Electrokinetic characterisation techniques on asymmetric microfiltration membranes

An investigation using the transversal streaming potential in asymmetric microfiltration membrane was undertaken. The results obtained were compared to other membrane surface characterisation techniques, namely electroviscous effects and electrophoresis. Ground particles of the composite membranes as well as particles ground from the membrane surfaces were used in electrophoretic mobility studies whilst streaming potential and electroviscous measurements were carried out on the membrane itself. The zeta potentials derived from each measurement show similar trends with increasing ionic strength and pH. Iso-electric points obtained from streaming potentials, electroviscous measurements and electrophoresis (particles from the membrane surface) were in agreement, i.e. pH 5.6 ± 0.1, however, an iso-electric point (i.e.p.) of pH 4.8 ± 0.1 was found with electrophoresis with the ground composite membrane. Different morphologies from the two different types of alumina layers within the asymmetric composite membrane could have attributed to this observed difference. The similarities in the observed i.e.p. of the in situ techniques and electrophoresis carried out using the particles from the membrane surface show that the transverse streaming potential is able to identify the specific alumina characteristics of the material that forms the membrane surface. The trend in calculated zeta potentials in ascending order is: electrophoresis carried out on particles obtained from the whole membrane, ζem(whole) < streaming potential, ζsp, < ζev, the zeta potential obtained from measurement of electroviscous effects < electrophoresis carried out on particles obtained from the membrane surface, ζem(surf). Surface chemistry accounts for the differences in the ζ obtained between ζem(surf) and the other two in situ techniques. The lowest zeta potential, ζem(whole), obtained arises from the combined measurements of the alumina particles of the support layer. It seems that the support layer does not contribute significantly to the streaming potential in the measurements made on the microfiltration membranes used in this study.