Steric stabilization, mechanism

Fig. 2.1. Steric stabilization mechanism of soot particles by formation of reverse micelles and solubilization...

Solubilization of insoluble oxidation products and soot particles. Reverse micelles (RMs) formations manage the prevention of agglomeration and the contamination process of insoluble oxidation particles and soot particles by both steric stabilization (Fig.2.1) and electrostatic stabilization mechanisms (Fig.2.2). The steric stabilization mechanism provides a physical barrier to agglomeration of particles by adsorption on particle surfaces. Adsorbed dispersant acts as a physical barrier to attraction between particles. 

Historically, ideas of casein micelle structure and stability have evolved in tandem. In the earlier literature, discussions of micellar stability drew on the classical ideas of the stability of hydrophobic colloids. More recently, the hairy micelle model has focused attention more on the hydrophilic nature of the micelle and steric stabilization mechanisms. According to the hairy micelle model, the C-terminal macropeptides of some of the K-casein project from the surface of the micelle to form a hydrophilic and negatively charged diffuse outer layer, which causes the micelles to repel one another on close approach. Aggregation of micelles can only occur when the hairs are removed enzymatically, e.g., by chymosin (EC 3.4.23.4) in the renneting of milk, or when the micelle structure is so disrupted that the hairy layer is destroyed, e.g., by heating or acidification, or when the dispersion medium becomes a poor solvent for the hairs, e.g., by addition of ethanol. 

The term steric stabilization has been used by colloid scientists to describe how a lyophilic substance, located on the surface of a lyo-phobic colloid, can prevent aggregation of the dispersion. The phenomenology of steric stabilization has been recognized and put to use over many millenia one notable example is the use, by the ancient Egyptians, of casein as a steric stabilizer of carbon (lamp black) in the production of inks for writing on papyrus. Only in the last 50 years or so has a scientific understanding of steric stabilization mechanisms emerged. 

Proteins, which are also surface active, can be used to prepare food emulsions. The protein molecules adsorb at the O/W interface and they may remain in their native state (forming a rigid layer of unfolded molecules) or undergo unfolding, forming loops, tails, and trains. These protein molecules stabilize the emulsion droplets, either by a steric stabilization mechanism or by producing a mechanical barrier at the O/W interface. 

In dilute aqueous solutions, polyelectrolyte block copolymers self-assemble into micelles consisting of a hydrophobic core and a polyelectrolyte shell. The study of their structural properties is expected to provide a basic understanding of the properties of dense polyelectrolyte layers, electro-steric stabilization mechanisms, and actuator functions based on variations in the electrostatic interactions. 

Steric stabilizing mechanisms are also quite important, and most currently used crankcase oil dispersants make use of both mechanisms. In general the steric mechanism favors stabilizing the smaller particles best and the electrostatic mechanism favors stabilizing the larger particles best, as explained by Koelmans and Overbeek some decades ago (21). In some oils one can actually see a double-peaked distribution of particle sizes, the smaller ones presumably stabilized by the steric mechanism and the larger by the electrostatic mechanism. 

Proteins adsorbed at an oil-water interface may stabilize the oil droplets by the Derjaguin, Landau, Verwey, and Overbeek (DLVO) interactions and/or the steric stabilization mechanism. The proteins may possess or be capable of adopting extended structures, which protrude into a solution for a considerable distance from the interface. This extended hydrated layer may form the basis for steric stabilization of the emulsion. Interactions between the adsorbed protein layers can involve a reduction in conhgurational entropy as molecular chains overlap (Darling and Birkett, 1987). In addition, hydration of adsorbed hydrophilic components can lead to an enthalpic repulsion when two particles are in close proximity. This tends to force the oil droplets apart (Darling and Birkett, 1987). 

Both charge-dependent and steric stabilization mechanisms prevent the close approach of particles, so that flocculation is prevented, and if flocculation (or aggregation) cannot occiu, then creaming will be slow, provided that the emulsion droplets are small to start with. If aggregation and creaming are slow, then coalescence is unlikely under nor-... 

FIGURE 12.5. Thin lamellar films in foams will develop a disjoining pressure, 7r h), between the opposing interfaces when electrostatic or steric stabilization mechanisms are operative. The stabiUty of the film (and the foam) will depend on the value of n, among other things. 

In this case, it is the steric stabilization mechanism that protects the interactive particles from coagulation. In addition, the use of non-ionic t)rpes improves the stability of latex product against electrolytes, freeze-thaw cycles, water and high shear rates. As a result of them, in many emulsion pol3merization recipes (particularly in industry), mixtures of anionic and non-ionic emulsifiers have been widely used together in a s)mergistic manner to control the particle size and to impart enhanced colloidal stability [33-35]. The cationic and zwitterionic emulsifiers are used infrequently in emulsion pol3metizafion applications. 

The distribution of polymer segments between trains, loops, and tails may be adjusted when the polymer-covered surfaces encounter each other and this, in turn, may interfere with the steric stabilization mechanism. The polymers best performing as stabilizers are block copolymers made of two kinds of monomers that are clustered in long sequences of one kind. The sequence of the one kind of monomer should have a high affinity for adsorption and, hence, firm attachment at the surface, whereas the sequence of the other type should prefer to stay in solution giving extended loops and tails. With homopolymers, affinity for the sorbent surface and extension into the solution are compromising tendencies. 

NOTE In the mosaic tile model, please note that the particle-particle ties provided by the binder not only hold the particles together but also hold them apart. This particle separation function of the binder is a steric hindrance or steric stabilization mechanism that aids and sometimes overshadows the dispersing property of the dispersant. It was mentioned in the Surfactants section that the binder is sometimes a more powerful dispersant than the dispersant itself. The mosaic tile model provides a good picture of the dispersing power of some binders. 

In the common procedure extremely large oil-water interfacial area is generated and the particle nuclei grow in size with the progress of the polymerization. Thus, effective stabilizers such as ionic and non-ionic surfactants and protective colloids e.g. hydroxyethyl cellulose and polyvinyl alcohol), which can be physically adsorbed or chemically incorporated onto the particle surface, are often required to prevent the interactive latex particles from coagulation. Under the circumstances, satisfactory colloidal stability can be achieved via the electrostatic stabilization mechanism [268], the steric stabilization mechanism [269] or both. 

The preceding descriptions of steric stabilization mechanisms assume that the time-scale for the particle encounter is much shorter than that for... 

Inversion emulsion polymerization involves the dispersion and then polymerization of hydrophilic monomers, normally in aqueous solution, in a nonaque-ous continuous phase. The emulsifier systems primarily based on the steric stabilization mechanism (see Section 1.3.3) are quite different from those of the more conventional oil-in-water emulsion polymerization processes. This is simply because the electrostatic stabilization mechanism (see Section 1.3.2) is not effective in stabilizing inverse emulsion polymerization comprising an aqueous disperse phase and a nonaqueous continuous phase with a very low dielectric constant. The unique anionic surfactant bis(2-ethylhexyl) sulfosuc-cinate (trade name Aerosol OT) that can be dissolved in both oil and water... 

In addition to the electrostatic stabilization mechanism, latex particles can be stabilized by adsorption of hydrophilic polymer chains on their particle surfaces. The physically adsorbed or chemically grafted polymer chains surrounding the colloidal particles and extending themselves into the continuous aqueous phase serve as a steric barrier against the close approach of the pair of particles. In this manner, coagulation of latex particles can be prevented via the steric stabilization mechanism [17,18]. Typical nonionic surfactants and surface-active, nonionic block copolymers are quite effective in imparting such a steric stabilization effect to colloidal dispersions. 

As an indirect method toward processability, conducting colloids of polypyrrole and polyaniline have been prepared via a dispersion polymerization route. In this approach, the conducting polymer is che-mically synthesized in the presence of a suitable polymeric surfactant. The surfactant adsorbs or is chemically grafted onto the conducting polymer particle or chain and prevents macroscopic precipitation by a steric stabilization mechanism. The result is a stable dispersion of submicronic conducting polymer particles which consist of a conducting core and a thin outer layer of the non conducting surfactant as shown schematically in Fig. 2. 

Figure 5.4 Different free energies involved in the steric stabilization mechanism as a function of distance (d).

Macromolecules adsorb onto interfaces and facilitate better coverage than monomeric emulsifiers. The adsorbed polymers are known to enhance steric stabilization mechanisms and were proved to be efficient emulsifiers in oil-in-water emulsions. 

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