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The solubility and aggregation process of polyethoxylated non-ionic surfactants, of general formula C i H 2i?1 -(O-CH 2 -CH 2 ) j -OH with i = 6, 8, 10 and j = 3-6 (C i EO j ), in heptane were studied. The aggregation of C i EO j surfactants in heptane was investigated by using methylene blue (MB) as an absorption probe. In solutions of MB in the presence of these surfactants in heptane, at concentrations larger than the re-dissolution concentration, the UV bands associated to free MB (A 1 ) and MB-EO complex (A 2 ) were detected. The ratio of these intensities A 2 /A 1 , was used to study the kinetics of the complex formation in pure surfactant. The value of A 2 /A 1 depends on the surfactant structure and the media wherein MB is dissolved, being larger in the pure surfactant than in heptane solutions. These results are explained in terms of solvent effect and aggregate structures on the complex formation.

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 (1)H-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.