Electron spin resonance study of the pH-induced transformation of micellesto vesicles in an aqueous oleic acid/oleate system

ESR (electron spin resonance) spectra of a fatty acid spin probe ( 16-doxyl stearic acid, 16-DS) incorporated into an aqueous surfactant system compose d of oleic acid and oleate molecules were measured between 10 and 50 degree sC up to a total oleic acid + oleate concentration of 50 mM. Depending on t he total concentration and the pH, different types of oleic acid/oleate agg regates formed. At the two ends of the pH range investigated (above pH 10.4 and below pH 6.4), the ESR spectra of 16-DS were highly symmetric, enablin g calculation of the microviscosities in the surfactant aggregates to be 4 cP and 6 cP, respectively. In the high pH range, the observed aggregates ar e micelles. On the other hand, in the low pH range the microviscosity was c onsiderably lower than that of neat oleic acid (measured to be 11 cP), indi cating that the obtained emulsion system was not composed of pure oleic aci d droplets. We postulate that the surfactant molecules at low pH form conde nsed aggregates of lamellar bilayers. Asymmetric high-field ESR lines were obtained at intermediate pH between pH 6.4 and pH 10.4. This indicates that the probe molecules were present in two physically different aggregation s tates. We assigned the two aggregation states to be vesicles and nonlamella r aggregates (most likely nonspherical micelles), based on the observation made by microscopy and light scattering techniques. The analysis of the ESR lines by spectral simulation using a modified Bloch equation supports the coexistence of vesicles and nonlamellar aggregates through the entire inter mediate pH range; the relative amount of the two aggregation forms depends critically on pH, temperature, and concentration. Furthermore, the spectral simulation indicated that particularly stable oleic acid/oleate vesicles a re formed around pH 8.5, where the protonated and ionized species exist in a stoichiometric ratio.