(39b) Modeling the Homogeneous and Heterogeneous Nucleation of Crystals of Ionic Liquids in Different Environments

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, in contact with graphitic surfaces, and inside slit-like graphitic nanopores. 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 results^5,6 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, "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 L. Maragliano, E. Vanden-Eijnden and B. Roux, "Free Energy and Kinetics of Conformational Transitions from Voronoi Tesellated Milestoning with Restraining Potentials", J. Chem. Theory Comput., 2009, 5, 2589

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

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

⁶ X. He, Y. Shen, F. R. Hung and E. E. Santiso, "Heterogeneous Nucleation from a Supercooled Ionic Liquid on a Carbon Surface", J. Chem. Phys. 2016, 145, 211919