Implicit-solvent Models for Micellization of Ionic Surfactants

Abstract

We propose a method for parameterization of implicit-solvent models for the simulation of self-assembly of ionic surfactants into micelles. The parameterization is carried out in two steps. The first step involves atomistic Molecular Dynamics simulations of headgroups and counterions with explicit solvent to determine structural properties. An implicit-solvent model of the headgroup/counterion system is ob- tained by matching structural quantities between explicit-solvent and implicit-solvent systems. In the second step we identify the solvophobic attractions between the tail beads. We determine the solvophobic parameters using Grand Canonical Monte Carlo simulations with histogram reweighting techniques. The matching objective for the identification of solvophobic attractions is the critical micelle concentration (cmc). We chose sodium dodecyl sulfate as the reference system. Based on hydrophobic parameters obtained from this particular model we study specific ion effects (lithium and potassium instead of sodium), as well as the effect of cationic headgroups (dodecyltrimethylammonium bromide/chloride). Furthermore, the chain length dependence of micellization properties is investigated for sodium alkyl sulfate, with alkyl lengths between 6 and 14. All cases considered give results in broad agreement to experimental data, confirming transferability of parameters and the generality of the approach.