Interaction of Povidone with Aromatic Compounds III: Thermodynamics of the Binding Equilibria and Interaction Forces in Buffer Solutions at Varying pH Values and Varying Dielectric Constant

The complex formation of a series of aromatic compounds with povidone was studied in buffer solutions and organic solvent mixtures by equilibrium dialysis. For all the ligand molecules studied, a linear relationship was found between r, the number of moles of bound ligand per mole of povidone, and the free ligand concentration. The binding constants and the free energies of binding (—∆F), were greater for compounds in the nonionic state and increased with the number of hydroxyl groups which were capable of forming hydrogen bonds. They decreased with temperature elevation. The thermodynamic data showed entropy gains during the binding process accompanied by small negative enthalpy values. The increased ability to form hydrogen bonds and the increase in ionization of the ligand molecule was reflected in more negative ∆H and decreasing ∆S values. (The thermodynamic values were interpreted on the basis of the "iceberg" concept of water structure.) From these entropy and enthalpy changes, hydrogen and hydrophobic bondings appeared to be the most important types of binding. In organic solvent mixtures, the association constants lowered with increasing ethanol or propylene glycol concentration; a line relationship between the free energy and the dielectric constant of the solvent mixtures was observed.