Electrokinetic characterisation of ultrafiltration membranes by streaming potential, electroviscous effect, and salt retention

The possibility of determining the zeta potentials of ultrafiltration membranes from measured electroviscous effects and salt retentions was investigated. Therefore, electroviscous effects, salt retentions and streaming potentials of various Millipore ultrafiltration membranes were measured at various salt concentrations and pH. Using highly simplified models, apparent zeta potentials could be calculated from the obtained data. Values of zeta potential thus obtained were in qualitative agreement, the values of iso-electric point obtained from the different methods being very similar. However, the absolute values of the zeta potentials obtained from the three different methods were quite different; results obtained from electroviscosity and salt retention measurements were often smaller than those obtained from streaming potential measurements. This seemed to depend on the accuracy of the equations, used to calculate the zeta potential, and thus not on inherent defects of the techniques.