Solubility and stability of b-cyclodextrin inclusion complex with terpineol

MAZZOBRE, MARÍA FLORENCIA; DOS SANTOS, CRISTINA; BUERA, MARÍA DEL PILAR

Trieste

Congreso; Frontiers in Water Biophysics; 2010

Cyclodextrins are able to form inclusion complexes with hydrophobic molecules. Complex formation in solution is a dynamic equilibrium process and the stability of the complex can be described in terms of the equilibrium constant. a-Terpineol is a terpenic alcohol used as perfume and antifungal [1]. Since it is poorly soluble in water the complexation can increase its solubility. In this work, inclusion complexes of a-Terpineol  (Terp) and b-cyclodextrin (BCD) were prepared. Physical properties and stability of the complexes were evaluated as a function of water content and storage time. The solubility of Terp was looked at by phase solubility studies. The formation of the inclusion complex Terp/BCD was confirmed by DSC. Terp was completely encapsulated in BCD at the preparation conditions and studied molar ratios. The complex was stable at different relative humidities (RH) (11-97% RH) for at least 250 days of storage at 25 °C. The sorption isotherms for the Terp/BCD systems, in 1:1 and 1:3 molar ratios, were obtained and compared with the original cyclodextrin. The presence of Terp greatly modified the BCD sorption curves being the amount of adsorbed water smaller in the combined systems. Since the “driving-force” of the complex formation is probably related with the substitution of high-enthalpy water molecules from the inner CD cavity by the guest molecule [5], these results confirm the encapsulation of the ligand at any of the studied molar ratio. Terp/BCD systems evidenced a well defined glass transition confirming their amorphous characteristics. For a certain water content, the Tg values dropped when increasing the Terp proportion in the CD matrix. This indicates that in the presence of Terp structural modifications take place affecting the physical characteristics of the matrix. The plots of Terp concentration in the solution vs BCD concentration showed a linear trend confirming the 1:1 stoichiometry for the complex. Phase solubility studies were carried out at different temperatures and the correspondent equilibrium constants were calculated. The constants decreased with increasing temperature, as expected for an exothermic process. The Van’t Hoff plot (LnKc vs 1/temperature) was a linear function and allowed to calculate thermodynamic parameters: enthalpy and entropy. 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 is another confirmation of the encapsulation of Terp. The limited water solubility of Terp could be overcome by the formation of BCD inclusion complexes, which are stable at different storage conditiosns (RH 11-97 and 25°). The obtained phase solubility data are useful if Terp/BCD is required as water soluble additive for special products.