Dissolution Study of Salt of Long Chain Fatty Acids (Soap Scum) in Surfactant Solutions. Part II: Kinetics of Dissolution

A study of the dissolution kinetics of soap scum, calcium octadecanoate (Ca(C18)2), by aqueous solutions was carried out by measuring the rate of dissolution of Ca(C18)2 using a flow cell apparatus. These solutions contained three different types of surfactants: dimethyldodecylamine oxide (DDAO), sodium dodecyl sulfate (SDS), or octyl polyglycoside (C8APG), in the presence of a chelating agent, disodium ethylenediaminetetraacetate (Na2EDTA). The resulting rate of dissolution corresponds well with the equilibrium solubility obtained in the part I of this series even though the dissolution is rate limited (far from equilibrium). High rates of Ca(C18)2 dissolution are achieved in solutions of zwitterionic DDAO or nonionic C8APG with Na2EDTA at high pH. From rate analysis, the Ca(C18)2 dissolution was found to be surface‐reaction limited rather than limited by solution processes or wettability.