Determination of Critical Micelle Concentration of Anionic Surfactants by Capillary Electrophoresis Using 2-Naphthalenemethanol as a Marker for Micelle Formation

A method for the determination of the critical micelle concentration (CMC) of anionic surfactants by capillary elec-trophoresis (CE) has been developed. The method is based on the measurement of migration time of 2-naphthalene-methanol as a marker compound in CE, using varying concentrations of a desired surfactant in phosphate buffer (pH 7.0) as electrolyte solution. When the migration time of 2-naphthalenemethanol was plotted against the surfactant concentration, an inflection point appeared which corresponded to the CMC. By the CE method, the CMC values of seven anionic surfactants including sodium salts of alkyl sulfate, alkylsulfonic acid, fatty acid and bile acid were successfully determined; the CMC of sodium dodecyl sulfate determined by the CE method was 3.92 mM, which agreed closely with those measured by a dye solubilization method (3.53 mM) and a conductometric method (4.02 mM). The present CE method enabled the determination of CMC, if required, using as little as 300 μl or smaller volumes each of varying concentrations of surfactant solutions.