2.6.4. Zeta Potential Measurement (ζ-Potential)

The electrochemical double layer (EDL) is a system used to explain the ζ-potential and consists of a system model in which a solid in contact with an aqueous solution assumes a surface charge; this charge provokes an interfacial charge distribution that is different from the charge distribution in the liquid phase. The EDL model compensates the surface charge and defines 2 layers: 1 stationary layer and 1 diffuse mobile layer. The ζ-potential is the potential at the boundary between the stationary and the mobile diffusive layers (shear plane).

This parameter can be measured experimentally through the gauge of the electrokinetic effect appropriate for solid samples. There are many different electrokinetic effects: electro-osmosis, electrophoresis, sedimentation potential, and streaming potential, and each of these effects needs the 2 phases to move relative to each other. This study used the streaming potential concept, which is generated by the tangential flow of liquid forced to pass through a granular powder (Figure 1a). The streaming potential is generated from the movement of a liquid in a capillary formed from the solid samples; in this situation the ions of the electrochemical double-layer are shared off their equilibrium position and shifted along the solid surface, resulting in a net charge separation leading to an electrical potential difference (Figure 1a). This so-called streaming potential is detected between electrodes located on both ends of the capillary.

Figure 1. ζ-Potential measurements of Synergoss Red and Bone filler Control. (a) Graphical images of the principle of ζ-potential measurement for solid surfaces; (b) pH scan curve from 8.5 to 5.5.

ζ-potential measurement was performed using the SurPass 3 equipped with a cylindrical cell for granulate materials (Anton-Paar GmbH, Graz, Austria).

Measurements were performed using a cylindrical cell, and for each bone filler material, 1.5 g of granules were transferred in the cylindrical compartment of the cylindrical cell and mounted between the support disk and filters (with 25 µm mesh) on both sides of the granular sample plug (Figure 1a). A volume of 0.001 M KCl was used as the electrolyte solution. Three replicates for each test sample were analyzed. The analysis was performed according to the pH scan method. It consists of the measurement of the streaming potential at different pH points, between 9 and 5. The pH of the electrolyte solution was automatically modified by the instruments using 0.05 M HCl and 0.05 M KCl. At each pH point, 3 measurements were performed in order to condition the sample, then the 4th value was taken and reported.

To calculate the ζ-potential, the Helmholtz-Smoluchowski approximated equation was used:

where dU_str/dΔp is the streaming potential coefficient, κ_B in the case of non-conducting samples is the electrical conductivity of the bulk electrolyte solution, and η and ε × ε₀ are the viscosity and dielectric coefficient of the electrolyte solution. For diluted aqueous solutions, the viscosity and the dielectric coefficient of water were used. All calculations were performed by the instrument software.