Dissolution kinetics of mixed soap scums at different molar ratios of Ca and Mg in an amphoteric surfactant system with different chelating agents

Abstract

Hard water generally contains divalent cations, especially calcium and magnesium ions, which have a typical molar ratio of 4:1. Soap or salt of fatty acid reacts with Ca and Mg ions, to form white precipitate known as soap scum. It is a sticky stain or filmy layer that can form on sanitary ware. In this work, the equilibrium solubility and dissolution rate of synthesized soap scum samples at different Ca:Mg molar ratios (1:1 and 4:1) in different systems (pure water, disodiumethylene diaminetetraacetate (Na₂EDTA), tetrasodium glutamatediacetate (Na₄GLDA), dimethyldodecylamine oxide (DDAO), DDAO/Na₂EDTA, and DDAO/Na₄GLDA) were investigated at different solution pH levels (4-11) and at a constant temperature of 25 ºC. The results showed that the DDAO/Na₄GLDA system provided the highest equilibrium solubility of the mixed soap scum, whereas the DDAO/Na₂EDTA system provided the highest dissolution rate of any mixed soap-scum at pH 11. For the 1:1 ratio, the equilibrium solubilities of the calcium and magnesium soap scum samples were not significantly different. For the 4:1 ratio, the equilibrium solubility of calcium soap scum was higher than that of the magnesium soap scums. The synthesized mixed soap scum samples were also characterized for particle size distribution, surface morphology, crystalline size, specific surface area, and functional groups.