Determination of the Second Critical Micelle Concentration of Aqueous Non-Ionic Surfactants: Measurement and Calculation of Various Physicochemical Properties above the First CMC Point

The second critical micelle concentration (SCMC) of non-ionic surfactants, including the Tween and Span series, was measured in an aqueous medium using the conductometric technique for the first time. The SCMC is a point at which the monodisperse equilibrium disks aggregated at the first critical micelle concentration are transmitted to flat ribbons (cylindrical micelles), which have a polydisperse length, uniform width, and swollen ends. So, considering the SCMC definition, the influence of temperature was examined on the micellization parameter (SCMC) of four consumed surfactants (Tween 60, Tween 80, Span 60, and Span 80) at T = (298-313) K. The structure of cylindrical micelles was determined via transmission electron microscopy at 298 K. Moreover, significant thermodynamic properties involving liquid mixture density, viscosity, osmotic coefficient, and water activity were measured at T = (293-313) K. In addition, the surfactant activity coefficient was evaluated using the NRTL GE model for the first time. The model successfully satisfied the Gibbs-Duhem equation. Eventually, it was demonstrated that the SCMC has a substantial influence on all measured thermodynamic properties, and the SCMC corresponds with the variation of the mentioned thermodynamic properties.