(174d) Implicit Water Model of Ionic Surfactant Micelles

We propose an implicit-solvent model for the simulation of self-assembly of alkali metal dodecyl sulfate micelles. Association of amphiphilic molecules entails multiple time- and length scales. Spontaneous self-assembly of ionic surfactants at low concentrations is currently computationally intractable for atomistic models with explicit solvent. In this work, atomistic Molecular Dynamics (MD) simulations of headgroup and Na counterions are

performed in order to coarse grain the headgroup ion and to trace out the solvent degrees of freedom.

The water-implicit ion interaction potential takes explicitly hydration barriers into account.

For the parameterization of the implicit model of the surfactant tails, we use Grand Canonical Monte Carlo simulations with histogram reweighting techniques.

The matching objectives for the coarse-graining procedure are agreement between simulation and experimental results with regard to their thermodynamic and structural quantities of NaDS micelles. In particular, we seek to reproduce the critical micelle concentration (cmc) and the aggregation number of the resulting micelles. Using this methodology,

we investigate the counterion impact on the micellization process by exchanging Na by Li and K ions. The cmc, aggregation numbers and the degree of counterion binding were obtained for all systems (Li-, Na-, and KDS). The results are in good agreement with experimental results and with recent atomistic MD simulations at high surfactant loadings. The computational methodology can be extended to

other ionic surfactants, including lipids and provides an important step towards large-scale simulations for the study of self-assembly of surfactant molecules.