Limits of alkanes confined in mesoporous silica as a probe for geometrical tortuosity. An NMR relaxation study

Abstract

Diffusion Nuclear Magnetic Resonance experiments offer a means to determine the geometrical tortuosity of a porous matrix, assuming that the interaction between the confined fluid and the confining matrix is negligible. In this, alkanes play a crucial role as molecular probes for polar mesoporous systems, with silica being particularly relevant due to its wide range of technological applications. Despite their inherent hydrophobicity, NMR measurements reveal that alkane/silica interactions become significant when the pore diameter approaches the size of the molecules. This fact is clearly demonstrated through the examination of the longitudinal (T1) and transverse (T2) relaxation times of linear and cyclic alkanes confined within silica mesopores of mean pore sizes: 3, 6, and 15 nm. Noticeably, the interactions are practically negligible in 6 and 15 nm pores, but they become relevant in the smallest ones. T1-T2 correlation maps evidence that relaxation times are influenced by molecular length, shape, and pore diameter. These empirical results not only corroborate previous theoretical findings but also highlight the ratio T1/T2 as a valuable, rapid, and cost-effective experimental tool for assessing the feasibility of a given solvent as a molecular probe for determining geometrical tortuosity.