Surfactant-free O/W emulsion formation of oleic acid and its esters with ultrasonic dispersion

Dispersibility and stabilizing factors for surfactant-free O/W emulsion were investigated with oleic acid (OA) and its esters, focusing on the effects of their weak polarity, molecular length and branched chain structure, in comparison to normal hydrocarbons. The droplet size distributions obtained by the dynamic light scattering method appeared to be discrete but almost singly peaked except for OA. For OA monoesters, the droplet growth was found to be continuous and retarded as the ester chain length increased, in contrast to the discrete, fast growth in OA dispersion. In the case of glycerol trioleate (GTO), a branched ester, aqueous dispersions of extremely fine droplets could be prepared and the number distribution of droplet diameters showed a single peak in the nanometer range. This high dispersibility remained unchanged for about a year after preparation to give the dispersions a good stability. Changes of the observed ξ potential, Fourier transform-infra red (FT-IR) spectrum, fluorescence spectrum of probes indicated that a particular carboxyl acid group network is formed in the droplet sphere to make it more stable than expected while the interior of oil droplets is hydrophobic. The ξ potential change, in particular, was found to be highly correlated with these of the carboxyl CO stretching frequency and the reciprocal droplet diameter. The droplet stability evaluated by increase in the diameter revealed a biphasic growth consisting of fast and slow modes. The fast growth at early stages (in hours) observed for OA and methyl oleate (MO) was found to proceed by the Ostwald ripening mechanism through rate analysis. On the other hand, the slow growth at later stages (in days) found for the other esters showed a semi-logarithmic dependence on the oil viscosity. This seems to be caused by an Arrhenius-type activation factor in the stepwise flocculation/coalescence rate.