Hydrosols irreversible

W.B. Hardy, Preliminary investigation of the conditions which determine the stability of irreversible hydrosols. J. Phys. Chem. 4(4), 235-253 (1900). doi 10.1021/jl50022a001... 

The preparation of irreversible hydrosols indicates that they differ in many respects from solutions of crystalloids. For, while the latter are obtained directly by dissolving the solute in the solvent, roundabout methods are necessary in the former case to procure a state of fine subdivision. In point of fact closer investigations have shown that irreversible hydrosols do not in general exist in a state of molec-... 

In the case of finer suspensions such as wheat starch or quartz powder, the particles of which have a diameter of 0.001 to 0.005 mm., the effect of electrolytes is more marked. The particles flock together to form larger aggregates, and these aggregates precipitate more rapidly than do the individual particles. The much investigated turbid solutions of clay exhibit this to a marked degree, as shown by the work of Schlosing f and Bodlander.f These solutions behave in a manner very similar to that of irreversible hydrosols such as colloidal metals, which are also very sensitive to the action of electrolytes. 

The statement so often met with in the literature that the precipitation of these finer suspensions is irreversible, as in the case of colloidal metals, is quite false. Schlosing has demonstrated that after the removal of the electrolyte the clay may be returned to its original suspended form. In this regard turbid clay solutions resemble hydro-phile colloids (reversible) much more than they do metal hydrosols, and from the properties of the precipitate should be classed with the former rather than with the latter. The similarity between turbid solutions of clay and metal hydrosols exists almost exclusively in the sensitivity of both toward electrolytes. This sensitivity, however, they have in common with certain ionogen disperse solutions such as Congo-red and Benzo-purple. 

Electrical charges on the particles operate in such a manner that many hydrosols behave during electrolysis precisely as electrolytes with large complex ions. Colloidal stannic acid, the purple of Cassius and many others may be cited as examples. The reactions of hydrosols are often determined by the electrical charge and are generally characteristic of the sign. They are dependent upon the nature of the disperse phase just as in the case of crystalloidal solutions. The particles of reversible and also the irreversible hydrosols behave similarly to molecules and ions in that they are adsorbed by various substances. 

With the irreversible colloids of the second class, on the other hand, it is possible to reform the hydrosol by the addition of a small amount of a suitable reagent provided that the residue has not been too thoroughly dehydrated. A too complete dehydration causes a continuous change to take place which may proceed so far that the dry residue will no longer undergo peptisation. (Example, colloidal stannic acid.)... 

Lying in a position intermediate between hydrosols of pure metals and irreversible oxides will be found the majority of well-dialyzed sulfide hydrosols. On evaporation these sometimes give jelly-like bodies and sometimes precipitate in a powder form. 

B. Reversible CoUoids. The reversible hydrosols are in general not very sensitive toward the salts of the alkali metals. Large quantities of these salts are usually necessary to produce precipitation. The reaction is mostly reversible. Toward the salts of the heavy metals these colloids act in a manner similar to that of the irreversible, inasmuch as often exceedingly small quantities are sufficient for precipitation. In all these cases specific effects are manifested that render a further subdivision into classes extremely difficult. 

The hypothesis, that the electric charge on irreversible hydrosols is due to the adsorption of ions on, or the giving up of ions by, the particles, seems capable of explaining an enormous number of experimental facts. The principles of this point of view, arising out of Hardy s work, were first presented by Bredig, and more fundamentally dealt with by Billitzer. The author had independentl3r employed the same hypothesis to explain peptisation and the reactions of the purple of Cassius. These considerations were not published until 1904. 

Transitional Stages Between EIectrol3rtic Solutions and Irreversible Hydrosols... 

There are so many transitional stages between electrolytic solutions and irreversible hydrosols that no sharp dividing line may be drawn. Let us consider an aqueous solution of iron chloride as it is diluted successively. At great dilution a yellowish brown precipitate of ferric hydroxide, or the gel of iron oxide is obtained. It has not yet been determined whether the precipitate is Fe(OH)3 or amicrons of FcaOs separated from each other by spaces filled with water. For the sake of simplicity we will assume the latter, as van Bemmelen has done, without committing ourselves to either hypothesis. 

The mutual adsorption of the ultramicrons results in the gold hydrosol losing its characteristic properties, viz., irreversible dehydration and sensitiveness to electrolytes, while the complex particles assume the properties and show all the reactions of the protective colloid. If the protective colloid can be precipitated by a given. reagent, so can the gelatin gold complex. On the contrary if the protective colloid is unaffected by the electrolyte in question, the complex will also be exempt from the influence. 

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