The protonation thermodynamics of ferulic acid/γ-cyclodextrin inclusion compounds

The physico-chemical properties of ferulic acid (FA) and its inclusion compounds with the γ-cyclodextrin (γ-CD) have been studied at 25 °C in aqueous solution (0.15 M NaCl) by potentiometry and solution calorimetry. The two basicity constants of the ionized FA, log K1 8.94 and log K2 4.48, in agreement with those of compounds of similar structure, were attributed to the protonation of hydroxyl and carboxyl ionized groups, respectively. Unlike log K2, the first basicity constant showed a slight decreasing pattern on increasing the amount of γ-CD. On the other hand, calorimetric data showed greater exothermicity during the protonation of the COO− group in FA when in the presence of the γ-CD. Any increase of the latter led to greater enthalpy (-ΔH°) and lower entropy (ΔS°) changes. The increase of -ΔH° and the decrease of ΔS° values reached a constancy only beyond γ-CD/FA molar ratios of 1. Thermodynamic data were consistent with the hypothesis of the inclusion complexation obtained by the penetration of the neutralized guest FA into the γ-CD host cavity.