Molecular simulation of crystal nucleation in n-octane melts

摘要

Homogeneous nucleation of the crystal phase in n-octane melts was studied by molecular simulationwith a realistic, united-atom model for n-octane. The structure of the crystal phase and the meltingpoint of n-octane were determined through molecular dynamics simulation and found to agree withexperimental results. Molecular dynamics simulations were performed to observe the nucleationevents at constant pressure and constant temperature corresponding to about 20% supercooling.Umbrella sampling Monte Carlo simulations were used to calculate the nucleation free energy forthree temperatures, ranging from 8% to 20% supercooling, and to reveal details of the criticalnucleus for the first time. The cylindrical nucleus model was found to provide a better quantitativedescription of the critical nucleus than the spherical nucleus model. The interfacial free energies ofthe cylinder model were calculated from the simulation data. As the temperature increased, theinterfacial free energy of the side surface remained relatively unchanged, at 7-8 mJ/ m2, whereasthe interfacial free energy of the end surface decreased significantly from 5.4 mJ/ m2 to about3 MJ/ m2. These results, and the methods employed, provide valuable and quantitative informationregarding the rate-limiting step during the solidification of chain molecules, with ramifications forboth short alkanes and polymers.