Surpass electrokinetic analyzer

The measuring device was a Surpass electrokinetic analyzer (Anton Paar, Graz, Austria) equipped with an adjustable “gap” measuring cell. The measuring surface was 20 mm × 10 mm. The fibers were placed on two sample holders using a double-sided adhesive and the gap between the two membrane sample surfaces was 100 µm. Three different solutions were prepared for the analysis: electrolyte solution (KCl 10-3 M at pH 11), acid solution (HCl 10-1M), and basic solution (NaOH 10-1M). The traffic flow of the electrolyte solution (10-3 M of KCl) was 0 to 500 ± 0.5 mL/min, with a pressure maximum differential of 300 mbar. All measurements were carried out with a flow of approximately 100 mL/min and a maximum pressure of 300 mbar. To vary the pH in the beaker containing the electrolyte solution, automatic titration was provided by injection of micro-volumes of acid (HCl) solution. Thus, the range of pH was from the initial pH of KCl around 11 to the acid pH. Before starting any measurement, the symmetry of the flow channel was verified by monitoring the evolution of the pressure as a function of the circulation flow in both directions of flow. The procedure was then validated by checking a flow perfectly proportional to the pressure (linearity of the flow).

Membrane charge properties were characterised from streaming current measurements performed with a SurPASS electrokinetic analyzer (Anton Paar GmbH, Austria) equipped with an adjustable-gap cell. All measurements were carried out by setting the distance between the two membrane coupons in the measuring cell to 100 ± 5 μm. Streaming current was measured with a pair of Ag/AgCl electrodes by applying pressure ramps of 300 mbar. Visiolab software was used for data analysis (zeta potential determination). All experiments were performed with 10⁻³ M KCl background solutions at room temperature (25 ± 2 °C) following the experimental protocol described by Mouhoumed et al.^{35 (link)} Membranes were first equilibrated with the background solution at pH ≈ 9 (pH was adjusted with 0.1 M KOH) and then streaming current measurements were performed by progressively decreasing the pH down to about 3 by additions of 0.1 M HCl.