Heats of mixing of mixed solvent solutions of alkali metal salts of substituted benzenesulfonic acids

The enthalpy change on mixing solutions of methyl-substituted benzenesulfon ic acids and their salts, with salts having a common cation or anion, at co nstant ionic strength in mixtures of 1,4-dioxane with water, were measured at 25 degreesC. The heat effects of mixing solutions having a common anion increase almost linearly with the reciprocal value of the dielectric consta nt of the solvent. The heat effects of mixing solutions having a common cat ion, which are all negative in water-rich solutions, become endothermic aft er a certain value of the dielectric constant is passed. The results are di scussed in terms of the solute-solvent structural properties.