6533b820fe1ef96bd1279875

RESEARCH PRODUCT

Polarity profiles in reverse micelles of Triton X-100, as studied by spin probe and absorption probe techniques

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The microenvironment characteristics — polarity, viscosity and order degree - in Triton X-100 reverse micelles (RM) in cyclohexane and benzene — n-hexane 30 : 70 (v/v), were investigated with the aid of a homologous series of cationic spin probes, CAT n, of 5- and 16-doxyl stearic acids and of a new absorption probe, 4-nitropyridine-N-oxide (NP). The spectral parameters were related to local hydration values by means of a series of poly(oxyethylene) (PEO)/water calibration mixtures. All results regarding the polarities in RM and calibration mixtures have been expressed in terms of Kosower's Z values, by determining the linear dependence of the transition energies of NP, ENP, on Z, for a series of polar solvents: Z = 3.008ENP − 178.4 (in kcal mol−1). This way the relative radial positioning of all probes was established and could be compared with those of 1-methyl-8-oxyquinolinium betaine (QB) and methyl orange (MO), previously used in the same systems. A continuous variation of the polarity in the RM was evidenced with different probes, from the most polar region in the center, with the polarity of ethanol/water mixtures (CAT 1, CAT 4 and QB) to a region corresponding to tetraethylene glycol (TG)/water mixtures and triethylene glycol monoethyl ether (TGME)/water mixtures (NP and CAT 8), to a region at the limits of the core, with the polarities of the triethylene glycol dimethyl ether (TGDE)/water mixtures (MO) and to still lower values, found with the 16-doxyl probe, in the corona. At the same time, the different hydration dynamics at various depths in the micelles were followed showing earlier saturation in the outer regions of the core as compared to the central regions. The spin probes evidenced the compacting and ordering effects of water on the surfactant chains in the polar core and corona. From this point of view the RM in benzene-hexane appear to have a much lower viscosity in the polar core (over the whole range of water contents) as compared to those in cyclohexane. The order degree of the surfactant chains is lower too and the corona appears to be penetrable by an oil-soluble Cu complex, at variance with the cyclohexane system. All this data can be consistently explained by the loosening effect of benzene, solvating the surfactant chains in the corona.