(655g) Simulation of Homogeneous Nucleation of Co-Crystals from a Binary Hard Sphere Fluid Mixture

Co-crystal formation from fluid-mixtures is quite common in a large number of systems. The simplest systems that shows co-crystal (also called substitutionally ordered solids) formation are binary hard sphere mixtures. In this work we study the nucleation of AB₂ type solid compounds using Monte Carlo molecular simulations in a binary hard sphere mixtures with size ratio of 0.55. The conditions chosen for the study lie in the region where nucleation of AB₂ type solid competes with that of pure A solid with FCC (face-centered-cubic) structure. The fluid phase compositon is kept equal to that of the AB₂ type solid. The nucleation free-energy barriers are computed using the seeding technique of Sanz et al. [J. Am. Chem. Soc. 135, 15008 (2013)]. Our computed free energy barriers for homogeneous nucleation show that the AB₂ type solid is always favored even under conditions where the pure A solid is more stable. This is primarily due to similarity in the composition of fluid-phase and the AB₂ type solid which in turn leads to much lower interfacial tension between the crystal nucleus and the fluid phase. This system is an example of how the fluid phase composition affect the structure of the nucleating solid phase during crystallization and has relevance to solvent-induced polymorphism seen in molecular crystals.