Non-negligible interactions of alkanes with silica mesopores affect self-diffusivity: Insights from first-principles calculations

Abstract

Alkanes, due to their hydrophobicity, are extensively used in NMR experiments as probe molecules for geometric tortuosity measurement of polar mesopores. In this work, ab-initio calculations results show that the interactions between linear and cyclic alkanes with the walls of silica mesopores are non-negligible. The dependence of the adsorption energy with the molecular length and shape is determined. Furthermore, a geometrical model is used to calculate a weighted adsorption energy that takes into account the fraction of molecules that reside close to the pore's wall compared to those in the pore's bulk. A dependence of the weighted energy with the pore diameter is found to correlate with restricted diffusion coefficients determined previously by nuclear magnetic resonance experiments. We conclude that the determination of the system's geometric tortuosity by this methodology is not accurate for pore diameters smaller than ca. 6 nm.