Effective steric stabilisation

Several criteria can be estabhshed for effective steric stabilisation ... 

As mentioned above, to prevent aggregation of the formed polymer particles one needs a dispersant (polymer surfactant) that must satisfy several criteria. The most effective dispersants are those of the block (A-B or A-B-A) or graft (BA ) type. The B chain is chosen to be insoluble in the medium and has high affinity to the surface of the polymer particles (or becomes incorporated within its matrix) (Chapter 5). This is usually referred to as the anchor chain. A chain(s) are chosen to be highly soluble in the medium and strongly solvated with its molecules. It should give a Flory-Huggins interaction parameter (/) < 0.5 to ensure effective steric stabilisation. 

Thus by measuring the 0-point (CFT or CFV) for the polymer chains (A) in the medium under investigation (which could be obtained from viscosity measurements) one can establish the stability conditions for a dispersion, before its preparation. This procedure also helps in designing effective steric stabilisers such as block and graft copolymers. 

If the polymer layers increases the stability of the dispersion, it is denoted steric stabilisation. The polymer must fulfil two key criteria (i) the polymer needs to be of sufficient coverage to coat all the particle surfaces with a dense polymer layer, and (ii) the polymer layer is firmly attached to the surface. How this is engineered is beyond the scope of this article, but the consequences of not satisfying these criteria are informative in understanding the effect that polymers have on the overall interparticle interaction. Since complete or incomplete coverage of the particles results in very different properties (i.e stability or instability), this is clearly one way in which minimal change in initial conditions can lead to major differences in product. 

Already the ancient Egyptians knew that one can keep soot particles dispersed in water when they were incubated with gum arabicum, an exudate from the stems of acaia trees, or egg white. In this way ink was made. The reason for the stabilizing effect is the steric repulsive force cause by adsorbed polymers. In the first case these are a mixture of polysaccharide and plycoprotein, in the second case it is mainly the protein albumin. Steric stabilisation of dispersions is very important in many industrial applications. Direct quantitative measurements were... 

Surfactants or polymers adsorbed on the particle surface are able to keep particles that far ap-part that the Van der Waals attraction cannot become effective. This phenomenon is called steric stabilisation. 

In contrast to ionic reactions, radical additions to unsymmetrical fluoroalkenes are frequently bi-directional factors that affect the rate and orientation of addition depend on, for example, polar effects, steric effects, radical stabilisation and the character of the attacking radical [182-185]. 

Polymers are also essential for the stabilisation of nonaqueous dispersions, since in this case electrostatic stabilisation is not possible (due to the low dielectric constant of the medium). In order to understand the role of nonionic surfactants and polymers in dispersion stability, it is essential to consider the adsorption and conformation of the surfactant and macromolecule at the solid/liquid interface (this point was discussed in detail in Chapters 5 and 6). With nonionic surfactants of the alcohol ethoxylate-type (which may be represented as A-B stmctures), the hydrophobic chain B (the alkyl group) becomes adsorbed onto the hydrophobic particle or droplet surface so as to leave the strongly hydrated poly(ethylene oxide) (PEO) chain A dangling in solution The latter provides not only the steric repulsion but also a hydrodynamic thickness 5 that is determined by the number of ethylene oxide (EO) units present. The polymeric surfactants used for steric stabilisation are mostly of the A-B-A type, with the hydrophobic B chain [e.g., poly (propylene oxide)] forming the anchor as a result of its being strongly adsorbed onto the hydrophobic particle or oil droplet The A chains consist of hydrophilic components (e.g., EO groups), and these provide the effective steric repulsion. 

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