Limits imposed by liquid/surface interactions in the determination of tortuosity in mesopores

Abstract

The restricted mobility of different liquids, polar and non-polar, were studied by means of 1H NMR diffusion and Larmor frequency dependent relaxation measurements, upon confinement in mesoporous silica with mean pore sizes ranging from 3–30 nm. The use of alkanes as a reference in the determination of the true tortuosity value was observed to be accurate for pores larger than 6 nm, while for smaller pore sizes geometrical restrictions affect the diffusion even for short chain linear alkanes. 1H NMR fast field cycling relaxometry was used to measure the relaxation times as a function of the Larmor frequency. We observed that the total observable dispersion (TOD), which is a measure of the surface/liquid interaction, is mainly driven by the liquid polarity for pores larger than 10 nm, while less dependence on polarity is observed for smaller pore sizes. Our results indicate that as confinement increases, liquid/surface interactions become more relevant than the structural properties of the porous network, imposing a limit to the use of alkanes to probe the geometrical tortuosity.