CO2-Propane Mixtures in Silica Pores: Relation Between Structure and Dynamics

We report here equilibrium molecular dynamics simulation results obtained for mixtures of carbon dioxide and propane confined in narrow slit-shaped silica pores. A wide range of thermodynamic conditions (from sub-critical to super-critical) has been considered, as well as a wide range of fluid compositions. The pore width was changed to quantify the effect of confinement on the results, and the pore surface properties were changed to modulate the preferential pore-fluid interactions. We observed evidence for preferential adsorption of carbon dioxide near the protonated –OH groups on the surface, which yields slow self-diffusion coefficients at low loadings. Non-monotonic changes in the self-diffusion coefficients as a function of pore loading are observed, and explained, tentatively, based on the competitive adsorption of the various fluids near the pore surfaces. We will discuss how these predictions compare with experimental NMR results. The results could be important for designing separation devices, and also for better understanding the behavior of fluids in sub-surface environments.