Monte Carlo simulations of disjoining-pressure isotherms for Lennard-Jones surfactant-stabilized free thin films

We present Monte Carlo simulations of disjoining-pressure isotherms for soluble- surfactant- stabilized free thin films. To mimic actual experiments, we define a disjoining-pressure isotherm such that films of all thicknesses on that isotherm are in equilibrium with the same infinite reservoir of surfactant and solvent. This necessitates use of inert-gas molecules to vary the film thicknesses while still maintaining equilibrium with the reservoir. All types of molecules in the system (solvent, surfactant, and inert gas) are defined by using Lennard-Jones (LJ) based potentials. For thick films, the center of the film is sufficiently far away from the interface for the surfactant orientation to be random, whereas at the interface, the surfactant molecules are elongated and oriented roughly perpendicular to the interface. As the films are thinned, the disjoining pressure (171) is positive and increases monotonically. Two bulk surfactant concentrations are considered that differ by a factor of about 2. For the isotherm with a higher surfactant concentration, the disjoining pressure is larger in magnitude due to higher surface coverage and concomitant stretching of the adsorbed surfactant, but the difference is not substantial within the accuracy of our simulations.