Solvent structure in aqueous mixtures. III. Ionic conductances in ethanol-water mixtures at 10 and 25°C

Viscosities at 10 and 25°C and dielectric constants at 10°C are reported for ethanol-water mixtures. Precise transference numbers for KBr at 10°C were determined by the moving-boundary method for five solvent mixtures containing up to 20 mole % ethanol. Conductance measurements for the alkali halides and tetraalkylammonium bromides were carried out across the complete range of solvent composition at 25°C, but for the water-rich region only at 10°C. The resulting limiting ionic Walden products confirmed the conclusions arrived at from earlier measurements. The maxima occurring in the water-rich region cannot be attributed solely to a structure-breaking effect owing to the ionic size dependence which is in the wrong direction. The tetraalkylammonium ions in these mixtures do not exhibit an enhanced hydrophobic effect, nor do they appear to enhance structure of any kind. Rather, the addition of alcohol to an aqueous Bu₄N⁺ solution merely reduces the magnitude of the hydrophobic effect, as illustrated by a steadily increasing Walden product in the water-rich region. A decrease in temperature has no effect on the Walden product for the Bu₄N⁺ ion but increases the magnitude of the maxima for the alkali-metal and halide ions. These maxima are attributed to sorting of the solvent components by the ionic charge, due to an acid-base type interaction, so that the proportion of water in the ionic cospheres is greater than in the bulk solvents. The sorting effect is shown to be temperature independent. The oxyanions are shown to exhibit a behavior in these mixtures that cannot be identified at this time.