Surfactant Exchange in Low-Symmetry Sphere Packings of Micelles

The hierarchical self-assembly of surfactants is an attractive route to the fabrication of functional soft materials. In this work, we study solutions of an ionic surfactant, which assembles into deformable spherical micelles in water, which further pack into low-symmetry Frank-Kasper (FK) phases. Starting with an initial structure in the experimentally observed phase we study the dynamics of surfactants using molecular dynamics simulation. We find that the diffusion of surfactant molecules is dominated by molecular exchange between micelles. Micelles temporarily touch neighbors during thermal motion, and surfactant exchange occurs via a flip-flop motion during this contact. This effect significantly enhances hopping rate of surfactant molecules, resulting in uncommonly short relaxation time for the change of the size of the micelles. We suggest this exchange mechanism makes FK phases dynamically inhomogeneous.