The pseudophase ion exchange (PIE) model provides a satisfactory quantitative interpretation of specific counterion effects on the apparent basicity constant, KB, of benzimidazole in 0.01 M CTAX solutions at several different OH- concentrations. The change in KB with added NaX follows the counterion order Cl- < Br- < NO3-. Values for the binding constant, Ks, of the neutral form of benzimidazole in the presence and absence of added salt and the ion exchange constant, KXB, between benzimidazolide anion were estimated spectrophotometrically. Average values of KB were computed from three sets of absorbance pairs measured at three different wavelengths permitting direct estimate of the benzimidazolide concentration in the aqueous and micellar pseudophases. These KB values are significantly lower at high concentrations of added salt compared to ones calculated at a single wavelength, assuming benzimidazolide is completely micellar bound at all NaX concentrations. The calculated average KB values are simulated with the same "best set" values of the parameters previously used to fit 1/KB vs. CTAX concentration plots, including the same value for the intrinsic basicity constant, KBm. The agreement between theory and experiment up to 0.2 M added NaX is excellent within experimental limits. Between 0.2 and 1.0 NaCl the fit is less satisfactory, and potential reasons for the discrepancy are discussed.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry