Dependence of the broad frequency dielectric spectra of colloidal polystyrene particle suspensions on the difference between the counterion and co-ion diffusion coefficients

Dielectric properties of four suspensions of spherical polystyrene particles were measured at 25 °C over a broad frequency range extending from 100 Hz to 10 MHz, using a HP 4192 A Impedance Analyzer. The instrument was coupled to a cell with parallel platinum black electrodes and variable spacing, and the quadrupole calibration method was used. The aqueous electrolyte solutions were prepared using equal concentrations of NaCl, KCl, NaAc, or KAc, so that the calculated Debye screening length and Zeta potential remained constant, while the conductivity changed. The polystyrene particles used (Interfacial Dynamics Corp., surfactant-free white sulfate latex) have a diameter of 1 micron and a surface charge density that is independent of the pH. The dielectric spectra were described using the Nettelblad–Niklasson expression combined with a Debye type high-frequency term and analyzed using the Shilov–Dukhin theory and numerical results. The theoretical prediction that the low-frequency dispersion parameters are determined by the co-ion diffusion coefficient was experimentally confirmed. This also allowed to justify an alternative definition of the characteristic time of the low-frequency dispersion. On the contrary, the prediction that the high-frequency dispersion parameters are determined by the diffusion coefficient of counterions could not be confirmed, possibly due to experimental problems. However, the ζ-potential values deduced from high-frequency data were compatible with values deduced from electrophoretic mobility measurements.

Theory predicts that the low-frequency dielectric spectra of colloidal suspensions are determined by the mobility of co-ions while the high-frequency by that of counterions. This prediction is partly confirmed by experimental results.Figure optionsDownload as PowerPoint slide