Exploring the effect of ethanol-water structuring on the transport properties of ethanol in porous silicas

The transport properties of liquid mixtures confined within porous media can change significantly from those observed for bulk mixtures due to changes in the liquid structuring within the pore space. Here, pulsed field gradient NMR was used to measure the diffusion coefficient of ethanol in ethanol-water liquid mixtures confined within silicas with pore diameters of 6 nm and 3 nm as a function of composition. For liquids imbibed within the 6 nm pores, the composition dependence of the ethanol diffusion coefficient closely followed that of the bulk liquid mixture and the absolute diffusion coefficients were reduced by a tortuosity factor of 3, with a minor contribution due to liquid-surface interactions. For liquids imbibed within the 3 nm pores, the diffusion coefficient of ethanol decreased as the composition of ethanol within the pore space increased and for single-component ethanol imbibition, the effective tortuosity was 63. Fast field cycling NMR experiments showed that the diffusion behaviour was not controlled by an increase in ethanol adsorption strength. A geometric analysis of the pore space was consistent with a highly confined system in which most molecules interacted with the pore walls. Under such confinement, the liquid structuring within the bulk pore space did not reflect that of the bulk liquid mixtures, and the observed decrease in diffusion coefficient as ethanol composition increased was consistent with an increase in confinement due to the larger size of the ethanol molecule.