Investigation of ceramic membrane materials by streaming potential measurements

Streaming potential measurements offer the possibility to investigate the surface charge of compact solids, fibres or powders in liquid media. This measuring method bases on the fact that an electrolyte solution, which is forced to flow through a capillary system by applying a hydraulic pressure, generates a so-called streaming potential. If the pore dimensions are much larger than the dimensions of the electrochemical double layer on the pore walls, the zetapotential which is commonly used to characterize the charge density of steady interfacial layers can be calculated from the streaming potential data using the well-know Helmholtz–Smoluchowski equation. Different ceramic ultra filtration membranes are investigated by these electrokinetic measurements under changing conditions (pH, ionic strength and ion concentration). The streaming potential is determined by forcing an electrolyte solution through a narrow slit, which is formed by the top surfaces of two similar flat membranes. The experiments show that the sign of the membrane surface and pore wall charge changes within a small pH range, but the magnitude of the streaming potential and the pH of the isoelectric point strongly depend on the pre-treatment of the membranes and on the time the membranes are exposed to changed solution parameters. The time dependency of adjusting surface charges in the case of the used membranes by adsorption and desorption mechanisms becomes evident. The investigations are of great practical importance for applying inorganic membranes under changing filtering or cleaning conditions.