Phase separation in confined systems

We review the current state of knowledge of phase separation and phase equi libria in porous materials. Our emphasis is on fundamental studies of simpl e fluids (composed of small, neutral molecules) and well-characterized mate rials. While theoretical and molecular simulation studies an stressed, we a lso survey experimental investigations that are fundamental in nature. Foll owing a brief survey of the most useful theoretical and simulation methods, we describe the nature of gas-liquid (capillary condensation), layering, l iquid-liquid and freezing/melting transitions. In each case studies for sim ple pore geometries, and also more complex ones where available, are discus sed, While a reasonably good understanding is available for phase equilibri a of pure adsorbates in simple pore geometries, there is a need to extend t he models to more complex pore geometries that include effects of chemical and geometrical heterogeneity and connectivity. In addition, with the excep tion of liquid-liquid equilibria, little work has been done so far on phase separation for mixtures in porous media.