Structure and Dynamics of Ionic Micelles: MD Simulation and Neutron Scattering Study

Fully atomistic molecular dynamics (MD) simulations have been carried out on sodium dodecyl sulfate (SDS), an anionic micelle, and three cationic (C(n)TAB; n = 12, 14, 16) micelles, investigating the effects of size, the form of the headgroup, and chain length. They have been used to analyze neutron scattering data. MD simulations confirm the dynamical model of global motion of the whole micelle, segmental motion (headgroup and alkyl chain), and fast torsional motion associated with the surfactants that is used to analyze the experimental data. It is found that the solvent surrounding the headgroups results in their significant mobility, which exceeds that of the tails on the nanosecond time scale. The middle of the chain is found to be least mobile, consolidating the micellar configuration. This dynamical feature is similar for all the ionic micelles investigated and therefore independent of headgroup form and charge and chain length. Diffusion constants for global and segmental motion of the different micelles are consistent with experimentally obtained values as well as known structural features. This work provides a more realistic model of micelle dynamics and offers new insight into the strongly fluctuating surface of micelles which is important in understanding micelle dispersion and related functionality, like drug delivery.