Ultrasonic, density, and potentiometric characterization of the interaction of gentisic and gallic acids with an apolar cavity in aqueous solution

Speed of sound, density and potentiometric measurements are used to analyze the thermodynamics of the aqueous solutions of gentisic and gallic acids, in their free form and as inclusion complexes with β-cyclodextrin (β-CD). The pH-potentiometric study, performed at several temperatures, yields the values of the dissociation constant of the acids, and the association constants of the β-CD–HA and β-CD–A- complexes, HA and A- being the non-ionic and ionic forms of the guest molecule, respectively. The changes on the enthalpy, ΔH°, entropy, ΔS° and heat capacity at constant pressure, ΔCp°, of the encapsulation processes, determined through van't Hoff analysis, give information about the driving forces governing the inclusion. A linear relationship holds between the decrease in enthalpy and the compensating decrease in entropy, this effect being mainly attributed to the solvent water and related solvation phenomena. The role of the solvent, and the contribution of solute–solute and solute–solvent interactions to the overall stability of the complexes, have also been analyzed through the speed of sound and density experiments, and the derived apparent molar properties. A comparison with previous results on different CD–hydroxyacid systems allows for the analysis of the effect of the number and/or position of the hydroxy groups of the benzoic acid ring on the phenomena studied herein.