Solvation and Protonation of 1,10-Phenanthroline in Aqueous Dioxane Solutions

Abstract

Abstract Solvation and protonation of 1,10-phenanthroline (phen) have been studied by potentiometry and calorimetry in various aqueous dioxane solutions. Protonation reactions were studied in the mixtures (the dioxane content x=0–0.4 mole fraction) containing 0.3 mol dm−3 NaCl as a constant ionic medium at 25 °C. Potentiometric and calorimetric titration curves obtained in the range of −log[H+]=2–7 were well explained in terms of the formation of Hphen+ and H(phen)2+, and the formation constants, enthalpies and entropies of formation of the species were determined. The formation constants of the both species gradually decreased with increasing x, and then the formation of H(phen)2+ became negligibly small at x>0.1, while the enthalpy and entropy of formation of Hphen+ first decreased and then increased with increasing x and thus minima were found at x=0.05. Enthalpies of solution of anhydrous 1,10-phenanthroline were also determined in both neutral (x=0–1) and acidic (0.1 mol dm−3 HCl, x=0–0.4) aqueous dioxane solutions. By knowing enthalpies of solution of 1,10-phenanthroline in acidic solutions and of formation of Hphen+ in the mixed solvents, enthalpies of solution of 1,10-phenanthroline in neutral solutions (ΔH°s(phen)) were evaluated and compared with those directly obtained in the neutral solutions (ΔH°s,neutral). A significant difference was found between ΔH°s(phen) and ΔH°s,neutral in the range x<0.3, which could be ascribed to the formation of stacked species of 1,10-phenanthroline molecules, (phen)n. The variation of ΔH°s(phen) with x was explained in terms of the enhanced solvation of dioxane to phenanthroline molecules owing to breaking of the hydrophobic water structure around them. Solvent effects on the formation of Hphen+ and H(phen)2+ were discussed in terms of different solvation behavior of 1,10-phenanthroline and proton with x.