Molecular-Microscopic Properties and Preferential Solvation in Protic Ionic Liquid Mixtures

Structural and molecular-microscopic properties of the solvatochromic probes 4-nitroaniline, 4-nitroanisole, and Reichardt's dye were investigated in binary mixtures of ethylammonium propionate with methanol, ethanol, 1-propanol and 2-propanol. Solvatochromic parameters (alpha, hydrogen-bond donor acidity; beta, hydrogen-bond acceptor basicity; pi*, dipolarity/polarizability; , normalized polarity parameter) in different binary mixtures of ionic liquid with molecular solvents were determined with UV-Vis spectroscopy. The parameters show nearly ideal trends in all solvent mixtures, but the other parameters show different behavior in the mixtures. The pi* parameters show a negative deviation from ideality in the ionic liquid/methanol system. In contrast, the alpha parameters have severe positive deviations from ideal behavior in ionic liquid/1-propanol and ionic liquid/2-propanol solvent mixtures. A synergistic solvation effect is observed for the pi* parameters in IL/methanol mixtures. Specific solute-solvent interactions or solvent-solvent interactions, which cause non-ideal trends in some parameters, are justified and interpreted by the preferential solvation model.