We employ molecular dynamics (MD) simulation and experiment to investigate the structure, thermodynamics, and transport of N-methyl-N-butylpyrrolidinium bis(trifluoromethylsufonyl)imide ([pyr14][TFSI]), N-methyl-N-propylpyrrolidinium bis(fluorosufonyl)imide ([pyr13][FSI]), and 1-ethyl-3-methylimidazolium boron tetrafluoride ([EMIM][BF4]), as a function of Li-salt mole fraction (0.05 ≤ xLi(+) ≤ 0.33) and temperature (298 K ≤ T ≤ 393 K). Structurally, Li(+) is shown to be solvated by three anion neighbors in [pyr14][TFSI] and four anion neighbors in both [pyr13][FSI] and [EMIM][BF4], and at all levels of xLi(+) we find the presence of lithium aggregates. Pulsed field gradient spin-echo NMR measurements of diffusion and electrochemical impedance spectroscopy measurements of ionic conductivity are made for the neat ionic liquids as well as 0.5 molal solutions of Li-salt in the ionic liquids. Bulk ionic liquid properties (density, diffusion, viscosity, and ionic conductivity) are obtained with MD simulations and show excellent agreement with experiment. While the diffusion exhibits a systematic decrease with increasing xLi(+), the contribution of Li(+) to ionic conductivity increases until reaching a saturation doping level of xLi(+) = 0.10. Comparatively, the Li(+) conductivity of [pyr14][TFSI] is an order of magnitude lower than that of the other liquids, which range between 0.1 and 0.3 mS/cm. Our transport results also demonstrate the necessity of long MD simulation runs (∼200 ns) to converge transport properties at room temperature. The differences in Li(+) transport are reflected in the residence times of Li(+) with the anions (τ(Li/-)), which are revealed to be much larger for [pyr14][TFSI] (up to 100 ns at the highest doping levels) than in either [EMIM][BF4] or [pyr13][FSI]. Finally, to comment on the relative kinetics of Li(+) transport in each liquid, we find that while the net motion of Li(+) with its solvation shell (vehicular) significantly contributes to net diffusion in all liquids, the importance of transport through anion exchange increases at high xLi(+) and in liquids with large anions.

Molecular dynamics simulations were performed to investigate the structural and dynamical properties of varying amounts of the ionic liquid (IL) [EMIM + ][TFMSI − ] confined inside slit-like graphitic pores of different widths, H. The ions distributed in layers inside the slit pores, with the number of layers depending on pore size. A reduction in pore loading leads to the formation of regions of high and low density of ions in the center of the pore. Variations in pore size and pore loading seem to induce only slight changes in the local liquid structure of [EMIM + ][TFMSI − ] in the different layers, as compared with the liquid structure of the bulk IL. This finding, when combined with our previous work for a different IL (Singh, R.; Monk, J.; Hung, F. R. J. Phys. Chem. C 2011, 115, 16544−16554), suggests that confinement inside slit-like nanopores may or may not induce changes in the local liquid structure depending on the specific IL. However, pore size and pore loading have a marked effect on the dynamics of confined [EMIM + ][TFMSI − ].The overall dynamics of the confined ions become faster with increasing pore size. The local dynamics of the IL are heterogeneous, with the ions exhibiting slower dynamics in the layers closer to the walls. When ρ = ρ bulk , the ions in the first layers (closest to the pore walls) and in the second layers of a pore of H = 5.2 nm have faster dynamics than those in the same layers of a pore of H = 2.5 nm; the ions in the center of a pore of H = 5.2 nm have dynamics similar to that of the bulk IL. For varying amounts of [EMIM + ][TFMSI − ] inside a pore of H = 5.2 nm, slight differences in the dynamics of the ions in the first and second layers are observed. In contrast, the dynamics of the ions in the center of the pore change markedly, with the fastest dynamics observed when ρ = 0.8ρ bulk (even faster than those of a bulk system). Marked deviations from Gaussian behavior (e.g., large secondary peaks) arise in the self-part of the van Hove correlation function with reductions in pore loading, which suggest that the local dynamics become more complex as regions of high and low density form in the center of the pore when pore loading is reduced.

We have performed molecular dynamics to study the structural and dynamical properties of the ionic liquid (IL) [BMIM+][PF6 −] confined inside multiwalled carbon nanotubes (MWCNTs) with inner diameters ranging between 2.0 and 3.7 nm. Our results indicate that the diameter of the MWCNT and the pore loading have a profound influence on the structural and dynamical properties of the confined IL. Regarding the structural properties, significant layering is observed in the mass density profiles of the cations and anions in the radial direction. The cations close to the pore walls tend to align with their imidazolium ring parallel to the surface. Regions of high and low density are observed in both the radial and the axial directions upon reduction in the pore loading. Regarding the dynamics, the confined cations move faster than the anions, in analogy to bulk systems, but the dynamics are much slower in confinement than in the bulk. The confined ions spend a larger time in the “cage regime” before finally reaching the Fickian (diffusive) regime. Our results also suggest that the cations in the center of the pore tend to move faster than those close to the pore walls; the anions exhibit a similar behavior, although the differences in dynamics are not as evident as those observed between layers of cations. Our results also suggest that, for some pore sizes, the axial mean square displacement (MSD) increases with decreasing pore filling; however, for some pore sizes, the axial MSD seems to have a nonmonotonic dependence with pore loading. Our results also suggest a nonmonotonic dependence of the axial MSD with pore size and similar pore loading. These nonmonotonic behaviors are possibly due to the presence of local variations in the axial density profile of the IL as the pore loading decreases for a given pore size. In analogy to bulk systems, there are large differences in the characteristic time scales for the translational and rotational motions of the confined cations.

Molecular dynamics simulation was used to investigate the dynamics of the ionic liquid [BMIM + ][PF 6 À ] when confined inside an uncharged slit-like graphitic pore of width H = 5.4 nm, in the temperature range 300À400 K. Our results indicate that the dynamics of the confined ions are highly heterogeneous and depend strongly on the distance of the ions with the pore walls. The ions in the center regions of the pore have dynamics and relaxation times that are very similar to those observed in a bulk IL at the same temperature. In contrast, the dynamics slow down appreciably as the ions get closer to the pore walls, and their relaxation times increase markedly. The overall diffusivities in the parallel (x, y) direction for the confined ions are found to be always smaller than the bulk diffusivities. Our results also suggest that these dynamical heterogeneities are linked to differences in structural properties. Radial distribution functions suggest that the structure of the ions in the center of the pore is similar to that observed in the bulk IL; however, significant structural differences are observed between the ions near the pore walls and those in the center of the pore.

The key to perfect radiation endurance is perfect recovery. Since surfaces are perfect sinks for defects, a porous material with a high surface to volume ratio has the potential to be extremely radiation tolerant, provided it is morphologically stable in a radiation environment. Experiments and computer simulations on nanoscale gold foams reported here show the existence of a window in the parameter space where foams are radiation tolerant. We analyze these results in terms of a model for the irradiation response that quantitatively locates such window that appears to be the consequence of the combined effect of two length scales dependent on the irradiation conditions: (i) foams with ligament diameters below a minimum value display ligament melting and breaking, together with compaction increasing with dose (this value is typically ∼5 nm for primary knock on atoms (PKA) of ∼15 keV in Au), while (ii) foams with ligament diameters above a maximum value show bulk behavior, that is, damage accumulation (few hundred nanometers for the PKA's energy and dose rate used in this study). In between these dimensions, (i.e., ∼100 nm in Au), defect migration to the ligament surface happens faster than the time between cascades, ensuring radiation resistance for a given dose-rate. We conclude that foams can be tailored to become radiation tolerant.