CHARGING OF OIL-WATER INTERFACES DUE TO SPONTANEOUS ADSORPTION OF HYDROXYL IONS

The electrophoretic mobility of oil droplets, dispersed without any su rfactant in the aqueous phase, was measured. Four different oils were studied: xylene, dodecane, hexadecane, and perfluoromethyldecalin. Spe cial precautions were undertaken to avoid artifacts caused by the pres ence of surfactant impurities. The results show that the oil droplets are negatively charged and the magnitude of their zeta-potential stron gly depends on pH and the ionic strength of the aqueous phase. The ele ctrophoretic mobility is almost independent of the type of specific no npolar oil. Series of experiments were performed to check different hy potheses about the origin of the spontaneous charging of the oil-water interfaces. The results lead to the conclusion that hydroxyl ions, re leased by the dissociation-association equilibrium of the water molecu les, adsorb at the oil-water interface. The specific adsorption energy was estimated to be 25kT per ion (kT is the thermal energy). The mole cular origin and the implications of this phenomenon are discussed. Th e zeta-potential decreases in magnitude when poly(oxyethylene) chain n onionic surfactants are adsorbed at the interface.