Preferential solvation of 6-propionyl(N,N-dimethylamino)naphthalene in binary, polar solvent mixtures

Using fluorescence spectroscopy, we have determined the extent of preferential solvation of the dye PRODAN (6-propionyl(N,N-dimethylamino)naphthalene) in binary mixtures where one component is capable of hydrogen bonding. In mixtures of methanol with acetone, acetonitrile, or pyridine the spectroscopically determined mole fraction of the PRODAN-methanol complex was much higher than the methanol mole fraction. The amount of preferential solvation follows the trend acetonitrile > acetone > pyridine with corresponding stoichiometries of 1.1, 1.2, and 0.93 and equilibrium constants of 12.0, 7.9, and 1.7. The amount of preferential solvation can be correlated to the heat of mixing of the solvent pair making up the mixture or to the donor number of the non-hydrogen-bonding solvent. This former correlation exists because preferential solvation of the probe involves unmixing of the solvents. For mixtures where hydrogen bonding between the probe and the cosolvent do not occur, we find no evidence of preferential solvation due to dielectric enrichment when the dielectric properties of the mixtures are taken into account.