(222q) Effect of pH On Surface Charge of Zwitterionic Surfactant and Its Hardness Tolerance

Precipitation of surfactant by hardness ions effects negatively on the cleaning result in the detergency industry since the unsightly soap scum precipitated removes the surfactant from the cleaning action. Zwitterionic surfactant has been synergistically mixed with anionic surfactant in order to serve as a foam booster and generate mildness at certain pH levels. However, the anionic form of this inner salt in hard water is often limited by its tendency to precipitate. Precipitation of zwitterionic surfactant in the anionic form with hardness ions depends on temperature, ionic strength and pH. Therefore, this work aims to study the effect of pH on the solubility of their precipitate and precipitation behavior of 3-(decyldimethylammonio)­ propane­sulfonate with calcium ions at 30°C. A series of zwitterionic surfactant used with varying concentration was prepared in order to measure the zeta potential and to determine its CMC as a function of pH at a certain amount of swamping solution of 0.01 M NaCl. The isoelectric point (pKa) was determined from the curve of zeta potential vs. pH at various surfactant concentrations. The solubility of calcium salt (soap scum) of zwitterionic surfactant was measured as a function of pH at a certain amount of swamping solution of 0.01 M NaCl. The experiments above provide informative data to construct the precipitation phase boundaries of 3-(decyldimethylammonio) propane­sulfonate precipitated with calcium ions as a function of pH at a certain amount of swamping solution of 0.01 M NaCl. These studies show that pH level is a key factor for zwitterionic surfactants when dealing with hardness ions. Knowledge gained from this work will be valuable in the understanding of hardness tolerance ability of zwitterionic surfactant in hard water.