Role of hydroxyl concentrations in solvatochromic measures of solvent polarity of alcohols and alcohol-water mixtures - evidence that preferential solvation effects may be overestimated

For single solvents, primary alcohols and water, there is a good linear correlation (r=0.994) between the solvent polarity index E-T(30) and the molar concentration of OH groups (or 1000/V-m, where V-m is the solvent molar volume). The corresponding correlations for alcohol-water mixtures are plots vs. the sum of molar concentrations of alcohol and water, alternatively expressed as plots of E-T(30) vs. volume fraction. Our quantitative treatment is an extension of recent theoretical and experimental results. In contrast, previous studies of alcohol - water mixtures have relied on plots of E-T(30) vs. mole fraction, and have overestimated the effect of preferential solvation of solvatochromic dyes by the more hydrophobic alcohols.