Non-negligible interactions of alkanes with silica mesopores affect self-diffusivity: a computational study

CHEVALLIER BOUTELL, IGNACIO JOSÉ; MONTI, GUSTAVO A.; ACOSTA, RODOLFO H.; OLMOS-ASAR JIMENA A.; FRANZONI, M. BELÉN

Congreso; #LatinXChem2021; 2021

LatinXChem

Mesoporous materials are of great scientific and technological interest, with numerous applications ranging from catalyst supports for heterogeneous catalysis and packing materials for separation processes to energy storage and carriers of drugs or even cells. The success in these applications mainly depends on the understanding of molecular transport inside the porous structures, being diffusion one of the most relevant.Confining liquids in a mesoporous structure may change observable properties such as spin relaxation and effective diffusion, which is very useful to characterize the morphology of the porous system in a non-invasive way, as in Nuclear Magnetic Resonance experiments.Due to their hydrophobicity, alkanes are extensively used as probe molecules for geometric tortuosity measurement of polar mesopores, since it is assumed a null interaction with the surface[1]. 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[2]. The dependence of the adsorption energy with the molecular length and shape is determined. Furthermore, in order to study confinement effects, 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. This led us to establish a dependence between the weighted energy and the pore diameter, which in turn correlates with the restricted diffusion coefficients determined previously by nuclear magnetic resonance experiments[3]. 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.