Solubility and Stability of β-Cyclodextrin–Terpineol Inclusion Complex as Affected by Water

Abstract

In the present work, inclusion complexes of α-terpineol (Terp) and β-cyclodextrin (BCD) were prepared by the coprecipitation method. Phase solubility studies were performed and thermodynamic parameters involved in the complex formation were calculated. The solubility of Terp increased linearly as the concentration of BCD was increased, confirming the 1:1 stoichiometry of the complex. The stability constants decreased along with increasing temperature. The negative value of the enthalpy and of the Gibbs free energy demonstrated that the process is exothermic and spontaneous. Since complexation gives more ordered systems, the negative value obtained for the entropy change evidenced the encapsulation of Terp. Terp was completely encapsulated in BCD at the preparation conditions and studied molar ratios, as confirmed in the freeze-dried samples by differential scanning calorimeter. The presence of Terp greatly modified the BCD water sorption curves, and the amount of adsorbed water was lower for the complexes. The limited water solubility of Terp could be overcome by the formation of BCD inclusion complexes, and the complexes were stable at different storage conditions (relative humidities 11–97% and 25 °C). The obtained phase solubility data are useful for food or pharmaceutical products formulation involving cyclodextrins and stability predictions.