Kamlet-Taft .pi.* polarizability/dipolarity of mixtures of water with various organic solvents

The Kamlet-Taft ..pi..* values of mixtures of water with four organic solvents over the entire range of composition have been estimated by solvatochromic measurement with a series of carefully selected indicators. The measured ..pi..* value of each indicator is collinear with the average ..pi..* value. The authors interpret these results as indicating that the indicators sense the polarizability/dipolarity and not the hydrogen bond acidity of the solvent. Examination of relationships between ..pi..* and epsilon and between E/sub T/ and ..pi..* and excess properties of ..pi..*, E/sub T/, and epsilon with respect to volume fraction of the organic cosolvent leads to the conclusion that the principal effect of changing the solvent composition on the observed ..pi..* values operates through the dielectric properties of the local medium about the solute. This also supports the previous observation that solvatochromic shifts of the indicators used in this study are not very sensitive to solvent hydrogen bond acidity. The measured ..pi..* values together with literature E/sub T/ values were used to estimate ..cap alpha.. values, the hydrogen bond acidity parameter, of aqueous organic solvents.