Determination of Critical Micellar Concentration in Mixed Surfactant Systems by A Kinetic Method

MARÍA FLORENCIA TORRES; MARIANA ADELA FERNÁNDEZ; RITA HOYOS DE ROSSI

Florianópolis, Brasil

Conferencia; 10th Latin American Conference on Physical Organic Chemistry; 2009

Surfactants are versatile compounds widely used in a variety of industrial and commercial applications. Mixtures of surfactants of different types form mixed micellar aggregates that frequently exhibit characteristic properties remarkably different and with superior performance than the individual components. Hence, the determination of the properties of mixed surfactant solutions as well as the effect of these systems on the reactivity of organic compounds is of current interest. In this work we studied the hydrolysis of phenyl trifluoroacetate in mixed micellar systems. The mixture contained a nonionic hydrocarbon surfactant (polyoxyethylene(23)lauryl ether, Brij-35) and an anionic perfluorinated surfactant (perfluorononanoic acid) in water. This mixture was previously studied using diverse techniques (UV-Visible spectrophotometry and fluorescence spectroscopy with molecular probes, and 19F Nuclear Magnetic Resonance (19F-NMR)), and it was established that two different aggregates are formed in this mixture: one rich in the hydrocarbonated surfactant and the other one rich in the perfluorinated one. The rate data for the hydrolysis reaction of phenyl trifluoroacetate have been obtained at 25° C over a wide range of total surfactant concentration and perfluorinated surfactant mole fractions. The kinetic data showed that there are two different zones: one at low total surfactant concentration, where the rate constant values are not significantly influenced by the increase in the concentration of surfactants; and another zone where a decrease of rate constants is observed when the total surfactant concentration continues raising. This change in behavior is coincident with the critical micellar concentration (cmc) determined using UV-Visible, which corresponds to the fluor-rich aggregate. The other cmc, is not detected in this way but it can be seen by fluorescence or 19F-NMR. This is probably due to the fact that in fluorescence we use a hydrocarbonated probe while in UV-Visible and in this study the probe is fluorinated. So, the hydrocarbon probes only sense the hydrocarbon-rich aggregate while the fluorinated compounds sense the fluor-rich aggregate. By NMR, which is independent of probes, it was possible to detect both simultaneously. Because of the good agreement between other techniques and this reaction is that we propose it as a good method for determining cmc in mixed micellar systems containing perfluorinated surfactants.