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, sigma (0), of AEROSIL OX 50 (Degussa) was deter mined by titration experiments as well as the electrophoretic mobility, mu, using the electrokinetic sonic amplitude method to characterize the electr ic double layer of spherical SiO2-particles in dependence on pH and electro lyte concentration. The electrokinetic experiments result in an isoelectric point pH(iep) 2.4. At the lowest electrolyte concentration investigated a minimum in the mu -pH-plots emerges. Three relationships between mu and the zeta -potential were examined for the evaluation of the experimental data: (i) the Smoluchowski formula, (ii) an equation from O'Brien et al. (J. Col loid 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 unde r application of the theory of O'Brien and Perrins (J. Colloid Interf. Sci. 99 (1984) 20) with numerical solutions of Kang and Sangani (J. Colloid Int erf. Sci. 165 (1994) 195). Here, the contribution of the ions in the stagna nt layer is included too. The Smoluchowski formula is not suitable to descr ibe the mu -pH plots in a quantitative manner, specially at low electrolyte concentration. The other two methods taking into account the surface condu ctivity give a comparable goodness of fit as a result of the multiparameter regression under application of the triple layer model. A significant cont ribution of the ions in the stagnant layer to surface conductivity follows from the conductivity measurements. This effect results in an increase of t he magnitude of the zeta -potential calculated from the electrophoretic mob ility in comparison to the Smoluchowski formula. (C) 2001 Elsevier Science B.V. All rights reserved.