Recharging of Colloids

Recharging of Colloids, change in sign of the charge of colloidal particles under the influence of ions. Colloidal particles usually carry an electric charge on their boundary surface. In solution, this is counterbalanced by an equal in magnitude and opposite in sign charge formed by ions, which constitute the outer layer of the electrical double layer (see). As a result, by changing the ionic composition of the solution, it is possible to change not only the magnitude but even the sign of the charge of the colloidal particle, causing its recharging. For example, salts of trivalent (lanthanum, cerium and other rare earths) and tetravalent cations (thorium) can impart a positive charge to negatively charged colloidal particles. Many anions act in the opposite direction, so that adding their salts [for example K₄Fe(CN₆)] allows recharging of positively charged colloids. Recharging of amphoteric colloids is distinguished by very characteristic features. The latter, depending on the reaction of the solution, in some cases have an acidic character, and in others - an alkaline character (see Ampholytes). In the first case they have a negative charge, in the second - a positive charge. Therefore, recharging of amphoteric colloids can easily be produced by changing the reaction of the solution (see also Isoelectric point). Many properties of colloids (stability, attitude toward electrolytes, etc.) depend on their electric charge and therefore can undergo profound changes during recharging.