Structure and phase behavior of two-Yukawa fluids with competing interactions in planar slit pores.

A density functional perturbation theory, which is based both on the modified fundamental-measure theory and on the first-order mean-spherical approximation for long-range attractive and repulsive interactions, has been developed for studying the structure and phase behaviors of a competing system restricted to slit pores. The hysteresis loop for the adsorption and desorption curves indicates that the system exhibits vapor-cluster and cluster-liquid transitions which depend on the pair potential parameters and the slit width (H). The periodic spacing (D) of the cluster is commensurate with the periodicity of modulation in the particle density distribution and more closely related to the vapor-cluster and cluster-liquid phase transitions of the system. For the cluster phase, we find the transition from a single liquidlike slab to a multi-liquidlike slab with increasing the slit width. The multi-liquidlike slab is formed depending on the periodicity of modulation by finite-size artifacts. The cluster-related phase transitions, such as the vapor-cluster or cluster-liquid transitions occur for H>D, while for H<D the system only exhibits the vapor-liquid transition. At a low amplitude, the vapor-liquid transition disappears and the cluster-liquid transition only occurs for H<D. The coexistence curves for the confined phase diagram are contained within the corresponding bulk liquid-vapor coexistence curve. For a wide slit pore (H>D), the system exhibits two tricritical points, joined to one another by the line of second-order transition. The results support the conclusion that the confinement effect plays an important role in determining the equilibrium phase transition.