Electrophoretic mobilities of cationic analytes in non-aqueous methanol, acetonitrile and their mixtures - Influence of ionic strength and ion-pair formation

The mobilities of cationic analytes in organic solvents and water are compa red, and the reasons for differences in the mobilities are discussed in det ail. Actual mobilities (at background electrolyte concentration 10 mmol/1) of anilinium ions were determined by capillary zone electrophoresis in wate r, methanol, acetonitrile and mixtures of methanol and acetonitrile (in vol ume ratios 1:1, 1:3 and 3:1). The actual mobilities correlated with the vis cosity of the organic solvent: the products of actual mobility and viscosit y were constant within 7%. However, these products were significantly large r in water. Larger products of mobility and viscosity in water were also fo und for unsubstituted anilinium when the absolute mobility (at zero ionic s trength) was taken into consideration. Thus, ion-solvent interactions must be responsible for the seemingly high mobility in water compared with that in organic solvents. This finding can be explained by the effect of the ion on the water structure. Based on equilibrium constant for ion-pair formati on given in the literature, about 20% of the main background electrolyte co nstituent (tetrapropylammonium perchlorate) is associated at 10 mmol/l conc entration in acetonitrile. Comparison of the plot of the measured mobilitie s of the analytes vs. the square root of the corrected ionic strength of th e background electrolyte in acetonitrile with the prediction based on the D ebye-Huckel-Onsager theory showed the measured mobilities deviate negativel y from the theoretical line. This is apparently due to ion pairing, which t akes place for the analytes as well. (C) 2001 Elsevier Science B.V. All rig hts reserved.