Separation of Dielectric Nonideality from Preferential Solvation in Binary Solvent Systems: An Experimental Examination of the Relationship between Solvatochromism and Local Solvent Composition around a Dipolar Solute

Abstract

Dielectric nonideality of a binary solvent system refers to the deviation of the Onsager reaction field function from linearity in the polar mole fraction of the solvent mixture. A dipolar fluorophore dissolved in an ideal dielectric mixture exhibits a solvatochromic shift that is linear in the solvent polar mole fraction in it's solvation sphere. As a result, the “local composition” can be easily determined from the peak shift. Here we identify the conditions under which this linear approximation is appropriate for estimating local compositions around dipolar solutes. In a previous study (Khajehpour, M. H.; Kauffman, J. F. J. Phys. Chem. A 2000, 104, 7151−7159), we have demonstrated the influence of dielectric nonideality on the observed emission peak shifts of the charge-transfer excited state of ADMA [1-(9-anthryl)-3-(4-N,N-dimethylaniline)propane] in hexanes−ethanol mixtures. The linear approximation fails for this binary solvent, and a more elaborate method of analysis such as Suppan's nonlinearity ratio method must be used to determine the local composition from solvatochromic shifts. In this work, we examine mixture nonideality and dielectric enrichment in hexane−tetrahydrofuran and hexane−dichloromethane mixtures. Our analysis demonstrates that the contribution of nonideality to the observed solvatochromic shifts cannot be neglected in these binary solvents. Using Suppan's theory of dielectric enrichment, we have calculated the local composition of ADMA's solvation sphere and find that it is enriched in the polar component by ∼30% over the bulk composition. This calculated value agrees with experimental measures of the local composition based on analysis of solvatochromic shifts using Suppan's nonlinearity ratio method which accounts for dielectric nonideality. The linear approximation overpredicts this composition by as much as 50%, even though these binary solvents are more nearly ideal than the hexane−ethanol system. Following this observation, we have identified conditions under which the linear approximation is justified, and find that for most cases of practical importance the linear approximation will not provide accurate estimates of the local solvent composition from solvatochromic studies. Similarly, solvatochromic shifts can only be accurately predicted from theoretical local compositions if dielectric nonideality is taken into account. These results along with our previous studies indicate that the charge-transfer excited state of ADMA behaves as an ideal dipolar solute.