Electroacoustic measurement

Rowell and co-workers [62-64] have developed an electrophoretic fingerprint to uniquely characterize the properties of charged colloidal particles. They present contour diagrams of the electrophoretic mobility as a function of the suspension pH and specific conductance, pX. These fingerprints illustrate anomalies and specific characteristics of the charged colloidal surface. A more sophisticated electroacoustic measurement provides the particle size distribution and potential in a polydisperse suspension. Not limited to dilute suspensions, in this experiment, one characterizes the sonic waves generated by the motion of particles in an alternating electric field. O Brien and co-workers have an excellent review of this technique [65]. 

The dynamic electrophoretic mobility of colloidal particles in an applied oscillating electric field plays an essential role in analyzing the results of electroacoustic measurements of colloidal dispersions, that is, colloid vibration potential (CVP) and electrokinetic sonic amplitude (ESA) measurements [1-20]. This is because CVP and ESA are proportional to the dynamic electrophoretic mobility of colloidal particles. In this chapter, we develop a theory of the dynamic electrophoretic mobility of soft particles in dilute suspensions [21]. 

Use of Ultrasonic Vibration Potential To Monitor Coalescence. The complex chemical nature of crude oils makes it difficult to relate the dispersion behavior to the physicochemical properties at the crude-oil-water interface. In addition, the nonpolar and nontransparent nature of the oleic phase provides significant obstacles for studies of the interactions of the suspended water droplets in real systems. Recent development (28, 29) of electroacoustical techniques has shown considerable promise for electrokinetic measurements of colloidal systems and the direct monitoring of the rate and extent of coagulation (flocculation and coalescence) of water droplets in nontransparent water-in-oil media. The electroacoustic measurement for colloidal systems in nonpolar media is based on the ultrasound vibration potential (UVP) mode, which involves the applica-... 

A typical preparation of dispersions for electrophoretic or electroacoustic measurements consists of the following steps ... 

The options for the preparation of dispersions for electroacoustic measurements are similar to those discussed above for electrophoresis, except that the experiments are usually carried out in titration mode and long equilibration before each measurement is not practiced, although long equilibration and/or sonication before the titration are commonplace. Various experimental protocols in electroacoustic measurements were compared in [423], for example, using a stirrer versus a pump and simultaneous measurements of the potential and size versus using a pre-assumed size. 

Mass Fractions of Solid Particles Used in Electroacoustic Measurements... 

Electroacoustic measurements are usually carried out in titration mode, and several papers report results of both acid and base titration. Substantial hysteresis (Figure 1.9) was found in [242,427,491-493]. Other studies in similar systems report negligible hysteresis [444,449,494,495]. Four cycles of titration in ESA measurements reported in [496] produced different values of the potential in the acidic range but a common IEP. Similar lEPs were observed in titrations of alumina from pH 2 to 12 and back, but in titrations from pH 1 to 12, the IEP was shifted to high pH [497]. Generally, more pronounced hysteresis is expected in titrations over a wider pH range. 

Only value, data points not reported the solid-to-liquid ratio (0.1% by volume) was below standard level used in electroacoustic measurements (Chapter 2). 

Costa, A.L., Galassi, C., and Greenwood, R., a-Alumina-IIjG interface analysis by electroacoustic measurements, 7. Colloid Interf. Sci., 212, 350, 1999. 

Greenwood, R. and Kendall, K., Effect of ionic strength on the adsorption of cationic polyelectrolytes onto alumina smdied using electroacoustic measurements, Powder Technol., 113, 148, 2000. 

The dynamic electrophoretic mobility is a useful system characteristic. For instance, it provides a means for determining the isoelectric point from electroacoustic measurements. Other properties of colloidal dispersions, namely -potential and particle size, can also be obtained from measurements of dynamic electrophoretic mobility. For a dilute dispersion (less then a few weight percent) of spherical particles, the dynamic electrophoretic mobility is given by a Smoluchowski-type relationship [37,38] ... 

It is also worth mentioning that particle size can also be estimated from electroacoustic measurements of the phase angle of CVP. In this case measurements done at a single frequency yield mean particle size, while in the case of multiple frequencies particle size distribution can be obtained. Acoustic spectroscopy is, however, more practical for particle size analysis than for electroacoustic CVP measurement. 

The possibility of making measurements directly on an emulsion of essentially any concentration up to around 60% is therefore a very appealing one. When the same measurement can yield a consistent measure of both the size and the electrokinetic charge on the particles, then the method becomes of unique value. That is the present situation with electroacoustic measurements of the ESA effect in the megahertz frequency range. 

After the electroacoustic measurements were finished, the sample was placed into a centrifuge tube and spun at 1500 rpm in a table top centrifuge (International Model HN). The amount of released water was recorded to confirm the relative dewatering efficiency of the demulsifiers. 

M. Rasmusson, Volume fraction effects in electroacoustic measurements. J. Colloid Interface Sci. 

Additional to the sedimentation behaviour, the zeta-potential was measured for each suspension. This was conducted by means of an electroacoustic measurement technique (Sect. 2.3.7.2). The technique yields an effective zeta-potential of the binary suspension, which is calculated from the electroacoustic raw signals by assuming effective particle properties (e.g. for the permittivity and density of the solid material). When the two particle components contribute independently to the electroacoustic signal and do not affect each other with regard to the interfacial properties, it is possible to calculate the effective zeta-potential from the zeta-potentials of the single components. The comparison between such calculated zeta-potential values with experimental ones allows a first evaluation of the interfacial phenomena in the binary suspension. 

This relation is only valid for acoustically thin films. Concretely, it means that the relation is not respected when thick layers are used because the influence of the viscoelastic properties of the layer may appear in the frequency change. This effect is, in general, amplified when polymer Aims are used. To determine the maximum useful thickness, electroacoustic measurements allowed a pertinent value to be evaluated. In a first step, a classical Butterworth-Van Dyke equivalent circuit of the loaded quartz (Fig. 13a) is extracted... 

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