Gold protective action

The word gold has been used in many different fields, mostly to evoke perfection, like for example the famous gold section in geometry. In chemistry, the concept of a gold number is used to represent the protective action of colloid (Ref. [10], p. 738). This concept was... 

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. 

The protective action of gelatin on the precipitation of colloidal gold was noticed by Faraday (see p, 729). Zsigmondy represented the protective action of a colloid by a gold number . Protective colloids (sodium lysalbinate, prepared from albumin, etc.) were used by C. Paal to prepare colloidal silver containing 93 per cent, of silver but soluble in water. The protective action is probably due to adsorption. 

The synalbumoses have no protective action on gold solutions but act in a manner very similar to the second albumin fraction. Zunzf has re-... 

In a later article Zunz f has pointed out that there is no relation between the effect of the albumoses on mastic solutions and the effect on gold solutions. Heteroalbumoses and sjmalbumoses precipitate mastic turbidities, while other albumoses do not. On the other hand heteroalbumoses have a protective action on gold solutions. 

Of general interest is the observation that urea, uric acid, urochrome, hippuric acid, and hypoxanthin have no protective action, while nu-cleinic acid is a protective colloid having a gold number 2.5. Urine albumin does not always operate as a protective colloid.J The cause is probably to be sought for in the size of the particles, which latter may be seen under the ultramicroscope as established by the work of Rsehl-mann. Much, Romer, and Siebert. , ... 

On the basis of an elaborate investigation the author If has expressed the opinion that the protective action is due to the union of several ultramicrons of the protective colloid with a particle of the gold or vice versa, the gold particles are adsorbed by a protective individual. 

The colloidal nature of soap solutions is manifested by the protective action on gold solutions. The gold number of sodium oleate lies between 0.5 and 2. The protective effect is therefore about as great as that of gum arabic, and much smaller than that of gelatin. The protective effect of sodium stearate increases with the temperature. 

Alkanethiolate coating of the gold particles protects them against action of chemical etchants. Protected colloids dissolve much more slowly in aqua regia than their unprotected counterparts. The level of protection observed from monolayers on the flat gold surface, however, is never achieved, probably because of the high curvature of the surface of gold... 

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