Adsorption, desorption, and exchange kinetics of mixtures on planar surfaces

A theory for the adsorption, desorption, and exchange kinetics of a multicomponent mixture onto planar surfaces for kinetic-diffusion-controlled models has been developed. Equations were derived to calculate (1) the rate constant for adsorption processes, (2) the effective coefficients of diffusion for the various mixture components, and (3) the time needed to attain the equilibrium states for the adsorption processes of mixtures by using the adsorption, desorption, and exchange kinetic data with associative polymers of different molecular weights at different adsorbate concentrations. It is shown that the adsorption kinetics for mixtures over a wide range of concentrations are governed by (I) the kinetic adsorption at the interface, (II) the kinetic adsorption at the interfaces as well as simultaneous diffusion in the adsorbed layer, and/or (III) diffusion in the adsorbed layer. Adsorption, desorption, and exchange kinetics for mixtures of water-soluble associative polymers with different molecular weights on a crystal of titanium dioxide [TiO2(001)] were studied by ellipsometry. The existence of regimes (I), (II), and (III) of adsorption and exchange kinetics for polymer mixtures is justified by using the experimental data for the adsorption and exchange kinetics of water-soluble associative polymers with different molecular weights. It is shown that the rate of the exchange kinetics of polymer mixtures for short times is increased or decreased depending on the values of effective coefficients of diffusion and adsorption isotherms of mixture components. (C) 1996 Academic Press, Inc.