  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.
