The potential at the slipping plane outside the layer that moves with a particle in a liquid. The larger its absolute value, the stronger the electrostatic repulsion between particles and the more stable the dispersion. It is nearly zero at the isoelectric point.
Japanese name: ゼータ電位
Zeta potential appears in 1 practice question across 1 topic, including Nanomaterials.
Field: Nanomaterials
The surface of a particle in a liquid becomes charged through dissociation of surface groups or adsorption of ions, and ions of opposite sign gather around it to form an electrical double layer. Ions close to the surface are held strongly and move with the particle, while those farther out are spread by thermal motion. When the particle moves, the boundary between the layer moving with it and the stationary liquid is called the slipping plane, and the potential there is the zeta potential. The potential of the surface itself cannot be measured directly, but the zeta potential can be found from the motion of particles, so it is used as a measure of the electrostatic repulsion particles feel as they approach each other.
The zeta potential is found from how fast particles move in an applied electric field (the electrophoretic mobility µ). When the double layer is much thinner than the particle, the Smoluchowski equation ζ = ηµ/ε is used, where η is the viscosity of the liquid and ε its permittivity. In water at 25°C, a mobility of 1 µm·cm/(V·s) corresponds to about 12.8 mV. The thickness of the double layer is indicated by the Debye length, which in aqueous solutions of a 1:1 salt at 25°C is about 9.6 nm at 1 mmol/L and about 0.96 nm at 100 mmol/L. Adding salt shrinks the double layer and shortens the range of the repulsion.
Unlike the surface potential, the zeta potential is the value at the slipping plane, so its magnitude is smaller than that of the surface potential. Also, the isoelectric point (the pH at which the zeta potential is zero) and the point of zero charge (where the surface charge is zero) coincide when no ions adsorb specifically, but they differ when such adsorption occurs. The rule of thumb that dispersions are stable when the magnitude exceeds 30 mV is widely used, but it depends on salt concentration and particle size. Steric repulsion from adsorbed polymers does not show up in the zeta potential, so a dispersion can be stable even when the magnitude is small.
Measurement commonly derives the mobility from the Doppler shift of scattered laser light. Because the result depends strongly on pH and salt concentration, these must always be recorded. The isoelectric point differs with the substance and its surface state; typically it is low for silica, at about 2 to 3, and high for alumina, at around 9. The zeta potential is used to design against agglomeration by choosing pH and dispersants for slurries for slip casting and tape casting of ceramics, CMP polishing slurries for semiconductors, paints and pharmaceutical dispersions. Conversely, in water treatment, coagulants bring the zeta potential close to zero so that particles agglomerate on purpose and settle.
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.
As a potential, its unit is the volt, usually expressed in mV. Its sign corresponds to the sign of the particle's surface charge; for oxides it is positive when the pH is below the isoelectric point and negative above it. Reports should ideally state the pH, salt concentration, temperature and conversion equation used.
Measuring the zeta potential while changing the pH, the pH at which it becomes zero is the isoelectric point. Near the isoelectric point there is almost no electrostatic repulsion, so particles agglomerate most easily. To disperse particles, choose a pH far from the isoelectric point; to agglomerate them, set it near the isoelectric point.
Not always. At high salt concentration the double layer shrinks and the range of repulsion becomes short, so particles can agglomerate even at the same zeta potential. Large, heavy particles also settle even when dispersed. Stability is confirmed by measuring settling and changes in particle size over time in addition to the zeta potential.
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).
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