Molecular Dynamics Simulation of the Kinetics of Spontaneous Micelle Formation

Using an atom based force field, molecular dynamics (MD) simulations of 54 dodecylphosphocholine (DPC) surfactant molecules in water at two different concentrations above the critical micelle concentration have been pcr formed. Starting from a random distribution of surfactants, we observed the spontaneous aggregation of the surfactants into a single micelle. At the higher DPC concentration (0.46 M) the surfactants aggregated into a worm-like micelle within 1 ns, whereas at lower concentration (0.12 M) they aggregated on a slower time scale (similar to 12 ns) into a spherical micelle. The difference in the final aggregate is a direct consequence of the system achieving the lowest free energy configuration for a given quantity of surfactant within the periodic boundary conditions. The simulation at low surfactant concentration was repeated three times in order to obtain statistics on the rate bf aggregation. It was found that the aggregation occurs at a (virtually) constant rate with a rate constant of k = 1 x 10(-4) ps(-1). This is an unexpected result. On the basis of Monte Carlo simulations of a stochastic description of the system, using diffusion rates and cluster radii as determined by separate MD simulations of single DPC clusters, a lower rate constant which diminishes in the course of the aggregation process had been predicted. Neglect of hydrodynamic interactions, of long-range hydrophobic interactions, or of spatial correlations in the stochastic approach might account for the descrepancies with the more accurate MD simulations.