New Experimental Approach for the Proper Consideration of Stagnant and Diffuse Layer Conductivity in the Zeta Potential Determination.

Accurate determination of the zeta potential in colloidal dispersions often requires consideration of the relaxation effect, which is associated with the polarization of the electrical double layer and the surface conductivity. In this study, we pursue a new approach that combines conductivity measurements of the dispersion and dispersion medium with the electroacoustic and electrophoretic zeta potential determination. The conductivity data are analyzed with the Maxwell-Wagner-O'Konski theory, providing the Dukhin number. Zeta potentials of highly concentrated polymer dispersions were determined using the colloid vibration current (CVI) method and compared with those obtained by electrophoretic light scattering (ELS) in diluted dispersions. In both cases, the relaxation effect was now taken into account on the basis of the experimentally determined Dukhin number. The evaluation of the Dukhin numbers revealed significant surface conductivity for all investigated polymer dispersions. In addition, it was often found that not only the diffuse layer but also the stagnant layer contributes considerably to the surface conductivity. Proper consideration of both effects is essential for the reliable determination of the zeta potential, as otherwise inconsistencies can be observed in the evaluated data. Moreover, we have validated for the first time that the advanced CVI theory takes the effect of surface conductivity properly into account for a wide range of particle volume fractions. These values agree well with those obtained by the ELS method using the Dukhin-Semenikhin theory or a modified theory of Ohshima, Healy, and White. This study thus shows that the Dukhin number can serve as a key parameter to reliably connect conductivity and electrophoretic and electroacoustic experiments.