A COMPARATIVE-STUDY OF THE DETERMINATION OF THE CRITICAL MICELLE CONCENTRATION BY CONDUCTIVITY AND DIELECTRIC-CONSTANT MEASUREMENTS

Conductivities and dielectric constant measurements in water at 25 deg rees C have been made on the amphiphilics sodium n-dodecyl sulfate, n- dodecyltrimethylammonium bromide, and chlorpromazine hydrochloride. By using the conductivity/concentration data, critical micelle concentra tions (cmc) have been determined by applying the Williams definition a nd two forms of the Phillips method. This first Phillips form consists of an approximation to Gaussians of the second derivative of the cond uctivity/concentration data followed by two consecutive integrations. The second form, which is proposed here, consists of the application o f a combination of the Runge-Kutta numerical integrations method and t he Levenberg-Marquardt least-squares fitting algorithm. The proposed m ethod permits the determination of the cmc in systems with low aggrega tion numbers and with slow variations of physical property/concentrati on curves allowing the determination of the so-called second cmc. A co mparative study with results obtained by dielectric constant measureme nts has been carried out. With this new technique, the cmc's (first an d second) are directly obtained as singular points in the dielectric c onstant/concentration curves, and thus, this technique is an alternati ve to the determination of cmc's from conductivities.