Thermodynamics of the effects of substituent, degree of substitution, and pH on complex formation of hydroxypropyl-α- and hydroxypropyl-β-cyclodextrins with ascorbic acid

Abstract

The interaction of ascorbic acid with hydroxypropyl-α- and hydroxypropyl-β-cyclodextrins of different degree of substitution was studied at 298.15 K and different pH using solution calorimetry. In an aqueous solution, only hydroxypropyl-β-cyclodextrins form weak molecular complexes with the nonionized form of ascorbic acid. The thermodynamic functions of complex formation and stability constants of the complexes were calculated. The systems with weak intermolecular interaction without complex formation were characterized by enthalpic virial coefficients. On the basis of the obtained thermodynamic characteristics it was shown that the selectivity of complex formation of hydroxypropyl-α- and hydroxypropyl-β-cyclodextrins with ascorbic acid is determined by the size of the macrocyclic cavity, the presence of the hydroxypropyl substituent, and the medium acidity. The degree of substitution of hydroxypropyl-β-cyclodextrins exerts no substantial effect on the thermodynamic parameters of interaction with ascorbic acid.