STRUCTURE OF THE POLAR CORE IN REVERSE MICELLES OF NONIONIC POLY(OXYETHYLENE) SURFACTANTS, AS STUDIED BY SPIN-PROBE AND FLUORESCENCE PROBE TECHNIQUES

Nonionic poly(oxyethylene) surfactants with four ethylene oxide units (i.e. monodisperse poly(oxyethylene) [4] lauryl ether (C(12)E(4)), Bri j 30, and poly(oxyethylene) [4] nonylphenol (NPE(4))) have been studie d in reverse micellar systems with two nonpolar solvents (cyclohexane and decane), at different concentrations, water contents, and temperat ures using two spin probes, utyl)ammonio]-2,2,6,6-tetramethylpiperidin e-1-oxyl bromide (I) and 5-doxyl stearic acid (II), and two fluorescen ce probes, 1-anilinonaphtalene-8-sulfonic acid (III) and 1-pyrenesulfo nic acid sodium salt (IV). Micellar radii lower than 20 Angstrom have been found for ''dry'' micelles from rotational correlation times of I , as determined by ESR, and from the fluorescence anisotropy decay of III, The study has focused on the determination of water distribution in the core by using the isotropic nitrogen hyperfine splitting consta nt, alpha(N), and the rotational correlation time tau(c), of the spin probe I along with fluorescence band maxima shifts of III and band int ensity ratios of IV, all data being calibrated vs the corresponding va lues in homogeneous poly(oxyethylene) (POE)/water mixtures with varyin g water proportion. A measure of the water segregation in the polar co re, with respect to the reference POE/water mixtures, is proposed. The results clearly indicate a low hydration degree of the POE chains wit h the resulting segregation of water in the polar core. The effect inc reases with water concentration and is more pronounced with cyclohexan e, as compared to decane, as solvent. Order degrees of the surfactant chains in the core have been determined from the components of the A t enser of probe II. The effects of increasing water content and tempera ture were sought. For comparison, lamellar structures have been also i nvestigated. Differences between micellar and lamellar structures, reg arding the water segregation and the dependence of order degree on wat er content and temperature, were noticed. These seem to rule out plana r structures (lamella, hanks) and to favor curved ones for the micelle s in the systems investigated.