Surface transition in athermal polymer solutions

According to a recently developed density functional theory, athermal polymer solutions, in which the solvent particles are smaller than the monomers, may undergo a bulk fluid-fluid phase separation, driven by excluded volume effects. In recent work, we showed that an inert surface immersed in the dilute polymer phase can, in principle, be wetted by the condensed phase. However, we show here that the "prewetting transition" we assumed in our earlier studies is in fact a different type of surface transition. Rather than completely wet the surface at coexistence, the condensed phase layer which forms in the presence of the dilute bulk remains globally stable (and is finite in width) even as the bulk coexistence conditions are approached. Hence, the adsorbed phase inhibits complete wetting of the surface by the dilute phase. The surface transition is first order for the systems we study here and, for longer polymers, the surface phase coexistence line meets the bulk coexistence curve nontangentially to give rise to a lower transition point. For short polymers, we find that the surface transition can occur for a supercritical bulk. We develop a simple one-component thermal model, which displays analogous behavior at an adsorbing surface and provides us with some insight into the qualitative mechanisms responsible.