Evidence for the demixing of miscible binary liquids in mesoporous glass

Abstract

Mixtures of cyclohexane and acetone are studied in bulk and under confinement inside the pores of Vycor porous glass by means of nuclear magnetic resonance (NMR). The dependence of longitudinal (T₁) and transverse (T₂) 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. While cyclohexane and acetone are fully miscible in bulk, which is supported by the results of T₁, T₂ and D, there is strong evidence that both liquids demix inside the pore space, which is attributed to the preferential adsorption of acetone to the polar pore walls. The relaxation times and their respective dependence on magnetic field strength, which is indicative of the characteristic reorientation mechanisms expected for polar and non-polar adsorbates, confirm this trend for both acetone and cyclohexane. The latter follows a bulk-like relaxation behavior in the presence of acetone concentrations as low as 10 vol%. The pronounced increase in the tortuosity of acetone, as determined from 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 profiles, while the average residence times and mean-squared displacements of both molecules are estimated for single-component liquids and mixtures.