Colloid electric 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. 

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. 

The repulsive property of colloid particles is due to electrical forces that they possess. The characteristic of these forces is indicated in the upper half of Figure 12.1b. At a short distance from the surface of the particle, the force is very high. It dwindles down to zero at infinite distance from the surface. 

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. 

For monodisperse or unimodal dispersion systems (emulsions or suspensions), some literature (28-30) indicates that the relative viscosity is independent of the particle size. These results are applicable as long as the hydrodynamic forces are dominant. In other words, forces due to the presence of an electrical double layer or a steric barrier (due to the adsorption of macromolecules onto the surface of the particles) are negligible. In general the hydrodynamic forces are dominant (hard-sphere interaction) when the solid particles are relatively large (diameter >10 (xm). For particles with diameters less than 1 (xm, the colloidal surface forces and Brownian motion can be dominant, and the viscosity of a unimodal dispersion is no longer a unique function of the solids volume fraction (30). 

I Hayati, AI Bailey, TF Tadros. Investigations into the mechanisms of electro-hydrodynamic spraying of liquids ILmechanism of stable jet formation and electrical forces acting on a liquid cone. J Colloid Interface Sci 117 222—230, 1987. 

Colloidal interaction forces act primarily in a direction perpendicular to the particle surface forces primarily acting in a lateral direction were discussed in Chapter 10. We merely consider internal forces, which find their origin in the properties of the materials present. This excludes forces due to an external field, such as gravitational, hydrodynamic, and external electric forces these are involved in some subjects of Chapter 13. 

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. 

Started to study the motion of colloidal suspensions of charged particles in solution in electrical forces. His discovery of electrophoresis would lead to his receipt of the 1948 Nobel Prize in chemistry. 

Acoustics has a related field that is usually referred to as electroacoustics (8). Electroacoustics can provide particle size distribution as well as zeta potential. This relatively new technique is more complex than acoustics because an additional electric field is involved. As a result, both hardware and theory become more complicated. There are even two different versions of electroacoustics depending on what field is used as a driving force. Electrokinetic sonic amplitude (ESA) involves the generation of sound energy caused by the driving force of an applied electric field. Colloid vibration current (CVC) is the phenomenon where sound energy is applied to a system and a resultant eleetrie field or eurrent is created by the vibration of the colloid electric double layers. 

This experiment is most significant because it gives a clue to the extraordinary lack of adhesion between many colloidal particles. If the particles are charged, then they will experience electrical repulsions which could be greate than the molecular adhesion. Then the particles are held apart by electrical forces. Adhesion can consequently be altered by varying the electrical nature ofthe fluid. 

At the point of zero charge, there is no repulsive electrical force on the particles and so the full adhesion between the grains is developed. If this adhesion is strong, then each Brownian collision between singlet particles will produce a doublet. This was the case considered by Smoluchowski in 1917 and extended by Fuchs in 1934. The theory was based on the idea that colloidal particles behave like molecules which can react to form abimolecular compound. Thus the rate of appearance of doublets dAf /df is proportional to the square of singlet concentration N per unit volume by the law of mass action, and is limited by the Brownian diffusion coefficient to give... 

Miller, C.A. and Scriven, L.E., Interfacial instability due to electrical forces in double layers I. General considerations, J. Colloid Interface Sci., 33, 360, 1970. 

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