Stabilization Against Aggregation

Concentrated silica sols are stabilized against interparticle siloxane bonding by either (1) an ionic charge on particles so that particles are kept apart by charge repulsion or (2) an adsorbed, generally monomolecular, layer of inert material which separates the silica surfaces to an extent that prevents direct contact of silanol groups. This has been referred to as steric stabilization. 

In the case of particles of appreciable size, and especially, at low pH where siloxane bond formation is slow, spontaneous interparticle bonding is usually not observed. Thus with particles more than 100 nm in diameter such bonding does not appear to occur even in concentrated sols over the whole pH range unless the sol is dried. With such large particles, even if some siloxane bonds are formed at the points of contact, they are probably insufficient to withstand the mechanical strain involved when a pair of such large particles collides with a third particle in the course of Brownian motion. 

There is one type of instability that is not encountered in this system, namely, crystallization. As pointed out by Walton (4 3), the higher the degree of supersaturation the smaller the size of the critical nucleus, which may be so small as not to cor- 

An ionic charge on the particles in the presence of alkali is the chief mechanism of stabilization in commercial sols. However, a completely satisfactory theory of stabilization has apparently not yet been developed. The basic principles of stabilization by the ionic double layer around particles were developed by Derjaguin and Landau (44) and Vcrwey and Overbeek (45), hence the DLVO theory it has been specifically applied to spherical particles (46a). An excellent summary of the forces affecting the stability of disperse systems was presented by Ottewill (46b). 

Napper has written a summary of colloid stability (47), including the principles, of both electrostatic and steric stabilization. A fundamental study of the van der Waals forces between amorphous SiOj surfaces was carried out by Roweler (48), who measured the attraction between two fused silica surfaces covered with thin films of chromium metal in a high vacuum. However, it is dilticult to translate these results to an aqueous silica system. The Hamaker constant in many colloid systems has been reviewed by Visser (49) including the SiOj-HjO system. 

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Any agent that acts to stabilize an emulsion. The emulsifier can make it easier to form an emulsion and to provide stability against aggregation and possibly coalescence. Emulsifiers are frequently but not necessarily surfactants. 

At higher ionic strength, NaCl stabilizes against aggregation. 

DLVO Theory An acronym for a theory of the stability of colloidal dispersions developed independently by B. Derjaguin and L. D. Landau in one laboratory and by E. J. W. Verwey and J. Th. G. Overbeek in another. The theory was developed to account for the stability against aggregation of electrostatically charged particles in a dispersion. 

No additional stabilization against aggregation of the gold nanoparticles is required - surface bound ions (citrate ions, chloroaurate ions etc.) normally stabilize the nanoparticles electrostatically in solution. 

By adsorption onto particles they may greatly affect colloidal interaction forces between those particles. Repulsive forces may provide long-term stability against aggregation attractive forces may allow the formation of continuous networks. 

The first useful theory of colloidal interaction forces and colloid stability (against aggregation) was developed independently by Deryagin and Landau and by Verwey and Overbeek. Hence it is called the DLVO theory. It takes into account the combined effects of van der Waals attraction and electrostatic repulsion. 

The DLVO theory has been very successful in predicting (in) stability against aggregation for many, especially inorganic, systems. Although developed for lyophobic colloids, the theory can often be usefully applied to lyophilic colloids these are often found in biogenic systems, including most foods. However, some complications and other interaction forces may come into play. 

The surfactant /1-lactoglobulin is a small protein molecule that does not strongly unfold upon adsorption at the O-W interface (Section 10.3.2). Consequently, stability against aggregation will primarily be due to electrostatic repulsion. The dilution with water will lower the ionic strength by about a factor of 2. It can be seen in Figure 13.4 that this will cause a considerable increase in W, e.g., by a factor of 50. Moreover, the larger W value will cause an increase in the value of D, which will also increase the gel time. 

Coating of silver nanoparticles with polyelectrolytes influences their stability against aggregation. Herein the silver nanoparticles were coated with positive weak poly(allylamine hydrochloride) (PAH, 70kDa or 15kDa) or neutral polyethylene glycol (PEG, 8 kDa) polyelectrolytes. 

The material can be released under defined conditions (e.g., photoinitiator) Dispersions containing the encapsulated material show improved stability against aggregation (e.g., pigments, carbon black, etc.)... 

Stability is important for cosmetic skin care products from the points of view of function and also shelf-life. Stability against aggregation is important but fairly easily dealt with because most of these products are formulated to have a yield stress [33]. Stability against coalescence is very important but less straightforward (see Chapter 5 for the factors involved). In the cosmetics industry, standardized tests have been developed to yield the net effect of all aspects of emulsion stability. 

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