Computer simulation of free, thin-liquid films and disjoining pressures

Many processes, from foodstuffs to environmental remediation, employ free, surfactant-laden, thin-liquid films such as those found in foams and emulsions. An understanding of the forces that control the stability of these films is important in system selection and design. The disjoining pressure is a direct measure of thin-film forces and their relationship to film thickness. This thesis analyzes and develops molecular simulation techniques to study free, thin-liquid films and to predict their disjoining pressures as a function of film thickness.;The initial work employs a simple molecular model and existing Monte Carlo methods. For a single-component, Lennard-Jones thin-film, canonical ensemble Monte Carlo is shown to be insufficient for predicting disjoining pressures. The magnitude of the disjoining pressure in these systems is so small that it is overwhelmed by fluctuations in the simulation data.;Next, a modified Gibbs ensemble Monte Carlo simulation is applied whereby the disjoining pressure is fixed and the resulting film thickness and composition are measured. The results from this simulation show an evaporation/condensation instability inherent to the simulation technique that prevents accurate calculation of the film thickness.;For this reason, we develop a new simulation technique that combines a canonical and a modified grand-canonical ensemble. We isolate the thin film from disturbances using a canonical ensemble simulation of the flat portion of the film. We employ a second simulation of the bulk-liquid meniscus to act as a sort of chemical potential "gauge" for the system. Using a modified grand-canonical simulation, the bulk-liquid meniscus equilibrates with the thin film without disturbing it. Once this is accomplished, the disjoining pressure of the system is calculated using our newly developed ghost-volume exchange method.;Using modified Lennard-Jones potentials, we apply this simulation new method to predict the disjoining pressure isotherm of a thin-film containing model chain surfactant and water molecules. For these simple systems, we find that a film thins in response to increased disjoining pressure by expelling both water and surfactant molecules in approximately equal proportions. While preliminary, we believe these results indicate that the modified grand-canonical Monte Carlo simulation is a promising tool for the study of free, thin-liquid films and the calculation of disjoining pressure as a function of film thickness.