Phase behavior of films adsorbed on model crystal surfaces

Effects of confinement on freezing and melting

We present a review of experimental, theoretical, and molecular simulation studies of confinement effects on freezing and melting. We consider both simple and more complex adsorbates that are confined in various environments (slit or cylindrical pores and also disordered porous materials). The most commonly used molecular simulation, theoretical and experimental methods are first presented. We also provide a brief description of the most widely used porous materials. The current state of knowledge on the effects of confinement on structure and freezing temperature, and the appearance of new surface-driven and confinement-driven phases are then discussed. We also address how confinement affects the glass transition.

The structure of fluids confined in crystalline slitlike nanoscopic pores

Grand canonical and canonical ensemble Monte Carlo simulation methods are used to study the structure and phase behavior of Lennard-Jones fluids confined between the parallel (100) planes of the face centered cubic crystal. Thin slit pores with a width allowing for the formation of only up to five atomic layers are considered. The phase diagrams of the systems characterized by different pore width as well as by different strength of the fluid-pore walls potential are determined. It is shown that an enormously large number of different phase diagram topologies can occur, depending on the parameters of the problem (pore width, strength of the fluid-pore walls potential, etc).

Monte Carlo simulations on the effect of substrate geometry on adsorption and compression

Canonical Monte Carlo simulations were used to study the adsorption and compression of fluid layers on model substrates with cubic, (111) fcc, and graphite geometries. The effect of the relative size of the fluid and substrate molecules on adsorption was considered for strong molecule-surface interactions. In the case of monolayer formation, it was found that the surface geometry and the size of the adsorbate molecules had a significant effect on the structure of the adsorbed layer. These structures varied from well-ordered, commensurate layers to liquid-like structures. Lateral compression was observed for certain fluid to substrate molecule sizes. For the interactions studied in this work, it was found that maximum lateral compression occurred on the cubic surface when adsorbate molecules had a diameter approximately 15% larger than the substrate diameter. In the case of multilayer formation, it was found that second and higher adsorbed layers could compress into the adsorbed layers below them. For cubic substrates, the interlayer compression was predicted analytically with reasonable accuracy, with maximum interlayer compression found for fluid diameters approximately 90% the size of substrate molecule diameters.

Lennard-Jones fluids confined in nanoscopic slits: evidence for reentrant filling transitions

By using grand canonical and canonical ensemble Monte Carlo simulations, the structure and phase behavior of a Lennard-Jones (LJ) fluid confined between the parallel (100) planes of a face-centered cubic crystal are studied. Slit pores with a width which allows three adsorbate layers to form are used. It is shown that the filled pore consists of three commensurate layers over a wide range of the surface potential strength, while the pore-filling mechanism and the topology of the phase diagram change when the strength of this fluid-wall potential is varied. Condensation may occur in one step or via two layering-like transitions. The structure of monolayer films depends on the strength and corrugation of the surface potential, and the condensation of the middle layer may induce a reentrant first-order transition.

The structure of fluids confined in crystalline slitlike nanoscopic pores: Bilayers

Grand canonical and canonical ensemble Monte Carlo simulation methods are used to study the structure and phase behavior of Lennard-Jones fluids confined between the parallel (100) planes of the face centered cubic crystal. Ultra thin slit pores of the width allowing for the formation of only two adsorbate layers are considered. It is demonstrated that the structure of adsorbed phases is very sensitive to the wall-wall separation and to the strength of the fluid-wall potential. It is also shown that the structure of low temperature (solid) phases strongly depends on the fluid density. In particular, when the surface field is sufficiently strong, then the high density phases may exhibit a domain wall structure, quite the same as found in monolayer films adsorbed at a single substrate wall. On the other hand, the weakening of the surface potential leads to the regime in which only the hexagonally ordered bilayer structure is stable. The phase diagrams for a series of systems are estimated. It is shown that, depending on the pore width and the temperature, the condensation leads to the formation of the commensurate or incommensurate phases. The incommensurate phases may have the domain-wall or the hexagonal structure depending on the pore width and the strength of the fluid-wall potential.