STRUCTURE OF REVERSED MICELLES IN THE CYCLOHEXANE POLYOXYETHYLENE(4)NONYLPHENOL WATER-SYSTEM, AS STUDIED BY THE SPIN PROBE TECHNIQUE

The cyclohexane/polyoxyethylene(4)nonylphenol (EO(4)NP)/water system (2.8% EO(4)NP) with [H2O]/[EO(4)NP] molar ratios between 0.1 and 6.9, has been investigated by electron spin resonance (ESR) spectroscopy using the following nitroxides: 3-carboxy-2,2,5,5-tetramethylpyrroline-1-oxyl (I), 2,5-dihydro-2,2,5,5-tetramethyl-3-[(triethylammonio)methyl]pyrrole-1-oxyl bromide (II), and 5-doxylstearic acid (IV) as spin probes. For radicals I and II the nitrogen hyperfine splitting, a(N), and the rotational correlation time, tau(c), have been analyzed, yielding information about the polarity and viscosity of their locations. At [H2O]/[EO(4)NP] less-than-or-equal-to 0.3, the presence of "waterless' micellar aggregates in fast exchange (nu(ex) > 10(6) s-1) with the monomer surfactant is evident. At [H2O]/[EO(4)NP] = 0.6, the core viscosity is at its maximum value and the exchange rate decreases below the limit of slow exchange (nu(ex) < 10(6) s-1). Upon further increasing the water content, the core becomes more polar and less viscous, with these parameters approaching their values in bulk water. The effect of Cu(II) ion (in an oil-soluble complex) on the spectra of radical II-confined to the water-pool-has been interpreted by using Leigh's theory, and the "distance of closest approach" between these two paramagnetic species has been evaluated. This distance is considered to represent a measure of the penetrability of the micellar shell. Radical IV yielded anisotropic spectra in the aggregates with slow exchange. From their ESR parameters the order degree of the surfactant chains in the micellar shell has been evaluated. All results are consistent, indicating reduced penetrability and increased order with increasing dimensions of the water-pool.