Self-assembly of alkylammonium chains on montmorillonite: Effect of chain length, head group structure, and cation exchange capacity
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 effects cause up to 45% gauche conformations as compared to only 5% gauche conformations in chains with crystalline order. A low CEC leads to stepwise increases of the basal plane spacing with increasing chain length, corresponding to the subsequent formation of alkyl monolayers, bilayers, trilayers, etc., while a high CEC leads to a continuous increase in basal plane spacing with increasing chain length. The density of the organic interlayer space between two silicate layers shows minima for emerging new layers of alkyl chains and maxima for densely packed layers. These fluctuations decrease as the number of layers increases. On an isolated clay mineral surface, layer formation of the surfactants is also found and lateral 2D diffusion constants range from 5 × 10-6 cm2/s to <10-9 cm2/s depending on the structure of the surfactant.