Aqueous solubility behavior of three cyclodextrins

Abstract The solubilities in water of α-, β-, and γ-cyclodextrins were measured over the temperature range 23.7–48°. Dissolution rate studies and microscopy showed that the stable solid phase was the hydrate. van't Hoff plots were linear, yielding these thermodynamic paramters of solution (in the order α, β, γ): Δ H 0 = 7.67, 8.31, 7.73 kcal/mol; Δ S 0 = 13.8, 11.7, 14.7 e.u. Methyl α- d -glucopyranoside, studied in the same way, gave Δ H 0 = 3.95 kcal/mol, Δ S 0 = 8.4 e.u. The cyclodextrins are less soluble than acyclic saccharides because of their relatively unfavorable enthalpies of solution, partially offset by more favorable entropies of solution. Among the cyclodextrins, cyclomaltoheptaose (β-cyclodextrin) is less soluble than the others because both its enthalpy and its entropy are less favorable. The following empirical equations satisfactorily correlate the thermodynamic parameters of the three cyclodextrins: Δ H 0 / n = −0.155 n + 2.232, and Δ S 0 / m = 1.95 n - 8.10, where n is the number of glucose residues per cyclodextrin molecule, and m is the number of cyclodextrin molecules per unit cell of the hydrate.