(247l) Molecular Modeling of Homogeneous Nucleation of [Bmim+][Cl-] from the Melt

The study of solid phases of ionic liquids (ILs) confined inside nanoporous materials is relevant for the development of IL-based nanomaterials with unique properties (fluorescence, magnetic susceptibility). Although nucleation is a critical step in the process of crystallization, a fundamental understanding of these phenomena at the molecular level is still challenging, especially in the field of ILs. 1-butyl-3-methylimidazolium chloride, [bmim⁺][Cl^-], a prototype ionic liquid, has a relatively simple structure and can form two crystal polymorphs. Here we performed molecular simulation to study the homogeneous nucleation of [bmim⁺][Cl^-] from its supercooled liquid phase to its orthorhombic or monoclinic crystalline structure. The string method in collective variables^1,2 was used in combination with Markovian milestoning with Voronoi tessellations^2,3 and order parameters for molecular crystals⁴  to sketch a minimum free energy path connecting the supercooled liquid and the crystal phases, and to determine  the rates involved in the nucleation process. The mechanism of nucleation and the free energy barriers associated with this process will be presented and discussed.

¹ L. Maragliano, A. Fischer, E. Vanden-Eijnden and G. Ciccotti, “String Method in Collective Variables: Minimum Free Energy Paths and Isocommittor Surfaces”, J. Chem. Phys. 2006, 125, 024106

² V. Ovchinnikov, M. Karplus and E. Vanden-Eijnden, “Free Energy of Conformational Transition Paths in Biomolecules: The String Method and Its Application to Myosin VI” J. Chem. Phys. 2011, 134, 085103

³ E. Vanden-Eijnden and M. Venturoli, “Markovian Milestoning with Voronoi Tessellations”, J. Chem. Phys. 2009, 130, 194101

⁴ E. E. Santiso and B. L. Trout, “A General Set of Order Parameters for Molecular Crystals”, J. Chem. Phys. 2011, 134, 064109