Predicting synergism in binary mixtures of surfactants

Abstract

Chemicals in the environment continue to cause concern. One method of decreasing the environmental impact of surfactants is to design new molecules that are more efficient and effective in their interfacial activity. New types of surfactants with two hydrophilic and two or three hydrophobic groups in the molecule, called gemini surfactants, exhibit these characteristics. When properly designed, they can be as much as three orders of magnitude more efficient in reducing the surface tension of aqueous solutions than comparable conventional surfactants. A second method is to use mixtures of known surfactants that exhibit synergism in their interfacial properties. This paper will discuss the second method.

The requirements, in quantitative terms, for the existence of synergy in mixtures of surfactants have been elucidated for several interfacial phenomena. They involve an interaction (β) parameter whose various values, for mixed monolayer formation at different interfaces and for mixed micelle formation in the solution phase, can be determined from surface, interfacial, or adhesion tension data on the individual interface-active components and at least one mixture of them. This, together with relevant data on the individual components, permits one to predict not only whether the mixture will exhibit synergy, but also the ratio of the two surfactants needed to reach the point of maximum synergism and the value of the investigated property at that point. Since synergy depends so greatly on the value of the β parameter, this discussion will also cover the effect on the β value of the chemical structures of the interface-active components of the mixture, the nature of the interface, and the molecular environment.