Preferential Adsorption in Ethane/Carbon Dioxide Fluid Mixtures Confined within Silica Nanopores

Molecular dynamics simulations have been performed to investigate the structural and dynamical properties of fluid ethane confined within cylindrical silica nano ores of 3.8 and 1 nm diameters. Pure ethane and equimolar mixtures of ethane and CO2 were considered for the analysis. Computer simulations were carried out along the supercritical isotherm T = 320 K, exploring densities within the range rho/rho(c)= 0.05-2.28. Density profiles along the axial and radial directions of the pore, orientational distribution functions, analysis of interfacial dynamic properties, and estimations of diffusion coefficients are presented. In agreement with experimental data, the results show the formation of a dense adsorption layer, with densities that may be up to 10 times larger than those of the bulk phase, in particular at low densities and under subnanometer confinement. The incorporation of CO2 changes the scenario, leading to preferential adsorption of CO2 over ethane species. The dynamics of ethane also change in mixtures, in agreement with experimental measurements. These changes are manifested by increments in the diffusion coefficients of confined ethane in the presence of CO2. However, in contrast with experimental data, our results for the diffusion coefficients of confined ethane showed a monotonic decreasing behavior with increasing bulk density. Plausible interpretations for this discrepancy are also discussed.