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Abstract
A 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 monomelic 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 pK^sub a^ 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.
Keywords Anionic surfactant * Amphoteric surfactant Mixed micelle formation * Protonation of surfactant * Titration of surfactant
Introduction
In consumer product applications, surfactant mixtures are often used to obtain various specific beneficial properties simultaneously and to gain general benefits from synergistic effects. Mixtures of amphoteric and anionic surfactants are often used in shampoo, hand-dishwash, and other liquid formulations. Anionic surfactants are well known for cleaning properties, while amphoteric surfactants are often used for skin mildness and good foam properties [1,2]. However, precipitation can be extremely detrimental to system performance if surfactants used have oppositely charged headgroups. Since precipitation is driven by monomer concentrations (i.e., thermodynamic activities) of each surfactant forming the precipitate, the activities of each surfactant must be known in order to determine whether precipitation occurs or not.
A mixture of amphoteric surfactant dimethyldodecylamine oxide (DDAO) and anionic surfactant sodium dodecyl sulfate (SDS) is studied here. Amphoteric surfactants may possess both positive and negative charges on their head groups and the net charge...