| Dissipative particle dynamics simulation with novel model is applied to predict aggregation in ionic surfactants. Anionic surfactant sodium dodecyl sulfate (SDS) and cationic surfactant cetyl trimethyl ammonium bromide (CTAB) are chosen as characteristic examples. Coarse-grained models are constructed from infinite dilution activity coefficient in reference solutions and atomistic simulations. Ionic groups are modeled as beads with charges distributed around the bead centers and placed in implicit dielectric medium. The model of ionic surfactant semi-quantitatively reproduces the dependence of self-assembly in solutions of CTAB and SDS on surfactant concentration, the molarity of added electrolyte. In particular, the decline of the free surfactant concentration with increasing total surfactant loading, as well as the aggregation transition in single-component surfactant cause by salt addition are predicted correctly. Addition of CTAB to a micellar solution of SDS causes a transition to worm-like micelles and then to mini-vesicles. This systematic methodology has been further extended to carbopol system. A stable carbopol film has been observed after equilibrium based on this model. Further investigation to study rheology of surfactant-carbopol system are required in future work, like add surfactant into this system. |
