Capillary zone electrophoresis in mixed aqueous–organic media: effect of organic solvents on actual ionic mobilities, acidity constants and separation selectivity of substituted aromatic acids.: I. Methanol

Abstract

The impact on the actual mobility, μ act, of 25 substituted aromatic single charged carboxylates (with hydroxy, methyl, nitro or chloro substituents) and benzoate by addition of up to 75% (v/v) of methanol to the background electrolyte (phosphate buffer, pH 7 in water, ionic strength 20 mmol l −1) was investigated systematically. Furthermore, the magnitude of the changes in the pK a values of five of the benzoic acids in the mixed aqueous–organic environment (up to about 80%, v/v, methanol), which was found to be 1.5–2 units higher compared to pure aqueous solutions, was determined by potentiometric titration. The solutes were classified based on the change of their actual mobility by cluster analysis with a hierarchical clustering procedure. The influence of methanol on the actual mobility was discussed concerning the viscosity of the solution, showing that Walden's rule is obeyed only roughly. A different behavior of the mobilities of hydroxy-substituted acids upon addition of methanol was found compared to the methyl, nitro and chloro derivatives, as their actual mobilities were reduced to a markedly larger extent. This effect was related to the hydrogen bonding ability of the OH groups of the former class of anions. Generally, enhancement in selectivity (expressed by the selectivity coefficient, the ratio of the actual mobility of benzoate, used as reference ion, to that of the particular separand) by adding methanol as organic modifier was found only for the acids with hydroxy substituents. For the other solutes in general selectivity decreased upon addition of methanol.