Molecular modeling for nonideal mixing of amphiphilic molecules and applications to ionic surfactant/salt solutions

Abstract

A new theory is developed to describe nonideal mixing behaviors and bulk and interfacial properties of surfactant mixtures, which cannot be described by regular solution theories. Within the framework of this model, the nature of nonideality is taken into account in the aspects of intermolecular interactions and the molecular arrangement of micelles and interfacial layers. In calculating the excess free energy of mixing, the approach separates the irregular effects, arising from changes in the size, shape, and structure of micelles and interfacial monolayers, from the exchange effect attributed to "regular" mixing. Molecular modeling is employed to calculate these effects based on the molecular structures of the constituents and equivalent information for the pure components. To check the model, the surface tension of mixed triethyleneglycal mono-n-dodecyi ether (C12EO3)/sodium dodecyl sulfate (SDS) aqueous solutions at the phase inversion point and the dependence of the critical micellization concentration and the surface tension of SDS aqueous solutions on the salt concentration were predicted based on our approaches. The predicted results are in excellent agreement with experiments.