Zetapotential Measurements of pnc-Si, SiN, and Stripped SiN Membranes

ByKarl Smith

My streaming potential device is now working consistently. It turns out that the problem was the concentrations of salt that I was using. The Debye Length (the distance over which charge-charge interactions are important in an electrolyte) is given as K^{-1} = frac{0.304}{sqrt{I(M)}}, where  K^{-1} is the debye length in nm and I(M) is the molar concentration of a monovalent salt at 25^circ C. The following table is illustrative:

KCl Concentration Debye Length
100 mM 1 nm
10 mM 3 nm
1 mM 9 nm

For a 30 nm diameter pore, the donut representing the debye length inside the pore takes up 13% of the pore volume at 100mM KCl, and 95% at 1 mM. Based on a Burns and Zydney paper (Burns EO paper) I went with 10 mM KCl.

The manual for the Malvern Zetasizer tells us that because the zetapotential is strongly dependent on the pH, a reported zetapotential without a pH is a useless number. I’ve spent a considerable amount of time trying to get our pH meter to work, but after doing all of the recommended cleans and replacing the electrode solutions I’ve continued to get drifting measurements. A new beckman coulter probe is in the mail. As a workaround, I used the Waugh lab’s pH meter to measure a large bottle of 0.2 micron filtered 10 mM KCl and have been working from that for all of the following measurements. That bottle was at 6.68 pH units and had a conductivity of 1.405 mS/cm (0.14 S/m). For reference, the 10 mM KCl solution that Burns and Zydney used had a pH of 6.5 and a conductivity of 1.41 mS/cm.

RESULTS:

I measured the zetapotentials of three different membranes – pnc-Si from wafer 1039, SiN from wafer 1015, and BOE stripped SiN from wafer 1055:

PressureVPotential

 

The formula for Zetapotential is zeta = frac{dE_z}{dDelta P} cdot frac{etaLambda_0}{epsilon_0epsilon_r}

pnc-Si has a slope of: -8.87E-8 V/Pa

SiN: -7.73E-8 V/Pa

BOE Stripped SiN: -6.70E-8 V/Pa

These correspond to zetapotentials of:

pnc-Si:  -17mV

SiN: -15mV

BOE Stripped SiN: -13mV

For reference, the Biomax membranes Burns and Zydney studied had zetapotentials of ~10mV at pH 6.5, and Jess has previously measured untreated pnc-Si zetapotential as -13.9mV, but I was not able to find the pH of her 100 mM KCl solution.

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One Comment

  1. 1. Great progress.
    2. Make sure you can get reproducible values on a single material type
    3. UV ozone of pnc-Si chips to make ‘standard’ material and remove any organic
    4. SiN and ‘stripped’ SiN may be the same material (see Josh)

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