Effect of ionic environment on the rheology of wormlike micelle solutions of mixtures of surfactants with opposite charge

Coarse-Grained Model Incorporating Short- and Long-Range Effective Potentials for the Fast Simulation of Micelle Formation in Solutions of Ionic Surfactants

A coarse-grained model comprising short- and long-range effective potentials, parametrized with the iterative Boltzmann inversion (IBI) method, is presented for capturing micelle formation in aqueous solutions of ionic surfactants using as a model system sodium dodecyl sulfate (SDS). In the coarse-grained (CG) model, each SDS molecule is represented as a sequence of four beads while each water molecule is modeled as a single bead. The proposed CG scheme involves ten potential energy functions: four of them describe bonded interactions and control the distribution functions of intramolecular degrees of freedom (bond lengths, valence angles, and dihedrals) along an SDS molecule while the other six account for intermolecular interactions between pairs of SDS and water beads and control the radial distribution functions. The nonbonded effective potentials between coarse-grained SDS molecules extend up to about 12 nm and capture structural and morphological features of the micellar solution both at short and long distances. The long-range component of these potentials, in particular, captures correlations between surfactant molecules belonging to different micelles and is essential to describe ordering associated with micelle formation. A new strategy is introduced for determining the effective potentials through IBI by using information (target distribution functions) extracted from independent atomistic simulations of a micellar reference system (a salt-free SDS solution at total surfactant concentration c_T equal to 103 mM, temperature T equal to 300 K, and pressure P equal to 1 atm) obtained through a multiscale approach described in an earlier study. It employs several optimization steps for bonded and nonbonded interactions and a gradual parametrization of the short- and long-range components of the latter, followed by reparametrization of the bonded ones. The proposed CG model can reproduce remarkably accurately the microstructure and morphology of the reference system within only a few hours of computational time. It is therefore very promising for future studies of structural and morphological behavior of various liquid surfactant formulations.

Self-assembly and viscosity changes of binary surfactant solutions: A molecular dynamics study

HYPOTHESIS

Structure and self-assembly of surfactants in the solution shows a fundamental influence on its viscosity. Through molecular simulations using Martini force field, synergistic effects in aggregation as well as the viscosity changes of a binary ionic surfactant system can be modelled. Simulations: Coarse-grained molecular dynamics simulations are performed to model the SDS/CAPB binary surfactant solution, and both equilibrium and non-equilibrium methods are used to calculate the viscosity of the equilibrated micellar systems.

FINDINGS

Our simulation results indicate that the new version of the Martini force field can provide more reasonable self-assembly of surfactant, both single and binary system. Synergistic effects in micelle formation for SDS/CAPB have been successfully reproduced, that is, the formation of cylindrical micelles or even wormlike micelles at a lower concentration when compared with the pure system. Meanwhile, both equilibrium and non-equilibrium methods provide quantitatively comparable viscosity for each system. For pure micellar system, the viscosity linearly increases with the total concentration. Nevertheless, our simulation fails to capture the viscosity enhancement of the solution in corresponding with the formation of rodlike or wormlike micelles, and a full parameter optimization of force field is still necessary.

1-Hexanol triggered structural characterization of the worm-like micelle to vesicle transitions in cetyltrimethylammonium tosylate solutions

Incorporation of 1-hexanol close to the palisade layer of CTAT micelles favors transition of worm-like micelles to vesicular structuresviaformation of elongated worm-like micelles.