Colloidal particles Conductivity

The most familiar type of electrokinetic experiment consists of setting up a potential gradient in a solution containing charged particles and determining their rate of motion. If the particles are small molecular ions, the phenomenon is called ionic conductance, if they are larger units, such as protein molecules, or colloidal particles, it is called electrophoresis. 

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. 

Rowell and co-workers [62-64] have developed an electrophoretic fingerprint to uniquely characterize the properties of charged colloidal particles. They present contour diagrams of the electrophoretic mobility as a function of the suspension pH and specific conductance, pX. These fingerprints illustrate anomalies and specific characteristics of the charged colloidal surface. A more sophisticated electroacoustic measurement provides the particle size distribution and potential in a polydisperse suspension. Not limited to dilute suspensions, in this experiment, one characterizes the sonic waves generated by the motion of particles in an alternating electric field. O Brien and co-workers have an excellent review of this technique [65]. 

Ohshima, H Healy, T White, LR, Approximate Analytic Expressions for the Electrophoretic Mobility of Spherical Colloidal Particles and the Conductivity of their Dilute Suspensions, Journal of the Chemical Society, Faraday Transactions 79, 1613, 1983. 

A similar study was conducted by Dimitrijevi6 et al. with neutral solutions of a-Fe203. The yield of Fe was found to be very low. However, a large Fe yield was found after dissolution of the colloid by hydrochloric acid under an argon atmosphere. This showed that electrons donated by the free radicals penetrated deep into the colloidal particles to reduce iron to F, . Buxton et al. observol in a study on the reductive dissolution of colloidal Fe304 that Fe ions in this material are less readily released into the aqueous phase than reduced Fe ions. 

Film Formation. A novel feature of these Au-organic solvent colloids is their film forming properties that can be induced simply by solvent stripping. In this sense they are "living" colloidal particles. Films formed in this way are conductive, but less so than pure metals. (in ) The higher resistance of the films is due to the incorporation of substantial portions of the organic solvent, which can partially be removed by heating, and resistivity then decreases.(M1)... 

Colloidal particles, foams used to collect and separate, 12 22 Colloidal powders, 23 55-56 Colloidal silica, 22 380, 382, 384 applications of, 22 394 modification of, 22 393-394 preparation of, 22 392-393 properties of, 22 391-392 purification of, 22 393 Colloidal silica gels, 23 60 Colloidal solids, 7 293-294 Colloidal stability, 7 286-291 10 116 22 55 Colloidal stabilizers, in polychloroprene latex compounding, 19 857 Colloid mills, 8 702 10 127 Colloids, 7 271-303 23 54. See also Polymer colloids analysis, 7 296 applications, 7 292-296 conducting, 7 524... 

Ionic micelles will migrate in an electric field, and the ion atmosphere of the colloidal particle is dragged along with it. Interpretation of micellar mobility (conductivity experiments) must take this into account. The same is true, however, of the mobility of simple ions, but the situation is more involved here since the micelle and the ion atmosphere have comparable dimensions. We see in Chapter 12 how particle and double-layer dimensions affect the interpretation of mobility experiments. 

The fact that positive ions migrate toward the cathode and negative ions migrate toward the anode is so well known as to be virtually self-evident. It seems equally evident, therefore, that positively and negatively charged colloidal particles should display similar migrations. Indeed, this is the case. Because we are relatively familiar with the conductivity of simple electrolytes, we start our discussion of electrokinetic phenomena with a comparison of the mobilities of the particles in the small ion and macroion size domains. 

The success and relative simplicity of conductivity as a method of study for small ions prompt us to extend these ideas to particles in the colloidal size range. For the purpose of our discussion here, we can treat a charged colloidal particle as an ion of large charge, hence the name macroion. However, we identify shortly some of the differences between such macroions and small ions with respect to their response to an applied electric field. For certain colloids the experimental aspects of studying mobilities are simpler than for small ions because of the... 

These electrons and holes can be trapped both at the interior sites and on the surface of colloidal particles. Then, electrons can be located in the conduction band (e"cb) or on Ti4+ ions, at the surface (Ti3+)surf and in the bulk lattice (Ti3+)Uttice. It follows from Table 8.1 that surface and lattice Ti3+ centers can be distinguished by difference it their EPR parameters. 

In an attempt to overcome the low infusible character and low solubility of aniline, dispersion polymerization of aniline was conducted in water-dispersible colloidal particles that can be cast as films or blended with other materials to prepare composites. HRP mediated polymerization of aniline in a mixture of phosphate buffer and organic solvent resulted in polyaniline composed of ortho-directed units and para-directed units. Increasing the pH or adopting an organic solvent with a high dielectric constant, enhanced the production of ortho-directed units [54]. These ortho-directed polyanilines were more thermally flexible and electrically conductive. 

Colloid characterization is not the classical application of Th-FFF. Nevertheless, Th-FFF was first applied to silica particles suspended in toluene testing a correlation between thermal diffusion and thermal conductivity [397]. Although a weak retention was achieved, no further studies were carried out until the work of Liu and Giddings [398] who fractionated polystyrene latex beads ranging from 90 to 430 nm in acetonitrile applying a low AT of only 17 K. More recently, polystyrene and polybutadiene latexes with particle sizes between 50 pm and 10 pm were also fractionated in aqueous suspensions despite the weak thermal diffusion [215] (see Fig. 30). Th-FFF is also sensitive to the surface composition of colloids (see the work on block copolymer micelles), recent effort in this area has been devoted to analyzing surfaces of colloidal particles [399,400]. 

A1 hydroxide are known to act as binding agents and induce flocculation [33], In all cases, eluent electrical conductivity values (EC), and therefore ionic strength, remained low (50-100 tS cm-1) during the course of the leaching experiment, suggesting that the electrochemical conditions were not conducive for adequate suppression of the thickness of the double layer that would sufficiently reduce the electrostatic repulsive forces between colloid particles and cause flocculation [34],... 

As already discussed in Sect. 2.2., the bandgap of semiconductor particles increases considerably when their size becomes smaller than about 100 A (Figs. 4 and 5). Accordingly, the position of energy bands is shifted, and it is expected that certain reactions should become possible with quantized particles which do not occur with bulk materials. This has been demonstrated for H2-evolution in 50 A PbSe- and HgSe-colloids, which has not been observed with large particles [181, 182]. An extreme negative shift of the conduction band by about 1.2 eV has been found with 50 A-CdTe-colloids due to their low effective mass. Since COa-reduction to formic add was observed with photoexcited CdTe-colloids, the conduction band must be at < - 1.9 eV, compared to the flatband potential of n-CdTe electrodes of — 0.6 V [181]. 

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. 

Carbon black [1333-86-4] is virtually pure elemental carbon (diamond and graphite are other forms of nearly pure carbon) in the form of near-spherical colloidal particles that are produced by incomplete combustion or thermal decomposition of gaseous or Uquid hydrocarbons. Its physical appearance is that of a black, finely divided pellet or powder, the latter sometimes small enough to be invisible to the naked eye. Its use in tires, mbber and plastic products, printing inks and coatings is related to the properties of specific surface area, particle size and structure, conductivity and color. 

By way of Introduction, consider first an uncharged colloidal particle in an applied homogeneous field. Depending on the conductivity K of the particle with respect to that of the solvent,, different situations can be created, as illustrated in fig. 3.84. 

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