Zeta potential measurement with patented measuring method

The zeta potential is a measurable quantity that characterises the charge situation on a solid surface in solution. It is measured at the boundary between the ion layer firmly adsorbed on the solid surface and the solution. Dr Astrid Drechsler, scientist at the Leibniz-Institut für Polymerforschung Dresden e.V. (IPF) and an expert in the characterisation of polymer interfaces, explains why the zeta potential is an important parameter: "When a solid surface is immersed in an aqueous solution, it becomes electrically charged due to dissociated surface groups and the adsorption of ions and molecules. The zeta potential provides information about these chemical processes and charge formation processes. For example, it can be used to predict interactions, such as adhesion between different surfaces.

Zeta potential measurements are important in a variety of applications, such as the investigation of fuel cell membranes, filter fouling processes, bacterial growth on food packaging and the washing of textiles.

Depending on the size and shape of the samples, the zeta potential can be determined using different measurement methods. For small particles in the nanometre to lower micrometre range, electrophoretic methods are often used. For larger samples in the millimetre and centimetre range, the analysis of the streaming potential or the streaming current has proven to be advantageous. The Zeta Potential Analyser ZPA 20 from DataPhysics Instruments uses a patented measuring method based on such an analysis. It is therefore particularly suitable for analysing samples in the macroscopic range. The device can be equipped with the MC-ZPA/S, a measuring cell for solids such as plates, membranes or films, and the MC-ZPA/PF, a measuring cell for fibres, powders and granulates.

Collaboration with the Leibniz Institute of Polymer Research

The manufacturer DataPhysics Instruments cooperated with the Leibniz-Institut für Polymerforschung Dresden e.V. (IPF) in the development of the analyser. Dr Astrid Drechsler works there as a scientist on the physical chemistry of polymer surfaces, including zeta potentials. Since 2017, Drechsler and her team have accompanied the development of the zeta potential analyser ZPA 20 in several project phases. „In the first phase, we validated data that was measured with the prototype“, says Drechsler. In a second project phase, not only the device but also the measuring cells were further developed. „The aim was to simplify the handling of the measuring cells and to achieve reproducible sample preparation“, explains Anja Caspari, a member of staff at the IPF.

On the measurement method

The current potential and current flow analysis for investigating the zeta potential works as follows: An electrolyte solution is pumped through a measuring cell containing the sample material. Specifically, it is passed between two solid samples or through fibre or powder packaging. The electrolyte solution separates ions that have collected near the surface and takes them with it. This creates a potential difference between the electrodes, which are located on both sides of the sample. Depending on the sample geometry, this difference is measured as the flow potential or the flow current. These values, together with the pressure difference in front of and behind the measuring cell, make it possible to calculate the zeta potential.

The Zeta Potential Analyser ZPA 20 uses a patented measuring method to achieve fast results with high accuracy. Schaubach explains: "The Zeta Potential Analyser ZPA 20 is the only measuring device on the market that works with a bidirectional and oscillating measurement of the current potential or current flow." The electrolyte solution is not only pumped in one direction, but alternately in opposite directions over or through the sample. In addition, the flow rate of the electrolyte liquid changes in repeated cycles, which leads to pressure changes. The analyser thus records up to 100 pressure values together with the corresponding flow potential or flow current in one second. This produces results of outstanding statistical quality in a short time. The analysis software can easily process and evaluate large amounts of data.

The patented measurement method not only saves time during measurements, but also helps to reduce sources of error. The oscillating measurement generates both positive and negative pressure differences and therefore positive and negative values of the flow potential or current. This can prevent polarisation of the device electrodes.

In addition, the bidirectional current can make asymmetries of the sample surface visible, such as inhomogeneous fibre or powder packaging or insufficiently fixed samples. The oscillating, bidirectional flow of the electrolyte allows such sources of error to be recognised and avoided.

Modular and open design

During the development of the Zeta Potential Analyser ZPA 20, particular attention was paid to ensuring that all surfaces are easy to clean in order to avoid cross-contamination. This aspect is particularly important in order to be able to carry out accurate, time- and pH-dependent zeta potential measurements.

In addition, the Zeta Potential Analyser ZPA 20 can be expanded with optional modules. With the LDU 25 liquid dosing system, for example, the electrolyte composition can be changed automatically. This allows the isoelectric point or the adsorption of surfactants to be determined quickly and accurately according to the manufacturer's specifications.

Order measurements possible

Interested parties who are not sure whether the Zeta Potential Analyser ZPA 20 is the right device for them can also request test measurements with their samples. Dr Sebastian Schaubach, Chief Innovation Officer at DataPhysics Instruments, says: „Our laboratory offers contract measurements for a wide range of applications, both with the ZPA 20 and with all other instruments in our portfolio.“

Source: LABO from 14 November 2022