Particle charge, nonaqueous

This expression can be used for arriving at the stability ratio for charged particles in nonaqueous media in which the repulsion can be modeled using a simple Coulombic expression (see Problem 3 at the end of the chapter). 

Recent advances made in measuring particle charge and mobility in nonaqueous suspensions are reviewed. Microelectrophoretic techniques have been used to determine zeta potential and the measurements related to particle stability. 

Recently, great strides have been made in developing electrical and optical transient methods for measuring particle charge and mobility in these nonaqueous dispersions. It has been possible to obtain particle charge/mass ratios as well as the field dependence of particle mobility. 

The purpose of this paper has been to present a short overview of the recent advances that have been made in determining particle charge and mobility in nonaqueous suspensions. Clearly the nature of the charge on the particles plays a major role in determining the strength of the adhesion of these particles to an electrode and the electrical transient methods described here may be used to determine the force of that adhesion. In this respect the reader is referred to three recent papers by Vincett (15, JJ3, 17). This work was oriented towards... 

Figure 7. Typical adsorption isotherm of charge control agent on oxide particles in nonaqueous media. Key and O, atomic absorption electrical conductivity A, IR absorption and ,... 

For both LC and IC colloids, it was demonstrated that highly charged colloids are possible even in nonaqueous media. When the fraction of ionized molecules on the particle surface is compared with the total number of adsorbed molecules on the surface, it is found that approximately 1 out of 10 adsorbed molecules is ionized. The low dielectric constant of these media compared with water is primarily responsible for a low dissociation rate of the ionizable molecules. The fact that the dissociation in the solution is also low ensures that high concentrations of ions are not available to effectively screen the particle charge and high charges or zeta potentials are attainable. 

With high particle charges and low ionic concentrations, electrostatic repulsion (39,4C) alone should be capable of stabilizing, the colloid. This Typically occurs when surface potentials are around lOOmV. Steric stabilization (41,42) in nonaq-ueous media is important to achieve stability in weakly charged systems. Direct experimental measurements of particle-particle forces or potentials in nonaqueous media are not yet available. 

Mechanism of Electric Charging of Particles in Nonaqueous Liquids... 

In a nonaqueous system with small closely spaced particles (s < t ) the electrostatic repulsion energy between two identical charged spheres may be approximated (1) ... 

Cross-flow-elec trofiltratiou (CF-EF) is the multifunctional separation process which combines the electrophoretic migration present in elec trofiltration with the particle diffusion and radial-migration forces present in cross-flow filtration (CFF) (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears later in this section) in order to reduce further the formation of filter cake. Cross-flow-electrofiltratiou can even eliminate the formation of filter cake entirely. This process should find application in the filtration of suspensions when there are charged particles as well as a relatively low conduc tivity in the continuous phase. Low conductivity in the continuous phase is necessary in order to minimize the amount of elec trical power necessaiy to sustain the elec tric field. Low-ionic-strength aqueous media and nonaqueous suspending media fulfill this requirement. 

The diffusive transport of M proceeds through both ions and ion pairs. In the conduction process, however, the situation is different (Fig. 4.64). The applied electric field exerts a driving force on only the charged particles. An ion pair as a whole is electrically neutral it does not feel the electric field. Thus, ion pairs are not participants in the conduction process. This point is of considerable importance in conduction in nonaqueous media (see Section 4.7.12). 

Surfactants are employed in emulsion polymerizations to facilitate emulsification and impart electrostatic and steric stabilization to the polymer particles. Sicric stabilization was described earlier in connection with nonaqueous dispersion polymerization the same mechanism applies in aqueous emulsion systems. Electrostatic stabilizers are usually anionic surfactants, i.e., salts of organic acids, which provide colloidal stability by electrostatic repulsion of charges on the particle surfaces and their associated double layers. (Cationic surfactants are not commonly used in emulsion polymerizations.)... 

The results obtained also are useful for the calculation of the ionic conductivity of nonaqueous electrolyte solutions. Several attempts exist for the calculation of the molar conductivity of associating electrolytes beyond the limiting law at the level of the MSA [3, 32, 33], where, however, only ion pairs were taken into account. Ion pairs and tetramers are electrically neutral, nonconducting species in the solution, by contrast to the ion trimers. The total concentration of charged particles is given by,... 

The description of a colloid should include particle size, mobility, charge and their distributions, charge/mass ratio, electrical conductivity of the media, concentration and mobility of ionic species, the extent of a double layer, particle-particle and particle-substrate interaction forces and complete interfacial analysis. The application of classical characterization methods to nonaqueous colloids is limited and, for this reason, the techniques best suited to these systems will be reviewed. Characteristic results obtained with nonaqueous dispersions will be summarized. Physical aspects, such as space charge effects and electrohydrodynamics, will receive special attention while the relationships between chemical and physical properties will not be addressed. An application of nonaqueous colloids, the electrophoretic development of latent images, will also be discussed. 

Figure 1. Schematic of low (left) and intermediate (right) conductivity nonaqueous colloids. Key 0, positively charged particles counterions +, positive excess ions and —, negative excess ions.

Figure 8. Dependence of average particle size on the concentration of charge control agent in nonaqueous media.

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