Contribution from non-ideality and preferential solvation to non-linear solvatochromism in binary mixtures

Abstract

The knowledge of preferential solvation is of critical importance in description of the solute behavior in mixed solvents. The analysis of solvatochromic shifts as a function of solvent composition is popular in this regard, since it gives molecular information on solute-solvent interactions. However, due to the complexity of the interactions and possible non-ideality of mixed solvents, implementing an efficient approach for interpretation of solvent-solvent interactions is an opening major challenge in modeling solvatochromism to get reliable results on preferential solvation. Here, we proposed a new methodology to treat solvatochromism in solvent mixtures in order to reach a more reliable picture of the local composition around the solute. As the main idea, the effect of solvent-solvent interactions on spectral shift was considered by formulating the non-ideality of solvent in solvation shell of the solute as a function of the local composition by exploiting the results obtained from the analysis of the bulk mixture non-ideality. From that, the non-linear variation in spectral shift can be well separated into contributions from preferential solvation and the local non-ideality. To test this approach, solvatochroimsm of 2-methyl-4-nitroaniline was studied in some mono-solvents and binary mixtures. The spectral properties of 2-methyl-4-nitroaniline and its response to solvent were rationalized by quantum chemical calculation and linear solvation energy relationships analysis in mono-solvents. Then, the observed strong non-linear solvatochromism of 2-methyl-4-nitroaniline in aqueous mixtures of ethanol, DMF and DMSO was treated by the proposed model that provided both physically and statistically more reliable results on preferential solvation and molecular interactions, with respect to the other widely-used solvent exchange models. As an interesting and important result, non-ideality, but not preferential solvation, was responsible for non-linear solvatochromism in aqueous solutions of ethanol and DMF.