Laser Doppler electrophoresis and electro-osmotic flow mapping for the zeta potential measurement of positively charged membrane surfaces

[Display omitted] •Laser Doppler electrophoresis was successfully applied to positively charged membranes.•Laser Doppler electrophoresis was successful for membrane charge characterization.•Results were compared to streaming potentials and found to correlate well.•The novel technique now spans the entire membrane range. Successful characterization of membranes is of paramount importance for the development and improvement of novel membranes and membrane processes. The characterization of membrane charge is key to understanding charge interactions between the process stream and the membrane and is typically represented by the surface zeta potential. In a previous paper (Thomas et al., 2017), a novel technique employing an Uzigirs dip cell arrangement used in conjunction with laser Doppler electrophoresis was used to characterize the surface of several negatively charged membranes. In this paper, positively charged modified PTFE membranes are fabricated and the novel zeta potential measurement technique is utilized to quantify the resultant membrane charge by use of a positively charged amidine tracer particle. The amidine particles were characterized and shown to have a positive zeta potential of 12.4mV for the experimental conditions used. A comparative analysis was made between the novel laser Doppler electrophoresis measurements and tangential streaming potential measurements for the positive membrane and the agreement was good. The phase plot and mobility-displacement were of good quality for the data set, with the surface equivalent mobility being 0.632μmcm/Vs with R2=0.977. In addition, a series of experiments were conducted to explore the operating envelope and highlight the pitfalls of the technique, i.e. oppositely charged particles to the surface should not be used. Overall, this work expands the application of the novel zeta potential measurement technique to span all membrane charge types. Thus providing a real benefit to the practicing scientist or engineer by having a reliable, fast and simple zeta potential technique that uses only a very small membrane sample.