Evidence for demixing of miscible binary liquids in mesoporous glass

Abstract

Mixtures of cyclohexane and acetone have been studied in bulk and under confinement inside the pores of Vycor porous glass by means of Nuclear Magnetic Resonance (NMR). The dependence of longitudinal (T_1) and transverse (T_2) relaxation as well as the self-diffusion coefficient D on the volume ratio of both liquids is investigated, with either one of the two liquids being replaced by its deuterated homologue. Whereas cyclohexane and acetone are fully miscible in bulk, which is supported by the results of T_1, T_2 and D, there is strong evidence that both liquids demix inside the pore space which is attributed to preferential adsorption of acetone to the polar pore walls. The relaxation times and their respective dependence on magnetic field strength, which is indicative for the characteristic reorientation mechanisms expected for polar and non-polar adsorbates, confirm this trend both for acetone and cyclohexane. The latter follows bulk-like relaxation behavior in the presence of acetone as low as 10 vol-%. The pronounced increase in tortuosity of acetone in the self-diffusion coefficient further supports this concept. Molecular dynamics simulations carried out for cylindrical silica pores show the separation of both liquids from their respective density profile, while the average residence times and mean-squared displacements of both molecules are estimated for single-component liquids and mixtures.