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

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Science

Background:

• Homogeneous nucleation is a fundamental process in crystallization.
• Understanding nucleation kinetics and thermodynamics is crucial for controlling material properties.
• Molecular simulations offer a powerful tool to investigate nucleation at the atomic scale.

Purpose of the Study:

• To investigate the homogeneous nucleation of n-eicosane crystals from a supercooled melt using molecular simulation.
• To determine nucleation rates, induction times, and nucleation free energies.
• To characterize the structure of critical nuclei and calculate interfacial free energies.

Main Methods:

• Molecular dynamics (MD) simulations were employed to observe nucleation events at constant pressure and temperature.
• Monte Carlo (MC) simulations with umbrella sampling were used to calculate nucleation free energy.
• A realistic, united-atom model for n-alkanes was utilized.

Main Results:

• Nucleation events were observed at approximately 19% supercooling.
• The induction time was measured as 80.6 ± 8.8 ns, yielding a nucleation rate of (6.59 ± 0.72) × 10^25 cm^-3 s^-1.
• Nucleation free energies ranged from 7.3 to 13.2 k(B)T, and critical nuclei were identified as cylindrical bundles of stretched segments with disordered interfacial layers.

Conclusions:

• The study successfully simulated homogeneous nucleation of n-eicosane, providing quantitative kinetic and thermodynamic data.
• The critical nucleus structure was characterized as a cylindrical bundle, with calculated solid-liquid interfacial free energies of approximately 10 mJ/m^2 (side) and 4 mJ/m^2 (end).
• These findings contribute to a deeper understanding of crystallization processes in molecular systems.