Multilayer theory for adsorption from solution. Mixtures of monomers + dimers

Abstract

A multilayer theory for adsorption from solutions composed of molecules of different size has been developed. The treatment is based on a quasi-crystalline statistical model of mixtures and a plane homogeneous interface, and considers all possible configurations of molecules in the interface region. The potential energy of the system is taken as the sum of interaction energies between different segments of liquid molecules, and between such segments and the surface. The model is applied to mixtures of monomers plus dimers and numerical results are presented for the surface excess over a range of conditions. The shape of adsorption isotherms and the orientations of dimer molecules are analyzed in terms of liquid-liquid and solid-liquid interactions. For dimers where both segment interact equally strongly with the surface the model predicts a predominant orientation of adsorbed molecules with their major axes parallel to the surface. For dimers where only one segment is preferentially adsorbed there is a change in the predominant orientation of adsorbed dimers from parallel to perpendicular as the concentration of the solution is increased. However, over a wide range of interaction energies a model which considers only one orientation of adsorbed molecules, either parallel or perpendicular to the surface, seems not to be justified.