Abstract
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 hig...
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