Molecular simulation of homogeneous crystal nucleation of AB₂ solid phase from a binary hard sphere mixture

Area of Science:

• Materials Science
• Chemical Physics
• Computational Chemistry

Background:

• Co-crystal formation is common in fluid mixtures, particularly in binary hard sphere systems.
• Substitutionally ordered solids, or co-crystals, form in simple binary hard sphere mixtures.
• Understanding nucleation is crucial for controlling crystallization processes and solid-state properties.

Purpose of the Study:

• To investigate the nucleation of AB2 type co-crystals in binary hard sphere mixtures using molecular simulations.
• To compare the nucleation of AB2 co-crystals with pure A solid formation under competing conditions.
• To elucidate the role of fluid phase composition in determining the nucleating solid phase structure.

Main Methods:

• Monte Carlo molecular simulations were employed to study binary hard sphere mixtures with a size ratio of 0.55.
• The seeding technique, as described by Sanz et al., was used to compute nucleation free-energy barriers.
• Simulations were conducted in a region where AB2 co-crystal nucleation competes with face-centered-cubic pure A solid nucleation.

Main Results:

• The nucleation of the AB2 type solid was found to be favored, even when the pure A solid is thermodynamically more stable.
• Lower interfacial tension between the AB2 crystal nucleus and the fluid phase was observed due to matching compositions.
• Fluid phase composition significantly impacts the structure of the nucleating solid phase.

Conclusions:

• Fluid phase composition plays a critical role in directing the outcome of crystallization, favoring co-crystal formation.
• This finding has implications for understanding and controlling crystal polymorphism during crystallization.
• The study provides insights into the fundamental mechanisms governing co-crystal nucleation in binary mixtures.