Stability of oil‐in‐water emulsions I: The electrical double layer at the oil‐water interface

Abstract

Abstract The electrostatic interaction between charged oil globules in water is determined by the potential drop at the aqueous side of the interface. This potential drop is, under the conditions of the present investigation, practically entirely determined by the amount of adsorbed soap ions and the electrolyte concentration in the aqueous phase. A model is described according to which the charge in the diffuse layer is compensated by the charge of the adsorbed soap anions minus the charge of the counter‐ions situated between the ionic heads of the adsorbed soap ions. The capacity of the double layer in the oil phase may be neglected. The amount of adsorbed soap ions has been determined by measuring the dependency of the interfacial tension on the soap concentration, followed by application of the Gibbs adsorption isotherm. The potential drop at the aqueous side of the interface has been calculated; it was found to be practically independent on the soap concentration in the range of concentrations investigated. The potential drop is decreased by increasing the salt concentration; the effect of an addition of salt on the potential drop is strongly dependent on the valency of the counter ions. Determination of the zeta‐potential by measurement of electrophoretic mobility of oil droplets indicates that the values of the surface potential as calculated are of the right order of magnitude.