high electrolyte concentrations, well above the critical coag-Streaming potential and conductivity measurements were perulation concentration. In microelectrophoresis, coagulation formed on porous plugs in a number of indifferent electrolytes. would interfere with the measurements.
The plugs were made of monodisperse spherical polystyrene sul-
As compared to microelectrophoresis, the electric conducfate latex with fixed surface charge density. The observed surface tivity of a plug yields additional information. The surface conductivity could be interpreted with the classical Bikerman exconductivity, which is calculated from the plug conductivity, pression extended with the contribution from conduction behind is an important double-layer characteristic. Knowledge of the shear plane. It was shown that electroosmosis contributes posithe surface conductivity is necessary to determine z-potentively, while the depletion of co-ions in the double layer contributes tials from streaming potentials (or particle mobilities) at negatively to the surface conductivity. It was further demonstrated that a significant part of the conduction is located behind the slip low electrolyte concentrations if conduction behind the shear plane. The ion mobility of the indifferent counterions behind, as plane takes place. Furthermore, from the combination of well as beyond, the shear plane is close to the bulk mobility. In conductivity and streaming potential experiments, informathe correct determination of z-potentials in the low electrolyte tion on the amount of conduction behind and beyond the concentration range, it is necessary to take surface conduction shear plane can be evaluated. behind the plane of shear into account. When this is done, the Although interpretation of conductivity data is more difmaximum in the curve of the z-potential versus electrolyte concenficult for plugs than for dilute dispersions, the measurements tration disappears. ᭧ 1998 Academic Press are extremely sensitive to the double-layer properties. The