Molecular simulation of crystal nucleation in n-octane melts

Homogeneous nucleation of the crystal phase in n-octane melts was studied by molecular simulation with a realistic, united-atom model for n-octane. The structure of the crystal phase and the melting point of n-octane were determined through molecular dynamics simulation and found to agree with experimental results. Molecular dynamics simulations were performed to observe the nucleation events at constant pressure and constant temperature corresponding to about 20% supercooling. Umbrella sampling Monte Carlo simulations were used to calculate the nucleation free energy for three temperatures, ranging from 8% to 20% supercooling, and to reveal details of the critical nucleus for the first time. The cylindrical nucleus model was found to provide a better quantitative description of the critical nucleus than the spherical nucleus model. The interfacial free energies of the cylinder model were calculated from the simulation data. As the temperature increased, the interfacial free energy of the side surface remained relatively unchanged, at 7-8 mJ/m(2), whereas the interfacial free energy of the end surface decreased significantly from 5.4 mJ/m(2) to about 3 mJ/m(2). These results, and the methods employed, provide valuable and quantitative information regarding the rate-limiting step during the solidification of chain molecules, with ramifications for both short alkanes and polymers.