Dissolution of Soap Scum by Surfactant Part I: Effects of Chelant and Type of Soap Scum

Abstract

The equilibrium solubilities of two model soap scums [calcium stearate and magnesium stearate: Ca(C18)2 and Mg(C18)2] were measured in aqueous solutions containing three different types of surfactants: methyl ester sulfonate (MES) as an anionic; alcohol ethoxylate (EO9) as a nonionic; and dimethyldodecylamine oxide (DDAO) as an amphoteric with and without a chelating agent [disodium ethylenediaminetetraacetate (Na2EDTA)]. The solubility of calcium soap scum was generally higher than that of magnesium soap scum, the exception being some DDAO systems. The use of the DDAO surfactant with the Na2EDTA chelating agent at high pH gives the highest solubilities of both studied soap scums. The soap scum solubility is on the order of 2,000 times that in water at high pH. The DDAO is the most effective surfactant under all conditions. The MES is more effective than the EO9 at low pH with the opposite trend observed at high pH. The synergism from added chelant is generally greater at higher pH and is greatest for DDAO followed by EO9.

Dissolution of Soap Scum by Surfactant. Part II: Effects of NaCl and Added Chelant on Equilibrium Solubility and Dissolution Rate of Calcium Soap Scum in Amphoteric Surfactant Solutions

“…At low pH, the stearate is predominately protonated to be a nonionic molecule while the DDAO is primarily cationic. First order kinetics describes the data remarkably well over the time period of the experiments with the greatest deviation at longest times as observed in previous work [4]. Added electrolyte can decrease the absolute value of the electrical potential at a charged micelle surface [6,7], thereby decreasing the electrostatic attractive forces between nonionic/cationic molecules at low pH or anionic/zwitterionic molecules at high pH in the mixed micelles [8,9], thus reducing mixed micelle synergism and solubility enhancement due to micellization.…”

“…3 to show the effect better. The greater effect of NaCl at high pH indicates that the synergism in formation of the anionic/zwitterionic mixed micelles is stronger than that in the nonionic/cationic mixed micelles [10] as previously discussed [4]. At a pH of 4, the solubility decreases by 13 % from no added salt to 0.1 M NaCl, while at pH of 11, the decrease is 45 %.…”

“…An effective soap scum cleaning system requires both the dissociation of calcium ions from soap scum and the formation of mixed micelles of the free fatty acids dissociated from soap scum with an added surfactant [3]. The dissolution rate of soap scum was shown to follow first order kinetics in one of our previous studies [5] of different surfactant systems that studied in Part I [4] of this work or in this study. The synergism in mixed micelle formation of the free fatty acids and added surfactant depends on the surfactant structure and pH, so different types of added surfactants affect the dissolution of calcium soap scum [3].…”

“…At high pH, the stearate is primarily anionic while the DDAO is primarily zwitterionic. It was shown [4] that the rate of dissolution is rate controlled as the systems are far from equilibrium (solution stearate concentrations are orders of magnitude lower than that corresponding to equilibrium solubilities). The greater effect of NaCl at high pH indicates that the synergism in formation of the anionic/zwitterionic mixed micelles is stronger than that in the nonionic/cationic mixed micelles [10] as previously discussed [4].…”

“…The large surfactant hydrophobe binding with calcium ion causes this molecule, indicated as Ca (C 18 ) 2 in what follows, to have very low solubility in water. From our previous paper (Part I in the series of the present study) [4], the equilibrium solubilities of calcium and magnesium soap scums in aqueous solutions containing three different types of surfactants with and without a chelating agent were studied. The calcium dissociation from soap scum molecules can be achieved by using a complexing or chelating agent such as ethylenediaminetetraacetate (EDTA) to form a stable water-soluble chelant-calcium complex.…”