Colloid stability destabilization

Colloid stability conferred by random copolymers decreased as solvent quality worsened and became increasingly solvent dependent around theta-conditions. However, dispersions maintain some stability at the theta-point but destabilize close to the appropriate phase separation condition. 

The new Chapter 13 collects the material on colloid stability previously distributed between old Chapters 11 and 12 and integrates it with new material on stability ratio and slow and fast coagulation, polymer-induced forces, and polymerinduced stabilization and destabilization. 

Addition of polymers can both stabilize and destabilize a solution. If the polymer contains ionizable units it is usually referred to as a polyelectrolyte. In this report we will focus on the effect from polyelectrolytes on the colloidal stability. In high dielectric media like water, where the monomers are ionized, the behavior of a polyelectrolyte is mainly governed by electrostatics and the connectivity of the monomers. Therefore, in theoretical studies, many important features of the polyelectrolyte behavior in water solution can be studied by a schematic description of the polyelectrolyte as a linear chain of charged monomers connected with springs. The bonding interaction between two monomers is Ub=K(r —a)2, where K is the spring constant, a is the equilibrium value and r is the distance between the two monomers (see Fig. 11). 

The use of chemicals to reduce the distance to a - a from the surface of the colloid is portrayed in Figure 12.4. The zeta potential is the measure of the stability of colloids. To destabilize a colloid, its zeta potential must be reduced this reduction is equivalent to the shortening of the distance to a - a and can be accomplished through the addition of chemicals. 

The molecules of both HSl and HS2 have a high colloid stability due to their very low collision efficiency and small size which prevent them from settling under normal gravitational acceleration. Their separation in a centrifuge with a g value of some thousands is still practically inefficient. This fact is used for sedimentation analysis to investigate the destabilization and aggregation rate of the coagulation process of humic substances. 

A polysaccharide can be added as a component in a protein system to produce a protein-polysaccharide composite structure. Tolstoguzov (2003) reviewed the main function of protein and polysaccharide in protein-polysaccharide food formulation. Generally, polysaccharides have less surface activity in comparison to proteins. This inferiority is related to their low flexibility and monotonic repetition of the monomer units in the backbone. The low surface activity of polysaccharides results in their inability to form a primary adsorbed layer in the system. The nature of interactions between polysaccharides and adsorbed proteins, as well as their influence on colloid stability, can either stabilize or destabilize the emulsions. Attractive protein-polysaccharide interactions can enhance the emulsion stability by forming a thicker and stronger steric-stabilizing layer. In contrast, the attractive interactions... 

In free disperse systems, in particular those with low concentration of dispersed phase, the nature of colloid stability and conditions under which the collapse occurs, are to a great extent dependent on thermal motion of dispersed particles, which may contribute to both stability and destabilization. For example, the necessary condition for sedimentation stability is sufficiently small particle size, so that the tendency of particles to distribute within the entire volume of disperse system due to the Brownian motion (an increase in entropy) would not be affected by gravity. As a quantitative criterion for the presence of noticeable amount of dispersed particles in equilibrium with a sediment one, for instance, may use the... 

The dimensions of a polymer chain in solution are important to the rheological properties of the system. More specific to the question of colloidal stability, however, such dimensions play a vital role in the ability of an adsorbed polymer to stabilize (or destabilize) a lyophobic colloid as discussed below and in Chapter 10. 

In hydrophobic colloidal systems, the water molecules have a higher affinity for one another than for the colloidal particles. Consequently, the particles would stick together on each encounter, nnless they repel each other. It was discovered by H. Schulze in 1883 that addition of electrolyte destabilizes hydrophobic colloidal dispersions, and W.D. Hardy in 1900 showed that the destabilization is accompanied by a reduction in the electrophoretic mobility of the particles. From this, it was inferred that colloidal stability is maintained by electrostatic repulsion between charged particles. 

It was generally accepted that the main forces involved in polymer colloids stabilization or destabilization are as follows ... 

Among different alternatives, a very effective way to operate an LRP in segregated systems is indeed miniemulsion. In this case, small monomer droplets are the primary locus of reaction and all the difficulties from interphase transfer vanish, since monomer and all the other hydrophobic species required to run an LRP are already in the main reaction locus. However, further difficulties have been reported, such as incomplete droplet nudeation and colloidal stability problems [74, 82, 85]. More subtle is the evidence of instabilities in the miniemulsion due to the kinetics of LRP. In contrast to conventional systems, where long chains are created from the beginning, in LRP all the polymer chains are short initially. This might lead to superswelling states of the droplets and, eventually, to destabilization [86]. 

The excellent colloidal stability of Latex 115 gives it exceptional resistance to shear and a broad tolerance to a variety of materials that would destabilize the anionic Neoprene latexes. 

Figure 10.1 Colloidal dispersions are Inherently unstable systems and in the long run the attractive forces will dominate and the colloidal system will destabilize. However, colloid stability depends on the attractive van der Waals and the repulsive electrical or steric (polymeric) forces. The repulsive forces stabilize a dispersion if they are larger than the van der Waals (vdW) ones (and the total potential is larger than the "natural" kinetic energy of the particles). Surfaces are Inherently unstable and the van der Waals forces "take the system" back to its stable (minimum surface area) condition and contribute to instability (aggregation)...

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian Hquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin Hquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a large number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the lUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

In steric stabilization the colloids are covered with a polymer sheath stabilizing the sol against coagulation by electrolytes. In sensitization or adsorption flocculation, the addition of very small concentrations of polymers or polyelectrolytes leads to destabilization (Lyklema, 1985). 

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