Cylindrical to spherical shape transformations of micelles using all-atom and coarse-grained molecular dynamics simulations

Micelles with different symmetries have immense applications on cosmetic formulations, oil recovery, drug delivery and so on. To understand the controlling factors responsible for shape transformations of micelles and to achieve the relevant time and length scale, a multi-scale approach is used where an all atomistic simulation is employed to derive a coarse-grained (CG) model for the micelles. Cationic surfactants, behenyltrimethyl ammonium chloride (BTMAC) in water self-assemble into a cylindrical micellar phase which transforms into a spherical micellar phase upon addition of a co-surfactant, stearyl alcohol, SA. The bonded distributions of the CG model are derived by the canonical sampling of their respective AA simulations. Martini non-bonded potentials are found to be suitable to obtain the cylindrical micellar phase as in the AA model for BTMAC/water system, but not for the mixed system. Parameterization of martini force field enables to obtain the size distributions of the spherical micelle consistent with the AA ones. Our simulations reveal that a correct interplay between the head-group size and hydrophilicity is crucial for obtaining the micellar size distributions. Thus, the current study provides insights on the controlling factors of the cylindrical to spherical shape transformations of the micelles and shows the suitability of multi-scale ansatz to achieve the relevant length and time scale, inaccessible to the experiments, otherwise.