Measuring the Kinetics of Surfactant Adsorptive - Desorptive Exchange: The Role of Surfactant Structure and Charge

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ABSTRACT CTS-9520972 Kathleen J. Stebe Johns Hopkins University The aim of this study is to probe the dependence of the absorption kinetics of surfactants on their structure and charge. Two methods will be used; the pendant bubble and the oscillating bubble techniques, which have precise theoretical descriptions in terms of surfactant mass transfer and thermodynamics. The shape of a pendant bubble in surfactant solution becomes increasingly elongated as surfactant diffuses and absorbs, reducing the surface profiles. These profiles are matched to theoretical diffusion controlled relaxation's to obtain diffusion coefficients. Equilibrium data establish the surface equation of state and the adsorption isotherm. In the oscillating bubble method a small spherical bubble is formed, equilibrated, and subsequently forced to oscillate by the action of a piezo-piston. Phase angles and amplitude ratios between the radial and gas phase pressure oscillations are analyzed to confirm the diffusivities and to obtain the adsorption-desorption coefficients. For non-ionic surfactants, the theoretical framework to use these techniques has been established. In the first part of this study, the equilibrium and dynamics of surfactant absorption of a homologous series of nonionic surfactants, the polyethoxylates, will be studied. The absorption of ionic surfactants used to describe the equilibrium and dynamics of surfactant absorption. Because Debye lengths are typically small, of order 1000A, an approach based on a thin Debye length will be adopted. Diffusive flux alone determines the sublayer concentration. Surfactant partitions between the interface and the bulk according to an absorption kinetic barrier which is caused, in part, by the surface potential. The potential gradient at the surface concentration. Instantaneous electrical equilibrium is assumed, so the potential obeys a Gouy-Chapman distribution within the double layer. A surface equation of state and adsorption isotherm tha t properly incorporate the surface potential will be adopted. This approach will be used to develop models for ionic surfactant adsorption in the pendant bubble and in the oscillating bubble methods. Theory will be compare to experiment for the a series of the long chain acids.