POTENTIOMETRIC STUDY OF CYCLODEXTRIN COMPLEXES OF WEAK ACIDS AND BASE

An exact and comprehensive theory has been developed for the potentiometry of a 1:1 + 1:2 cyclodextrin complexation system with the ultimate goal of extracting the values of all the complexation constants from the system. The technique works best in the neutral pH region. Several computational methods were suggested to treat the data obtained with this technique. It is concluded that the potentiometric method, though generally incapable of providing a unique solution for this system, has useful advantages when used in conjunction with other techniques.;Following a stoichiometric model introduced earlier by this laboratory, group contributions of the substitutent groups of the substrates to the complexation constants were calculated for a 1:1 + 1:2 stoichiometry. for the para-substituted benzoic acids, the isomeric complexation constants for complexing at the COOH site correlate reasonably well with Hammett's (sigma) ((rho) = -.7), and constants for complexing at the para-substituent site correlate with the group dipole moments of the substituent. For para-substituted phenols, where extensive charge relocation can occur in their phenolate forms, the isomeric complexation constants correlate fairly well with the molar refraction of their corresponding monosubstituted benzenes. Based on these findings, the following postulates were made: (A) Cyclodextrin complex stability is diminished by high polarity at the substrate binding site; (B) Complex stability is enhanced by high polarizability and electron density at the substrate binding site. These two factors may operate in concert or in opposition. Thus in principle, the complexation constants of any substrate can be calculated once the complexation constants of its component groups are known. Moreover, on the basis of these postulates, it is possible to account for the general observations that the conjugate acid forms of carboxylic acids complex more strongly than do the conjugate bases, whereas the reverse is seen for phenols and amines.