  Constant pressure Monte Carlo simulations of a coarse grained off-lattice
model for monolayers of amphiphilic molecules at the air/water interface are
presented. Our study focusses on phase transitions within a monolayer rather
than on self aggregation. We thus model the molecules as stiff chains of
Lennard-Jones spheres with one slightly larger repulsive end bead (head)
grafted to a planar surface. Depending on the size of the head, the temperature
and the pressure, we find a variety of phases, which differ in tilt order
(including tilt direction), and in positional order. In particular, we observe
a modulated phase with a striped superstructure. The modulation results from
the competition between two length scales, the head size and the tail diameter.
As this mechanism is fairly general, it may conceivably also be relevant in
experimental monolayers. We argue that the superstructure would be very
difficult to detect in a scattering experiment, which perhaps accounts for the
fact that it has not been reported so far. Finally the effect of varying the
chain length on the phase diagram is discussed. Except at high pressures and
temperatures, the phase boundaries in systems with longer chains are shifted to
higher temperatures.
