Analysis of the phase transitions in alkyl-mica by density and pressure profiles

In a previous work [Heinz, Castelijns, and Suter, J. Am. Chem. Soc. 115, 9500 (2003)], we developed an accurate force field and simulated the phase transitions in C-18-mica (octadecyltrimethylammonium-mica) as well as the absence of such transitions in 2C(18)-mica (dioctadecyldimethylammonium-mica) between room temperature and 100degreesC. 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 C-18-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 head group, as well as decreasing tensile and shear stress in the disordered phase. In 2C(18)-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 C-18 backbones and minor head group displacements at 100degreesC. Close contacts between the bulky head groups with sidearms cause significant local pressure which is in part not relieved at 100degreesC. 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 C-18-mica only, a stronger upward orientation of the C-18 chain at the ammonium head group. The likelihood for chain interdigitation between the two hydrocarbon layers is 24%-30% for C-18-mica, and 65%-26% for 2C(18)-mica (for 20-100 degreesC). (C) 2004 American Institute of Physics.