Axial Diffusion in Liquid-Saturated Cylindrical Silica Pore Models

Abstract

Molecular dynamics simulations of solvent-filled cylindrical silica mesopores of 5 nm diameter are carried out to obtain a detailed molecular-level picture of the fluid structure and the self-diffusion coefficient in confinement. Two approaches are compared to calculate the self-diffusion coefficient. The first is based on a linear fit to the mean square displacement according to the Einstein relation. The second relies on the analysis of the discretized Smoluchowski diffusion equation. The focus of the study is to obtain the ratio between the bulk and the pore self-diffusion coefficient, a quantity often used in experimental work to draw conclusions on the tortuosity of a given material. For the straight pores studied in the present work, this ratio is between 2 and 3 for all 14 solvent molecules considered independent of the computational approach used to obtain the average self-diffusion coefficient in the pore or whether the pore was carved out from crystalline or amorphous bulk silica. Therefore, it merely reflects the influence of the confinement perturbing the fluid relative to that of the bulk phase.