Colloids mutual diffusion

Equation 10.26 would be valid if colloidal diffusion processes were exactly analogous to those for individual molecules. However, the interactions between particles in colloidal systems tend to extend over distances much greater than those involved in the formation of atomic or molecular activated complexes (say, 10-100 run vs. O.l-l.O nm). As a result, the effects of those interactions will begin to be felt by the particles well before they approach to the critical distance r. Their mutual diffusion rate will therefore be reduced and the collision frequency will drop accordingly. The collision frequency will also be reduced by the hydrostatic effect mentioned above for rapid coagulation. 

Section 10.3 treated mutual diffusion of colloidal spheres. To summarize significant results the low-concentration dependence of D on (j) is close to zero, in reasonable agreement with models based on the assertion that Dm 4>) of colloidal spheres is determined by the direct and hydrodynamic interactions of the spheres, and that (as also found for Ds) dynamic friction corrections are not large at short times. The value of Dmiinitial slope ko decreases markedly with increasing q at large Dm (q) approaches Ds, as expected theoretically. 

Finally, it is critical to emphasize the lack of similarity between the mutual diffusion coefficient of a colloid suspension and the mutual diffusion coefficient of a two-component fluid near a consolute point. Light scattering spectra of concentration fluctuations in binary mixtures approaching their consolute points find a mutual diffusion coefficient that may be written... 

In contrast, the concentration dependence of the Dm of a colloid suspension represents a balance between hydrodynamic and direct interactions(64-68). This balance leads to a mutual diffusion coefficient that may increase or decrease with increasing colloid concentration. Consider the diffusion of a protein in water. At large ionic strength, for a nearly neutral protein. Dm does not differ greatly from its value at very small protein concentration, so the corresponding nominal f is approximately the protein s hydrodynamic radius. However, as the protein charge z is increased and the solvent ionic strength I is reduced, the diffusion coefficient... 

By comparing colloid dynamics, probe dynamics (sedimentation, electrophoresis, probe diffusion), and chain dynamics (single-chain diffusion, mutual diffusion, solution viscosity, and viscoelasticity), information on the importance of topological constraints becomes accessible. Any properties found equally for colloid and chain dynamics cannot be created by topological constraints, because colloidal... 

The list of experimentally accessible properties of colloid solutions is the same as the list of accessible properties of polymer solutions. There are measurements of single-particle diffusion, mutual diffusion and associated relaxation spectra, rotational diffusion (though determined by optical means, not dielectric relaxation), viscosity, and viscoelastic properties (though the number of viscoelastic studies of colloidal fluids is quite limited). One certainly could study sedimentation in or electrophoresis through nondilute colloidal fluids, but such measurements do not appear to have been made. Colloidal particles are rigid, so internal motions within a particle are not hkely to be significant the surface area of colloids, even in a concentrated suspension, is quite small relative to the surface area of an equal weight of dissolved random-coil chains, so it seems unlikely that colloidal particles have the major effect on solvent dynamics that is obtained by dissolved polymer molecules. 

These examples describe low segregation at the colloidal level. Other examples [168] describe processes of long-lasting equilibration initially turbid blends become transparent in several months or even years due to low mutual diffusion rate. Therefore, thermodynamieally compatible systems [118, 129,168] were considered incompatible. 

The sorption mechanism includes diffusion motion, facilitating the penetration of molecules and ions to the active surface of colloids, their release into the medium and mutual exchanges. The diffusion is manifested during the motion of ions in the electric double layer of colloids as well as during the motion of ions and molecules to the surface of plant and microbial cells. 

Fair, B., D. Chao, and A. Jamieson, Mutual translational diffusion coefficients in bovine serum albumin solutions measured by quasielastic laser light scattering. Journal of Colloid and Interface Science, 1978, 66, 324-330. 

DLS is certainly the foremost method to obtain the diffusion coefficient D of colloidal particles in the dilute regime. Since the measurements are performed at high dilution, a possible influence of mutual interaction of the particles can be safely dismissed. Hence, the diffusion coefficient D may directly be converted into the hydrodynamic radius Rh by the well-known Stokes-Einstein relation ... 

The territory of an expanded single coil is much larger than the volume the monomers actually occupy. Therefore, in comparison to the collisions of small molecules, the probability of mutual collisions between two coils is significantly enhanced. When such fluffy coils diffuse, the mutual friction yields a solution with a high viscosity. In the early history of polymer science, the high viscous polymer solution was misimderstood as a colloidal gel. However, polymer solutions are actually the molecular dispersions of long chains in the solvent molecules. With the increase of the polymer concentration, the coils start to interpenetrate into each other. We can define an illusive critical overlap concentration C, as illustrated in Fig. 4.2. Then, polymer solutions with the concentrations beyraid C are called concentrated solutions. 

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