Abstract
Boundary-Driven type Non-Equilibrium Molecular Dynamics (BD-NEMD) simulations have been carried out to investigate effusion processes of pure gas from the inside of silicalite membrane pores to the gas phase. Two types of model outer surfaces, SC and IS models, were devised to see whether any structural change in the outer surface affects the effusion flux of permeating gas. An SC model is made to have the outer surface terminated at the cross section passing through the center of straight channels of silicalite, while the outer surface of an IS model is terminated to pass through the center of intersections. Propane was chosen as a permeating gas. In all simulations, the silicalite crystal atoms and the propane molecules were represented by flexible (movable) models so as to exchange their kinetic energy without any artificial velocity scaling near the membrane exit. The present simulations have provided an interesting result in effusion flux; that is, the effusion flux of propane across the IS surface model is larger than that across the SC surface model. This enhanced effusion of propane is ascribed to the local density enhancement at the surface of the IS model; that is, according to energetic analyses, the larger the local density at the outer surface, the larger the opportunity for molecules to take extra kinetic energy necessary for desorption. This simulation result will be informative for molecular design of the structure of membrane exit.
