Zeta Potential Testing

Zeta potential (sometimes written as ζ-potential), also known as electrokinetic potential, is a physical property exhibited by any (nano) particle either suspended in solution or upon a surface. It is a powerful technique that can be used to predict the long-term stability of an emulsion or a suspension. The units used for zeta potential are usually Volts (V) or millivolts (mV). Zeta potential is usually used within the field of colloid chemistry, where a colloid is defined as a dispersion of particles in a medium with the size of the dispersed particles ranging from 1 nm to 1 μm. Theoretically there is no upper limit to the size of particle that can be measured by zeta potential, but over a certain particle size gravitational settling can become an issue and impacts validity of results. Using our instrument, samples with particles in the size range 3 nm to 100 μm can be analysed.

Zeta potential, what is it?

A zeta potential is established on the surface of any material when it comes into contact with a liquid medium (e.g. liquid-liquid, solid-liquid). Zeta potential is therefore classified as an interfacial property. The size of the interface can be huge and will have a significant bearing upon the overall behaviour of a system. For example, a cube with sides of 1 cm has surface area of 6 cm2. When this cube is dispersed as 1018 little cubes with sides of 10 nm the total surface area within the sample is 6 x 106 cm2, which is about the size of a tennis court!

When a material comes into contact with a liquid, any chemical functionalisation on the surface will react with the liquid medium and lead to the formation of a surface charge on the particle. This surface charge leads to a reorganisation of ions that are nearby; oppositely-charged ions are attracted and particles of the same charge are repelled. With increasing distance from the surface of the particle the influence of the charged surface on the ion reorganisation decreases. This therefore leads to a gradient, or change in potential, over distance.

The Stern layer and the slipping plane

The layer of ions that are directly interacting with the surface of the particle is called the Stern layer. These ions are bonded to the surface fairly strongly. Outside the Stern layer there is another diffuse layer of more loosely bound ions. The edge of this diffuse layer is called the ‘slipping plane’ and zeta potential measures the voltage at this slipping plane i.e. the point at which the central particle no longer affects the ions in the surrounding medium. The inner shell of charge (Stern layer) and the outer ionic atmosphere (diffuse ion layer) is known as the electrical double layer.

Schematic to show the electrical double layer
Zeta potential measures the voltage at the slipping plane, i.e. where the influence of the central particle upon the ions in solution is minimal.

What the numbers mean

During zeta potential testing, the sample is loaded into a cell that has conductive points. A charge is applied to the sample and a laser measures how fast the particles are moving when they are charged. The faster the particles move, the higher the value of zeta potential. Zeta potential can be either negative or positive and can range from –200 to 200 mV, depending upon the electrochemical behaviour of the particle interface.

Generally speaking, a large value (i.e. over 30 mV or below –30 mV) indicates that a sample is stable and unlikely to coalesce. In addition, a large value indicates that the surfaces of particles are highly charged and repel each other. The nearer to 0 the zeta potential value, the less stable the particles in solution are and the more likely they are to coalesce or flocculate. The sign of the zeta potential also provides information about the surface of the particles. A positive zeta potential value shows that the dispersed particles have a positive charge and a negative value shows that the dispersed particles have a negative charge.

Case study: Emulsions

An example of where this is useful is when considering the structure of an emulsion. Emulsions are usually oil-in-water (o/w) or water-in-oil (w/o) mixtures. Within an emulsion there are numerous particles in suspension. All these particles are surrounded by ions, generating an electrical double layer per particle. Emulsions are thermodynamically unstable, and reversion to the original bulk phases can occur rapidly and irreversibly. The main ways that an emulsion can break down are by flocculation, creaming, coalescence and breaking. As noted earlier an unstable emulsion can fall apart rapidly, however zeta potential measurements can help predict the stability of an emulsion. If two particles have high enough zeta potential they repel so coalescence is much less likely; the emulsion’s stability is increased.

Schematic showing degradation pathways of emulsions
Once formed, emulsions can break down in several different ways. (a) an ideal emulsion; (b) flocculation of droplets; (c) creaming; (d) coalescence; (e) breaking.

Case study: isoelectric point

Another aspect that zeta potential can be used for is determination of an isoelectric point. The isoelectric point is the pH at which a particular molecule contains no overall net charge. By measuring the zeta potential of a sample over a pH range, the isoelectric point and the pH range over which a sample is stable can be determined.

Schematic showing IEP for a hypothetical sample
Monitoring zeta potential over a pH range can identify the isoelectric point and a range of pH over which a sample is stable

To summarise, zeta potential provides formulators with key information relating to stability of a formulation and R&D specialists with vital guidance for next steps. It can help identify a stable emulsion formulation, the isoelectric point of a peptide or the aggregation point of coffee granules.

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