Determination of critical micelle concentration by capillary electrophoresis. Theoretical approach and validation

The aim of this study was the determination of the critical micelle concentration (CMC) of the most commonly employed anionic surfactant in micellar electrokinetic chromatography, sodium dodecyl sulphate (SDS), using capillary electrophoresis, i.e., under the operating conditions of the electrophoretic separation (pH, ionic strength, temperature, etc.). The effective electrophoretic mobility of a neutral compound, resulting from the solvophobic and micellar contributions, appeared to be a well adapted parameter for the study of the micellization process of anionic surfactants. In fact, the theoretical treatment of the evolution of this parameter as a function of the total surfactant concentration allowed the identification of a sharp change in slope at the cmc, and therefore to establish capillary electrophoresis as a new analytical tool for the determination of critical micelle concentrations. In the experimental case of an electrolytic solution consisting of 5 mmol l(-1) borax, the observed value of the cmc of SDS (5.29 mmol l(-1)) was in total agreement with the literature data when the sodium concentration in the solution was the same.