Effect of surfactant interfacial orientation/aggregation on adsorption dynamics

The application of new thermodynamic adsorption isotherms allow to improve the description of surfactant adsorption kinetics based on a diffusional tr ansport. While the consideration of interfacial reorientation corrects appa rently too high diffusion coefficients, interfacial aggregation avoids too small diffusion coefficients or the assumption of adsorption barriers. The adsorption kinetics of alkyl dimethyl phosphine oxides is influenced by int erfacial reorientation. While the lower homologues (C-8-C-12) follow the cl assical diffusion model, the higher homologues (C-13-C-15) yield diffusion coefficients several times larger than the physically reasonable values. As suming two different adsorption states, the resulting diffusion coefficient s agree with those expected from the geometric size of the molecules. The m odel also works well for oxyethylated non-ionics, such as C10EO8. As a seco nd example, a good theoretical description is obtained for experiments of 1 -decanol solutions when a mean surface aggregation number of n = 2.5 is ass umed. The same n was obtained from the description of the equilibrium adsor ption isotherm of 1-decanol. Assuming that the transition from one into the other state is controlled by a rate constant (change in orientation, forma tion or disintegration of two-dimensional aggregates) significant changes i n the kinetics curves can result. The use of additional rate constants yiel ds an improved fitting to experimental data. (C) 2000 Elsevier Science B.V. All rights reserved.