The concept of zeta potential has been long established. We give here a very brief synopsis of the concept. Detailed discussions can be found in a large selection of texts, for example (1, 2).

When fine particles are dispersed in liquids a charged interface between the surface of the particle and the bulk liquid develops in most instances. In the picture below is shown a microscopic portion of a particle surface immersed in water.

Zeta potential is defined as the electrical potential at the shear plane with respect to the bulk liquid. See Figure 1.

Figure 1

Measurements of zeta potential before 1985 were carried out with considerable difficulties, using “microelectrophoresis”, which requires much sample handling, including large sample dilutions, injection of small samples into well defined sample cell geometries and making optical measurements at special locations under special conditions. These difficulties often resulted in data of poor precision and with considerable error.

In 1985 a patent was granted (3) which became the foundation of the modern electroacoustic (ESA) method for measuring zeta potential in non-diluted samples. This method is now well established, based on many publications of measurements and theoretical work (4).

The ESA method can be briefly explained as follows: When an oscillating electric field is applied to a collection of fine particles in a dispersion the field exerts a force on the particles which is proportional to the effective particle charges. When the particles move in one direction the displaced fluid moves in the opposite direction. If the particles have a density different from that of the fluid there will be a net momentum transfer in the direction of (or opposite to) the applied electric field. The momentum transfer at the electrode/dispersion interface results in a sound wave which is then detected by an acoustic transducer behind the electrode. The generated sound wave is at the same frequency as the applied electric field. The measured amplitude of the sound wave is simply connected to the colloid particle mobility and hence to the zeta potential.

Dispersions are stable, that is particles do not aggregate (or unstable, particles aggregate) depending on the magnitude of the zeta potential of the particles. In many instances stable dispersions are required. In other cases (e.g. waste water treatment, paper manufacturing) unstable dispersions are desired. Stable/ Unstable dispersion are produced in a variety of ways using different chemicals,

1. Changing pH
2. Adding electrolytes
3. Adding dispersants/coagulants
   
   

Figure 2

In Figure 2 is shown the zeta potential as a function of pH of the aqueous dispersions of four different metal oxides. Most metal oxides can be characterized with an isoelectric point (i.e.p.) at this point the zeta potential is zero and in the neighborhood of the i.e.p. the dispersion is unstable and will aggregate or flocculate to form larger particles. The exception is Silica. A pure silica surface does not have an i.e.p and the zeta potential vs. pH curve approaches the zero of zeta potential asymptotically at pH~2. The i.e.p. of metal oxide surfaces are very sensitive to impurities which arise generally from the production process of the material. For example, one can purchase a 99.9 % pure Titania that should have an i.e.p. around pH=6 that nevertheless, has an i.e.p. around pH=2 indicating a silica surface coverage. Even within one type of pure metal oxide there are variations of the i.e.p. depending on the crystal structure of the oxide. As an example, the Anatase crystal structure of Titania will yield an i.e.p in the high pH=4 range while the Rutile structure will yield an i.e.p. in the pH=7 range. Most commercial Titania will be a mixture of the two crystal structures and give an i.e.p. around pH=6. Similar discussions will apply to Alumina and other metal oxides.

In Figure 3 is shown the results of the addition of a dispersing agent (Darvan C) to a Mullite dispersion at pH~4.7 at about maximum positive zeta potential.

Figure 3

Mullite is a clay mineral of an Aluminum Silicate form with composition (Al6Si2O13).
Darvan C is a short chain polymer deflocculant (polyacrylic acid) frequently used to optimize the specific gravity of clay slips. The resulting high negative zeta potential (~ -34 mV) implies that the clay slip is well dispersed and stable against aggregation and hence the slip can be made up to high specific gravity (1.7-1.8) suitable for casting. It is also evident in this instance that more than about 0.8 ml. of Darvan C would be a waste of expensive chemical for this particular quantity of Mullite.

Figure 4

In Figure 4 is shown the results of a potentiometric titration on a corn oil in water micro emulsion. The oil emulsion droplets are initially stabilized at a high negative zeta potential with a protein (β Lactoglobulin) coating at pH~7.0.

β Lactoglobulin is a compact globular protein (molecular mass = 18.3 kDa) obtained from cow’s milk. Its i.e.p. is reported to lie between pH=4.7 to5.2. Upon titrating with an acid the negatively charged oil droplets becomes positively charged at around pH =4.6 in Figure 4 implying an almost complete coverage of the oil droplet.

Figure 5

In Figure 5 is shown potentiometric titrations of various proteins of interest in food science. A large fraction of Whey protein is composed of β Lactoglobulin, hence almost coincident i.e.p.’s and very similar zeta potential dependencies. In this case the β Lactoglobulin by itself has an i.e.p. at pH=5.1. Pectin remains negative over the whole practical pH range. Of greatest interest is that the Field ESA technique can be used to measure the state of surface charge on very small particles.

Conclusions

The acoustic zeta potential measurement method (Field ESA) has many advantages over the traditional microelectrophoresis methods;

1. No sample handling is required meaning measurements can be made on native concentrations of colloids.
2. Measurements are made on stirred , pumped, or rapidly flowing systems.
3. No critical optical alignments or adjustments. No optics.
4. Since the colloid is constantly stirred during measurements titrations can be carried out automatically and very rapidly with a computer controlled burette. Most of the data shown here were obtained in about a half hour with pH resolution of 0.2 units. pH resolution of 0.1 units is easily attainable. Conductivity and temperature are also measured simultaneously with pH.
5. The Field ESA detector is a dip in probe which is extremely rugged, easy to clean and not susceptible to any external influences such as noise or ambient vibrations.
6. No low end particle size limitations. Only requirement is that there be a density difference of at least 2% between the particle and the liquid in which it is immersed.
7. Field ESA measurements can be made on any colloidal particles in any liquids including organic and apolar liquids.
8. The whole system weighs only 8 lbs. and hence highly portable.
   
   

1. R.J.Hunter, Zeta Potential in Colloid Science, Academic Press, New York, 1981.  Also, R.J. Hunter, Introduction to Modern Colloid Science, Oxford University Press, New York, 1993.
2. S.Ross and I.D. Morrison, Colloidal Dispersions, Wiley Interscience, New York, 2002.
3. T.Oja, D.Cannon, G.L.Petersen, US Patent 4,497208, 1985.
4. R. Greenwood, “Review of the measurement of zeta potentials in concentrated aqueous suspensions using electroacoustics”  Advances in Colloid and Interface Science, 106 (2003), 55-81 Elsevier, New York.