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 surfa ctant (poly(oxyethylene) sorbitan monolaurate), was monitored using di fferential scanning calorimetry to measure the time-dependent changes of the crystallization behavior of emulsions initially containing a mi xture of pure n-hexadecane droplets (10 wt%) and pure octadecane dropl ets (10 wt%). The rate of oil exchange increased linearly with the con centration 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 sod ium 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 emuls ions containing additional surfactant in the aqueous phase increased s lightly (10-15%) during the course of the experiment, possibly due to enhanced Ostwald ripening caused by the increased solubility of the oi l in the aqueous phase. The thermodynamic driving force for oil exchan ge is the free energy of mixing, which is much greater than that for O stwald ripening. This explains why there was only a slight increase in droplet size even though complete mixing of the oils in the droplets occurred.