Axial Diffusion in Liquid-Saturated Cylindrical Silica Pore Models

Molecular dynamics simulations of solvent-filled cylindricalsilicamesopores of 5 nm diameter are carried out to obtain a detailed molecular-levelpicture of the fluid structure and the self-diffusion coefficientin confinement. Two approaches are compared to calculate the self-diffusioncoefficient. The first is based on a linear fit to the mean squaredisplacement according to the Einstein relation. The second relieson the analysis of the discretized Smoluchowski diffusion equation.The focus of the study is to obtain the ratio between the bulk andthe pore self-diffusion coefficient, a quantity often used in experimentalwork to draw conclusions on the tortuosity of a given material. Forthe straight pores studied in the present work, this ratio is between2 and 3 for all 14 solvent molecules considered independent of thecomputational approach used to obtain the average self-diffusion coefficientin the pore or whether the pore was carved out from crystalline oramorphous bulk silica. Therefore, it merely reflects the influenceof the confinement perturbing the fluid relative to that of the bulkphase.

Automated summary

Molecular dynamics simulations were conducted on solvent-filled cylindrical silica mesopores with a diameter of 5 nm to obtain a detailed understanding of the fluid structure and self-diffusion coefficient in confinement. Two approaches were compared to calculate the self-diffusion coefficient, and the ratio between the bulk and pore self-diffusion coefficient was obtained. This ratio, which reflects the influence of confinement on the fluid, was found to be between 2 and 3 for all solvent molecules studied, regardless of the computational approach or the type of silica used.