Solvatochromic hydrogen bond donor acidity of aqueous binary solvent mixtures for reversed-phase liquid

Solvatochromic hydrogen bond (HB) donor acidity (α_m) values for aqueous mixtures of methanol, ethanol, isopropanol, acetonitrile and tetrahydrofuran were measured by electronic absorption spectroscopy employing bis[α-(2-pyridyl)-benzylidine-3,4-dimethylaniline]bis(cyano)iron(II) as an indicator, and compared to available literature values. The HB donor acidity of all the aqueous organic mixtures increases in a nonlinear fashion as water is added to the organic solvent. For mixtures of acetonitrile and tetrahydrofuran, which are not acidic or are only slightly acidic, with water α_m increases rapidly as a small amount of water is first added, then shows a plateau region, and finally slowly rises to the acidity value of water. In contrast, the α_m for the mixtures of methanol, ethanol and isopropanol, which are quite acidic, increase slowly as the first small amount of water is added and then increase rapidly as the mixtures approaches pure water. Literature values of α_m for water-methanol mixtures, which were determined from the D_T(30) values of Reichardt's betaine dye, showed a minimum in the plot of α_m vs. volume fraction of organic component. It was also shown that the solvophobicity parameter (S_p) of methanol- and ethanol-water mixtures can be described by a combination of the dipolarity (π_m^*) and HB donor acidity (α_m) of the mixtures.