Mechanism of Electrokinetic Separations of Hydrophobic Compounds with Sodium Dodecyl Sulfate in Acetonitrile−Water Mixtures

The effect of the addition of 0−70% (v/v) acetonitrile (AcCN) as organic modifier, into a buffer solution containing 0−60 mmol L - 1 sodium dodecyl sulfate (SDS), on the electrokinetic behavior of neutral hydrophobic compounds has been studied experimentally. The effective mobilities of the hydrophobic compounds sharply decreased above a certain AcCN concentration, depending to some extent on the SDS concentration. In solutions with up to 40% AcCN, with SDS concentrations below the literature values of the critical micelle concentration, neutral hydrophobic analytes still showed a considerable electrophoretic mobility. The observed mobilities were too high to be attributed to solvophobic interaction of the analytes with single SDS ions. From conductivity measurements, a strong indication was found that increasing the fraction of AcCN in the SDS solution does not lead to a complete disintegration of the micelles. Even with high AcCN concentrations, the mechanism of the separation was found to be a distribution between the aqueous phase and micelle-like SDS aggregates. The influence of the addition of AcCN on the mobilities could be explained as the combined effect of a decrease of the micellar volume fraction, a change of the micellar properties, and a decrease of the distribution constants of the analytes.