Modeling Micellar Growth and Branching in Mixtures of Zwitterionic with Ionic Surfactants.

Zwitterionic surfactants are widely applied as drag-reducing or thickening agents because their aggregation patterns may drastically change in response to variations of the system composition or external stimuli, which provides controllable viscoelasticity. For predicting aggregation behavior of surfactant mixtures, classical molecular thermodynamic models have been widely used. Particularly, the results of modeling have been reported for zwitterionic/ionic surfactant mixtures. However, for solutions containing a zwitterionic surfactant, no molecular thermodynamic model has been proposed for a micellar branch. In this work we extend the classical molecular thermodynamic aggregation model to describe aggregation in the aqueous mixtures that contain a zwitterionic and an ionic surfactant. We derive analytical expressions (1) for the contribution of dipoles to the electrostatic term of the standard free energy of aggregation into micellar branches and (2) for the dipolar contribution to the persistence length of wormlike micelles. The dependence of micellar branching on the surfactant concentration is taken into account by including the population of micellar branches in the material balance equations. This model is applied to predict aggregation equilibrium in aqueous salt solutions of betaine (oleoylamidopropyl-N,N-dimethylbetaine) mixed with sodium dodecyl sulfate (SDS) and the longer tail sodium n-alkyl sulfates. We discuss the predicted properties of the aggregates and micellar networks and compare our predictions with available experimental data.

Self-assembly in aqueous solutions of imidazolium ionic liquids and their mixtures with an anionic surfactant.

Experimental data on micellization in aqueous solutions of 1-alkyl-3-methylimidazolium salts [C(n)mim]X and their mixtures with sodium dodecyl sulfate (NaDS) are reviewed. New results (the critical micelle concentration and enthalpy of micellization) are presented for mixtures of [C(4)mim]PF(6), [C(6)mim]BF(4), [C(6)mim]Br and [C(10)mim]Br with NaDS. Our data cover a wide range (from 0 to 0.9) of solvent-free based mole fractions of ionic liquid (IL). Even very small addition of ILs substantially decreases the cmc of NaDS due to the combined effect of electrostatic and hydrophobic interactions, and formation of mixed micelles. It is shown that the quasichemical aggregation model by Nagarajan and Ruckenstein may be successfully applied to aqueous solutions of long-chain ILs and their mixtures with NaDS. The local structure of micelles is obtained from all-atom MD simulations for [C(n)mim]Br and [C(n)mim]X+NaDS in aqueous medium.