What is the zeta potential?

Figure 1: Wet chemical processes are used during the production of wafers. Measuring the zeta potential helps to prevent the deposition of impurities during such processes.

The zeta potential is the electrical potential near a solid surface in aqueous solution. The zeta potential influences the stability of colloidal suspensions and gives an indication of the adhesion between solids. The zeta potential is of great importance in food technology, the development of biomaterials, filtration materials, the textile industry and the preparation of silicon wafers.

Why is the zeta potential interesting?

The zeta potential ζ is a parameter for analysing the surface charge. More precisely, the zeta potential characterises the electrochemical properties near a solid surface in an aqueous solution. These electrochemical properties are caused by functional groups at the surface. The surface charge determines whether and how the surface interacts with other materials.

If the zeta potential is known, it is possible to estimate whether attractive or repulsive forces occur between two surfaces. In practical terms, the zeta potential can therefore help to answer questions such as: Will proteins attach to the membrane? or How quickly do the surface properties change when a surfactant is added?.

The zeta potential is not identical to the surface charge. The surface charge describes the charge directly on the surface. The surface charge cannot be measured experimentally. The zeta potential, on the other hand, describes the charge situation at the so-called shear plane, near the solid surface. In contrast to the surface charge, the zeta potential can be determined experimentally and is more relevant in practice.

What generates surface charges?

When a surface meets an aqueous solution, it generally loses its electrical neutrality. The surface charge can be caused by various chemical processes, such as the adsorption of ions as well as the protonation and the deprotonation of functional groups. Over time, a pH-dependent equilibrium is established. Some examples of this are:

Deprotonation, i.e., the release of a hydrogen ion, from a carboxyl group (COOH) at the solid surface
[surface]-COOH + H₂O ⇌ [surface]-COO^- + H₃O⁺_(aq)
Deprotonation of a hydroxyl group (OH)
[surface]-OH + H₂O ⇌ [surface]-O^- + H₃O⁺_(aq)
Deprotonation of a thiol group (SH)
[surface]-SH + H₂O ⇌ [surface]-S^- + H₃O⁺_(aq)
Protonation, i.e. absorption of a hydrogen ion, of an amino group (NH₂)
[surface]-NH₂ + H₂O ⇌ [surface]-NH₃⁺ + OH^-_(aq)

The sign and magnitude of the surface charge allow conclusions to be drawn about the type and number of functional groups present on the surface. These largely determine the interaction of the surface with other surrounding substances.

									
										
											
											
										
									
								
Figure 2: Electrical potential near the solid surface according to the GCSG model
The electrochemical double layer at the surfaceThe surface charge generates an electric field near the solid surface. The charge attracts ions from the liquid, which carry the opposite charge compared to the solid surface. This causes an electrochemical double layer to form in front of the surface. Various models exist to describe the structure of the electrochemical double layer. The Gouy-Chapman-Stern-Grahame model (GCSG model) will be discussed here.
According to the GCSG model (see Figure 2), the electrochemical double layer consists of an immobile, i.e., unmoveable, layer directly at the solid surface as well as a diffuse, i.e., mobile, layer. The immobile layer is made up of the inner Helmholtz sublayer and the outer Helmholtz sublayer. The inner Helmholtz sublayer (IHP) contains adsorbed ions, which are strongly bound to the surface over a short distance. These ions are partially dehydrated. The inner Helmholtz sublayer is followed by the outer Helmholtz layer (OHP), which consists of ions of the opposite charge that are non-specifically adsorbed and are fully hydrated. 
The immobile layer with the inner and outer Helmholtz sublayers is followed by the diffuse layer with mobile, hydrated ions with both positive and negative charges. The number of ions is influenced by the surface charge and decreases with larger distance to the solid surface.
The zeta potential at the shear planeThe electrical potential of the double layer can also be subdivided (see green line in Figure 2). Along the immobile layer, it is assumed that the value of the potential decreases linearly. In the diffuse layer, the electrical potential is defined by a Boltzmann distribution.
The potential in the immobile layer is experimentally inaccessible and generally not relevant for practical applications. The potential at the transition between the immobile and diffuse layer, on the other hand, can be measured and is relevant in practical applications. By moving the liquid relative to the surface, the ions in the diffuse layer can be displaced or sheared off. The electrical potential at this shear plane is referred to as the zeta potential.
Application examples for zeta potential measurementsThe zeta potential influences the stability of colloidal suspensions and provides indications of the adhesion between solids. The zeta potential can also be used to observe adsorption and chemical reactions between solids and ions in aqueous solutions.
Additional parameters such as the isoelectric point and the adsorption kinetics of the solid can be derived with the zeta potential analysis. This means that application-related questions from a wide range of fields can be answered.
In food technology, one focus is filtration, in which membranes are used for selective fractionation. The zeta potential can characterise these membranes in detail.
Another focus in the development of filtration membranes is on the filtration of viruses, where the modification of the surface charge of filter media plays a decisive role.
When developing antimicrobial surfaces, the measurement of the zeta potential can provide important insights.
In the field of medical technology, the measurement of the zeta potential can help to characterise the deposition of proteins (biofilms) on implants..
Contact lenses are also the subject of biomaterial-based research. The focus here is on reducing biofilms and bacterial adhesion on the surface of the lenses.
The challenge of so-called membrane fouling, in which impurities cover the membrane surface, is also being addressed. Measuring the zeta potential makes it possible to better recognise fouling.
When producing silicon wafers, wet chemical processes such as etching are used. Measuring the zeta potential, these processes can be modified by adding surfactants to prevent the deposition of impurities.

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