Colloidal dispersions electrical forces

Lyophobic colloids (sols) may be prepared by grinding crystalline materials or running an electric arc between metallic electrodes, both in the dispersing medium. More commonly, they are prepared by precipitating the solid from a strongly supersaturated solution, which produces a large number of precipitation nuclei. Because there is little attractive interaction between the particles and the medium, attractive forces between the particles would soon lead to their aggregation flocculation). This tendency, however, is counterbalanced by repulsive electrical forces between the particles. 

Colloidal dispersions, in general, are rendered stable either by electrostatic stabilization or by steric stabilization. In the former case, the repulsive electrical double layer forces between two particles counteract the attractive van der Waals forces and generate a potential barrier between the primary and secondary minima. If the potential barrier is sufficiently higher than the... 

Addition of soluble macromolecules (polymers) in the colloidal dispersion can stabilize the colloidal particles due to the adsorption of the polymers to the particle surfaces. The soluble polymers are often called protective agents or colloids. If the protective agents are ionic and have the same charge as the particles, the electrical double-layer repulsive forces will be increased and thus the stability of the colloidal particles will be enhanced. In addition, the adsorbed polymers may help weaken the van der Waals attraction forces among particles. However, the double-layer repulsion and the van der Waals attraction cannot account for the entire stabilization of the particle dispersions. 

Electroacoustics — Ultrasound passing through a colloidal dispersion forces the colloidal particles to move back and forth, which leads to a displacement of the double layer around the particles with respect to their centers, and thus induces small electric dipoles. The sum of these dipoles creates a macroscopic AC voltage with the frequency of the sound waves. The latter is called the Colloid Vibration Potential (CVP) [i]. The reverse effect is called Electrokinetic Sonic Amplitude (ESA) effect [ii]. See also Debye effect. 

In the theory developed by Derjaguin and Landau (24) and Verwey and Overbeek (25.) the stability of colloidal dispersions is treated in terms of the energy changes which take place when particles approach one another. The theory involves estimations of the energy of attraction (London-van der Walls forces) and the energy of repulsion (overlapping of electric double layers) in terms of inter-oarticle distance. But in addition to electrostatic interaction, steric repulsion has also to be considered. 

The electrical forces are produced due to the charges that the particles possess at their surfaces. These charges called primary charges are, in turn, produced from one or both of two phenomena the dissociation of the polar groups and preferential adsorption of ions from the dispersion medium. The primary charges on hydrophobic colloids are due to preferential adsorption of ions from the dispersion medium. 

The high pressures, so conveniently applied to surface films, would be much more difficult to attain in bulk systems. A force of 20 dynes cm. acting on a monomolecular film is equivalent to a pressure of 10 dynes cm., about 100 atmospheres. The high concentrations in the surface are obtained in bulk phases only in pure liquids and sohds. The high electrical fields near charged surfaces are probably never found in bulk solutions except for colloidal dispersions. 

Electrodecantation A separation process in which a colloidal dispersion is separated from a noncolloidal solution by an applied electric field together with the force of gravity. Also called electrophoresis convection. 

Interfacial phenomena at metal oxide/water interfaces are fundamental to various phenomena in ceramic suspensions, such as dispersion, coagulation, coating, and viscous flow. The behavior of suspensions depends in large part on the electrical forces acting between particles, which in turn are affected directly by surface electrochemical reactions. Therefore, this chapter first reviews fundamental concepts and knowledge pertaining to electrochemical processes at metal oxide powder (ceramic powder)/aqueous solution interfaces. Colloidal stability and powder dispersion and packing are then discussed in terms of surface electrochemical properties and the particle-particle interaction in a ceramic suspension. Finally, several recent examples of colloid interfacial methods applied to the fabrication of advanced ceramic composites are introduced. 

This method of formulation by von Smoluchowski and Fuchs is limited to small concentrations of particles. Then the fixed particle can at most feel the presence of one other particle, and (p is equal to the sum of the van der Waals attraction and the electrical double-layer repulsion poteitial, or, as discussed in previous sections. In this limit it is also legitimate to model the reaction as a second-order reaction (i.e., only two-particle collisions can occur and the higher body collisions are virtually nonexistent). In aerosols, which arc colloidal dispersions in air, there is no significant electrical repulsion betwerai particles. Hence the effect of interparticle forces on the initial coagulation rate is negligible, and we find... 

The concentration and nature of the electrolyte also has a significant impact on the stability of charged colloid dispersions. This was discussed in Section 3.3.2, where the concept of electric double layers was introduced. The electric double layer results from the atmosphere of counterions around a charged colloid particle. The decay of the potential in an electric double layer is governed by the Debye screening length, which is dependent on electrolyte concentration (Eq. 3.8). In the section that follows, the stability of charged colloids is analysed in terms of the balance between the electrostatic (repulsive) forces between double layers and the (predominantly attractive) van der Waals forces. 

Most suspension particles dispersed in water have a charge acquired by specific adsorption of ions or ionization of surface groups, if present. If the charge arises from ionization, the charge on the particle will depend on the pH of the environment. As with other colloidal particles, repulsive forces arise because of the interaction of the electrical double layers on adjacent particles. The magnitude of the charge can be determined by measurement of the electrophoretic mobility of the particles in an applied electrical field. 

Stabilization of colloidal dispersions can be divided into the two basic mechanisms electrostatic and steric (Fig. 4) [57]. With the van der Waals-London attractive forces acting continuously between colloidal particles, it is necessary, in order to maintain stabiUly, to introduce a repulsive force (electrostatic and steric) to outweigh the attractive force. The electrostatic stabilization provides the repulsive forces between similarly charged electrical double layers to the interactive particles [58, 59] (Fig. 4). Thus, the electrical double layer imparts the electrostatic stabilization. The steric stabilization becomes important when there are hydrophilic macromolecules or chains adsorbed or bounded to the particle surface [60]. When the layers of two interacting particles overlap the concentrahon of these macromolecules (chains) increases as weh as free energy. The molecules of good solvent enter the overlap layer and then separate the particles. This phenomenon is accompanied with the increased osmohe pressure. 

Figure 10.1 Colloidal dispersions are Inherently unstable systems and in the long run the attractive forces will dominate and the colloidal system will destabilize. However, colloid stability depends on the attractive van der Waals and the repulsive electrical or steric (polymeric) forces. The repulsive forces stabilize a dispersion if they are larger than the van der Waals (vdW) ones (and the total potential is larger than the "natural" kinetic energy of the particles). Surfaces are Inherently unstable and the van der Waals forces "take the system" back to its stable (minimum surface area) condition and contribute to instability (aggregation)...

The electrical force is due to the electrical double layer that exists around the particles in nearly all colloidal dispersions. Almost all particles are charged in aqueous (or other polar) media (Figure 10.9). The interfaces may have a positive or negative charge, but the latter (negative) is the most common. We will see that the charge depends on pH, nature of surface groups and salt concentration. 

The understanding and control of the electrical forces and thus the stability of a colloidal dispersion depends a great deal on the Debye length and the value of the surface potential as well as the charging mechanism. It is especially cmcial to control the diffuse double layer through control of the ionic strength of the solution and the adsorption of charged... 

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