ADSORPTION-DESORPTION KINETICS AT LIQUID SURFACES

[摘要] The adsorption-desorption kinetics of surfactant molecules at liquid surfaces are studied. We find bulk-surface systems are naturally classified as either ''strongly adsorbing'' or ''weakly adsorbing,'' depending on the ratio of the desorption time Q-1 to the adsorption depth diffusion time t(h). In ''weak'' systems (Qt(h) much-less-than 1) surface-bulk exchange kinetics are rapid; the interface releases its adsorbed molecules before diffusion can disturb the bulk density profile. Surface coverage GAMMA relaxes exponentially in a time Q-1. Exchange kinetics for ''strong'' systems (Qt(h) much-greater-than 1) are by contrast sufficiently slow that diffusion-controlled effects develop; the bulk is strongly disturbed near the surface with GAMMA relaxing algebraically in a time t(h). Relaxation of surface density inhomogeneities also proceeds very differently in the two cases. Of particular interest is the ''anomalous'' (non-Fickian) relaxation in the strong case; surface perturbations of wavevector k(T) relax in a time t approximately 1/k(T). We find that in strong systems the surface furnishes bulk diffusive dynamics with absorbing (reflecting) boundary conditions for t < t(h) (t > t(h)), while for weak systems boundary conditions are always reflecting. We discuss dynamical correlation functions which reflect the above behaviors and are in principle available through scattering measurements. (C) 1994 Academic Press, Inc.

[发布日期] 1994-10-01