Study on surfactant adsorption kinetics: Effects of interfacial curvature and molecular interaction

A concept on the surfactant adsorption kinetics was explored and illustrated with 1-octanol and C12E8 (Langmuir, 1997, 13, 6204): using only a limited range of equilibrium surface tension data to determine the adsorption isotherm can cause a serious mistake in the determination of adsorption kinetics and in the evaluation of diffusion coefficient from the dynamic surface tension data. A systematic theoretical study was also reported recently for surfactants adsorbing onto a clean planar air--water interface (Langmuir, 1999, 15, 1952). To learn the effect of curvature of the fluid interface on this issue, a theoretical study is performed for surfactants adsorbing onto a clean spherical air-water interface with different curvature. Data from this simulation indicate that (a) for any interfacial curvature, the stronger the surfactant interactions (i.e., with a larger absolute K value), the greater the deviation in apparent diffusivity (D) or in sorption rate constants (h(a), k(d)) obtained from a Langmuir analysis; (b) at the same K, the larger the interfacial curvature, the smaller the deviation in D or in k(a) and k(d) and (c) a larger deviation in diffusivity results at more dilute concentrations.