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.
This paper reports the development and testing of atomistic models of silica MCM-41 pores. Model A is a regular cylindrical pore having a constant section. Model B has a surface disorder that reproduces the morphological features of a pore obtained from an on-lattice simulation that mimics the synthesis process of MCM-41 materials. Both models are generated using a similar procedure, which consists of carving the pore out of an atomistic silica block. The differences between the two models are analyzed in terms of small angle neutron scattering spectra as well as adsorption isotherms and isosteric heat curves for Ar at 87 K and Xe at 195 K. As expected for capillary condensation in regular nanopores, the Ar and Xe adsorption/desorption cycles for model A exhibit a large hysteresis loop having a symmetrical shape, i.e., with parallel adsorption and desorption branches. The features of the adsorption isotherms for model B strongly depart from those observed for model A. Both the Ar and Xe adsorption branches for model B correspond to a quasicontinuous pore filling that involves coexistence within the pore of liquid bridges and gas nanobubbles. As in the case of model A, the Ar adsorption isotherm for model B exhibits a significant hysteresis loop; however, the shape of the loop is asymmetrical with a desorption branch much steeper than the adsorption branch. In contrast, the adsorption/desorption cycle for Xe in model B is quasicontinuous and quasireversible. Comparison with adsorption and neutron scattering experiments suggests that model B is too rough at the molecular scale but reproduces reasonably the surface disorder of real MCM-41 at larger length scales. In contrast, model A is smooth at small length scales in agreement with experiments but seems to be too ordered at larger length scales.
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