Study of the double layer at an insulator/liquid interface by step voltage transients

The electrical behaviour of dielectric liquids is strongly influenced by the double layer at solid/liquid interfaces. When the solid is a metal, the injection of ions depends on the structure of this layer. A new method is proposed which evidences the presence of such a layer at an insulator/liquid interface. A liquid containing a known amount of electrolyte is subjected to a voltage step, the electrodes being covered with polymer films. From the shape of the transient currents, the total charge displaced across the liquid gap is measured, and the corresponding zeta potential calculated. zeta depends mainly on electrolyte dissolved in the liquid for a given insulator/liquid pair and is much higher than the known values at a metal/liquid interface. A simple model considering the diffuse layer as a charged sheet may be applied. However, at voltages high enough, electrohydrodynamic motion of the liquid contributes to the convective transport of charge in a similar way to that already studied in the case of unipolar injection in insulating liquids.