MD simulation studies for effect of membrane structures and dynamics on gas permeation properties through microporous amorphous silica membranes

Imaginary amorphous silica membranes were prepared on a computer and gas permeation simulations were conducted using a dual control plane non-equilibrium molecular dynamics (DCP-NEMD) method. The Melt-Quench technique was employed to prepare various types of imaginary amorphous silica membranes which had different densities (from 1.3 to 2.2 g/cm3) and different mean pore sizes. Helium was adopted as a permeating gas species and its permeability was calculated at temperatures from 300 to 800 K. The Knudsen diffusion-like temperature dependencies of permeability could be observed for densities below 1.7 g/cm3, while the activated diffusion for the higher density models. We have also examined the effect of 3-body membrane potential parameters on membrane dynamics and gas permeation properties. The larger thermal vibration of oxygen atoms both in siloxane bonds and silanol groups on membranes could be observed for greater 1 parameter in the SW potential function, which might result in the change of activation energy for gas permeation.

Virtual amorphous silica membranes were prepared and gas permeation simulations were conducted using DCP-NEMD method. The Knudsen diffusion-like temperature dependencies of permeance could be observed for membranes whose density was smaller than 1.7 g/cm3, while the activated diffusion for denser membranes. The size of pores from about 0.25 to 0.3 nm would be effective for activated diffusion behavior of helium, and pores of 0.35 nm or larger ones would lead to Knudsen-like diffusion. It was also suggested that both the static amorphous silica structure and dynamical characteristics of a membrane depended on the value of 1 parameter in the SW potential function, and that gas permeation properties through microporous silica membranes might be influenced by the thermal movement of silica network.