  In a previous work [H. Heinz, H. J. Castelijns, U. W. Suter, J. Am. Chem.
Soc. 115, 9500 (2003)], we developed an accurate force field and simulated the
phase transitions in C18-mica (octadecyltrimethylammonium-mica) as well as the
absence of such transitions in 2C18-mica (dioctadecyldimethylammonium-mica)
between room temperature and 100 deg C. Here we analyze (i) average z
coordinates of the carbon atoms and interdigitation of the hydrocarbon
bilayers, (ii) density profiles and (iii) pressure profiles of the structures
along all Cartesian axes. In C18-mica, the standard deviation in the z
coordinate for the chain atoms is high and more than doubles in the disordered
phase. The order-disorder transition is accompanied by a change in the
orientation of the ammonium headgroup, as well as decreasing tensile and shear
stress in the disordered phase. In 2C18-mica, the standard deviation in the z
coordinate for the chain atoms is low and does not increase remarkably on
heating. The backbones display a highly regular structure, which is slightly
obscured by rotations in the C18 backbones and minor headgroup displacements at
100 deg C. Close contacts between the bulky headgroups with sidearms cause
significant local pressure which is in part not relieved at 100 deg C. An
increase of the basal-plane spacing at higher temperature is found in both
systems due to larger separation between the two hydrocarbon layers and an
increased z spacing between adjacent chain atoms (=decreased tilt of the chains
relative to the surface normal), and, in C18-mica only, a stronger upward
orientation of the C18 chain at the ammonium headgroup. The likelihood for
chain interdigitation between the two hydrocarbon layers is 0.24 to 0.30 for
C18-mica,and 0.65 to 0.26 for 2C18-mica (for 20 deg C to 100 deg C).
