Phase Separation in Nano-Porous Materials
Keith E. Gubbins
North Carolina State University, Raleigh, NC
Phase transitions and phase separation in micro-and meso-porous materials are strongly influenced by finite size effects, changing dimensionality, and strong solid-fluid forces. Our current understanding of some of these phenomena will be reviewed for gas-liquid (capillary condensation) and liquid-liquid phase equilibria, and for freezing in narrow pores. The emphasis will be on molecular simulation of these systems, but examples drawn from experimental results on well characterized materials will be shown, particularly for controlled pore and vycor glasses. The simulations and experiments are in good agreement for gas-liquid phase coexistence, and in qualitative agreement for liquid-liquid and solid-fluid systems. In the case of liquid-liquid separation in pores, results will be presented for the effects of confinement on the kinetics of phase separation, as well as for equilibrium phase coexistence. In the case of melting/freezing phenomena, the factors influencing the freezing temperature (for both the fluid in the pore interior, as well as that near the pore walls) will be addressed for simple fluids. While some progress has been made in our understanding of each of these three areas, there remain numerous important, but unanswered questions.