We performed molecular dynamics (MD) simulations to study the structure and dynamics of the deep eutectic solvent (DES) choline iodide-glycerol (CI.G) at a molar ratio of 1:3, inside slitlike titania [rutile (110)] and graphitic nanopores of width H = 5.2 and 2.5 nm, and at a temperature T = 333 K. DESs share many of the remarkable properties of ionic liquids (ILs) while being more inexpensive; furthermore, and in addition to their fundamental scientific interest, the systems modeled here are relevant to dye-sensitized solar cells and gas separations. Our results show that glycerol can form stable hydrogen bonds with the oxygen atoms in the rutile walls, which account for ∼86% of the total number of hydrogen bonds involving DES species (choline, iodide, and glycerol) in the first layers (near the rutile walls), and for ∼24% of the total hydrogen bonds observed for the DES inside a rutile pore of width H = 5.2 nm. As a result, in these systems, the rutile walls are coated by glycerol layers that are almost depleted of ions, have a liquid structure that departs from that of the bulk DES, and have very slow dynamics. In contrast, for DES inside graphitic pores, all species are present in the first layers near the carbon walls, the local liquid structure everywhere is similar to that of a bulk DES, and the overall dynamics are faster than those observed inside rutile pores of the same pore size; however, the DES species in the center of both rutile and graphitic pores have comparable mobilities. When the pore size is reduced, a larger proportion of the hydrogen bonds involve the walls (in the case of a rutile pore), the overall dynamics of the confined DES become slower, and in general the hydrogen bonds formed are present during longer times in the simulation trajectories. These observations are in general similar to the results obtained by us for the IL [EMIM + ][TFMSI − ] inside the same rutile and graphitic pores; however, both of the IL ions are present in the layers near the walls, and the ions in the center of a rutile pore have dynamics that were 2−4 times slower than those observed for the same ions in the center of a graphitic pore.

The structure and dynamics of the ionic liquid (IL) [EMIM(+)][TFMSI(-)] inside a rutile (110) slit nanopore of width H = 5.2 nm at T = 333 K are studied using classical molecular dynamics (MD) simulations. These results are compared against those obtained in our previous study (N. N. Rajput et al., J. Phys. Chem. C, 2012, 116, 5169-5181) for the same IL inside a slit graphitic nanopore of the same width. Electrostatic and dispersion interactions are present between the IL and the rutile walls, whereas only weaker van der Waals interactions are present between the IL and the graphitic walls. Our results suggest that the strength of the interactions between the pore walls and the IL can significantly affect the structure and dynamics of the confined IL. Layering effects were more pronounced for the IL inside a rutile pore as compared to inside a graphitic pore. The ions near the rutile pore walls had a liquid structure that was significantly different from that of the bulk IL; in contrast, the same ions near graphitic pore walls had a liquid structure that was similar to that of the bulk IL. Cations and anions adopted multiple orientations near the rutile walls, which contrast with the parallel orientations that were uniformly observed for the same ions near graphitic walls. The dynamics of [EMIM(+)][TFMSI(-)] inside a slit rutile pore are significantly slower than those observed inside a slit graphitic pore. Near the rutile walls, the dynamics of the ions were about an order of magnitude slower than those of ions near graphitic walls. The ions in the center of a rutile pore exhibit enhanced mobilities, but still about 2-4 times slower than those observed for ions in the center of a graphitic pore. The effects of variations in the amount of IL on the dynamics were very marked inside a rutile pore, with reductions of up to 4 times in the mobilities of the ions in the different regions of the pore; in contrast, pore loading seems to cause smaller variations in the dynamics of ILs inside a graphitic slit nanopore.

The homogeneous nucleation of crystals of the ionic liquid [dmim(+)][Cl(-)] from its supercooled liquid phase in the bulk (P = 1 bar, T = 340 K, representing a supercooling of 58 K) was studied using molecular simulations. The string method in collective variables [Maragliano et al., J. Chem. Phys. 125, 024106 (2006)] was used in combination with Markovian milestoning with Voronoi tessellations [Maragliano et al., J. Chem. Theory Comput. 5, 2589-2594 (2009)] and order parameters for molecular crystals [E. E. Santiso and B. L. Trout, J. Chem. Phys. 134, 064109 (2011)] to sketch a minimum free energy path connecting the supercooled liquid and the monoclinic crystal phases, and to determine the free energy and the rates involved in the homogeneous nucleation process. The physical significance of the configurations found along this minimum free energy path is discussed with the help of calculations based on classical nucleation theory and with additional simulation results obtained for a larger system. Our results indicate that, at a supercooling of 58 K, the liquid has to overcome a free energy barrier of the order of 60 kcal/mol and to 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 × 10(10) 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. This study represents our first step in a series of studies aimed at understanding the nucleation and growth of crystals of organic salts near surfaces and inside nanopores.

Classical molecular dynamics simulations were used to study the nucleation of the crystal phase of the ionic liquid [dmim][Cl] from its supercooled liquid phase, both in the bulk and in contact with a graphitic surface of D = 3 nm. By combining the string method in collective variables [Maragliano et al., J. Chem. Phys. 125, 024106 (2006)], with Markovian milestoning with Voronoi tessellations [Maragliano et al., J. Chem. Theory Comput. 5, 2589-2594 (2009)] and order parameters for molecular crystals [Santiso and Trout, J. Chem. Phys. 134, 064109 (2011)], we computed minimum free energy paths, the approximate size of the critical nucleus, the free energy barrier, and the rates involved in these nucleation processes. For homogeneous nucleation, the subcooled liquid phase has to overcome a free energy barrier of ∼85 kcal/mol to form a critical nucleus of size ∼3.6 nm, which then grows into the monoclinic crystal phase. This free energy barrier becomes about 42% smaller (∼49 kcal/mol) when the subcooled liquid phase is in contact with a graphitic disk, and the critical nucleus formed is about 17% smaller (∼3.0 nm) than the one observed for homogeneous nucleation. The crystal formed in the heterogeneous nucleation scenario has a structure that is similar to that of the bulk crystal, with the exception of the layers of ions next to the graphene surface, which have larger local density and the cations lie with their imidazolium rings parallel to the graphitic surface. The critical nucleus forms near the graphene surface separated only by these layers of ions. The heterogeneous nucleation rate (∼4.8 × 10 cm s) is about one order of magnitude faster than the homogeneous rate (∼6.6 × 10 cm s). The computed free energy barriers and nucleation rates are in reasonable agreement with experimental and simulation values obtained for the homogeneous and heterogeneous nucleation of other systems (ice, urea, Lennard-Jones spheres, and oxide glasses).

Molecular modelling of ionic liquids in the ordered mesoporous carbon CMK-5" (2015). NASA Publications. 174. http://digitalcommons.unl.edu/nasapub/174 ], confined in a model CMK-5 material, which consists of amorphous carbon nanopipes (ACNPs) arranged in a hexagonal array. We compare our findings against the behaviour of the same ILs inside an isolated ACNP (i.e. no IL adsorbed on the outer surface of the ACNP) and inside a model CMK-3 material (which is similar to CMK-5, but is formed by amorphous carbon nanorods). Our results indicate that the presence of IL adsorbed in the outer surface of an uncharged ACNP in CMK-5 affects the dynamics and the density of an IL adsorbed inside the ACNP and vice versa. ILs adsorbed outside the nanopipes in CMK-5 (i.e. with IL also adsorbed inside the nanopipes) have faster dynamics and remain closer to the carbon surfaces when compared to the same ILs adsorbed on CMK-3 materials. ] moves faster when it is inside an isolated ACNP than when it is inside the ACNPs in CMK-5 (i.e. with IL adsorbed outside the nanopipes).