ADSORPTION OF AMPHIPHILES AT WATER AIR INTERFACES

The phase diagrams of H2O-alcohol (C(i)E0), and H2O-nonionic amphiphile (C(i)E(j)) mixtures show some striking similarities. This holds, in particular, for the concentration dependence of the interfacial tension sigma against air. Even short-chain alcohols show an indication for the onset of association of the alcohol molecules in the bulk phase that makes drawing a borderline between associated and micellar solutions difficult. Between zero concentration and the cmc, the concentration dependence of sigma of aqueous solutions of both short-chain alcohols and medium- and long-chain amphiphiles can be fitted surprisingly well with a single parameter-alpha (= RTT) to the well-known equation for the interfacial tension of ideal binary mixtures assuming an unimolecular interfacial layer with equal areas per molecule of the two components. This also holds for the dependence of the interfacial tension of water/oil interfaces on the concentration of both ionic and nonionic amphiphiles. This raises the question as to why such a simple model suffices for describing the experiments, in particular, why hardly measurable changes of gamma lead to a strong decrease of sigma (in the case of water/oil interfaces by several orders of magnitude).