Surfactant Aggregation in the Bulk Fluid and at the Fluid-Wall Interface

225 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1994.Molecular simulation methods are used to quantitatively describe the equilibrium thermodynamics of surfactant aggregation, both in the bulk phase and by surface adsorption, and characterize the formation of hemimicelles at the solid-fluid interface. Using a simple surfactant model consisting of Lennard-Jones sites connected by non-harmonic springs many features of real bulk surfactant solutions, including the critical micelle concentration (cmc), are reproduced. A series of NpT molecular dynamics simulations are performed for solutions with different surfactant concentrations. Solvent and surfactant chemical potentials are determined using both the Widom test particle and the Powles f-g sampling methods. Cluster size distributions are used to demonstrate the thermodynamics of cluster equilibria. The effect of bulk surfactant concentration on the structure of surfactants at a solid surface is explored. The system used to study surfactant adsorption at the fluid-wall interface consists of a mixture of solvent and surfactants confined between two parallel smooth walls. The external pressure is controlled by applying a force to one wall which acts like a piston. The surfactant adsorption behavior for the two different wall fields modeled is characterized by L2 type adsorption isotherms. The surface cluster distribution is determined. Using a clustering analysis, the adsorption isotherm is predicted using bulk and surface cluster standard potentials. A new analytical model is proposed which can be used to characterize an L2, L4 or hybrid adsorption isotherm. The effect of cluster size and wall field on the shape of the adsorption isotherms is explored using this analytical model. The effect of wall field on the structure of surfactants at a solid surface is characterized. The bulk state is fixed by controlling the solvent chemical potential using Grand Canonical molecular dynamics confined to a local control volume. The surface grand potential densities are obtained using statistical mechanical sum rules. By calculating the surface grand potential density as a function of the wall field, the optimal conditions for hemimicelle formation are identified. Results are presented for both two- and three-dimensional systems.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD