G Adsorption from Mixtures of Two Surfactants

Mixtures of two or more different types of surfactants often show a synergistic interaction, i.e., the interfacial properties of the mixture are more pronounced than those of the individual components by themselves. As a result, in many industrial products and processes, mixtures of different types of surfactants, rather than individual materials, are used. A study of the adsorption of the individual surface-active components in the mixture and of the interaction between them affords an understanding of the role of each and makes possible the selection in a rational, systematic manner of components for optimal properties. 

The Gibbs adsorption equation (2.17) for two surface-active solutes in dilute 

Therefore, the concentration of each surfactant at the interface can be calculated from the slope of a y-ln C (or log C) plot of each surfactant, holding the solution concentration of the other surfactant constant. 

When the absolute concentrations of the surfactants at the interface are not required, but only their relative concentrations, i.e., their relative effectiveness of adsorption, then these can be obtained in convenient fashion by use of non-ideal solution theory. 

From the thermodynamics of the system, it has been shown (Rosen and Hua, 1982) that the molar concentrations of the two surfactants in the solution phase are given by the expressions 

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