Effects of solution pH and salinity on equilibrium solubility and dissolution rate of calcium soap scum in amphoteric surfactant solutions with Na2EDTA chelant

Abstract

A sticky stain or filmy layer around sanitary wares results from the reaction between soap and divalent cations in hard water. The objective of this study was to investigate equilibrium solubility and dissolution rate of calcium soap scum using an amphoteric surfactant with a chelant. The equilibrium solubility and dissolution rate of soap scum increased with increasing solution pH when the disodium ethylenediaminetetraacetate (Na2EDTA) chelant was added in the dimethyldodecylamine oxide (DDAO) system but in chelant-free systems, the opposite trend was observed. The added NaCl significantly increased the dissolution rate of soap scum at a high solution pH and reduced the effect of chelant on the equilibrium solubility of soap scum at high solution pHs. However, an addition of NaCl had insignificant effect on equilibrium solubility of soap scum under in the DDAO system. Furthermore, an increase in NaCl concentration had no effect on both equilibrium solubility and dissolution rate of soap scum in the DDAO system without Na2EDTA.