Global phase diagrams for freezing in porous media

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 alpha that measures the ratio of the fluid-wall to the fluid-fluid attractive interaction. The value of the alpha 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 alpha 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 alpha 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 alpha 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 alpha values; the ACF have slit-shaped pores with average pore width 1.2 nm. The experimental and simulation results show qualitative agreement. (C) 2002 American Institute of Physics.