Streaming potential -> Zeta potential

In this post I will describe recent streaming potential measurements for PES membranes and the calculated apparent zeta potentials from these measurements.

Setup:

1. Fabricate Ag/AgCl electrodes: Silver wire at the anode in 1M KCl at 1mA current for 20 minutes.

2. Assemble streaming potential device: Electrodes are inserted into silicone chord o-rings and chamber is filled with salt of interest.  Membrane is sandwiched between two viton o-rings and chamber is sealed.

3. Chamber is pressurized: water filled tubing is connected to chamber and nitrogen pressurization system.  Manometer reads pressure in system.  Pressure is changed over a range and chamber is allowed to stabilize before voltage reading (5-15 min).

Getting good electrodes was the hardest step in all of this.  Current densities must be kept very low to get a good coating of AgCl on the wires.  Here are some results for 30kDa PES in 10mM KCl:

Here are results from 100mM KCl:

The trendlines are offset, but it’s the slope that matters for our equations.  The offsets in the initial potential may be due to wires or the chamber environment.

Zeta potential is calculated from Smoluchowski equation: zeta = slope * conductivity * viscosity / permittivity

Zeta results:

10mM KCl: -5.08 +- .17 mV

100mM KCl: -7.00 +- .38 mV

These both fall in the expected range for PES (as referenced by Zydney and others).  Next up is trying this setup with pnc-Si.