Electroacoustics for concentrated dispersions

Theories describing the dynamic electrophoretic mobility in concentrated dispersed systems have been discussed. There are two different theories derived recently using two different versions of a cell model: We point out here that these two versions created by H. Ohshima (first) and A. Dukhin with V. Shilov (second) agree with each other. We also provide explanations of this agreement. The cell-model-based theory describes motion of the particle in a frame of references associated with liquid. To compare this theory with experiment, the electrophoretic mobility must be recalculated into a frame of reference peculiar to the experimental technique. There are two possibilities suggested in the literature: a frame based on zero momentum flow or a frame based on zero volume flow. The first was introduced by O'Brien. Both possibilities are discussed and compared to experiment. This experiment was an equilibrium dilution of a rutile dispersion. The following parameters were measured for volume fractions from 40 vol % down to 5 vol %: attenuation spectra, sound speed, phase and amplitude of the colloid vibration current (CVI), temperature, and pH. The attenuation spectra provide independently measured particle size distribution. The sound speed is a measure of the system stability. Temperature and pH confirm the equilibrium status of the dilution. CVI is used to calculate the zeta-potential applying a new cell model theory. Since this was an equilibrium dilution, in principle, the zeta-potential should remain constant over this volume fraction range unless a double-layer overlap becomes important. Indeed, it is shown that the zeta-potential does remain constant up to 35 vol % with variations of only +/-2 mV if the value is calculated using a zero volume flow frame of reference. In comparison, the zeta-potential calculated following O'Brien in a zero momentum frame of reference increases more than 2 times at 35 vol %.