Electrophoretic mobility of colloids

There are a number of complications in the experimental measurement of the electrophoretic mobility of colloidal particles and its interpretation see Section V-6F. TTie experiment itself may involve a moving boundary type of apparatus, direct microscopic observation of the velocity of a particle in an applied field (the zeta-meter), or measurement of the conductivity of a colloidal suspension. 

Ohshima, H. and Kondo, T. (1989). Approximate analytic-expression for the electrophoretic mobility of colloidal particles with surface-charge layers, J. Coll. Interf. Sci., 130, 281-282. 

Electrokinetic equations describing the electrical conductivity of a suspension of colloidal particles are the same as those for the electrophoretic mobility of colloidal particles and thus conductivity measurements can provide us with essentially the same information as that from electrophoretic mobihty measurements. Several theoretical studies have been made on dilute suspensions of hard particles [1-3], mercury drops [4], and spherical polyelectrolytes (charged porous spheres) [5], and on concentrated suspensions of hard spherical particles [6] and mercury drops [7] on the basis of Kuwabara s cell model [8], which was originally applied to electrophoresis problem [9,10]. In this chapter, we develop a theory of conductivity of a concentrated suspension of soft particles [11]. The results cover those for the dilute case in the limit of very low particle volume fractions. We confine ourselves to the case where the overlapping of the electrical double layers of adjacent particles is negligible. 

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 first experiments demonstrating field-dependent electrophoretic mobility of colloids (Stotz-Wien effect) were reported by several... 

Ohshima, H. and Makino, K., Electrophoretic mobility of colloidal particles covered with a partially ion-penetrable surface layer. Colloids Surf. A, 13, 277, 1999. 

Varoqui, R., Effect of polymer adsorption on the electrophoretic mobility of colloids, Nouv. J. Chim.,... 

Phase analysis light scattering (PALS) is a technique that is very similar to laser Doppler electrophoresis (LDE) and is used to measure the electrophoretic mobilities of colloidal particles. This technique is particularly suited to measurements of charged particles suspended in nonpolar media and sensitivity of the method is far superior to that which could be obtained by LDE measurements. It makes use of a cross-beam technique and offsets one of the laser beams relative to the other by several kEIz. ... 

The first experiments demonstrating field-dependent electrophoretic mobility of colloids (Stotz-Wien effect) were reported by several groups in the 1970s [14], and the possibility of using this effect for particle separation using unbalanced AC fields has begun to be explored [14]. This work focused on nonlinear corrections to the classical phenomenon of electrophoresis, where a particle moves in the direction of the applied electric field, U = b(E)E, rather than on the associated ICEO flows and more complicated ICEP motion. 

Zeta Potential Zeta (Q potential is a parameter used to describe the electrophoretic mobility of colloidal particles. Charged colloidal particles are slightly different from ions in that colloidal particles are surrounded by an electric double layer which is similar but not identical to the ionic atmosphere. The inner part of the double layer moves as a unit in transport experiments. The ( potential is the surface potential of the inner part of the double layer, as shown in Figure 13.10. It is defined as... 

Grosse, C., Shilov, V. N., Electrophoretic Mobility of Colloidal Particles in Weak Electrolyte Solutions, J. Colloid Interface Sci, 1999,211, 160-170. 

O Brien, RW White, LR, Electrophoretic Mobility of a Spherical Colloidal Particle, Journal of the Chemical Society, Faraday Transactions 74, 1607, 1978. 

Weirsema, PH Loeb, AL Overbeek, JTG, Calculation of the Electrophoretic Mobility of a Spherical Colloid Paricle, Journal of Colloid and Interface Science 22, 78, 1966. 

The electrophoretic mobility of an ion is inversely related to the ionic strength of the buffer rather than to its molar concentration. The ionic strength (ytt) of a buffer is half the sum of the product of the molar concentration and the valency squared for all the ions present in the solution. The factor of a half is necessary because only half of the total ions present in the buffer carry an opposite charge to the colloid and are capable of modifying its charge ... 

Particle mobility and zeta potential can now be measured by more sophisticated techniques. With photoelectrophoresis, particle mobility is measured as a function of pH under the influence of ultraviolet radiation. At pH < 8, the electrophoretic mobility of irradiated hematite particles (A = 520 nm) was markedly different from that measured in the absence of UV irradiation. This was attributed to the development of a positive surface charge induced by photo-oxidation of the surface Fe-OH° sites to (Fe-OH) sites (Zhang et al., 1993). The electroacoustic technique involves generation of sound waves by the particles in the colloidal dispersion and from this data. 

The electrophoretic mobility of a protein solution may also be measured as a function of pH. By this technique it may also be observed that the colloid passes through a point of zero net charge at which its mobility is zero. The point at which charge reversal is observed electrophoretically is called the isoelectric point. 

Chen, J. and Dickinson, E. 1995b. Protein/surfactant interfacial interactions. Part 2. Electrophoretic mobility of mixed protein + surfactant systems. Colloids Surf. A Physicochem. Engin. Aspects 100 267-277. 

Ohshima, H. (2002). Electrophoretic mobility of a charged spherical colloidal particle covered with an uncharged polymer layer. Electrophoresis 23,1993-2000. 

In the early work of Schulze ( 0, Linder and Picton (2) and Hardy (3) the sensitivity of colloidal dispersions to the addition of electrolytes was clearly demonstrated. Then in 1900 Hardy (4) showed that the stability of sols was connected with the electrophoretic mobility of the particles and he demonstrated, i) that the valency of the ion opposite in charge to that of the sol particles determined the ability of an electrolyte to coagulate a sol and that, ii) the effectiveness of the electrolyte increased rapidly with increase in valency of the counter-ion. These observations formed the basis of the so-called Schulze-Hardy rule. 

During the past two decades, much attention has been drawn in this area and advances have been made in theoretical analysis concerning the applicability of Eq. (1) in a variety of systems. This chapter presents the state of understanding of the electrophoretic motion of colloidal particles under various conditions. We first introduce the basic concept and fundamental electrokinetic equations for electrophoretic motion. Then, we review some recent studies on the mobility of a single particle, the boundary effects and the particle interactions in electrophoresis. In addition, a few theoretical methods, which have been used to investigate the boundary effects and particle interactions, will be highlighted and demonstrated in the context. 

The electrophoretic mobilities of small colloidal particles, as well as of larger quartz particles, oil drops and air bubbles, are always about 2 to 4 X 10 cm. per sec. in water hence, in accordance with the requirements of equation (25), rj being 0.01 e.g.s. unit (poise) and D approximately 80, the value of the zeta-potential is between 0.03 and 0.06 volt in each case. 

The movement of colloidal particles in an electric field is termed electrophoresis. Most commercially available zetameters are designed to measure the electrophoretic mobility of particles. The potential is not measured directly, but is calculated from u. The Smoluchowski equation... 

Sidorova, M.P. et al.. Surface charge and electrophoretic mobility of boehmite particles in electrolyte solution. Colloid J., 59,495, 1997. 

Mishra, R.K., Chander, S., and Fuerstenau, D.W., Effect of ionic surfactants on the electrophoretic mobility of hydroxyapatite. Colloids Surf, 1, 105, 1980. 

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