The Role of the Solvent in Polymer Adsorption: Displacement and Solvency Effects

In adsorption, the solvent always plays a double role, affecting both lateral interactions between the adsorbate molecules and determining the effective interaction between the surface and the adsorbate. For polymers, this means that they adsorb strongly from some solvents, whereas from others they do not at all. As a consequence, mixed solvents can give rise to an adsorption/desorption transition: the polymer is desorbed by a so-called displacer. We investigate theoretically how the adsorption of the polymer varies with the displacer concentration. A simple analytical expression for the critical displacer concentration is derived, which is found to agree very well with numerical results from recent polymer adsorption theory. One of the applications of this expression is the determination of segmental adsorption energies from experimental desorption conditions and the adsorption energy of the displacer. Illustrative experiments and other applications are briefly discussed.