Dynamic molecular ordering in multiphasic nanoconfined ionic liquids detected with time-resolved diffusion NMR

Abstract

Molecular motion in nanosized channels can be highly complicated. For example, water molecules in ultranarrow hydrophobic nanopores move rapidly and coherently in a single file, whereas by increasing the pore size they organize into coaxial tubes, displaying stratified diffusion. Interestingly, an analogous complex motion is predicted in viscous charged fluids, such as room temperature ionic liquids (RTILs) confined in nanoporous carbon or silica; however, experimental evidence is still pending. Here, by combining 1H NMR diffusion experiments in different relaxation windows with molecular dynamics simulations, we show that the imidazolium-based RTIL [BMIM]+[TCM]−, entrapped in the MCM-41 silica nanopores, exhibits an intricate dynamic molecular ordering; adsorbed RTIL molecules form a fluctuating charged layer near the pore walls, while in the bulk pore space they diffuse discretely in coaxial tubular shells, with molecular mean square displacement following a nearly ∼τ0.5 time dependence, characteristic of single file diffusion.

Investigation of Dynamic Behavior of Confined Ionic Liquid [BMIM]+[TCM]− in Silica Material SBA-15 Using NMR

“…Furthermore, 1 H NMR spin-spin relaxation time measurements in the low-temperature regime of region II (inset of Figure 2 and ref. [ 32 ]) appear to be temperature independent. This is because, at low temperatures, all the translational motions of the IL cations are frozen out.…”

“…2D NMR diffusion-relaxation D - T 2eff experiments were conducted in the fringe field of a 4.7 T superconducting magnet providing a 34.7 T/m linear magnetic field gradient (the details of the pulse sequence can be found in Refs. [ 15 , 32 , 64 ]).…”

“…Under this context, a great number of experimental and theoretical efforts have been employed in order to examine guest–host molecular systems [ 15 , 16 , 17 ], including structural relaxation times and transport coefficients of confined glass-forming materials [ 18 ]. From the experimental point of view, the dynamical properties of many ILs have been investigated with several techniques, such as dielectric spectroscopy [ 19 , 20 ], neutron scattering [ 21 , 22 , 23 , 24 ], light scattering [ 19 ], and NMR spectroscopy [ 17 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ]. Theoretical studies using Molecular Dynamics (MD) simulations also reveal complex phase transitions of nano-confined ILs [ 32 , 33 , 34 ].…”

“…From the experimental point of view, the dynamical properties of many ILs have been investigated with several techniques, such as dielectric spectroscopy [ 19 , 20 ], neutron scattering [ 21 , 22 , 23 , 24 ], light scattering [ 19 ], and NMR spectroscopy [ 17 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ]. Theoretical studies using Molecular Dynamics (MD) simulations also reveal complex phase transitions of nano-confined ILs [ 32 , 33 , 34 ]. We refer readers to Ref.…”

“…In a recent publication [32], we combined MD simulations with 1 H NMR and showed that [BMIM] + [TCM] − entrapped into the narrow MCM-41 silica nanopores exhibited a complex dynamic ordering and stratified diffusion close to single file diffusion in the liquid state high-temperature regime. This effect was not observed in the wider SBA-15 nanoporous silica, where almost normal diffusion was detected.…”