ON THE COLLOIDAL STABILITY OF WATER-IN-OIL EMULSIONS A SELF-CONSISTENT-FIELD APPROACH

We apply the self-consistent field (SCF) theory for chain molecules in inhomogeneous systems, originated by Scheutjens and Fleer, to study t he interaction between two water droplets in oil in the presence of su rfactants. The modelling of interacting polymer-covered rigid interfac es is the classical application in the SCF approach. In this paper we study two surfactant-covered liquid-liquid interfaces near contact. Th is can be done by an osmotic pressure 'experiment'. Our analysis indic ates that for (surfactant-stabilised) water-in-oil emulsions two types of colloidal instability can be distinguished: (1) coalescence (flocc ulation in the primary minimum leading to a reduction of the number of droplets) and (2) aggregation (flocculation in secondary minimum, for mation of clusters of droplets, i.e. a reduction of the number of kine tically active droplets but retaining their individuality). The surfac tant adsorbed at the water-oil interface influences these two types of instabilities in different ways. A high surfactant concentration effe ctively protects against coalescence, whereas, especially when the mol ecular weight of the oil molecules is larger than that of the surfacta nts, aggregation is promoted. Analysis of the interaction curves for l amellar interfaces enables the prediction of the macroscopic contact a ngle between two droplets. We show that this angle can vary strongly w ith the type of oil used. Protection against aggregation can be obtain ed by carefully choosing the surfactant in connection with the oil use d.