Excess Sorption of Supercritical CO2 within Cylindrical Silica Nanopores

Abstract

Using Molecular Dynamics simulations, we examine structural and dynamical properties of supercritical CO2 confined within cylindrical hydrophobic nanopores of diameters 38 and 10 Å. Computer simulations were performed along the isotherm T = 315 K, spanning CO2 densities from ρ/ρc = 2.22 down to ρ/ρc = 0.22. Radial and orientational distribution functions, analysis of interfacial dynamic properties, and estimatons for local diffusion and orientational relaxation times are presented. In agreement with previous experimental data, our simulation results reveal the presence of a dense phase adsorbed within the pores. The combination of low CO2 bulk densities and narrow pores leads to ρint/ρblk ≈ 5-fold enhancement of the global density of the confined fluid. These density increments gradually become much less marked as the external phase becomes denser. Contrasting, in that latter limit, we found that the trapped fluid may become less dense than the bulk phase. Adsorption behavior of CO2 onto hydrophilic-like and rugged pore surfaces were also exmined. In these cases, we observed a global slowdown in both translational and rotational motions for the trapped CO2, the largest retardations being those associated with spatial domains of the fluid located near the silica interface.