Molecular simulation study of homogeneous crystal nucleation in n-alkane melts

Abstract

This work used molecular dynamics (MD) and Monte Carlo (MC) method to study the homogeneous crystal nucleation in the melts of n-alkanes, the simplest class of chain molecules. Three n-alkanes with progressive chain length were studied, n-octane (C8), n-eicosane (C20), and C150, using a united atom force field, which is able to reproduce physical quantities related to the solid-liquid phase transition in n-alkanes. Using a 3D Ising model, we proved that the size of the largest nucleus in the system, nmax, is the controlling reaction coordinate during the nucleation process. We have made direct observation of the homogeneous crystal nucleation using MD simulation at as small as 15% under-cooling. We calculated the nucleation rate and identified the critical nucleus through a mean-first-passage time (MFPT) analysis. At about 20% under-cooling, the critical nucleus size n* is around 100 united atoms, and is slightly decreasing as the chain length increases. Abnormal temperature dependence of n* against classical nucleation theory was found in C150 system. This behavior could possibly be explained by the high viscosity of the melt formed by long chain molecules. The crystal nucleus has a cylindrical shape. We have observed the change of the structure of the crystal nucleus as the chain length increases. For C8, the chains attach to and detach from the crystal nucleus as a whole, and the chains end at the end surface of the cylindrical nucleus. For C20, the partial participation of chains in the crystal nucleus became apparent, where the critical nucleus consists of a bundle of crystal segments with the tails on the same chains extending into the amorphous melt. For C150, chain folding was observed during the nucleation stage. A cylindrical nucleus model was adopted to characterize the crystal nucleus. The nucleus free energy [Delta]G(n) was sampled using MC, and was used to calculate the solid-liquid interfacial free energies based on classical nucleation theory. The end surface free energy [sigma]e is about 4 mJ/m2 and the side surface free energy [sigma]s is about 10 mJ/m 2 . Their values are insensitive to the chain length.