Hi,
Anyone has compared the zeta potential of PS beads in PEG aqueous solution with the zeta potential of PS in water? As a dispersant, PEG increase the stability of microparticles. Will PEG reduce the absolute value of zeta potential?
It will depend on if the PEG adsorbs onto the particles. If it does, then the change will depend on the molecular weight of the PEG.
Zeta potential is defined as the electrical potential at the region around the particle where there is slipping between the solution and the particle with all of its associated material (like adsorbed ions, water molecules, surfactants polymers) when the particle moves relative to the liquid. Beyond this location, the electric potential continues to decrease (due to electrolyte etc).
So, if you adsorb a macromolecule onto the particle, you push the slipping region further away from the particle and so the zeta potential is also further away. Since the potential decays, the zeta potential will be lower.
(If you have a polyelectrolyte or an ionic surfactant then the zeta potential will change in a manner to reflect the ionic nature of that adsorbed material.)
This illustrates a very important that is so often ignored: the zeta potential does NOT give you the charge at the particle's surface, only where the particle (+adsorbed material etc) moves relative to the bulk liquid.
The change in zeta potential this way is a very useful way to confirm adsorption.
(I describe the original of zeta potential in a simple way in a video - https://tinyurl.com/zetaorigin )
Arun Iyer Thanks a lot for your answer, Dr. Lyer. I agree with your point. Do you have any idea what will happen when PS beads get hotter and hotter? Will the adsorption of PEG be reduced? Thanks again!
John Francis Miller Many thanks for your detailed answer! Your video is great. So PEG with smaller molecular weight usually has weaker affect? Also, I wonder what will happen when PS beads get hotter and hotter? Will the adsorption of PEG be reduced?
To add to John's excellent answer and video, (the appropriate molecular mass of) PEG is often used as a steric stabilizer in many formulations. This means that the zeta potential isn't usually a good marker of stability in the system as the slipping plane can lie buried in the polymer chain and a calculated ZP of 0 mV (no mobility) can be perfectly stable in this situation. We also need to remember that the markers of stability (the 30 mV 'rule') are constantly misinterpreted. For example at 100 nm, charge stabilization (I don't like the term electrostatic here as this implies tribology to me) probably requires closer to 100 mV. There's plenty of discussion on this topic on Research Gate and it should be easy to locate. We must also mention that it's mobility that is the unambiguous parameter and that the conversion to ZP is not easy for non-aqueous systems especially.
To answer the last question. Kinetics of absorption are usually speeded up with temperature increase. This also implies no chain folding or other artifacts.
Alan F Rawle Hi Dr. Rawle, recently I measuring the zeta potential of Polystyrene microbeads in CTAC solution. So in the solution, there exist two particles: one is Polystyrene microbeads, another is CTAC micelle. But I can only obtain one peak. Do you know how I can get the accurate zeta potential of Polystyrene microbeads in CTAC solution?
The intensity of light scattered by particles in a sample scales with the fourth power of the radius. The CTAC micelles are much smaller than the PS microbeads, so the light will be dominated by the microbeads. You can confirm this by trying to measure a solution of CTAC without the microbeads present. Most likely, the instrument will complain of insufficient count rate or reduce the attentuation to try to collect more light.
John Francis Miller Dr. Miller, I really appreciate your speedy reply!!! You are absolutely right! Indeed, when concentration is smaller than 100mM, the instrument complains "insufficient count rate". When the concentration is larger than 100mM, sometimes "result meets quality creiteria". However, no matter "insufficient..." or "result meets...", zeta potential is around + 40-50mV. But when I put PS microbeads in CTAC solution, I find a clear decreasing trend when I increased concentration from 50mM to 500mM. I'm not sure what I measured is the zeta potential of PS beads or not.
Hongru Ding
You have not measured zeta potential. You have measured mobility or diffusion and some will say that you can convert the measured mobility to zeta potential using an appropriate equation and assumptions.
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