A theoretical and experimental study of surfactant transport from a micellar solution to a clean air/water interface

This dissertation studies the transport dynamics of a water soluble nonionic surfactant C14E6 at the air/water interface at concentration below the critical micelle concentration (CMC) and above the CMC. A method which involves using the pendant bubble as a monolayer film balance is used to directly measure the surface tension as a function of surface concentration. The equilibrium parameters could be uniquely determined by fitting the directly measured equation of state and adsorption isotherm. Instead remarkably different equilibrium parameters for the same surfactant system fit experimental data equally well in the traditional treatment. The kinetic constants are obtained by numerically solving the transport equations for surfactant transport from aqueous solution at concentration below the critical micelles concentration. Theoretically, a model was developed to describe the surfactant transport from micellar solution for the case in which the kinetic rate of breakdown of micelles into monomers is much faster than the rates of bulk diffusion of monomers and micelles. In this limit, the analysis becomes a singular perturbation one, in which micelles diffuse from the bulk to a boundary near the surface, where they breakdown and monomers then diffuse from the boundary onto the surface through a micelle-free zone. Numerical solutions were obtained for the transport equations using a front trapping scheme to solve for the time dependent movement of the boundary between the micelle and micelle-free zones. Experimentally, dynamic reductions in surface tension were measured for adsorption onto a clean bubble surface by using the pendant bubble technique, and these were compared to the transport solutions. For low micellar concentrations, the experimental results were in perfect agreement with theory with no adjustable constants; for larger micellar concentrations the dynamic tension reductions become much faster then the theoretical predictions. The more rapid experimental relaxation in tension suggests that micelles may be adsorbing directly on the surface, a process not taken into account in the modeling and a significant conclusion of this study. Experimentally, visual evidence of the micelle-free zone next to the surface is provided by using fluorescence microscopy.