Surface Structure of Organoclays

The properties of organically modified clay minerals determine essentially the quality of polymer-clay nanocomposites. We investigate alkylammonium-micas with different alkyl chain length by molecular dynamics simulation as homogeneous mixtures and separated island structures on the mica surface (80 percent alkali exchange, 20 percent of the alkali ions remain), including a detailed model of the Si --> Al...K charge defects on the surface of phyllosilicates. By comparison with experiment, we find that long chains (C18) lead to a mixed phase of alkali ions and organic ions on the surface, while chains of a medium length (~C12) give rise to phase separation. Very short chains, by thermodynamical arguments, prefer again a homogeneously mixed surface. We explain this interesting observation with a basic free energy model and present a diagram of the phase behaviour as a function of surface saturation with alkyl chains and the chain length. Besides, an order-disorder transition of the tethered alkyl chains is found on heating when ~20 percent of the torsional angles along the hydrocarbon backbones are gauche. At higher temperature, also rearrangements of the ammonium headgroups on the surface are possible, which lead to metastable structures after quenching.