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

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

MICROPOROUS AND MESOPOROUS MATERIALS

ELSEVIER SCIENCE BV

Año: 2024 vol. 364

1387-1811

Diffusion Nuclear Magnetic Resonance experiments offer a means to determine the geometrical tortuosityof a porous matrix, assuming that the interaction between the confined fluid and the confining matrixis negligible. In this, alkanes play a crucial role as molecular probes for polar mesoporous systems, withsilica being particularly relevant due to its wide range of technological applications. Despite their inherenthydrophobicity, NMR measurements reveal that alkane/silica interactions become significant when the porediameter approaches the size of the molecules. This fact is clearly demonstrated through the examinationof the longitudinal (T1) and transverse (T2) relaxation times of linear and cyclic alkanes confined withinsilica mesopores of mean pore sizes: 3, 6, and 15 nm. Noticeably, the interactions are practically negligiblein 6 and 15 nm pores, but they become relevant in the smallest ones. T1-T2 correlation maps evidence thatrelaxation times are influenced by molecular length, shape, and pore diameter. These empirical results notonly corroborate previous theoretical findings but also highlight the ratio T1/T2 as a valuable, rapid, and costeffectiveexperimental tool for assessing the feasibility of a given solvent as a molecular probe for determininggeometrical tortuosity.