FACTORS THAT AFFECT THE RATE OF OIL EXCHANGE BETWEEN OIL-IN-WATER EMULSION DROPLETS STABILIZED BY A NONIONIC SURFACTANT - DROPLET SIZE, SURFACTANT CONCENTRATION, AND IONIC-STRENGTH

Oil exchange between emulsion droplets, stabilized by a nonionic surfactant (poly(oxyethylene) sorbitan monolaurate), was monitored using differential scanning calorimetry to measure the time-dependent changes of the crystallization behavior of emulsions initially containing a mixture of pure n-hexadecane droplets (10 wt%) and pure octadecane droplets (10 wt%). The rate of oil exchange increased linearly with the concentration of surfactant added to the aqueous phase (0-4 wt%) and with the droplet surface area (for d32 = 0.4-1.0 muM). The addition of sodium chloride (0-2 wt%) to the emulsions slightly decreased the rate of oil exchange. These observations support the hypothesis that the oil exists in a dynamic equilibrium between that in emulsion droplets and that solubilized by surfactant micelles. The droplet size of the emulsions containing additional surfactant in the aqueous phase increased slightly (10-15%) during the course of the experiment, possibly due to enhanced Ostwald ripening caused by the increased solubility of the oil in the aqueous phase. The thermodynamic driving force for oil exchange is the free energy of mixing, which is much greater than that for Ostwald ripening. This explains why there was only a slight increase in droplet size even though complete mixing of the oils in the droplets occurred.