A molecular dynamics simulation of pressure-driven gas permeation in a micropore potential field on silica membranes

摘要

The mechanisms involved in pressure-driven gas permeation through a micropore on vitreous SiO_2 membranes were examined molecular dynamics (MD) simulation. Virtual amorphous SiO_2 membranes were prepared by the melt-quench method utilizing modified Born-Mayer-Huggins (BMH) pair potential and Stillinger-Weber (SW) three-body interactions. A dual control plane non-equilibrium MD (DCP-NEMD) technique was employed to simulate gas permeation phenomena under a constant upstream pressure, in which the permeating molecules were modeled as Lennard-Jones particles. The dependencies of the permeance of helium and CO_2 molecules on temperature and pore size were examined. For cylindrical pores about 8 and 6 A in diameter, the calculated temperature dependencies for the permeance of helium molecules were similar to the tendencies predicted by the normal Knudsen permeation mechanism, while in the case of CO_2 permeation, a temperature dependency larger than helium and a significant deviation from the Knudsen mechanism were observed. The deviation was more obvious for the smaller 6 A pore model. A simple gas permeation model that takes the effect of the pore wall potential field into consideration satisfactorily explained the permeation properties of CO_2 in the high temperature region. The permeation mechanism was also examined from the viewpoint of the lateral potential and density distribution in a micropore. The values for the potential within micropores, predicted from the observed temperature dependencies of the gas permeation rate and using the simple gas permeation model, were in good agreement with the depth of the potential field resulting from the given potential parameters. The findings also indicate that the density (pressure) difference in a micropore between the pore entrance and exit, which could be enhanced by an attractive pore wall potential, might be the true driving force for permeation, particularly in the high temperature region.