What is Zeta Potential? A brief description.

Zeta potential is an electrostatic potential that exists very near the surface of particles suspended in liquids1. Zeta potential (ζ) is responsible for particle-particle repulsion forces in colloidal suspensions and thus can be used to predict colloid stability against particle aggregation. Figure 1 illustrates a particle suspended in a liquid along with various notional regions around it. The“slipping plane” or “shear plane” is where Zeta potential is located versus the potential in the bulk solution. Within this slipping plane, the liquid is bound to the particle while it moves freely outside this boundary. The net potential far from the particle (in the bulk of the liquid) is zero.

Figure 1. A negatively charged particle suspended in a liquid. Notional boundaries are shown.

In aqueous media, Zeta potential is typically generated as the ions on the particle surface dissociate, leaving a net electric charge near the surface surrounded by a cloud of counter-ions. Various types of ions can diffuse in and out through the slipping plane which allows Zeta potential to vary depending on the ion composition in the liquid such as pH. Ions may also participate in chemical reactions within the slipping plane which can affect the Zeta potential. Sample dilution can significantly shift the Zeta potential as ions may adsorb or desorb from the particle. Thus, Zeta potential can be positive or negative, or zero (Iso-Electric Point, IEP) depending on the liquid (solvent) pH or ion type and concentration.

Particle-filtration systems may benefit from low Zeta potential levels as aggregated particles are easier to remove. Most other colloidal systems require higher Zeta potentials, e.g. over +/- 20 millivolts in order to maximize shell life. Coatings tend to be more efficient when the particles and coated surface have opposite polarities. Zeta potential normally cannot be directly measured. For example, one cannot place a voltmeter probe against a particle surface in order to measure its surface potential. Instead, Zeta potential is calculated from electrophoretic measurements which measure particle velocity under an applied electric field, i.e. make the particles move and measure their particle mobility (see www.matec.com/mas). Thus, the calculated Zeta potential depends on the theory used in these computations to relate particle mobility to Zeta potential. An alternative measurement for large particles or surfaces is to move the liquid against stationary particles, fibers, or surfaces and measure the resulting streaming potential2.

Potentiometric and Volumetric Titrations

Potentiometric Titrations are useful for determining a sample's IEP. As mentioned above, the IEP may be desirable or otherwise. Potentiometric Titration plots may display plateau regions for Zeta potential vs. pH. Such measurements enable manufacturers to optimize use of acids or bases for transportation and storage. Figure 2 shows a potentiometric titration on a Ludox-TM silica sample by automatic addition of 1N HCl. This titration was performed automatically by a Zeta-APS, Zeta Acoustic Particle Sizer, instrument from Matec Applied Sciences, Northborough, MA USA3. Figure 2 shows that below pH 4, the Zeta curve approaches a plateau region while Conductivity increases more rapidly. This suggests that the silica particles are becoming saturated with H+ ions as the pH is lowered. Conductivity increases more rapidly as more of these H+ ions stay in the continuous water solvent as opposed to diffusing through the slipping plane toward the particle surface.

Figure 2. Automatic potentiometric titration of 10%-vol Ludox-TM by 1N HCl addition.

The Zeta-APS instrument is also capable of performing automatic Volumetric titrations whereby a reagent such as a surfactant is added into a colloid in dosages as small as 1 μL. The Zeta-APS then produces titration graphs showing plots such as Zeta potential, pH, Conductivity, and Temperature vs. added reagent volume. Plots of Zeta vs. reagent volume would be flat if the added surfactant is not adsorbed by the particles, i.e. it does not supply potential-determining ions1,2.

References
1. Hunter, R. J., Introduction to Modern Colloid Science, Oxford Science Pub., 1993.
2. Morrison, I. D., Sydney, R., Colloidal Dispersions. Suspensions, Emulsions, and Foams. John Wiley and Sons, 2002.
3. www.matec.com/mas