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 ζ-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 ζ-potential decreases in magnitude when
poly(oxyethylene) chain nonionic surfactants are
adsorbed at the interface.
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