Tilt-ordering in Langmuir monolayers.

A topic related to the previous one concerns the molecular mechanisms which produce tilted structures in Langmuir monolayers, which are monolayers of surfactant molecules deposited on a water surface. These essentially two-dimensional systems also exhibit a significant variety of phases, differing in the nature of both translational and orientational ordering. In tilt-ordered phases, which are similar to smectic-C phases, the molecules are on average tilted uniformly with respect to the surface normal: the azimuthal direction of the tilt can be either toward nearest-neighbors, next-nearest neighbors, or in some intermediate direction. We have studied the origin of tilt-ordering using a rigid-rod model, where each surfactant molecules is composed of a chain of Lennard-Jones atoms, and replacing the aqueous substrate by a continuum (described by external fields). These studies have examined the roles of various features of the molecular architecture in producing tilt-ordering, particularly the effects of size mismatch between the hydrophillic head groups and hydrophobic tail units. Here we have used both ground-state analysis (See details) and Monte-Carlo simulation methods (See details).

Simulated Langmuir monolayer