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

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 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 pK 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.

Dissolution Study of Salt of Long Chain Fatty Acids (Soap Scum) in Surfactant Solutions. Part I: Equilibrium Dissolution

“…Extrapolating from reported CMC values with added electrolyte [11,12] to pure surfactant values, the DDAO at low pH where it is in cationic form has a CMC of 7.0 9 10 -3 M, and the DDAO at high pH where it is in zwitterionic form has a CMC of 5.4 9 10 -3 M. The dissolved calcium and stearate ions will reduce the CMC values below these pure surfactant values.…”

Interaction Between an Anionic and an Amphoteric Surfactant. Part II: Precipitation

“…monomer, respectively. According to the previous paper in this series [43], the extended Debye-Hückel equation can be used to calculate the activity coefficient of charged ions in solution. The ionic strength is considered constant throughout the study since the swamping electrolyte approximation is always valid due to added salt (NaCl) at high concentrations compared to the ionic surfactant and acid or base concentration in the system.…”

“…The pK a of DDAO is around 4.9, which is a true thermodynamic pK a [38,39,43,45]. The pK a indicates the dissociation of DDAO in monomer form so it is appropriate to use this value in Eqs.…”

“…18. As discussed in detail in part I of this series [43], in our model, the micellar environment effect on protonation of the DDAO is accounted for by the modeling of nonideal mixing in the micelles above the CMC.…”

“…The K SP was obtained from this study as already discussed while all other parameters were obtained from the first paper in this series [43] which addressed protonation and monomermicelle equilibrium. In that paper, it was shown that premicellar aggregation (e.g., ion-pair formation) is insignificant in this system, so that phenomenon does not need to be included in the model.…”