clay minerals (5), and oil-in-water emulsions (4). Since it The dynamic mobility and dielectric response of Na-bentonite does not involve the use of light scattering or absorption it have been measured at megahertz frequencies. The particles conis possible to use it with systems with concentrations as sist of thin, approximately hexagonal plates. Information about high as 60% by volume, in some cases (6). The instrument the particle size and the electrokinetic charge or z potential can measures the dynamic mobility of the particles from which be obtained from the dynamic mobility spectrum. Interpretation it is possible to calculate both the z potential and particle of the dynamic mobility spectrum is complicated by the presence size in the range from about 0.1 to 10 mm median diameter.
of anomalously high surface conductance, as also occurs in the
The only input required is a knowledge of the volume fraccase of kaolinite. This profoundly affects the shape of the dynamic tion of particles and their density. The dielectric permittivity mobility spectrum at low (Γ΅5 1 10 03 M) electrolyte concentrations. The approximate formula derived by Loewenberg to take is also required but the calculation is insensitive to that paaccount of the effect is found to be in excellent agreement with rameter. Although the commercial instrument is calibrated the results, the agreement being rather better than that found in for salt concentrations only up to 1 S/m, we have used it, the case of kaolinite. The surface conductances of the particles, after suitable calibration, to as high as 20 S/m (7). Particles determined directly by dielectric response measurements, compare of any shape can be measured, since disk-shaped (oblate) almost quantitatively with those needed to fit the dynamic mobility and cigar-shaped (prolate spheroids) particles behave like data using the theory described above. The results also suggest spheres with an equivalent radius, rather like the equivalent that the ratio of the mobility of ions in the Stern layer compared settling radius familiar in treatments of the sedimentation with those in the diffuse double layer (at the high frequencies process.
involved here) is about 0.6 for bentonite, which is similar to the To calculate the effective size of nonspherical particles value for kaolinite.