SOLVATOCHROMIC STUDY ON NITROANILINES - PREFERENTIAL SOLVATION VS DIELECTRIC ENRICHMENT IN BINARY SOLVENT MIXTURES

The preferential solvation approach and the dielectric enrichment model have been applied to explain the solvatochromic behavior of o-, m- and p-nitroaniline (oNA, mNA and pNA) in several binary solvent mixtures. Cyclohexane was used as the ''inert'' nonpolar cosolvent in every mixture. The other solvents were chosen trying to vary their polarity as much as possible as well as their hydrogen bond donor or acceptor capabilities. Preferential solvation is detected in every solvent mixture studied. These global interactions were quantified by calculating the preferential solvation constant, K. Also, by using the previously developed model, we calculated for each pair of solvent mixtures a theoretical curve and the corresponding K(D) due to dielectric enrichment. Non hydrogen bond acceptor solvents (beta = 0), give values of K quite similar to those of K(D), indicating that the preferential interaction is practically dielectric in nature. When the interacting solvent is a hydrogen bond acceptor, the values of K are higher than K(D) according to the acidity of the H in the amino group in the solutes. The values of K as well as of K(D) for any solvent mixtures in general follow the order pNA > mNA > oNA as expected, considering the values of mu(g) and mu(g) - mu(ex). Studies in pure solvent support previous conclusions.