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J. Phys. Chem. B 2001, 105, 7446-7451
Coupled Diffusion of Mixed Methanol + Ethanol Clusters in Carbon Tetrachloride Solutions Derek G. Leaist* and Ling Hao Department of Chemistry, UniVersity of Western Ontario, London, Ontario N6A 5B7, Canada ReceiVed: February 8, 2001
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, methanol gradients produce counterflows of ethanol. Binary diffusion in methanol + carbon tetrachloride and ethanol + carbon tetrachloride solutions is also discussed.
Introduction Solutions of alcohols in hydrocarbons and other nonpolar solvents have received considerable attention.1-11 Extensive selfassociation of the alcohol molecules in these solutions produces large deviations from ideal behavior.1-8 Stokes4-6 has developed an especially accurate and detailed treatment of the thermodynamic, spectroscopic, and dielectric properties of ethanol + cyclohexane mixtures in terms of the formation of hydrogenbonded ethanol dimers, trimers, and higher ethanol “polymers”. The deviations from ideal solution behavior for this system are remarkably large, rivaling the strongly nonideal behavior of aqueous electrolyte solutions. The thermodynamic properties of multicomponent solutions of alcohols in nonpolar solvents have also been investigated.10,11 Molecular association also has important consequences for the transport properties of solutions, such as viscous flow,12-14 diffusion,13-19 and thermal diffusion.20-23 In a study of the role of hydrogen bonding in diffusion processes, Longsworth19 measured binary mutual diffusion coefficients for alcohols and carboxylic acids in carbon tetrachloride solutions. For comparison, diffusion coefficients for nonassociating solutes were also measured. Whereas the carboxylic acids diffused as monomers and dimers with no indication of further association, the sharp drop in the diffusion coefficients of the alcohols with increasing concentration suggested association well beyond the trimer stage. In this paper, the coupled diffusion of mixed alcohols in a nonpolar solvent is studied by measuring ternary mutual diffusion coefficients for methanol + ethanol in carbon tetrachloride solutions. The reported diffusion coefficients give the fluxes of the total methanol (A) and total ethanol (B) components produced by gradients in the total alcohol concentrations. * To whom correspondence should be addressed. Fax: (519) 661-3022. E-mail:
[email protected].
JA(total methanol) ) -DAA∇CA - DAB∇CB
(1)
JB(total ethanol) ) -DBA∇CA - DBB∇CB
(2)
To interpret the results, the measured fluxes of the total alcohol components are related to the fluxes of free methanol and ethanol molecules (A1 and B1) and clusters of hydrogen-bonded alcohol molecules (AnBm) produced by the multiple association reactions nA1 + mB1 h AnBm. The coupling of the methanol and ethanol fluxes caused by association of the diffusing alcohol molecules is investigated by estimating the contributions to cross-diffusion coefficients DAB and DBA from the transport of the mixed AnBm species. Experimental Section Solutions were prepared in calibrated volumetric flasks by mixing carbon tetrachloride (BDH reagent grade) and weighed amounts of methanol (EM Science,
