Potentiometric study of molecular complexes of weak acids and bases applied to complexes of alpha-cyclodextrin with para-substituted benzoic acids.

The theory of the potentiometric methods of studying complexes of ionizable substrates was developed, nd graphical techniques are described for obtaining stability constant estimations from the data. The method described is for a system in which the conjugate acid and base forms of the substrate (S), are capable of forming 1:1 (SL) and 1:2 (SL2) complexes with the ligand (L). It was applied to complexes of alpha-cyclodextrin (cyclohexaamylose) with 10 para-substituted benzoic acid derivatives. Letting K11a and K12a be stability constants for the conjugate acid forms of the substrates, and K11b, K12b for the conjugate base forms, it was found that K12b is zero for all substrates, K12a is zero for seven of the substrates, and K11a greater than K11b in every case. Hammett plots yielded p11a and p11b values of -0.31 and 0.77, respectively, which was interpreted to mean that K11a mainly represents binding at the carboxylic acid site, and K11b describes binding at the site of the para-substituent. This model of the complexing suggests that K12a represents binding at the para-substituent, and therefore K12a should vary roughly with substituent as K11b does; this trend was observed.