Coupled Diffusion of Mixed Methanol + Ethanol Clusters in Carbon Tetrachloride Solutions

Abstract

Ternary mutual diffusion coefficients are measured for methanol + ethanol + carbon tetrachloride solutions at 25 °C and 0.200 mol dm-3 of total alcohol. The alcohol components in these solutions diffuse as free MeOH and EtOH molecules in local equilibrium with hydrogen-bonded alcohol clusters. Although each alcohol cotransports the other alcohol in the form of mixed (MeOH)n(EtOH)m clusters, negative cross-diffusion coefficients for this system indicate that each mole of diffusing alcohol drives counterflows of up to 0.5 mol of the other alcohol. The results are interpreted by relating the measured total alcohol fluxes to the fluxes of the alcohol monomers and associated species. Added ethanol increases the concentrations of the mixed clusters but reduces the concentrations of the MeOH monomers. The resulting flux of MeOH monomers up the ethanol gradient exceeds the flux of the larger, less-mobile associated methanol species down the ethanol gradient, producing a net counterflow of methanol. Similarly, met...