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The stability of negative silver iodide sols as a function of the concentration of added cationic surface-active agents has been examined spectrophotometrically. The influence of PI, temperature, particle size, and the nature of the surface-active agent, has been studied. The surface-active agents used were octyl-, dodecyl-, and cetylpyridinium bromides, dodecyltrimethylammonium bromide, dodecylamine hydrochloride, dodecylquinolinium bromide and 1,lO-dipyridinium decane bromide. The stability of the sols decreased rapidly with increasing concentration of surface-active agent, reaching a minimum value at concentrations in the region of 10-6-10-4 M. At higher concentrations the stability rose again, reached a maximum value, and then fell continuously until the critical micelle concentration (c.m.c.) was reached. At the second stability minimum the rate constant indicated a mutual coagulation process. The slow fall in stability prior to the c.m.c. could be interpreted on the basis of pre-micelle formation in the surface-active agent solution in this region.

Large organic ions, such as ionic surface-active agents, produce coagulation of hydrophobic sols at very low concentrations, generally with reversal of charge of the sol particle. A quantitative theory is developed to account for this phenomenon. Initially, an expression is derived for the stability ratio of spherical sol particles in terms of that part of the potential drop in the electrical double layer which is important from the point of view of coagulation. For hydrophobic sols this potential is most likely the Stern potential, #a, and hence by considering the influence of the surface-active agent on this potential an expression is derived relating t,bd to the concentration of surface-active agent and the electrochemical free energy of adsorption. By elimination of the potential term a relationship is obtained between the stability ratio and the concentration of surface-active agent. The coagulation concentration is found to be primarily dependent on the electrochemical free energy of adsorption of the surface-active agent.

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