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

The preferential solvation approach and the dielectric enrichment mode l have been applied to explain the solvatochromic behavior of o-, m- a nd p-nitroaniline (oNA, mNA and pNA) in several binary solvent mixture s. Cyclohexane was used as the ''inert'' nonpolar cosolvent in every m ixture. The other solvents were chosen trying to vary their polarity a s much as possible as well as their hydrogen bond donor or acceptor ca pabilities. Preferential solvation is detected in every solvent mixtur e studied. These global interactions were quantified by calculating th e preferential solvation constant, K. Also, by using the previously de veloped model, we calculated for each pair of solvent mixtures a theor etical 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 interactio n is practically dielectric in nature. When the interacting solvent is a hydrogen bond acceptor, the values of K are higher than K(D) accord ing to the acidity of the H in the amino group in the solutes. The val ues 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 conclus ions.