gauche form (1.83-1.90 D). The molecule was oriented with the N-N bond parallel or nearly parallel to the (001) surface of the mineral and the four H atoms of each hydrazine molecule reacted with the basal siloxane surface. When a suitable amount of water was present, it promoted the configuration change of the hydrazine molecules from the eclipsed form to the common gauche form. This gauche form was stabilized by transforming to a more polarized NH3NH tautomer structure (5.4 D). To promote an optimal interaction between hydrazine and the mineral surface, the N-N bond of the hydrazine was tilted about 30degrees from the (001) plane and caused the intercalation complex to expand from 0.96 to 1.03 nm. The eclipsed form and the tautorner were stabilized by the asymmetric interlayer environment of kaolinite. The two proposed models and reaction mechanisms match the high dipole moment requirement as found for other entraining agents. Further investigation is needed to confirm the exact configuration of hydrazine molecules and whether or not the tautomer exists. (C) 2003 Elsevier Science (USA). All rights reserved.