Theory and experiment on the measurement of kinetic rate constants for surfactant exchange at an air/water interface

The paper focuses on the measurement of the rate constants for the kinetic steps of adsorption and desorption of surfactant between an air/water surface and the aqueous bulk sublayer adjacent to the surface. Kinetic constants are determined in nonequilibrium experiments in which either a clean surface is contacted with a bulk solution and surfactant diffuses toward and adsorbs onto the interface, or the area of an established monolayer in equilibrium with an underlying solution is changed, and surfactant exchanges between the surface and bulk. The dynamic tension change due to the surfactant exchange is measured, and compared to predictions of kinetic-diffusive transport models in order to infer the kinetic coefficients as well the diffusion coefficients. Model comparisons for highly surface active surfactants have resolved only the diffusion coefficient as the transport was found to be diffusion controlled; kinetic constants have only been established for less active materials such as alcohols or bolaform surfactants. In this study, we demonstrate that kinetics can be differentiated from diffusion in clean interface adsorption and re-equilibration if high bulk concentrations of the surfactant are used, or in re-equilibration, if the surface is compressed sufficiently. We first establish theoretically that mass transfer shifts from diffusion-limited to mixed as the bulk concentration increases in clean interface adsorption, or the surface compression is increased in re-equilibration. We then experimentally verify this idea by using the polyethoxylated surfactant C12E6 (C12H25 (OCH2CH2)(6)-OH) and by measuring dynamic surface tensions in clean interface adsorption and re-equilibration, respectively by the shape analysis of pendant bubbles. We find values of 6 x 10(-10) m(2)/s for the diffusion coefficient, and 1.4 x 10(-5) m/sec and 1.4 x 10(-4) s(-1) for the adsorption and desorption rate constants, respectively, in a Frumkin kinetic formulation. While the adsorption constant is comparable to previously measured values for the less surface active surfactants, the desorption rate constant is a few orders of magnitude smaller. This indicates that the more surface active materials may have much smaller desorption rate constants than had been previously anticipated based on the studies of the less surface active materials. (C) 1998 Academic Press.