Electrokinetic Sonic Amplitude measurements

Sonnefeld, J., Lobbus, M., and Vogelsberger, W.. Determination of electric double layer parameters for spherical silica particles under application of the triple layer model using surface charge density data and results of electrokinetic sonic amplitude measurements, Colloids Surf. A, 195, 215, 2001. 

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]. 

The colloid vibration potential (difference) E or CVP is the a.c. potential difference measured between two Identical relaxed electrodes, placed in the dispersion if the latter Is subjected to an (ultra)sonlc field. CVP Is a particular case of the more general phenomenon, ultrasonic vibration potential (UVP), applying to any system, whether or not colloids are present. This field sets the particles into a vibrating motion, as a result of which the centres of particle charge and countercharge are periodically displaced with respect to each other. This phenomenon is the a.c. equivalent of that observed in the Dorn effect. Counterpart to this is the electrokinetic sonic amplitude, ESA, the amplitude of the (ultra)sonlc field created by an a.c, electric field in a dispersion. 

Lobbus, M. et al.. An improved method for calculating zeta-potentials from measurements of the electrokinetic sonic amplitude, J. Colloid Interf. Sci., 229, 174, 2000. 

The kinetic potential is usually denoted as the zeta (0 potential and it is determined from the electrophoretic mobility of the extremely dilute particles in an electric field. More recently, the nse of electrokinetic sonic amplitude (ESA), acoustosizer (AZR), or colloid (or ultrasonic) vibration potential (CVP) has become available for the determination of the potential in rather concentrated particle suspensions. Again the potential may be measured as a function of either the metal concentration or the pH. In the latter case the point where the mobility ceases is denoted the isoelectric point (pH,Ep Fignre 8.27). It correlates particnlarly well with the stability of the sol. 

When an alternating voltage is applied to a colloid, the particles move back and forth with a velocity that depends on their size and zeta potential and on the frequency of the applied field. As they move, the particles generate sound waves. This phenomenon is called the electroacoustic effect, which can be measured and what was named electrokinetic sonic amplitude (ESA) [5],... 

To measure the particle size and -potential of pigment TiC>2 by the ESA (Electrokinetic Sonic Amplitude) method, the Acoustosizer 2 device ( Colloidal Dynamics , Sydney) was used. 

More recendy, Matec has introduced their AcoustoSizer, which measures electrokinetic sonic amplitude as a function of frequency (dififusion wave spectroscopy). Rather large samples (400 ml) are required in the concentradon range 1-40 vol%. The particle size range is 0.1-10 pm. The Pen Kem AcoustoPhor 8000 is based on the measuimient of attenuation of sound as it passes through the dispersion. It is claimed to have a range of 0.1 -10 pm plus, requires a sample volume of 1(K) ml and is applicable to dispersions greater than 40 voI%. 

Electroacoustic phenomena. They are electrokinetic phenomena that have recently gained interest, both experimentally and theoretically. In the ESA (electrokinetic sonic amplitude) technique, an alternating electric field is applied to the suspension and the sound wave produced in the system is detected and analyzed. The colloid vibration potential (CVP) or colloid vibration current (CVI) is the reciprocal of the former a mechanical (ultrasonic) wave is forced to propagate through the system, and the resulting alternating potential difference (or current) is measured. 

Another method for the determination of electrophoretic mobility which has emerged in recent years is that of the measurement of the electrokinetic sonic amplitude (ESA) for a particle subjected to an alternating current (8). This electroacoustic effect is a result of the oscillation of the particles near the electrodes where a sound wave is produced that can be picked up by a pressure transducer located behind the electrode. The ESA pressure signal is simultaneously proportional to the dynamic mobility of the particle, the particle volume fraction and the density difference between particle and solvent. Thus, the electroacoustic effect is appropriate for concentrated dispersions where conventional electrophoretic methods are inappropriate. However, one disadvantage of the method is that it is not appropriate to systems having low density differences between the particles and suspending liquid. 

Electrokinetic sonic amplitude (ESA) measurements (water / ethanol 4 1, Matec MBS 8000) show a very similar picture for the two quinophthalone pigments. They carry a very small negative charge and their isoelectric points are close to pH 3. Both pigments can be used in aqueous and solvent-borne systems. Erom the ESA-data (Table 19-4) it can be concluded that electrostatic forces play only a minor role in the stabilization of these systems. 

Electrokinetic Sonic Amplitude (ESA), for measurement of surface charges on pigments. 

Different from acoustic attenuation spectroscopy, in electroacoustic spectral analysis, sound waves are generated by an applied high frequency electric field across a colloidal suspension and subsequently detected. This is called the electrokinetic sonic amplitude effect (ESA) [38]. These sound waves arise because the alternating electric field pushes the suspended particle forwards and backwards. By measuring the magnitude and phase angle of the sound waves at multiple frequencies (typically from 1-10 MHz), the particle dynamic mobility, Pd, can be determined, provided the concentration and the density of the... 

Similarly to LFDD, there is a set of electrokinetic techniques that involves ac fields and that can be applied to suspensions of arbitrary particle concentration, as they do not rely on optical techniques of evaluation. These are the so-called electroacoustic techniques, which enable the determination of the dynamic or ac mobility, u, of colloidal particles (the ac counterpart of the dc or classical electrophoretic mobility) as a function of frequency. There are basically two such techniques. One is based on the determination of the electric potential difference induced by the passage of a sound wave through the system it is called colloid vibration potential (CVP) or colloid vibration current (CVI), depending on the quantity measured. In the second technique, reciprocal of CVP or CVI, the basic process is the generation of a pressure wave when an ac electric field is applied to the suspension the amplitude of the sound wave, A sa is known as electrokinetic sonic amplitude, and so we speak of the ESA effect. After the very early works in the subject, O Brien [27,28] was the first author to perform a rigorous investigation on the physical foundations of electroacoustic techniques, and he found that Me is in fact proportional to [28] ... 

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