(161b) Vapor Liquid Surface Tension of Variable Square-Well Fluids and Normal Alkanes in Slit Pores

Investigation of phase coexistence, adsorption, diffusion and reaction of fluids in porous materials is of much interest from both scientific and industrial point of view. In general, molecules confined within narrow pores, with pore widths of a few molecular diameters, can exhibit a wide range of physical behavior. We examine the vapor-liquid phase transition of variable square-well fluid (λσ=1.25, 1.5, 1.75, 2.0 and 3.0) under slit pore using grand-canonical transition matrix Monte Carlo with histogram reweighting technique. We examine the critical temperature shift as a function of slit width, H, varying from 12.0σ to 2.5σ. Critical exponent, β, approaches to two dimensional value as H approaches 1.0. Critical temperature shift varies exponentially with H. Similar behavior is observed for normal alkanes confined in mica and graphite slit pores. In graphite slit pore critical density continuously decreases with the decrease in slit width while for lager slit width in mica pores critical density has shown larger values than the one in bulk. However critical density, in mica pores, displays a maxima and it eventually decreases with the reduction of the slit width. We also examine the layering and orientation behavior of normal alkanes in mica and graphite pores. Vapor-liquid interfacial tension for variable square-well fluids and n-alkanes in variable slit pore of different surface characteristics is examined using Binder's finite size scaling technique. We observed dramatic difference in the value of vapor-liquid surface tension inside the pore. Our results of n-alkanes and square-well fluids indicate that under the slit pores of nano confinements vapor-liquid interfacial tension substantially decreases by many times from its bulk value.