Surfactant Structure and Emulsion Performance

It would be nice if the world of emulsion formulation were such that a simple correlation could be obtained between the chemical structure of a surfactant and its performance in practice. Unfortunately, the complicated nature of typical emulsion formulations (the nature of the oil phase, additives in the liquid phases, specific surfactant interactions, etc.) make correlations between surfactant structure and properties in emulsification processes very empirical. 

In the absence of a handy quantitative and absolute method for choosing a surfactant for a given application, it is possible to outline a few rules of thumb that have historically proved useful for narrowing down the possibilities and limiting the amount of experimentation required for the final selection of surfactant(s) for a given application. First and foremost, of course, the 

The surfactant (or surfactants) employed in an emulsion formulation should produce as strong an interfacial film as possible, consistent with their ability to produce the required droplet size under the conditions of emulsification. It is useful, therefore, to choose a surfactant system with maximum lateral interaction among the surfactant molecules concurrent with efficient and effective lowering of the interfacial tension. 

At the molecular level, the choice of surfactant for a given application must take into consideration the type of emulsion desired and the nature of the oil phase. As a general rule, oil-soluble surfactants will preferentially produce w/o emulsions while water-soluble surfactants yield o/w systems. Because of the role of the interfacial layer in emulsion stabilization, it is often found that a mixture of surfactants with widely differing solubility properties will produce emulsions with enhanced stability. Finally, it is usually safe to say that the more polar the oil phase, the more polar will be the surfactant required to provide optimum emulsification and stability. Such rules of thumb, while having great practical utility, are less than satisfying on a theoretical level. One would really like to have a neat, quantitative formula for the design of complete emulsion systems. A number of attempts have been made over the years to develop just such a quantitative approach to surfactant selection. Some such approaches are briefly discussed. 

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