Molecular Simulation of Homogeneous Crystal Nucleation of n-Alkane Melts

One of the most important phenomena in molecular systems is homogeneous nucleation of the crystal phase from a melt.  This phenomenon is particularly interesting for chain molecules due to their strong anisotropy and their conformational flexibility.  In this work we report the results of molecular simulations of homogeneous crystal nucleation of n-eicosane (C20) from the melt.  We employed a realistic united atom force field which reproduces the experimental melting temperature.  The nucleation trajectory was then sampled using MD simulations at about 20% supercooling; and the nucleation free energy was sampled using Monte Carlo umbrella sampling method for three temperatures, ranging from 10% to 20% supercooling.  Detailed examination of the simulations reveals the critical nucleus to be a bundle of stretched segments about 8 CH2 groups long, organized into a cylindrical shape.  The remaining CH2 groups form a disordered interfacial layer.  The nucleation rate is calculated through a mean-first-passage-time analysis to the MD simulations.  By fitting the free energy curve to the cylindrical nucleus model, the crystal-melt interfacial free energies are calculated to be about 10 mJ/m2 for the side surface and 4 mJ/m2 for the end surface, which are in reasonable agreement with experiments.  We also studied the homogeneous crystal nucleation from C150 melts, where chain folding occurs in experiments.  Nucleation was directly observed at 20%-30% supercooling.  The critical nuclei are cylindrical with the stem length of around 10 CH2 groups.  We also found that each critical nucleus contains multiple chain folds with fold length of around 30 CH2 groups.