Self-Assembly in Surfactant Oligomers: A Coarse-Grained Description through Molecular Dynamics Simulations

We have developed a simple microscopic model of surfactant oligomers. Surfactant oligomers are made up of x (≥2) single-chain surfactants connected at the level of, or close to, the headgroups by spacer groups. We have studied the formation and morphologies of the supramolecular aggregates of these model surfactant oligomers in an aqueous medium by constant pressure molecular dynamics simulations. In particular, we have investigated the effect of the degree of oligomerization x on the self-assembling properties and diffusivity in bulk. For dimeric (x = 2) and trimeric surfactants (x = 3), our model simulations exhibit a transition from spherical micelles to cylindrical micelles with increase in surfactant concentration. With further increase in concentration, these cylindrical micelles transform into extremely long "wormlike" or "threadlike" micelles. These findings are in qualitative agreement with experimental results. Various pair correlation functions and eigenvalues of the moment of inertia tensor of the micellar aggregates provide valuable insight into the structure of the micelles. For dimeric and trimeric surfactants, at intermediate concentration, our simulation results give direct evidence of the formation of closed-loop micelles. The self-diffusion coefficients of both dimeric and trimeric surfactant solutions show remarkably similar behavior, indicating some universality in the behavior of such wormlike micellar solutions.