(432b) Computer Simulation of Nucleation of the Ionic Liquid [Dmim+][Cl-] in the Bulk and Near Graphitic Surfaces

Computer simulation of nucleation of the ionic liquid [dmim⁺][Cl^-] in the bulk and near graphitic surfaces

Xiaoxia He,¹ Erik E. Santiso² and Francisco R. Hung¹

¹Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803, U.S.A.
²Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695, U.S.A.

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 crystal phases, especially for ILs as most of their applications involve liquid phases. Here we studied the nucleation of crystals of the IL 1,3-dimethylimidazolium chloride, [dmim⁺][Cl^-] from its supercooled liquid phase in the bulk; we also examined systems where the liquid phase is 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. Our homogeneous nucleation results indicate that, at a supercooling of 58 K, the subcooled liquid has to overcome a free energy barrier of the order of 60-70 kcal/mol and form a critical nucleus with an average size of about 3.6 nm, before it reaches the thermodynamically stable crystal phase. A simulated homogeneous nucleation rate of 5.0 × 10¹⁰ cm^-3 s^-1 was obtained for our system, which is in reasonable agreement with experimental and simulation rates for homogeneous nucleation of ice at similar degrees of supercooling. Similar results for the nucleation of crystals of [dmim⁺][Cl^-] in contact with graphitic surfaces will be presented and discussed. This study represents our first steps in a series of studies aimed at understanding the nucleation and growth of crystals of organic salts 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 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