College of Science & Engineering
Twin Cities
A feedback loop is used to maintain a constant bubble area in a capillary pressure microtensiometer during LysoPC adsorption to determine the dynamic surface tension as a function of concentration above and below the CMC. New direct numerical solutions of the spherical diffusion equations coupled to a new three parameter virial equation of state relating surface tension, surface concentration, and bulk concentration are used to model the dynamic surface tension experiments. Above the CMC, a two-zone model is proposed in which depletion of the local monomer concentration drives the dissociation of micelles near the bubble interface. LysoPC adsorption is shown to be diffusion-limited over concentrations ranging from well below to well above the CMC. Obtaining adsorption data at constant interfacial area helps make fits of the models more precise using parameters of the virial equation of state extracted from equilibrium surface tension vs concentration data to obtain monomer and micelle diffusivity as fitting parameters. The dynamic models provide estimates of the micelle diffusivity and size at concentrations near the CMC that are difficult to obtain by conventional light scattering or other methods.