Investigations dealing with the state of aggregation. Part IV.—The flocculation of colloids by salts containing univalent organic ions

In an earlier paper the action of salts in a heterogeneous system was discussed, and the factors which exert influence on the state of hydration and aggregation of the “colloid phase” were summarised. The present communication deals with only one of these factors, viz., the surface-tension of the solutions and the relationship between this property and the capacity of salts for flocculating colloids. The mechanism by means of which this flocculation is produced is still a matter of considerable controversy. The view which appears, up to the present, to be most widely maintained is that the precipitation depends, in the first instance, on the adsorption of the active (precipitating) ion by the colloid. According to Freundlich, who is mainly responsible for what may be termed the adsorption hypothesis, the action of the ions can be explained in the following manner: Before coagulation takes place, the colloidal particles must lose a certain part of their charge, so that the difference of potential between colloid and dispersion medium does not exceed a certain critical maximum (Powis). This partial discharge can be brought about, when the precipitating capacity of various salts is considered, by the adsorption of equivalent quantities of ions with a charge opposite to that of the colloids; if one particular active ion is more readily adsorbed by a colloid than another of equal valency, a salt containing the former ion will produce the degree of discharge necessary for flocculation in a lower concentration of solution than will the salt which contains the ion that is less readily adsorbed. By means of the adsorption hypothesis, Freundlich has endeavoured to explain the numerical relationship between the flocculating capacities of salts containing ions of different valencies. The greater precipitating capacity of salts containing organic ions as compared with those containing inorganic ions has also been explained by the assumption that the former are more readily adsorbed than the latter. Furthermore, typical adsorption isotherms have been obtained, when the distribution of salts between colloid and dispersion medium has been determined.