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 wer e 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 s ingly peaked except for OA. For OA monoesters, the droplet growth was found to be continuous and retarded as the ester chain length increased, in cont rast to the discrete, fast growth in OA dispersion. In the case of glycerol trioleate (GTO), a branched ester, aqueous dispersions of extremely fine d roplets could be prepared and the number distribution of droplet diameters showed a single peak in the nanometer range. This high dispersibility remai ned unchanged for about a year after preparation to give the dispersions a good stability. Changes of the observed xi potential, Fourier transform-inf ra red (FT-IR) spectrum, fluorescence spectrum of probes indicated that a p articular carboxyl acid group network is formed in the droplet sphere to ma ke it more stable than expected while the interior of oil droplets is hydro phobic. The xi potential change, in particular, was found to be highly corr elated with these of the carboxyl C=O stretching frequency and the reciproc al droplet diameter. The droplet stability evaluated by increase in the dia meter revealed a biphasic growth consisting of fast and slow modes. The fas t growth at early stages tin hours) observed for OA and methyl oleate (MO) was found to proceed by the Ostwald ripening mechanism through rate analysi s. On the other hand, the slow growth at later stages tin days) found for t he other esters showed a semi-logarithmic dependence on the oil viscosity. This seems to be caused by an Arrhenius-type activation factor in the stepw ise flocculation/coalescence rate. (C) 2001 Elsevier Science B.V. All right s reserved.