MECHANISM OF ELECTROKINETIC SEPARATIONS OF HYDROPHOBIC COMPOUNDS WITHSODIUM DODECYL-SULFATE IN ACETONITRILE-WATER MIXTURES

The effect of the addition of 0-70% (v/v) acetonitrile (AcCN) as organ ic modifier, into a buffer solution containing 0-60 mmol L-1 sodium do decyl sulfate (SDS), ion the electrokinetic behavior of neutral hydrop hobic compounds has been studied experimentally. The effective mobilit ies of the hydrophobic compounds sharply decreased above a certain AcC N concentration, depending to some extent on the SDS concentration, In solutions with up to 40% AcCN, with SDS concentrations below the lite rature values of the critical micelle concentration, neutral hydrophob ic analytes still showed a considerable electrophoretic mobility; The observed mobilities were too high to be attributed to solvophobic inte raction of the analytes with single SDS ions, From conductivity measur ements, 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 th e 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 decr ease of the micellar volume fraction, a change of the micellar propert ies, acid a decrease of the distribution constants of the analytes.