Colloidal aggregation

FIGURE 5.6 Polymer brushes on the surface of colloids result in a steric repulsion between the particles when they approach each other. 

FIGURE 5.7 Surface modification using polymers in the brush phase can be used to reduce protein absorption in surfaces by steric repulsion. This technique is useful in the treatment of biomedical implants. Protein fouling can shorten the life of an implant considerably, but PEG coatings reduce this problem by slowing the rate of nonspecific surface absorption. 

FIGURE 5.8 This SEM image shows a colloidal aggregate formed from 2- jm polystyrene beads. The beads are packed in an apparently disordered arrangement within the cluster. 

FIGURE 5.9 Ballistic aggregation is often used as a model system for colloidal cluster growth. The cluster is randomly bombarded by successive new particles that stick and gradually increase the size. 

Induced flocculation is important industrially in the purification of drinking water and for wastewater treatment. Suspended particles in water are often too tiny to be filtered out of the water directly, so to solve this problem flocculation agents can be added to the water. Flocculation agents often contain multivalent charged ions (alum, ferric sulfate, or aluminum chlorohydrate, for example) or charged polymers, and they induce aggregation of suspended particles into floes by electrostatic screening. 

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For compact, homogeneous objects in tliree dimensions, p= 3. Colloidal aggregates, however, tend to be ratlier open, fractal stmctures, witli 3. For a general introduction to fractals, see section C3.6 and [61]. 

Poon W C K and Haw M D 1997 Mesoscopic structure formation in colloidal aggregation and gelation Adv. Colloid Interface Sc/. 73 71-126... 

Schaefer D W, Martin J E, Wiltzius P and Cannell D S 1984 Fractal geometry of colloidal aggregates Phys. Rev. Lett 52 2371-4... 

Meakin, P., Models for colloidal aggregation. Ann. Rev. Phys. Chem. 39, 237-269 (1988). Meakin, P Aggregation kinetics. Physica Scripta 46, 295-331 (1992). 

One major question of interest is how much asphaltene will flocculate out under certain conditions. Since the system under study consist generally of a mixture of oil, aromatics, resins, and asphaltenes it may be possible to consider each of the constituents of this system as a continuous or discrete mixture (depending on the number of its components) interacting with each other as pseudo-pure-components. The theory of continuous mixtures (24), and the statistical mechanical theory of monomer/polymer solutions, and the theory of colloidal aggregations and solutions are utilized in our laboratories to analyze and predict the phase behavior and other properties of this system. 

Colloidal aggregation is an application naturally suited to a CA approach because each cell can take on the role of a small volume element in a liquid, which may or may not contain a colloidal particle. Appropriate rules... 

If the excess of lanthanide is sufficiently great, overloading of the transport system occurs and colloidal aggregates of large size are formed by hydrolysis. The interstitial or intracavitary formation of immobilized lanthanide colloids labeled with relatively short-lived radioisotopes was the basis for the attempted use of radioactive lanthanides as internal sources of therapeutic radiation (Kyker, 1962a, 1962b). 

An electron microscopy study by Mullen et al. (1989) showed that Cd2+, Cu2+ and La3+ accumulated on the cell surface of Bacillus cereus, B. subtilis, E. coli and Pseudomonas aeruginosa as needle-like, crystalline precipitates, while Ag+ precipitated as discrete colloidal aggregates at the cell surface and occasionally in the cytoplasm. The needle-like and hexagonal precipitates were also found for the biosorption of Ni2+ on the cell surface of P. fluorescens 4F39 at pH 9 and it was suggested as a microprecipitation process that followed on ion exchange (Lopez et al. 2000). 

The use of micelles or similar aggregates in isotopic separations on the basis of nuclear spins is an especially interesting example of the way in which colloidal aggregates can keep reactive intermediates in close proximity (Turro and Kraeutler, 1980 Gould et al., 1984b Herve et al., 1984). 

The most rigorous evidence that proteins had defined structures was probably the molecular weight determinations of Adair and Svedberg. From 1900, however the crystallization of increasing numbers of proteins, while not a very reliable indication of purity, suggested to Schulz that proteins were not colloidal aggregates but large molecules with definite structures. 

A.E. Gonzalez and G. Ramirez-Santiago Spatial Ordering and Structure Factor Scaling in the Simulations of Colloid Aggregation. Phys. Rev. Lett 74,1238 (1995). 

The highly dynamic colloidal structures described in this chapter result in considerable complexity in behaviors. This complexity has resulted in relatively slow improvement in our understanding of colloidal systems despite the fact that the structure of micelles was in essence described almost a century ago already. Results from a series of relatively recent approaches to describe colloidal aggregates are now beginning to coalesce into a model of colloidal structures incorporating the dynamic and nonhomogeneous structures of these aggregates. 

In contrast, the three- or two-dimensional morphologies of colloidal aggregates via Brownian particle trajectories show a fractal-like structure. One of the most prominent features of the surface deposits formed by the diffusion-limited aggregation mechanism is the formation of isolated treelike clusters (9). In our experiments, the surface morphology of the silica-coated polyethylene composite prepared by... 

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