Propane simulated in silica pores: Adsorption isotherms, molecular structure, and mobility

Abstract

Molecular dynamics simulations were conducted for bulk propane in contact with fully protonated slit-shaped silica pores. The effective pore width was of either 0.8 or 2.7nm. The temperature was set at 343, 368, and 373K. The TraPPE-UA and CLAYFF force fields were implemented to model propane and silica, respectively. Each individual simulation yields the density of confined fluid as a function of the bulk pressure. For a given temperature, adsorption isotherms were estimated by repeating the simulations at various bulk pressures. The results qualitatively agree with available experimental data; namely, at fixed temperature the excess sorption is found to show a maximum near the pressure at which the pores fill; at fixed pressure the excess adsorption is found to decrease as the temperature increases and as the pore width expands. At equilibrium, pronounced layering was observed for propane near the pore surface, especially in the narrower pore and at the highest densities considered. The propane molecules at contact with silica tend to lay with their CH3-CH3 vector parallel to the pore surface. The mean square displacement as a function of time was used to quantify the self-diffusion coefficient of confined propane as a function of temperature, pressure and pore width. These results will be useful for enhancing the interpretation of experimental data.