Tangential streaming potential/current measurements for the characterization of composite membranes

Abstract

The zeta-potential is an important and reliable indicator of the electrokinetic charge density. Streaming potential measurement is the most commonly used tool for determining the zeta-potential of membranes. Measurements of streaming potential can be performed in a transversal or tangential mode. Even if the tangential mode is used for some 15yrs (and also applied for a few years to tangential streaming current measurements), these measurements could have been misinterpreted for a number of studies due to the fact that the contribution of the membrane porous body to the streaming current was not considered. In this paper, this phenomenon is illustrated by performing streaming potential, streaming current and electric conductance measurements at various channel heights with three composite polymer membranes. It will be shown that: (i) the correct value of zeta-potential of external membrane surface can be determined either from coupled streaming potential coefficient (Δφs/ΔP) and electric conductance (Gt) measurements or directly from streaming current coefficient (Is/ΔP) measurements, both being necessarily performed at various channel heights, (ii) the measurement of the streaming potential coefficient alone is inefficient even if the channel height is varied, (iii) the combination of streaming potential coefficient measurements with electric conductance measurements or streaming current coefficient measurements for a single channel height is also inefficient due to the possible contribution of the membrane porous body to the streaming current (this procedure being only efficient if the membrane porous body makes a contribution only to the electric conductance), and (iv) the use of “conventional” methods to convert electrokinetic measurements into zeta-potential (by means of the Helmholtz–Smoluchowski relation, from the product (Δφs/ΔP)×Gt or Is/ΔP) can lead to an erroneous zeta-potential of external surface. In the present study, overestimated values were even found by using these classical methods, which has not been yet reported in the literature. Last, it will also be shown that the zeta-potential of pore surface of the membrane support can be determined from tangential electrokinetic measurements, which is not possible for the active layer pores.