Competitive adsorption of emulsifiers 1. Theory for adsorption of small and large molecules

Abstract

Emulsifiers can be effective only if adsorbed at the interface between oil and water, and the outcome of any competition between different emulsifiers can be of crucial importance. An extreme example is a duplex emulsion, with competition between an oil-soluble emulsifier, added to stabilise the water drops against coalescence, and a water-soluble emulsifier needed for the stability of the oil. A quantitative theory is presented for the competitive adsorption of small and large emulsifier molecules, enabling us to evaluate both the adsorptions and the interfacial tension in terms of measurable characteristic parameters of the emulsifiers present. The non-ideal entropy of mixing of small and large molecules at the oil/water interface is accounted for by introducing a simple model into the two-dimensional solution theory developed previously. The competitive adsorption of different emulsifiers can be described quantitatively, accounting for different adsorption energies and for the entropic effect of mixing different-size molecules. Characteristic parameters of each emulsifier are its saturation adsorption, which reflects the molecular size at the interface, and its half-saturation concentration, which measures the bulk-to-surface distribution constant. Any emulsifier can be displaced from the interface, with consequent loss of its functionality, by another emulsifer added in sufficiently high concentration, but small emulsifier molecules are shown to be far more efficient at displacing large ones than vice versa. The theory presented is for the non-ideal entropy of mixing in its simplest form, accounting only for the effect of different surface areas occupied by large and small molecules, but it is capable of being generalised for more complex situations through the interfacial activity coefficients. The treatment proposed covers the mixed adsorption of any surface-active materials. It should be applicable to competition between emulsifiers, such as proteins and lecithins in food emulsions, as well as to the displacement of, for example, lipolytic enzymes by detergents.