Suspensions sedimentation potential

Sedimentation potential A potential is created when charged particles settle out of a suspension. This gives rise to sedimentation potential,... 

Good descriptions of practical experimental techniques in conventional electrophoresis can be found in Refs. [81,253,259]. For the most part, these techniques are applied to suspensions and emulsions, rather than foams. Even for foams, an indirect way to obtain information about the potential at foam lamella interfaces is by bubble electrophoresis. In bubble microelectrophoresis the dispersed bubbles are viewed under a microscope and their electrophoretic velocity is measured taking the horizontal component of motion, since bubbles rapidly float upwards in the electrophoresis cells [260,261]. A variation on this technique is the spinning cylinder method, in which a bubble is held in a cylindrical cell that is spinning about its long axis (see [262] and p.163 in Ref. [44]). Other electrokinetic techniques, such as the measurement of sedimentation potential [263] have also been used. 

SEDIMENTATION POTENTIAL AND VELOCITY IN A SUSPENSION fundamental electrokinetic equations can be expressed in terms of h and as... 

In the limit a = b, the polyelectrolyte layer vanishes so that 4 c = 4 and (J)s = 0. In this case, Eq. (24.40) gives the following result for sedimentation potential for concentrated suspension of hard particles of radius a for low zeta potentials, as expected ... 

Here Vp has been replaced with the pressure difference between the two points is AP, K°, and K are, respectively, the usual conductivity and the complex conductivity of the electrolyte solution in the absence of the particles, (f> is the particle volume fraction, (j)c is the volume fraction of the particle core, Vc is the volume of the particle core, volume fraction of the polyelectrolyte segments, I4 is the total volume of the polyelectrolyte segments coating one particle, and po, are respectively, the mass density of the particle core and that of the electrolyte solution, and ps is the mass density of the polyelectrolyte segment, V is the suspension volume, and p(cai) is the dynamic electrophoretic mobility of the particles. Equation (26.4) is an Onsager relation between CVP and pirn), which takes a similar form for an Onsager relation between sedimentation potential and static electrophoretic mobility (Chapter 24). 

The influence of charged acrylamide graft copolymers on the flocculation process of 5% (w/w) kaolin suspensions were investigated by Serita and Murai [17]. The flocculation ability of different copolymers was linked to their structure by measuring the sedimentation potential. An iep of kaolin particles was observed for the concentration of approximately 20 ppm polymer, and they could additionally show that copolymers were more effective for flocculation than typical commercial flocculants. 

Very finely divided particles suspended in a liquid carry an electrical charge which is equivalent to the charge on the particle itself plus the charge on the fixed portion of the double layer. If an electrical field is applied to such a suspension, the particles move in the field in the direction determined by the charge on the particle (electrophoresis). The diffuse part of the double layer, since it is mobile, has the opposite sign and is attracted to the other electrode. Conversely, if a suspension of particles is allowed to settle, they carry their charge toward the bottom of the vessel and leave the charge on the diffuse layer in the upper portion of the vessel. A potential difference, the sedimentation potential, develops between the top and bottom of the container. 

THE SEDIMENTATION POTENTIAL. When a soil particle bearing an electrified interface settles in an aqueous solution under the force of gravity, a plane of shear develops around the particle just as it does in electrophoresis. As the particle settles, the portion of the interfacial region enclosed by the plane of shear moves with it but the remainder is left behind, and this separation of interfacial charge gives rise to an elegtric potential difference called the sedimentation potential. For a suspension soil particles that do not interact with one another (i.e., that settle independently), the force per unit volume acting on the particles is... 

Sedimentation potential. It is the potential difference, l/sed. sensed by two electrodes placed at a known vertical distance in the suspension, subjected to a gravitational (or, equivalently, centrifugal) field, g. If the density of the particles is lower than that of the supporting fluid, we can also... 

Like in the case of electrophoresis, the Smoluchowski equation is only valid for particles with thin double layers and negligible surface conductance (low zeta potentials). The theory was later generalized to arbitrary Ka values by Booth [43] for low zeta potentials, and was developed for arbitrary by Stigter [44], Considering the fact that rather concentrated suspensions are often used in sedimentation potential determinations, theories have also been elaborated to include these situations [45-47]. 

A careful account of the problem can be found in Ref. [95]. Ohshima et al. [96] first found a numerical solution of the problem, valid for arbitrary values of the zeta potential or the product Ka. In the same paper, they dealt with the problem of finding the sedimentation potential and the DC conductivity of a suspension of mercury drops. The problems are solved following the lines of the electrophoresis theory of rigid particles previously derived by O Brien and White [18]. The liquid drop is assumed to behave as an ideal conductor, so that electric fields and currents inside the drop are zero, and its surface is equipotential. The main difference between the treatment of the electrophoresis of rigid particles and that of drops is that there is a velocity distribution of the fluid inside the drop, Vj, governed by the Navier-Stokes equation with zero body force (in the case of electrophoresis), and related to the velocity outside the drop, v, by the boundary conditions ... 

Ohshima, H., Sedimentation potential in a concentrated suspension of spherical colloidal particles, J. Colloid Interface Sci., 208, 295, 1998. 

The second technique, known as electro-osmosis, causes a fluid in contact with a stationary charged surface to move in response to an applied electric field. The third technique, which can be regarded as the converse of electro-osmosis, is known as the streaming potential technique. The streaming potential involves the measurement of the electric potential generated when a liquid is forced under pressure to move in contact with a stationary charged surface. Finally, the sedimentation potential technique is a method that is, in a way, the converse of electrophoresis. A sedimentation potential arises across the suspension when charged particles settle in a stationary liquid. Adamson (29) explained the basic principles of these electrokinetic techniques in a nutshell as shown in Table 2. 

Sedimentation potential A potential is created when charged particles settle out of a suspension. This gives rise to sedimentation potential, which is the opposite of the streaming potential. The reason for investigating electrokinetic properties of a system is to determine the quantity known as the zeta potential. 

The most popular and straightforward way to determine zeta potential is to apply an electric field to a colloidal suspension. In the case of neutral particles nothing happens, while particles carrying surface charges will have an oriented motion dependent on the direction of the electric field. Several phenomena (collectively known as electrokinetic effects) are observed i.e., electrophoresis, electroosmosis, streaming potential, and sedimentation potential. In this chapter we will discuss the first two effects. 

Most authors who have studied the consohdation process of soflds in compression use the basic model of a porous medium having point contacts which yield a general equation of the mass-and-momentum balances. This must be supplemented by a model describing filtration and deformation properties. Probably the best model to date (ca 1996) uses two parameters to define characteristic behavior of suspensions (9). This model can be potentially appHed to sedimentation, thickening, cake filtration, and expression. 

Sedimentation systems with the particle size of 0.1-1.0 pm were obtained by mechanical dispersion of the copolymers with subsequent washing, fractionation, and separation of fractions. Micrograin forms (gr.) were synthesized by suspension copolymerization and fractionated. For the description of properties of weakly swelling and weakly dissociating gels, Katchalsky [36] has proposed an equation which contains the electrostatic potential eip... 

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