Problem: The majority of the zetameters are based on the electrophoresis measuring principle where the zeta potential for a colloidal system is analyzed by measurement of the electrophoretic mobility of the particles. However, the measurement of the charged macroscopic surfaces is not possible with electrophoresis. Electrokinetic measurements will enable users to analyze the charged macroscopic surfaces where the zeta potential will give information regarding the adsorption and adhesion processes as well as hydrophobic and hydrophilic nature of these surfaces.

SurPASS (Surface Potential Analyzer for solid samples)

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Solution: With the SurPASS, the zeta potential becomes accessible by the streaming potential or (alternatively) the streaming current method. A dilute aqueous electrolyte solution (such as 1mM KCl) is circulated through the measuring cell that contains the sample, which may be a porous sample (permeation method) or a pair of planar surfaces separated by a small gap (tangential method). The sample arrangement represents a mechanical resistance and the liquid flow generates a differential pressure between the inlet and outlet of the measuring cell. Electrical charges, which are accumulated at the interface between the solid surface and the surrounding liquid, are sheared off and accumulated on one end of the measuring cell. The generated potential difference is detected by the Ag/AgCl electrodes.

Electrochemical double layer

The zeta potential itself is defined as the electrical potential at the interface between the immobile layer of solid surface charges and the diffuse layer of counter ions and is proportional to the streaming potential coefficient dU/ dp. The complete equation for the zeta potential ζ reads

Besides the solid surface properties, the viscosity η, the dielectric coefficient ε ε0 and the electrical conductivity κ of the liquid phase contribute to the magnitude and sign of the zeta potential. The conductivity and the pH of the electrolyte are measured externally in the electrolyte reservoir. SurPASS also features a built in automatic titrator that ensures that the zeta potential can be measured as a function of varying pH or conductivity of the electrolyte. It is also possible to add small quantities of additives to detect the changes to the sample surface due to adsorption processes.

This electrokinetic analyzer enables the measurement of the zeta potential for a variety of solid samples that can range from powder, fibers and planar samples such as membranes, filters, polymers, biomaterials, silicone nitride wafers etc. These samples can be mounted into the SurPASS by using three types of measuring cells.

For more information, go to www.anton-paar.com