Steric and electrosteric stabilizations

An important factor that is not taken into account in the DLVO theory is adsorption, on the particle s surface, of long polymeric chains. The adsorption of a non-ionic polymer or a polyelectrolyte on the solid surface can cause, not only a modification of the zeta potential, but also a critical difference between the value of the zeta potential and the state of dispersion. Steric repulsion is associated with the obstmction effect of these polymers that are capable to form a sufficiently thick layer to prevent the particles from approaching one another in the distanee of influence of the Van der Waals attractive forces. Steric stabihzation will therefore depend on the adsorption of the polymeric dispersant and the thickness of the layer developed. Several interpretation models for stabilization by steric effect have been put forward. They rely either on a statistical approach, or on the thermodynamics of solutions. Steric stabilization is particularly useful in organic, fairly non-polar or non-polar environments, as in the case of tape casting (see section 5.4.3). 

The adsorption of charged polymers (polyelectrolytes) leads to a mixed steric/electrostatic mechanism (electrosteric mechanism). The expression of total potential energy becomes  

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Capek I. Sterically and electrosterically stabilized emulsion polymerization. Kinetics and preparation. Adv Colloid Interface Sci 2002 99 77-162. 

The major route to colloidal (effectively water soluble) PAn has been through the chemical oxidation (S2082-) of the monomer in the presence of polymeric steric stabilizers and electrosteric stabilizers (polyelectrolytes), such as poly(vinyl alcohol), polyGV-vinyl pyrrolidone), polyethylene oxide), polystyrene sulfonate), dodecylben-zene sulfonate, and dextran sulfonate. It has been found that the stabilizer can act simultaneously as a dopant, imparting new functionality to the polymer or additional compatibility for the final application. 

Surfactants in Aqueous Solution A very important component that is usually present in the lyophobic colloids is the surfactant. These molecules are amphiphilic, that is, a part of the molecule is much more polar than the other part. On the basis of the nature of the polar groups in the surfactant molecule, they are classified as ionic (anionic or cationic) and nonionic. When ionic-type surfactants are adsorbed onto polymer particles, they provide stabilization by electrostatic repulsion between them and when the nonionic type are adsorbed instead the mode of stabilization is by steric repulsion. Electrosteric stabilization is provided by polyelectrolyte chains that give place to both modes of repulsion electrostatic and steric. 

The idea to employ bulky and highly charged adsorbents in the stabilization of nanopartides rather than a neutral stabilizer results in the so-called electrosteric stabilization. The best stabilizers for nanopartides are those, which are ionic, adsorb strongly enough and meet the conceptions of steric and electrostatic stabilization simultaneously. 

Scheme 9.3 N-alkyl-N,N-dimethyl-N-(2-hydroxyethyl)-ammonium chloride salt, a typical cationic surfactant which combines electrostatic and steric stabilizations (electrosteric stabilization).

Polyelectrolytes provide excellent stabilisation of colloidal dispersions when attached to particle surfaces as there is both a steric and electrostatic contribution, i.e. the particles are electrosterically stabilised. In addition the origin of the electrostatic interactions is displaced away from the particle surface and the origin of the van der Waals attraction, reinforcing the stability. Kaolinite stabilised by poly(acrylic acid) is a combination that would be typical of a paper-coating clay system. Acrylic acid or methacrylic acid is often copolymerised into the latex particles used in cement sytems giving particles which swell considerably in water. Figure 3.23 illustrates a viscosity curve for a copoly(styrene-... 

The lattice stabilization through steric or electrosteric mechanisms, resulting in electrolyte and freeze-thaw stability... 

With polymers that have ionizable groups, adsorption of a polymer will alter the charge of the surface altering the electrostatic interaction energy and also provide steric protection for the colloid, because the ionized groups will give better than theta conditions for the poisoner in an aqueous solution. This type of polymer stabilization is called electrosteric stabilization because both the electrostatic and the steric play a role in stabilization. The equations for this total interaction are simply the sum of electrostatic and steric terms as well as the van der Waals attraction. 

The hairy particles stabilized by non-ionic emulsifier (electrosteric or steric stabilization) enhance the barrier for entering radicals and differ from the polymer particles stabilized by ionic emulsifier [35]. For example, the polymer lattices with the hairy interface have much smaller values of both the radical entry (p) and exit (kdes) rate coefficients as compared to the thin particle surface layer of the same size [128,129]. The decrease of p in the electrosterically stabilized lattices is ascribed to a hairy layer which reduces the diffusion of oligomeric radicals, so that these radicals may be terminated prior to actual entry. For the electrostatically stabilized lattices with the thin interfacial layer, exit of radicals occurs by the chain transfer reaction [35]. This chain transfer reaction results in a monomeric radical which then exits out of the particle by diffusing through the aqueous phase and this event is competing with the propagation reaction in the particle [130]. The decrease of kdes in the electrosterically stabilized latex... 

Electrosteric Stabilization The stabilization of a dispersed species by a combination of electrostatic and steric repulsions. 

It is possible to have combinations of electrostatic and steric stabilization, which has been termed electrosteric stabilization. The electrostatic component may originate from a net charge on the particle surface (see Fig. 1.7a) and/or charges associated with the polymer attached to the surface (i.e. through an attached polyelectrolyte) (see Fig. 1.7b). Electrosteric stabilization is common in biological systems. 

In addition to electrosteric stabilization, it is possible to have combinations of depletion stabilization with both steric and/or electrostatic stabilization. The combination of depletion and steric stabilization is quite common at high concentrations of free polymer in the dispersion medium. 

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