Colloidal separation

However, as follows from the results presented in Fig. 1(b), the behavior of the PMF for the case of adsorbed dispersion in the matrix at Pm< m — 0.386 contains interesting features in addition to those shown in Fig. 1(a). We observe that the PMF is modulated by the presence of solvent species and in addition is modulated by the presence of matrix particles. The structural repulsive barrier appears, due to matrix particles. An additional weak attractive minimum exists at separations corresponding to matrix-separated colloids. It is interesting that the effects of solvent modulation of the PMF in the adsorbed dispersion are seen for matrix separated colloids. The matrix particles are larger than colloids adsorption of solvent species on the surface of a matrix particle is stronger than on the surface of a colloid. Therefore, the solvent modulating effects of the PMF result from colloids separated by a matrix particle covered by a single layer of solvent species. 

The effects of confinement due to matrix species on the PMF between colloids is very well seen in Fig. 1(c). At a small matrix density, only the solvent effects contribute to the formation of the PMF. At a higher matrix density, the solvent preserves its role in modulating the PMF however, there appears another scale. The PMF also becomes modulated by matrix species additional repulsive maxima and attractive minima develop, reflecting configurations of colloids separated by one or two matrix particles or by a matrix particle covered by the solvent layer. It seems very difficult to simulate models of this sort. However, previous experience accumulated in the studies of bulk dispersions and validity of the PY closure results gives us confidence that the results presented are at least qualitatively correct. 

The most important application of semi-permeable membranes is in separations based on reverse osmosis. These membranes generally have pores smaller than 1 nm. The pressure across the semi-permeable membranes for reverse osmosis is generally much larger than those for ultrafiltration, for example. This is because reverse osmosis is usually used for small molecules which have a much higher osmotic pressure, because of the higher number density, than the colloids separated in ultrafiltration. As a result reverse osmosis membranes have to be much more robust than ultrafiltration membranes. Since the focus of our discussion in this chapter will be on reverse osmosis based separations, we will describe these membranes in greater detail. 

Initial tests of Brownian pumping required the measurement of Th in colloids separated from seawater samples. " Th proved to be an especially useful tracer of colloidal uptake of metal species because of its constant source and relative abundance. Baskaran et al. (1992) and Moran and Buesseler (1992) used cross-flow filtration to separate the colloidal fraction and both studies reported significant (up to 78% of total) " Th in this fraction. Subsequent work largely supported these observations (Moran and Buesseler 1993 Huh and Prahl 1995) and suggested the importance of colloidal organic matter in scavenging Th (Niven et al. 1995). 

The terms used to distinguish colloidal particles on the basis of their affinity to the fluid in which they are dispersed are lyophilic and lyophobic. These terms mean, literally, solvent loving and solvent fearing, respectively. When water is the medium or solvent, the terms hydrophilic or hydrophobic are often used. This terminology is very useful when considering surface activity such as wettability of a surface however, when used to classify colloids, the distinction is not always clear-cut. We consider these two types of colloids separately in the following subsections. 

On irradiated powder suspensions or on colloids, separation of the electron-hole pair can often translate into formation of an adsorbed radical ion pair, for the hole can oxidize... 

If one starts from a sol, that is the solution of the colloid in an appropriate solvent, then according to the nature of the colloid, various changes (temperature, pH, addition of a substance) can bring about a reduction of the solubility as a result of which a laig er part of the colloid separates out in a new phase. 

We may now ask if this applies also for a mixture of negative colloids if not pH is altered, but the concentration of an added salt is increased, which salt is supposed to be capable of bringing about reversal of charge of each of the colloids separately. 

A relatively new type of closed twill weave is available for colloidal separations. Whilst such fabrics have lower clean permeabilrties than plain or Dutch weaves, they are more easily cleaned and withstand heavy use. 

For context, we point out here that FFF is a broad family of techniques applicable to macromolecules, colloids, and particles of diverse types extending over a mass range from a few hundred to 10 dalton [3-8]. FFF has perhaps become best known for high-resolution colloid separation and characterization [5,7,9]. For this task, a specific subtechnique termed sedimentation FFF is most often utilized [10]. For polymer analysis, another subtechnique, thermal FFF, is most commonly employed [11,12]. 

Gueguen, C., Belin, C., and Dominik, J., Organic colloid separation in contrasting aquatic environments with tangential flow filtration. Water Res., 36,1677, 2002. 

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