Charging of oil-water interfaces due to spontaneous adsorption of hydroxyl ions

The electrophoretic mobility of oil droplets, dispersed without any surfactant in the aqueous phase, was measured. Four different oils were studied: xylene, dodecane, hexadecane, and perfluoromethyldecalin. Special precautions were undertaken to avoid artifacts caused by the presence of surfactant impurities. The results show that the oil droplets are negatively charged and the magnitude of their zeta-potential strongly depends on pH and the ionic strength of the aqueous phase. The electrophoretic mobility is almost independent of the type of specific nonpolar oil. Series of experiments were performed to check different hypotheses about the origin of the spontaneous charging of the oil-water interfaces. The results lead to the conclusion that hydroxyl ions, released by the dissociation-association equilibrium of the water molecules, adsorb at the oil-water interface. The specific adsorption energy was estimated to be 25kT per ion (kT is the thermal energy). The molecular origin and the implications of this phenomenon are discussed. The zeta-potential decreases in magnitude when poly(oxyethylene) chain nonionic surfactants are adsorbed at the interface.