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Which is an appropriate way to keep oxide nanoparticles dispersed in water from agglomerating and settling?

  1. ASet the pH to the isoelectric point of the particles to make the surface charge zero, so the particles no longer attract each other electrically
  2. BMove the pH well away from the isoelectric point to charge the surface and make the absolute value of the zeta potential large
  3. CAdd a generous amount of an electrolyte such as sodium chloride to raise the ionic strength and thicken the electric double layer
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Answer: B. Move the pH well away from the isoelectric point to charge the surface and make the absolute value of the zeta potential large

Nanoparticles have a large specific surface area and readily stick together and agglomerate through van der Waals attraction. Oxide particles in water become charged as surface hydroxyl groups accept or release protons depending on the pH, forming an electrical double layer around them. Moving the pH away from the isoelectric point to make the absolute value of the zeta potential large (as a rough guide, several tens of mV or more) lets the electrostatic repulsion between particles outweigh the attraction, stabilizing the dispersion (DLVO theory). At the isoelectric point the surface charge is nearly zero and the repulsion disappears, so agglomeration is most likely. Adding a lot of electrolyte compresses the double layer and shortens the range of the repulsion, so agglomeration actually increases. Steric repulsion from adsorbed polymeric dispersants is another commonly used method.

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