Capillary zone electrophoresis in mixed aqueous-organic media: effect of organic solvents on actual ionic mobilities and acidity constants of substituted aromatic acids - III. 1-Propanol

Apparent mixed-mode pK(a) values of five and actual mobilities of 26 substituted benzoates were investigated in mixed aqueous-propanolic solvent systems containing up to 80% (v/v) alcohol. The pK(a) values (determined by potentiometric titration) increase by 2 to 2.5 pK units compared to water, similar to ethanol. The change of the pK(a) values upon addition of the alcohol is interpreted by the aid of the transfer activity coefficient or medium effect, and discussed in the context of the previously investigated alcohols. The actual mobilities (measured by capillary zone electrophoresis, 25.0 degrees C, phosphate buffer, pH 7 in water, ionic strength 20 mmol/l) decrease with increasing propanol content. As in previous papers with methanol and ethanol as cosolvent the solutes investigated can be differentiated concerning their dependence of the relative Walden product analogue on the alcohol content: one group is formed by the methyl, nitro and chloro derivatives of benzoate, whereas to the other group all OH-substituted anions belong. Finally the change in actual mobility of the substituted aromatic anions (related to unsubstituted benzoate) is found to be more pronounced (similar to the ethanolic systems) for most of the OH-substituted carboxylates, as compared to the methyl, chloro and nitro derivatives. (C) 1998 Elsevier Science B.V. All rights reserved.