Homogeneous nucleation of n-eicosane crystals from the supercooled melt was studied
by molecular simulation using a realistic, united-atom model for n-alkanes. Using
molecular dynamics simulation, we observed nucleation events directly at constant
pressure and temperature, corresponding to about 19% supercooling. Under these conditions,
the induction time is found to be 80.6 ± 8.8 ns for a system of volume (1.882 ± 0.006)
× 10(-19) cm(3), corresponding to a nucleation rate of (6.59 ± 0.72) × 10(25) cm(-3)
s(-1). The nucleation free energy was calculated separately for three temperatures,
ranging from 10% to 19% supercooling, by a Monte Carlo method with umbrella sampling.
Values for the nucleation free energy range from 7.3 to 13.2 (in units of k(B)T).
Detailed examination of the simulations reveals the critical nucleus to be a bundle
of stretched segments about eight methylene groups long, organized into a cylindrical
shape. The remaining methylene groups of the chains that participate in the nucleus
form a disordered interfacial layer. By fitting the free energy curve to the cylindrical
nucleus model, the solid-liquid interfacial free energies are calculated to be about
10 mJ/m(2) for the side surface and 4 mJ/m(2) for the end surface, both of which are
relatively insensitive to temperature.