Thermodynamic parameters and counterion binding to the micelle in binary anionic surfactant systems

Competitive counterion binding of sodium and calcium to micelles, and mixed micellization have been investigated in the systems sodium dodecylsulfate (NaDS)/sodium decylsulfate (NaDeS) and NaDS/sodium 4-octylbenzenesulfonate (NaOBS) in order to accurately model the activity of the relevant species in solution. The critical micelle concentration (CMC) and equilibrium micelle compositions of mixtures of these anionic surfactants, which is necessary for determining fractional counterion binding measurements, is thermodynamically modeled by regular solution theory. The mixed micelle is ideal (the regular solution parameter βM = 0) for the NaDS/NaOBS system, while the mixed micelle for NaDS/NaDeS has βM = −1.05 indicating a slight synergistic interaction. Counterion binding of sodium to the micelle is influenced by the calcium ion concentration, and vice versa. However, the total degree of counterion binding is essentially constant at approximately 0.65 charge negation at the micelle’s surface. The counterion binding coefficients can be quantitatively modeled using a simple equilibrium model relating concentrations of bound and unbound counterions.

Graphical abstractCounterion binding of Na+ and Ca2+ to micelles, and thermodynamic parameters of mixed micellization in mixed-surfactant systems have been measured. Binding has been modeled using a chemical equilibrium model.Figure optionsDownload full-size imageDownload high-quality image (76 K)Download as PowerPoint slideResearch highlights► A study of micellization behavior in mixed anionic surfactant systems. ► Measurement of Na+ and Ca2+ binding to the micelle using ion-selective electrodes. ► An original model predicts the competitive binding of Na+ and Ca2+. ► The model is able to predict counterion binding of the two ions very successfully.