Self-assembly of charged surfactants: full comparison of molecular simulations and scattering experiments

We present a study of the self-assembly of charged surfactants by a combination of molecular simulations and anomalous small-angle X-ray scattering (ASAXS). Solvent-free grand canonical Monte Carlo simulations are used to obtain the equilibrium structure of tetradecyltrimethylammonium bromide (TTAB) micelles. Subsequent molecular dynamics simulations of multiple micelles were used to calculate the scattering intensity obtained at low surfactant concentrations (17 mM). In particular, the partial intensities of the macroion formed by the surfactant and the counterions were derived and compared directly to experimental data obtained from ASAXS, which revealed reasonably good agreement. Emphasis is put on the fluctuations of the spatial distributions of the surfactant molecules and the counterions, respectively. Criteria for the assessment of these fluctuations are given and compared to simulations and experiments. We found that fluctuations are mainly caused by counterions and need to be accounted for the correct interpretation of scattering data. It is demonstrated that the combination of molecular simulations with ASAXS leads to a comprehensive understanding of self-assembly in systems of charged surfactants.