DETERMINATION OF C8-C20-SATURATED ANIONIC AND CATIONIC SURFACTANT MIXTURES BY CAPILLARY ISOTACHOPHORESIS WITH CONDUCTIVITY DETECTION

Isotachophoresis combined with conductivity detection was systematically investigated as an alternative method for the determination of various classes of C-8-C20 saturated ionic surfactants, including sulphates, sulphonates and quaternary ammonium cations. Its major interest is the ability to perform separations in free electrolyte media (i.e., without any stationary phase) and in capillaries of fairly large inner diameter (typically 0.5 mm), which reduces the possibility of deleterious adsorption, often encountered in other techniques with ionic surfactants. Methanol-water mixtures of appropriate compositions were used to prevent micellization of the surfactant from occuring, thus yielding separations according to their free-form electrophoretic mobilities. Compounds differing in one carbon atom only can be fully resolved. The steady-state zone stability was ascertained by plotting analyte zone lengths versus injected amounts and by determinations of separation capacities for standard binary mixtures in various proportions. Conductimetry provides a convenient and universal detection method, with acceptable detection limits (ca. 0.1-1 nmol for a 10-mul volume injected), owing to the absence of background electrolyte in analyte zones, inherent in isotachophoretic principles. The determination of the quaternary ammonium ions contained in an industrial surfactant formulation is presented as an example.