Molecular simulation of bundle-like crystal nucleation from n-eicosane melts.

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.