Stability of oil-in-water emulsions III: Measurement of the rate of coagulation of an oil-in-water emulsion

Dilute oil-in-water emulsions stabilised with anionic soaps were coagulated by the addition of salt. After a predetermined time coagulation was stopped by diluting with a solution of a nonionic surface active agent. The rate of coagulation was measured by counting, after varying times of coagulation, the number of oil drops under a microscope. The rate of coagulation increases with increasing salt concentration. An upper limit in the rate of coagulation, where “rapid” flocculation occurs, was observed when salts with polyvalent cations were used as the coagulating electrolyte. Increasing the concentration of Aerosol OT caused a higher rate of coagulation in dilute salt solutions and a lower rate at higher salt concentrations. Variation of the concentration of the more soluble soaps had little effect on the rate of coagulation. The influence of the valency of the coagulating cation was investigated in emulsions stabilised with a soap giving no insoluble compounds with the cations used. A behaviour as predicted by the rule of Schulze and Hardy was observed. Variation of the anion in the coagulating electrolyte had no effect on the rate of coagulation. The rate of coagulation decreases with increasing initial particle concentration. This is explained by considering coagulation as caused by flocculation followed by coalescence. From experiments in which the initial particle concentration was varied, estimations could be made of both the rate of flocculation and of coalescence. The rate of coalescence had a value of the order of 0.001 per second. For the rate of flocculation a higher value was found than that predicted by Von Smoluchowski's theory for “rapid” flocculation. Several effects are present which operate simultaneously to increase the measured rate of flocculation; the influence of these effects is discussed.