Predicting the effect of nonionic surfactants on dispersed droplet radii in submicron oil-in-water emulsions.

A novel theoretical model which describes the mass mean radius of oil droplets in an oil-in-water emulsion is described. A modified form of the Langmuir adsorption isotherm is used to account for nonlinear adsorption of surfactant to an oil-water film and its effect on interfacial tension and oil droplet radius. On the basis of this model, the mass mean oil droplet radius may be related to bulk surfactant concentration for a nonionic surfactant. An analysis of the mineral oil-Triton X405-water system shows that aqueous solutions of Triton X405 against mineral oil behaved in accord with the proposed model. gamma m, the maximum interfacial tension lowering by Triton X405, was estimated to be 20.9 dyne/cm. A surfactant specific apparent constant (B) which relates the rate constant for adsorption to the interface to the rate constant for desorption from the interface into the aqueous bulk was estimated as 5.44E4 cm3/g. The pressure across the curved interface, delta P, was estimated as 1.05E6 dyne/cm2. The theoretical model appears to be consistent with experimentally observed oil droplet radii and is considered to be an accurate representation of the mechanics of dispersed droplet radii under conditions of moderate load of nonionic surfactant.