“Derivation of micelle size-dependent free energies of aggregation for octyl phosphocholine from molecular dynamics simulation” by Ph.D. student Xiaokun Zhang, EC ’19 student Jorge Arce Nunez, and J.T. Kindt has been accepted for publication in the journal Fluid Phase Equilibria as part of a special issue on the state of the art in molecular simulation approaches to phase equilibria and physical properties.  In this paper (pdf here, free link until Feb. 9 2019) we use the PEACH approach to derive free energies of aggregation of a simple zwitterionic surfactant (OPC) using two sets of force field parameters, and show that using parameters that are properly chosen it is possible to derive a critical micelle concentration in good agreement with experiment.

As this is the first (to our knowledge) complete determination of micellization free energy versus micelle size from an atomically-detailed simulation model, we were interested to see how the form of this function compared with theoretical models.  A significantly better fit was obtained with the “quasi-droplet” model of Grinin et al. (link), which is designed to take into account the roughness of the micelle arising from its assembly by discrete molecules that may protrude into solvent, than with models that assumed a spherical shape.  We look forward to performing further studies to see whether this model is generally successful.