Electron Spin Resonance Study of the pH-Induced Transformation of Micelles to Vesicles in an Aqueous Oleic Acid/Oleate System

ESR (electron spin resonance) spectra of a fatty acid spin probe (16-doxylstearic acid, 16-DS) incorporated into an aqueous surfactant system composed of oleic acid and oleate molecules were measured between 10 and 50 °C up to a total oleic acid + oleate concentration of 50 mM. Depending on the total concentration and the pH, different types of oleic acid/oleate aggregates 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, enabling calculation of the microviscosities in the surfactant aggregates to be 4 cP and 6 cP, respectively. In the high pH range, the observed aggregates are micelles. On the other hand, in the low pH range the microviscosity was considerably lower than that of neat oleic acid (measured to be 11 cP), indicating that the obtained emulsion system was not composed of pure oleic acid droplets. We postulate that the surfactant molecules at low pH form condensed 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 states. We assigned the two aggregation states to be vesicles and nonlamellar 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 intermediate 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 are formed around pH 8.5, where the protonated and ionized species exist in a stoichiometric ratio.