Non-negligible interactions of alkanes with silica mesopores affect self-diffusivity: a combined experimental and theoretical approach

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

Hangzhou

Congreso; 15th International Bologna Conference on Magnetic Resonance in Porous Media (MRPM15); 2022

Zhejiang University

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 developed to calculate a weighted adsorption energy (EW) that takes into account the fraction of molecules that reside close to the pore´s wall compared to those in the pore´s bulk. Calculations regarding how the coverage degree of alkanes on silica surfaces affects the adsorption energy support our findings.This led us to establish a dependence between EW and the pore diameter. Our numerical results are contrasted to NMR experiments from [3] and good agreement is observed. In the left panel of Fig. 1 (no se muestra acá) the trend of EW for a 3 nm pore is compared to the effective diffusion coefficients (Deff) reported for a few alkanes, while in the right panel EW is compared to Deff as a function of the pore diameter for n-heptane. In both cases, the proposed model predicts correctly the trend of the effective diffusion coefficients obtained by NMR. 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.References [1] D’Agostino C. et al., Phys. Chem. C, 2012, 116 (16) 8975-8982.[2] Chevallier-Boutell IJ et al., Microporous and Mesoporous Mater., 2021, 326, 111315.[3] Garro Linck L et al., Microporous and Mesoporous Mater., 2020, 305, 110351.