Strong Electrostatics, Slow Kinetics: Pre-Assembly as a Practical Route to Equilibrated Ionic Micelles in MD

Self-assembly of molecules into nanostructures is an essential strategy that enables the predictable fabrication of functional materials for applications ranging from targeted drug delivery and biosensing to energy storage and nanoelectronics. A significant amount of experimental and computational research has been conducted and is ongoing in this area. However, in the context of classical computational simulations, it is not always feasible to allow molecules to self-assemble due to the method's timescale limitations. Known structures can be pre-assembled beforehand while saving a considerable amount of computation. In this research, the timescale issues associated with the self-assembly of surfactants have been investigated. A series of long-scale molecular dynamics (MD)simulations has been performed using different starting configurations in all-atom, united-atom, and coarse-grained approaches. A preassembled rod-like micelle composed of 200 sodium oleate molecules remains relatively stable throughout a 5-μs run. Meanwhile, starting from a randomly distributed configuration, a 5μs time frame is far from equilibration when using both all-atom and united-atom approaches. The twofold increase in system size, while maintaining the surfactant concentration, did not yield success, and the aggregation number was not in line with the experimental value. However, switching to a granular or coarse-grained approach, we observe that the formation of an ellipsoidal micelle occurs when starting from a randomly dispersed configuration, and the clustering and formation of the final ellipsoidal micelle takes place very quickly.