Particles, colloidal colloid stability

The materials we studied are non-aqueous dispersions of polymer particles. Colloidal stability of these particles in hydrocarbon solvents is conferred by a surface covering of a highly swollen polymer (the stabilizer) on a second polymer, insoluble in the medium (the core polymer), which comprises 90 % of the material (11). These particles are prepared by dispersion polymerization polymerization of a monomer soluble in the medium to yield an insoluble polymer, carried out in the presence of a soluble polymer which becomes the stabilizer. In the examples discussed here, the core polymer is formed by free radical polymerization. Hydrogen abstraction from the soluble polymer present in the reaction medium... 

The remainder of this contribution is organized as follows. In section C2.6.2, some well studied colloidal model systems are introduced. Methods for characterizing colloidal suspensions are presented in section C2.6.3. An essential starting point for understanding the behaviour of colloids is a description of the interactions between particles. Various factors contributing to these are discussed in section C2.6.4. Following on from this, theories of colloid stability and of the kinetics of aggregation are presented in section C2.6.5. Finally, section C2.6.6 is devoted to the phase behaviour of concentrated suspensions. 

Anotlier standard metliod is to use a (high-speed) centrifuge to sediment tire colloids, replace tire supernatant and redisperse tire particles. Provided tire particles are well stabilized in tire solvent, tliis allows for a rigorous purification. Larger objects, such as particle aggregates, can be fractionated off because tliey settle first. A tliird metliod is (ultra)filtration, whereby larger impurities can be retained, particularly using membrane filters witli accurately defined pore sizes. 

The first case is relevant in the discussion of colloid stability of section C2.6.5. It uses the potential around a single sphere in the case of a double layer that is thin compared to the particle, Ka 1. Furthennore, it is assumed that the surface separation is fairly large, such that exp(-K/f) 1, so the potential between two spheres can be calculated from the sum of single-sphere potentials. Under these conditions, is approximated by [42] ... 

For a more complete understanding of colloid stability, we need to address the kinetics of aggregation. The theory discussed here was developed to describe coagulation of charged colloids, but it does apply to other cases as well. First, we consider the case of so-called rapid coagulation, which means that two particles will aggregate as soon as they meet (at high salt concentration, for instance). This was considered by von Smoluchowski 1561 here we follow [39, 57]. 

Based on the application of the established theory of colloid stability of water treatment particles [8,85-88], the colloidal particles in untreated water are attached to one another by van der waals forces and, therefore, always tend to aggregate unless kept apart by electrostatic repulsion forces arising from the presence of electrical charges on the particles. The aggregation process... 

The particles therefore lose their charge. Since the charge provides the colloidal stability, the colloidal paint destabilises and deposits on the nearest surface, the car body. Primer coatings 12-35 /im thick are applied according to primer type. Each particle also contains a crosslinker for the resin, usually a blocked isocyanate. After rinsing, the primed article is passed into a hot... 

The mechanism of formation of Pt particles by the or-ganometallic reduction route, however, was found to proceed differently, for example in the reductive stabilization of Pt nanoparticles produced by reacting Pt-acetylacetonate with excess trimethylaluminium. Here, derivates of aluminium alkyls act as both reducing agents and colloidal stabilizers. As was shown by a combination... 

Testing of G-l, G-2, and G-3 dendrimers in this application provided insight into the density of surface modification needed to passivate completely the particles and prevent aggregation. The G-l dendron was insufficient in this regard, but both the G-2 and G-3 dendron were big enough to create a surface barrier, which resulted in excellent colloidal stability of the particles in solution. 

By contrast, relatively hydrophilic particles like those made of pHEMA may maintain colloidal stability even at small size due to the repulsive effects of a water of hydration layer,... 

---