Prof. Dr. Anthony SZYMCZYK
Electrokinetic characterization of membranes: past, present and future
The zeta potential of membranes can be inferred from electrokinetic methods such as streaming potential and streaming current. Since these techniques permit to consider the compensation of the surface charge by the ions within the electrical double layer on the liquid side of the interface, they are particularly useful when investigating problems of practical relevance. Assessing the zeta potential of membranes is particularly attractive to provide insight into the charge formation process of membrane surfaces but also to investigate surface functionalization, fouling, ageing... From the experimental point of view, the easiest way to perform the electrokinetic characterization of membranes is to implement transversal (i.e. through-pore) measurements. However, the multilayer structure of many membranes makes the determination of the skin layer properties quite tricky. In order to overcome this issue, tangential measurements (streaming potential or streaming current) based on the application of a pressure gradient along the membrane skin layer have been developed and have become the most widely used method for determining the zeta potential of membranes.
However, complications have been pointed out when such tangential electrokinetic measurements are applied to membranes for which a part of the experimental signal is likely to pass through the membrane matrix (and not only on the external surface of the skin layer) [1]. This parasitic contribution to the experimental signal, referred to as electrokinetic leakage [2], has been largely overlooked in the literature although it may lead to a relatively inaccurate determination of the zeta potential or even to misleading conclusions about the surface properties of membranes. However, relevant information on the membrane matrix can extracted from electrokinetic leakage since the membrane is crossed by this parasitic phenomenon. Thus, we will show that electrokinetic leakage can be used as a probe for investigating various phenomena occurring inside the membrane matrix, paying special attention to fouling and ageing issues.
[1] A. Szymczyk, Y. Ibrahim Dirir, M. Picot, I. Nicolas, F. Barrière, J. Membr. Sci. 429 (2013) 44-51.
[2] C. Rouquié, S. Liu, M. Rabiller-Baudry, A. Riaublanc, M. Frappart, E. Couallier, A. Szymczyk, J. Membr. Sci. 599 (2020) 117707.