Monte Carlo Simulation for Distribution Behavior of Benzene between Slit Pores and Supercritical CO₂ at Infinite Dilution

Abstract

The distribution coefficients of benzene between slit pores and supercritical carbon dioxide at infinite dilution are calculated by the Monte Carlo simulation method over a wide temperature range. The Lennard-Jones potential function is used for describing interactions between benzene, CO₂, and graphitic carbon. The distribution coefficient, K₂, which is defined as the ratio of the concentration of benzene in a pore to that in the fluid phase, is calculated from the residual chemical potentials of benzene, μ₂^res, for both phases by applying Widom’s test particle insertion method. Under the condition of the constant fluid density, the logarithm of K₂ increases linearly with increasing reciprocal temperature, while it shows a maximum in the constant pressure condition. We observed a remarkable difference between the influence of density on μ₂^res: that is, μ₂^res in the fluid phase gets stabilized with an increase in fluid density due to the increase of number of CO₂ molecules interacting with the benzene molecule, while μ₂^res in the pore becomes unstable with an increase in pore-phase density after the first monolayer near the wall has been occupied by CO₂ molecules. The radial distribution functions of CO₂ surrounding a benzene molecule in several pores of different slitwidths are displayed in three-dimensional graphics and they indicate how the benzene molecule and the surface walls influence the microstructure of CO₂ in pores.