Phase behavior of grafted chain molecules: Influence of head size and chain length

Constant pressure Monte Carlo simulations of a coarse-grained off-lattice m odel for monolayers of amphiphilic molecules at the air-water interface are presented. Our study focuses on phase transitions within a monolayer rathe r than on self-aggregation. We thus model the molecules as stiff chains of Lennard-Jones spheres with one slightly larger repulsive end bead (head) gr afted to a planar surface. Depending on the size of the head, the temperatu re and the pressure, we find a variety of phases, which differ in tilt orde r (including tilt direction), and in positional order. In particular, we ob serve a modulated phase with a striped superstructure. The modulation resul ts from the competition between two length scales, the head size, and the t ail diameter. As this mechanism is fairly general, it may conceivably also be relevant in experimental monolayers. We argue that the superstructure wo uld be very difficult to detect in a scattering experiment, which perhaps a ccounts for the fact that it has not been reported so far. Finally the effe ct of varying the chain length on the phase diagram is discussed. Except at high pressures and temperatures, the phase boundaries in systems with long er chains are shifted to higher temperatures. (C) 1999 American Institute o f Physics. [S0021-9606(99)50617-X].