(323d) 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.