ABSTRACT
Recent molecular simulation studies for pores of simple geometry have shown a rich phase behavior associated with melting and freezing in confined systems [1-9]. A review of the simulation and experimental work in this area up to 1999 has been given by Gelb et al. [8]. The freezing temperature may be lowered or raised relative to the bulk freezing temperature, depending on the nature of the adsorbate and the porous material. In addition, new surface-driven phases may intervene between the liquid and solid phases in the pore. “Contact layer” phases of various kinds often occur, in which the layer of adsorbed molecules adjacent to the pore wall has a different structure from that of the adsorbate molecules in the interior of the pore. For materials having walls that are weakly attractive (e.g., glasses, silicas) this contact layer is usually fluid-like while the interior molecules have adopted a crystalline structure. For materials such as carbon, which has walls that are strongly attractive, the contact layer is usually crystalline while the interior layers remain fluid. These contact layer phases have been predicted theoretically, and confirmed experimentally for several systems [3,7]. In addition, for some systems in which strong layering of the adsorbate occurs (e.g., slit pore models of activated carbon fibers), hexatic phases can occur; such phases have quasi-long-ranged orientational order, but positional disorder, and for quasi-two-dimensional systems occur over a temperature range between those for the crystal and liquid phases. These are clearly seen in molecular simulations [2,8], and preliminary experiments seem to confirm these phases [7,9]. Recently it has been shown [7,10] that this apparently complex phase behavior results from a competition between the fluid-wall and fluid-fluid intermolecular interactions. For a given pore geometry and width, the phase diagrams for a wide range of adsorbates and porous solids can be classified in terms of a parameter
α
that is the ratio of the fluid-wall to fluid-fluid attractive interaction [7,8,10].
