Interaction Between an Anionic and an Amphoteric Surfactant. Part I: Monomer–Micelle Equilibrium

Abstract

AbstractA mixture of anionic and amphoteric surfactants is composed of three components at intermediate pH levels: anionic, cationic (protonated amphoteric), and zwitterionic (unprotonated amphoteric). Knowledge of the composition of each surfactant in both monomer and micellar forms (monomer–micelle equilibrium) is important in applications using this mixture. Hydrogen ion titration of the mixed surfactant solution as a function of surfactant composition is combined with the pseudophase separation model and regular solution theory for the three‐surfactant mixture to calculate the concentration of each surfactant in monomer and in micelle forms at different pH levels. The specific systems studied here contain sodium dodecyl sulfate (SDS) and dimethyldodecylamine oxide (DDAO), which are used in a wide range of consumer products. The degree of protonation of monomeric DDAO is not affected by the presence of SDS, indicating an insignificant formation of ion pairs between these monomers. However, the presence of SDS in micelles shifts the micellar pKa of DDAO protonation significantly and the method used here allows the quantification of partial fugacities of each individual surfactant in micelle form. The composition in the monomer phase at each pH will aid in understanding and predicting solution compositions corresponding to anionic/amphoteric surfactant precipitation boundaries, which is the focus of the subsequent paper in this series.