Conformation heterogeneity of alkylammonium surfactants self-assembled on montmorillonite: Effect of head-group structure and temperature

Abstract

The understanding of the organic cations arrangement on the surface of clay mineral is an important step toward optimizing the functionality of organoclays for industrial applications. In this study the effect of alkyl chain length, head-group structure (N(CH3)3, vsNH3), and temperature on the conformation of the cationic surfactants in a organo-montmorillonite (O-Mt) interlayers was systematically studied using XRD, near-IR and 13C NMR spectroscopy. Decreasing amount of the disordered gauche conformers, replaced by the ordered all-trans conformers, was observed with increasing alkyl chain length. A higher population of gauche conformers for cations with NH3 than for N(CH3)3 head-group resulted from a different mode of head-group binding to basal oxygen atoms. On increasing the temperature up to 205 °C the alkyl chains adopted a more disordered, liquid-like arrangement. In contrast, the surfactants in O-Mt cooled to −196 °C adjusted more ordered conformation, characteristic of a crystalline-like state.