An extension to the stability theory of lyophobic colloids

Abstract

The stability theory of lyophobic colloids of Derjaguin and Landau and Verwey and Overbeek (D.L.V.O. theory) is extended to include the so-called discreteness-of-charge effect. The interaction of two parallel charged colloidal plates in a large volume of aqueous binary symmetrical electrolyte is considered and the approximation of linear superposition of the double layer potential distributions is employed. A new term which corrects for the discrete-ion effect and is proportional to the density of adsorbed counter-ions in the Stern layer occurs in the adsorption energy of these ions. As a consequence the theory predicts that the electrolyte concentration required to produce coagulation increases with the potential at the colloidal surface for small potentials, but reaches a maximum and then decreases with further increase in surface potential. This is in general accord with the experimental results for silver halide sols, although the experimental rate of decrease in the case of silver iodide is smaller than the theoretical one. The theory also accounts for the different formulations of the Schulze-Hardy rule, as given by the D.L.V.O. theory and by Težak and his school.