2.10. Surface Charge Evaluation

The Zeta potential ζ was measured on the surface of flat samples with a ZetaCAD^® (CAD Instruments, Les Essarts le roi, France) setup. The various samples were immersed for one hour in KNO₃ 10⁻³ M before being inserted in the flow cell. A pressure ramp (from 20 to 235 mBar with a step of 20 mBar) was applied to the flow cell. At each pressure step the electrolyte flows in two directions (from tank 1 to 2 and inversely). For each step and each flow direction, the resulting streaming potential was measured. The streaming potential is linked to the zeta potential ζ through the Helmholtz–Smoluchowski equation [50]. Electrolyte pH, temperature and conductivity were measured for each experiment.

The values of the zeta potential are robust as far as both repeatability and reproducibility are concerned: each sample was tested three times and each kind of surface was evaluated with three couples of samples. Therefore, the streaming potential values given for each material variation result from nine measurements. The global deviation includes both correlation and experimental deviation.