Kinetic investigations of the interaction between sodium dodecyl sulfate and the nonionic surfactants CmEn. Electrical conductivity and fluorescence probe measurements

Abstract

Electrical conductivity, steady-state quenching fluorescence measurements, and the kinetic study of the acid hydrolysis reaction of 1-phenylethyl nitrite were used to investigate the structure of micelles arising from mixed surfactant solutions. The mixed micellar systems investigated were sodium dodecyl sulfate (SDS) with the poly(ethylene oxide) surfactants of type CmEn, i.e., CH3–(CH2)m-1–(OCH2CH2)n–OH, (C12E4, C12E9 and C16E10) in aqueous medium at a fixed and variable molar ratio: r=[CmEn]/[SDS]. The conductivity results indicate a change in the nature of the mixed micelles as r increases. At high r values and high surfactant concentration (0.10 mol dm-3) solvent effects due to intermicellar interactions, together with micellar effects, seem to arise. Fluorescence measurements do not indicate important changes in the polarity and size of the mixed micelles which are formed at low r values; conductivity and kinetic measurements are consistent with the formation of mixed micelles more ionized and more hydrated than pure SDS micelles. At high r values, the complexity of the system increases; it is possible that poly(ethylene oxide) surfactants of large polar head groups destroy the ordered structure of water. The important catalysis observed in the acid hydrolysis of 1-phenylethyl nitrite in the presence of pure SDS micelles becomes a strong inhibition when the content of the nonionic surfactant in the mixed surfactant system is high.