Formation and Morphology of Reverse Micelles Formed by Nonionic Surfactants in “Dry” Organic Solvents
Sofía V. Pérez, Andres F. Olea and M. Pilar Gárate
Affiliation: Departamento de Industrias, Universidad Técnica Federico Santa María, Av. Espana 1680, Valparaíso, Chile.
The formation of reverse micelles by nonionic alcohol ethoxylates surfactants in two “dry” non polar solvents,
heptane and dibutoxymethane (DBM), has been studied. These surfactants are formed by a linear hydrocarbon chain consisting
of i carbons, and a poly(ethylene oxide) chain with j ethoxylate units (EO) ending with a hydroxyl group, CiEOj.
The study is focused on the determination of the critical micelle concentration CMC and the size and morphology of the
formed aggregates. The CMC was obtained from the decreasing of interfacial tension with increasing surfactant concentration
and by using pyrene sulfonic acid sodium salt as fluorescence probe. The results show that the CMC in heptane is
one order of magnitude higher than in DBM and two orders of magnitude higher than those determined in aqueous solution.
The self-diffusion coefficients D of C8EO5, C8EO4 and C10EO6 in heptane, were obtained by diffusion ordered spectroscopy
(DOSY 1H-NMR). The experimental values of D were then fitted to four different configurations to determine
the most probable morphology of the formed aggregates. In all cases the presence of large and compact aggregates, with
aggregation numbers going from a few dozens of monomers to a hundred of them, was shown.
Keywords: Reverse micelles, nonionic surfactants, non-polar solvents, NMR.
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