Determination of electric double layer parameters for spherical silica particles under application of the triple layer model using surface charge density data and results of electrokinetic sonic amplitude measurements

The surface charge density, σ0, of AEROSIL OX 50 (Degussa) was determined by titration experiments as well as the electrophoretic mobility, μ, using the electrokinetic sonic amplitude method to characterize the electric double layer of spherical SiO2-particles in dependence on pH and electrolyte concentration. The electrokinetic experiments result in an isoelectric point pHiep 2.4. At the lowest electrolyte concentration investigated a minimum in the μ–pH-plots emerges. Three relationships between μ and the ζ-potential were examined for the evaluation of the experimental data: (i) the Smoluchowski formula, (ii) an equation from OʹBrien et al. (J. Colloid Interf. Sci. 173 (1995) 406) for the Dukhin number which includes the contribution of the mobile layer to surface conductivity only, (iii) Dukhin numbers were determined from the results of conductivity measurements under application of the theory of OʹBrien and Perrins (J. Colloid Interf. Sci. 99 (1984) 20) with numerical solutions of Kang and Sangani (J. Colloid Interf. Sci. 165 (1994) 195). Here, the contribution of the ions in the stagnant layer is included too. The Smoluchowski formula is not suitable to describe the μ–pH plots in a quantitative manner, specially at low electrolyte concentration. The other two methods taking into account the surface conductivity give a comparable goodness of fit as a result of the multiparameter regression under application of the triple layer model. A significant contribution of the ions in the stagnant layer to surface conductivity follows from the conductivity measurements. This effect results in an increase of the magnitude of the ζ-potential calculated from the electrophoretic mobility in comparison to the Smoluchowski formula.