Computer simulation of pressure-area isotherms of monolayers of charged fatty-acid molecules

We present a new Monte Carlo algorithm for simulating the conformational behaviour of surfactant soap molecules tethered to a planar interface. The extension to other geometrical arrangements is straightforward. The algorithm makes use of the superposition of local structural rearrangements for the description of conformational changes in the chains. The displacements of pairs of atoms subject to the geometrical constraints can be achieved in a computationally efficient way by the application of least-squares search methods. It is shown that a balance of the attractive interactions between the methylene chains and longer-range repulsive interactions between the charged headgroups determines the form of the spreading pressure isotherms. The experimental isotherms cannot be reproduced if the headgroups interact through purely repulsive potentials. We find it necessary either to modify these potentials by invoking a short-range attraction or to assume that the charge of a headgroup decreases with average area-per-molecule in the monolayers. Our model tracks the experimentally observed spreading pressure-area isotherm in Langmuir monolayers of ionised and non-ionised fatty acids. Importantly, we were able to reproduce the dependence of the isotherms on the chain length.