Coagulation by electrolytes

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). 

The process in which small amounts of added hydrophilic colloidal material make a hydrophobic colloid more sensitive to coagulation by electrolyte. Example the addition of polyelectrolyte to an oil-in-water emulsion to promote demulsification by salting out. Higher additions of the same material usually make the emulsion less sensitive to coagulation, and this is termed protective action or protection . The protected, colloidally stable dispersions that result in the latter case are termed protected lyophobic colloids . 

Mchedlov-Petrossyan, N.O., Klochkov, V.K., Andrievsky, G.V. (1997) Colloidal dispersions of fullerene C6o in water some properties and regularities of coagulation by electrolytes. J.Chem.Soc., Faraday Trans. 93(24), 4343-4346. 

As shown by Ono et al, (1974, 1975) decrease of the HLB of a mixed emulsifier by use of an increasing proportion of a nonionic emulsifier increases the stability of the latex to coagulation by electrolyte addition despite an increase in its average particle size. This is because purely eledrostatic stabilization by adsorbed ionic emulsifier is supplemented by steric stabilization by the adsorbed nonionic emulsifier which effectively decreases the van der Waals attractive force between the latex particles (which causes them to coalesce), thereby increasing their stalrility. 

Pauli et al. studied all of these reactions quantitatively but interpreted the results erroneously in terms of the presence of H2S2O3 [40]. They also observed that the conductivity of the acidoid sol increased with aging which can be understood in terms of the reaction at Eq. (6) in which lower polythionate anions are formed which are soluble in water in contrast to the very long-chain anions in the sol particles. Coagulation by electrolytes was most efficient with AgNOg and La(N03)3 followed by BaCl2. 

In the case of fine colloidal particles, coagulation by electrolytes exhibits a strong effect so that inorganic electrolytes and ionic polymers perform well. Also, surfactants and oils are found to be effective for agglomeration of fine particles. 

I. Coagulation by electrolytes These are reactions taking place in the double layer of the mineral surface, and can be classified into two categories. 

The electrostatic interaction between diffuse layers of ions surrounding particles is one of the most thoroughly theoretically developed factors of colloid stability. The theory of electrostatic factor is, essentially, the basis for the quantitative description of coagulation by electrolytes. This theory was developed in the Soviet Union by B.V. Derjaguin and L.D. Landau in 1935 -1941, and independently by the Dutch scientists E.Verwey and T. Overbeek, and is presently known by the initial letters of their names as the DL VO theory [44,45]. The DLVO theory is based on comparison of molecular interaction between the dispersed particles in dispersion medium and the electrostatic interaction between diffuse layers of ions, with Brownian motion of particles taken into account (in the simplest version of theory this is done on a qualitative level). 

These observations clearly point to an electrostatic interpretation of the properties of aqueous lyophobie dispersions. Thus early attempts to understand coagulation by electrolytes related it to the adsorption of counter-ions and the neutralisation of the surface charge, a view supported by the empirical observation that coagulation often occurs when the /eta-potential has been reduced to some critical value around 30 mV. However, a quantitative theory has to be based on the more general concept of the electrical double layer and of the influence of electrolyte concentration on its properties. 

Clearly, the smaller is the gold or rubin number, the more effective is the biopolymer at shielding the dispersion from coagulation by electrolytes. To describe this shielding phenomenon, Zsigmondy coined the term protective colloid ( Schutzkolloide ), which functions by protective action. From the results recorded in Table 2.1, it can be inferr that sodium casemate is considerably more effective in its protective action than is, say, dextrin or potato starch. Note that both of these latter polymers are nonionic in character. 

Two important conclusions emerge from the data of Heller and Pugh. First, there is no doubt that poly(oxyethylene) can protect the gold sol from coagulation by electrolyte. Second, the protective action is enhanced by increasing the molecular weight of the polymer. 

The stabilization of colloidal dispersions by polyelectrolytes is of considerable practical concern. We have already mentioned the early studies of Faraday (1857) and Zsigmondy (1901) wherein it was shown that proteins could protect gold sols against coagulation by electrolytes. Apart from such studies of the protective action of biopolymers, there have been few systematic investigations of the stabilizing effects of polyelectrolytes, especially those of synthetic origin. 

Sensitization. The process in which small amounts of added hydrophilic colloidal material make a hydrophobic colloid more sensitive to coagulation by electrolyte. Example the addition of polyelectrolyte to an oil-in-water emulsion to promote demulsification by salting out. 

For most dilute silica sols around pH 2, where there is little ionic charge on the particle, no coagulation by electrolyte is observed, presumably because of the hydration layer. However, Harding (237) has called attention to the fact that relatively large colloidal silica particles 50-100 nm or more in diameter flocculate at low pH, whereas small particles do not. It remains to be determined whether the flocculation Is due to the van der Waals attractive energy between the particles or to the formation of multiple hydrogen bonds between the silanol-covered surfaces over the area of contact at collision. 

No definitive work has yet been done to relate rates of coagulation by electrolytes to particle size and concentration of silica. Also, no detailed study has been made of the amount of coagulant in the coagulate. 

Frens (283) has noted that sols of metal particles can actually be fractionated according to particle size by coagulation by electrolyte concentration. The results are explained by the lower van der Waals attraction between small particles of metal. Harding (237) further investigated this phenomenon in connection with suspensions by pyrogenic silicas. 

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