Self-Assembly of Alkylammonium Chains on Montmorillonite: Effect of Chain Length, Head Group Structure, and Cation Exchange Capacity

Abstract

The structure and dynamics of alkylammonium-modified montmorillonites with different cation exchange capacity (CEC), ammonium head groups, and chain length is investigated by molecular dynamics simulation for a large set of structures and compared to a wide array of experimental data. In the 44 systems, the relationship between computational (molecular dynamics) and experimental data (X-ray, IR, NMR, and DSC) was found to be very complementary. Much of the properties appear to be dictated through the inorganic−organic interface, which in addition to electrostatic interactions involves hydrogen bonds between primary ammonium head groups (NH3+−R) and oxygen on the silicate surface (O···H distance ∼150 pm) or flexibly attached quaternary ammonium head groups (NMe3+−R) of higher lateral mobility (O···H distance ∼290 pm). The gauche content is a function of the packing density of the head groups on the surface, the preferred orientation of the head groups, and the interlayer density of the alkyl chains. These ...