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

Abstract

Dissolution of calcium salt of a long chain fatty acid or soap scum is a major challenge for hard surface cleaners since soap scum forms when soap is exposed to hard water and has very low water solubility. In this paper, the aqueous equilibrium solubility of calcium octadecanoate (or calcium stearate) was measured as a function of pH as well as chelating agent (ethylenediaminetetraacetate disodium salt) and surfactant concentrations. Anionic, nonionic, and amphoteric surfactants were studied. The highest soap scum solubility was observed at high pH with an amphoteric surfactant. Under this condition, the chelant effectively binds calcium, and the stearate anion forms mixed micelles well with the amphoteric surfactant, which is in zwitterionic form at high pH.

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

“…So, the nonionic surfactant is the most synergistic in improving the rate of dissolution of Ca(C 18 ) 2 for a given thermodynamic driving force for dissolution (the solubility), followed by the cationic surfactant (DDAO at pH 4 where solubilities are comparable), followed by the anionic surfactant. At a pH of 4, the stearate is protonated to form the nonionic stearic acid [1]. The highest dissolutions for pH 11 at a given time with DDAO or C 8 APG are driven by high Ca(C 18 ) 2 solubility or moderate solubility/rapid kinetics, respectively.…”

“…The highest dissolutions for pH 11 at a given time with DDAO or C 8 APG are driven by high Ca(C 18 ) 2 solubility or moderate solubility/rapid kinetics, respectively. From the solubility study of Ca(C 18 ) 2 in Part I of this series [1], the high solubility of pH 11 is due to the chelant most effectively complexing calcium at that pH and the anionic stearate anion co-micellizing with the zwitterionic DDAO, and to a lesser extent, the nonionic C 8 APG.…”

“…Also, since the time for dissolution of a specified fraction of the Ca(C 18 ) 2 would be halved by doubling the flow rate if the dissolution were at equilibrium, the independence of the rate of dissolution on flow rate proves that the dissolution is not at equilibrium. Table 5 shows the duration of time for the systems of 0.1 M surfactant/0.1 M chelant at 1 ml/min to dissolve 100% of Ca(C 18 ) 2 initially placed (0.01 g) if the system were to reach equilibrium instantaneously (from equilibrium solubility data from [1]). It is clearly shown that all the systems are far from equilibrium.…”

“…The first part of this series [1] measured the equilibrium solubility of calcium di-stearate or Ca(C 18 ) 2 in surfactant/ chelant solutions as a function of pH. It was found that both complexation of calcium by the chelant and formation of mixed micelles between stearic acid/stearate anion and added surfactants are important in order to obtain high solubility of Ca(C 18 ) 2 .…”

“…For the systems studied, the highest equilibrium solubility was obtained when an amphoteric surfactant, dimethyldodecylamine oxide (DDAO), with chelant, disodium ethylenediaminetetraacetate (Na 2 EDTA), was used at high pH. The high pH solution promotes both the dissociation of calcium from a molecule of Ca(C 18 ) 2 through the chelation reaction and the formation of mixed micelles between the zwitterionic DDAO and the stearate anion [1].…”