COMPOSITION OF MIXED ADSORPTION LAYERS OF NONIONIC SURFACTANTS ON OIL-WATER INTERFACES

The subject of this work is the adsorption of binary mixtures of non-ionic surfactants whose molecules differ considerably in size. Since the surface layers are known to behave ideally, we investigate the role of the configurational entropy of mixing. We adopt the Gibbs model for the interface and propose a statistical treatment of the Flory-Huggins type for the case of incomplete coverage. Expressions for the surface chemical potentials are derived from the canonic partition function. The solvent-surfactant interactions are also discussed briefly and it is confirmed that for non-ionic surfactants on the oil/water boundary the mixing entropy effects are predominant. The theory allows us to develop a method for obtaining the composition of the adsorption layer using data for the interfacial tension isotherms in mixed systems and for the individual surfactants. We study xylene-water systems containing Span 20, Tween 20 and their mixtures in proportions of 1: 1 and 9: 1 (molar ratio). From the equilibrium interfacial tensions we deduce the surface composition in a wide range of concentrations. It turns out that the adsorption layer is always enriched with Tween 20 owing to its higher surface activity. The results can be helpful in connection with emulsion stability.