Interaction of hydrocarbon and perfluorinated surfactants in mixed systems. Effects on the reactivity of organic compounds.

MARIANA ADELA FERNÁNDEZ; PABLO SEBASTIÁN SALES; MARÍA FLORENCIA TORRES; RITA HOYOS DE ROSSI

Los Cocos, Córdoba, Argentina

Conferencia; 9th Latin American Conference on Physical Organic Chemistry; 2007

Latin American Comitee

Surfactants are versatile compounds widely used in a variety of industrial and commercial applications. Surfactant mixtures 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. Interaction between binary surfactant mixtures containing a nonionic hydrocarbon surfactant (polyoxyethylene(23)lauryl ether, Brij-35) and an anionic perfluorinated surfactant (perfluorononanoic acid) in water were studied at different mole fractions (0-1), measuring fluorescence of pyrene as molecular probe. The composition of mixed micelles and the interaction parameter, b, were calculated from the critical micellar concentration (CMC) of the different systems using the regular solution model for non-ideal mixing. The b value is a measure of the intensity of the interaction between two surfactants. In our experiments the b value obtained is –3.08. The negative sign implies attractive interactions and synergism in the system studied. The synergistic behavior can be explained assuming that in the mixed micelle, ethoxylated chains of the nonionic surfactant coil around the charged head groups of the anionic surfactant, screening the electrostatic repulsions and favoring micelle formation, which results in low CMC values. Rate data for the hydrolysis reaction of p-nitrophenyl perfluorooctanoate in the presence of the same mixed systems has been obtained at 25° C over a wide range of total surfactant concentration and perfluorinated surfactant mole fractions. The kinetic data showed that at surfactant concentrations below the CMC of the mixture and at all mole fraction examined, the rate constant values are not significantly influenced by the increase in the concentration of surfactants. Beyond the CMC of the mixed system, the rate constant increases with the increase of total concentration of surfactant, until the CMC of pure perfluorononanoic acid is reached. After that, a decrease of rate constants is observed when the total surfactant concentration continues raising. The different behavior observed under several reaction conditions will be explained taking into account the different types of aggregates involved and discussing their role in the hydrolysis reaction of the studied substrate. b, were calculated from the critical micellar concentration (CMC) of the different systems using the regular solution model for non-ideal mixing. The b value is a measure of the intensity of the interaction between two surfactants. In our experiments the b value obtained is –3.08. The negative sign implies attractive interactions and synergism in the system studied. The synergistic behavior can be explained assuming that in the mixed micelle, ethoxylated chains of the nonionic surfactant coil around the charged head groups of the anionic surfactant, screening the electrostatic repulsions and favoring micelle formation, which results in low CMC values. Rate data for the hydrolysis reaction of p-nitrophenyl perfluorooctanoate in the presence of the same mixed systems has been obtained at 25° C over a wide range of total surfactant concentration and perfluorinated surfactant mole fractions. The kinetic data showed that at surfactant concentrations below the CMC of the mixture and at all mole fraction examined, the rate constant values are not significantly influenced by the increase in the concentration of surfactants. Beyond the CMC of the mixed system, the rate constant increases with the increase of total concentration of surfactant, until the CMC of pure perfluorononanoic acid is reached. After that, a decrease of rate constants is observed when the total surfactant concentration continues raising. The different behavior observed under several reaction conditions will be explained taking into account the different types of aggregates involved and discussing their role in the hydrolysis reaction of the studied substrate.