Subjecting charged colloidal particles to a compressional sound wave gives rise to a periodic polarization of the ionic atmosphere surrounding the particles. This periodic polarization causes each particle to act as a vibrating dipole resulting in an alternating voltage, termed the colloid vibration potential (CVP), between any two points in space separated by a phase distance other than an integral multiple of the wavelength and normal to the propagation direction. The present work shows that the CVP is analogous in many respects to the Dorn effect (sedimentation potential) and reflects the same intrinsic phenomena where double-layer relaxation is the dominant process. Both Smoluchowski's theory of the Dorn effect and Enderby's treatment of a charged particle in a sound field are reviewed. Expressions are presented showing the relationship of the CVP to the f potential for dilute colloids. It is also shown how the theory can be extended to particle concentrations as high as 50% by volume using the Levine et al. cell model theory. An apparatus for making electrokinetic measurements using continuous wave ultrasonics is described in detail Data are presented comparing mobilities obtained from CVP measurements and microelectrophoresis. Data are also shown for the dependence of the CVP on particle concentration and compared to predictions based on the proposed modifications using the cell model theory. Also included are data showing the versatility and advantages of acoustical electroki * Author to whom all correspondence should be addressed.

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The validity of the Smoluchowski treatment of the sedimentation potential (Dorn effect) is explored in relation to the recent cell-model treatment of Levine et al. which is applicable at higher volume fraction where both electrical and hydrodynamic interactions must be taken into account. An apparatus was constructed for the rapid successive measurement of the sedimentation potential gradient, the particle volume concentration, and the specific volume conductivity of the medium. Measurements on a model system of 97-p.m. glass microspheres dispersed in aqueous electrolytes showed that the Smoluchowski theory was followed below a volume fraction of 0.018 and that the potentials obtained compared to those obtained from streaming potential measurements at the glass-water interface characterized by an iep at pH 3.69. At higher volume fraction where the surface conduction correction was -1.1% the Levine cell-model theory was required to explain a -28% deviation from the linear approximation theory.

The electrokinetic characterization of coal dispersions with ultrasonics is demonstrated. Ultrasonics offer several advantages over conventional electrokinetic techniques such as microelectrophoresis, mass transport, or streaming potential, which require extreme dilution, a narrow concentration range, or packed beds. Some of the more important advantages of ultrasonics are (i) a wide range of particle sizes can be utilized, from ions to aggregates, (ii) coal concentrations can range from a few tenths of a percent to volume-filling systems, (iii) samples can be optically opaque or photosensitive, and (iv) measurements can be made on flowing streams applicable to on-line applications. It is demonstrated that colloid vibration potentials or acoustic mobility measurements can be successfully utilized to characterize the electrokinetic properties of concentrated coal dispersions. Experimental results for dilute coal dispersions are compared to the theory of Enderby and Booth and an independent technique, namely, microelectrophoresis. It is also demonstrated that colloid vibration potential measurements in concentrated coal dispersions can be explained by using the Levine, Neale, and Epstein cell model approach proposed by Marlow, Fairhurst, and Pendse. Evidence suggests that measurements in dilute coal dispersions cannot be extrapolated with reasonable confidence to particle concentrations typical of process conditions, an effect observed by others. The focus of this work, however, is to describe a technique that can be used to explore the reasons for this discrepancy because of its versatility with coal concentration. At coal concentrations comparable to process conditions significant deviations from ppm studies result. Deviations include the shifting of isoelectric points as well as the appearance of maxima in the mobility versus pH dependence. The deviations are proposed to result from the dissolution of the coal matrix with subsequent deposition onto the coal surface, although this work is not definitive in this regard.The coal-water interfacial chemistry is controlled by coal rank, mineral content, surface functional groups, pore structure, adsorption, pH, ionic strength, etc. These interactions can be probed by using electrokinetic techniques.1-12 Electrokinetic techniques presently used to study the coal-water interface include microelectrophoresis, mass transport, and streaming potential.13 Although these techniques are invaluable to the study of coal-water interactions, they cannot be used for the wide range of conditions that occur in processing coal water dispersions.+Presented at the Symposium on the Surface Chemistry of Coals, 193rd National Meeting of the American Chemical Society, Denver, CO, April 5-10 1987.These techniques require extreme dilution (microelectrophoresis), a narrow concentration range (mass transport), or packed beds (streaming potential). As a result, questionable extrapolations (see below) to process conditions must be performed.Below we describe a relatively new electrokinetic tech...