A STUDY OF CYCLODEXTRIN COMPLEX-FORMATION BY A FREEZING-POINT DEPRESSION METHOD

The interaction and the self-association of a variety of compounds in aqueous solution were investigated by the freezing point depression me thod using a commercially available osmometer. This method, described in our earlier publication, was used to determine the stability consta nt and self-association constant, which were based on the colligative properties of solution and calculated from the decrease of osmotic pre ssure obtained in terms of the freezing point depression. Stability co nstants were measured for complexes of alpha-, beta-, and gamma-cyclod extrin with non-aromatic carbonic acids (acetic acid, formic acid, oxa lic acid, succinic acid, malic acid, tartaric acid, maleic acid, citri c acid), anions (I-, NO3-, ClO4-, CNO-, SCN-, Cr2O72-) and amino acids (leucine, isoleucine, methionine, phenylalanine, tryptophan), and the self-association constant for caffeine was measured. These values gav e good reproducibility and rapid estimation with a simple procedure. I t was concluded that the freezing point depression method is useful fo r estimation of the stability constant and the self-association consta nt for drugs in aqueous solution.