(436d) Modeling the Nucleation of Crystals of Ionic Liquids in the Bulk and Near Graphitic Surfaces

Solidification of ionic liquids (ILs) is used in the synthesis of optically-active and magnetic nanomaterials based on ILs. However, very little is understood about the mechanism of nucleation of solid phases of ILs, mainly because most of their applications involve liquid phases. Here we studied the nucleation of crystals of two common ILs, 1,3-dimethylimidazolium chloride [dmim⁺][Cl^-], and 1-butyl-3-methylimidazolium chloride [bmim⁺][Cl^-], from their supercooled liquid phases in the bulk and in contact with graphitic surfaces. 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. This particular combination of methodologies is well suited to study nucleation of ILs, as these systems exhibit very slow dynamics and very large simulation box sizes are required in order to avoid finite-size effects. We will present and discuss results⁵ obtained for the free energy profiles, mechanisms and rates involved in the nucleation processes of these systems, aiming at understanding the nucleation and growth of crystals of organic salts in the bulk, near surfaces and inside nanopores.

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

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

³ L. Maragliano, E. Vanden-Eijnden and B. Roux, â??Free Energy and Kinetics of Conformational Transitions from Voronoi Tesellated Milestoning with Restraining Potentialsâ?, J. Chem. Theory Comput., 2009, 5, 2589

⁴ E. E. Santiso and B. L. Trout, â??A General Set of Order Parameters for Molecular Crystalsâ?, J. Chem. Phys. 2011, 134, 064109

⁵ X. He, Y. Shen, F. R. Hung and E. E. Santiso, â??Molecular Simulation of Homogeneous Nucleation of Crystals of an Ionic Liquid from the Meltâ?, J. Chem. Phys. 2015, 143, 124506