Determining the ζ-potential of plane membranes from tangential streaming potential measurements: effect of the membrane body conductance

The ζ-potential is an important and reliable indicator of the surface charge of membranes and its knowledge is essential for the design and operation of membrane processes. The streaming potential remains the most widely used technique for determining the ζ-potential of membranes. In the case of asymmetric or composite membranes, the ζ-potential is now frequently determined from tangential flow streaming potential measurements. However, the interpretation of data in terms of ζ-potential is usually carried out by doing the implicit assumption of non-conducting substrates. In this paper, we investigate the electrokinetic properties of a ceramic membrane close to the NF range, the porous structure of which affects the streaming potential because streaming and conduction currents involved in the streaming potential process do not flow through identical paths (the streaming current flows only through the slit channel formed by the two membrane samples facing each other whereas a non-negligible part of the conduction current is likely to flow through the membrane pores filled with electrolyte solution). In this case, the so-called Fairbrother and Mastin (F–M) procedure accounting for the surface conductance to correct the Helmholtz–Smoluchowski (H–S) relation is not suitable and leads to an erroneous ζ-potential. The correct ζ-potential value is determined from an extrapolation method for which a set of measurements with various channel heights is required. A very good agreement is obtained with ζ-potential values deduced from consecutive streaming potential and total conductance measurements. The contribution of the membrane body to the total conductivity of the membrane/channel/membrane system appears to be independent of the pH of solution, probably due to the fact that the electric conduction takes place through the membrane porous body inside a thick layer dominated by the support layer which then enforces its conductivity.