Fluid-Fluid Transitions at Bulk Supercritical Conditions

We use three different polymer solvent mixture models to theoretically determine the existence of capillary-induced phase separation in simple pores under supercritical bulk conditions. These models undergo bulk demixing, due to quite different mechanisms, yet readily display supercritical transitions without the use of esoteric interactions in the capillary. The theoretical method used to analyze these systems is density functional theory. We find that capillary demixing is not reliant on the presence of a pure surface transition but may occur in the absence of the latter. This is shown by considering cases where the surface enhancement factor is too weak to cause demixing at a single surface or else the bulk conditions are supercritical to both bulk and surface transitions. This phenomenon may prove useful in applications involving adsorption from mixtures into porous particles