Global phase diagrams for freezing in porous media

Abstract

Using molecular simulations and free energy calculations based on Landau theory, we show that freezing/melting behavior of fluids of small molecules in pores of simple geometry can be understood in terms of two main parameters: the pore width H* (expressed as a multiple of the diameter of the fluid molecule) and a parameter α that measures the ratio of the fluid-wall to the fluid–fluid attractive interaction. The value of the α parameter determines the qualitative nature of the freezing behavior, for example, the direction of change in the freezing temperature and the presence or absence of new phases. For slit-shaped pores, larger α values lead to an increase in the freezing temperature of the confined fluid, and to the presence of a hexatic phase. For pores that accommodate three or more layers of adsorbate molecules several kinds of contact layer phase (inhomogeneous phases in which the contact layer has a different structure than the inner layers) are observed. Smaller α values lead to a decrease in the freezing temperature. The parameter H* determines the magnitude of shift in the freezing temperature, and can also affect the presence of some of the new phases. Results are presented as plots of transition temperature vs α for a particular pore width. Experimental results are also presented for a variety of adsorbates in activated carbon fibers (ACF) covering a wide range of α values; the ACF have slit-shaped pores with average pore width 1.2 nm. The experimental and simulation results show qualitative agreement.