Surface and capillary transitions in an associating binary mixture model.

We investigate the phase diagram of a two-component associating fluid mixture in the presence of selectively adsorbing substrates. The mixture is characterized by a bulk phase diagram that displays peculiar features such as closed loops of immiscibility. The presence of the substrates may interfere with the physical mechanism involved in the appearance of these phase diagrams, leading to an enhanced tendency to phase separate below the lower critical solution point. Three different cases are considered: a planar solid surface in contact with a bulk fluid, while the other two represent two models of porous systems, namely, a slit and an array on infinitely long parallel cylinders. We confirm that surface transitions, as well as capillary transitions for a large surface area to volume ratio, are stabilized in the one-phase region. Applicability of our results to experiments reported in the literature is discussed.