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-i onic surfactants whose molecules differ considerably in size. Since th e 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-Huggin s type for the case of incomplete coverage. Expressions for the surfac e 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 d evelop 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 containi ng Span 20, Tween 20 and their mixtures in proportions of 1: 1 and 9: 1 (molar ratio). From the equilibrium interfacial tensions we deduce t he surface composition in a wide range of concentrations. It turns out that the adsorption layer is always enriched with Tween 20 owing to i ts higher surface activity. The results can be helpful in connection w ith emulsion stability.