Application of the density functional method to study adsorption and phase transitions in two-site associating, Lennard-Jones fluids in cylindrical pores

Abstract

A density functional approach is applied to study the adsorption of an associating model fluid in narrow cylindrical capillaries. The model with non-associative Lennard-Jones (LJ), attraction between fluid particles and the site-site association, permitting the formation of chains of LJ monomers, i.e. the two-site model for monomers, is investigated. The strength of associative interactions is varied in the model to obtain an insight into the role of the associative interactions on the phase diagrams of confined fluids. The fluid-pore walls interaction is chosen in the form of the Yukawa-type potential. The wetting properties of the confining solid surface is studied first. Next, we describe the first-order layering transitions in cylindrical pores and the phenomenon of capillary condensation in capillaries of molecular dimensions. We also analysed the structural changes in the adsorbed fluids accompanying layering transitions and capillary condensation. A comparison of the phase diagrams for the fluid in the cylindrical pores with two different radii and in the slit-like pores, with the same nominal width as the cylindrical pores, is performed. We have also compared the capillary evaporation phase diagram for the model in question in cylindrical and slit-like pores. The method and the results represent a useful basis for the development of inhomogeneous statistical associating fluid theory for several practical applications.