Dynamic molecular ordering in multiphasic nanoconned ionogels detected with time-resolved diffusion NMR

Molecular motion in nanosized channels can be highly complicated. For example, water molecules in hydrophobic nanopores move rapidly and coherently in a chain, following the so-called single file motion 1 . Surprisingly, fast molecular motion is also observed in viscous charged fluids, such as room temperature ionic liquids (RTILs) confined in a nanoporous carbon or silica matrix 2 – 6 . The microscopic mechanism of this intriguing effect is still unclear. Here, by combining NMR diffusion experiments in different relaxation windows with ab-initio molecular dynamics simulations, we show that the imidazolium-based RTIL [BMIM] + [TCM] - , entrapped in the MCM-41 silica nanopores, exhibits a complex dynamic molecular ordering (DMO); adsorbed RTIL molecules near the pore walls orient almost vertically to the walls, while at the center of the pores anion-cation pairs diffuse collectively in a single file (SFD). Enlightening this extraordinary effect is of primary importance in designing RTIL-based composite materials with tuned electrochemical properties.

RTILs are organic salts with low melting points, usually consisting of large asymmetric organic cations, such as imidazolium and pyridinium, quaternary ammonium, or phosphonium, paired with inorganic or organic anions exhibiting a varying degree of complexity 7 . In recent years, they have attracted much attention due to their unique and tunable properties, which render them promising alternatives for a broad range of applications in lithium-ion batteries, supercapacitors, solar cells, and gas absorption [8][9][10] . Similar to other liquids, spatial restriction and low dimensionality significantly affect the RTILs properties; they alter phase transition properties 11 , wetting, layering near-surface walls 7 , and ionic mobility [2][3][4][5][6] . Notably, both experiments and theory show that when the pore-sizes of the host material approach the size of the RTIL molecules, their transport properties are sufficiently enhanced [2][3][4][5][6] . However, the molecular mechanism underlying this kind of fast self-diffusion is still obscure.
One of the most common methods to study fluid diffusion in porous systems is the pulsed-field gradient (PFG) NMR 12 . An alternative method is to use the strong stray field gradient (SFG) of a superconductive magnet instead of the PFG [13][14][15] . For unrestricted 3D diffusion, by applying a Hahn echo π/2-τ-π pulse sequence the nuclear spin echo decays according to formula 13,14 , where γ is the nuclear gyromagnetic ratio, G is the SFG, T 2 the spin-spin relaxation time, and D is the selfdiffusion coefficient. The mean square displacement along the magnetic field gradient is given by the relation 16 〈 〉 . However, in the case of diffusion in 1D narrow channels, such as CNTs, biological porins 17 , or cylindrical silica nanopores 18 , 〈 〉 , and the NMR signal attenuation in the SFG is given by relation [19][20][21] (2), where ( ) , a is the mobility factor, i.e. the 1D analog of the self-diffusion coefficient , and [ ] is the error function. In the extreme case of ultra-narrow pore-channels comparable to the size of the diffusing molecules, the so-called single file diffusion (SFD) takes place 16,22,23 , with .
The presence of SFD has been detected with both PFG and SFG methods, by varying the magnetic field gradient amplitude and recording the spin-echo decay (SED) signal. In this way, SFD of water in single-wall carbon nanotubes (CNTs) 16   To acquaint with these intriguing results, we have performed a series of CPMG experiments by varying at each step the time interval τ between the π/2 and first π pulse, as explained in Supplementary Fig. 4. In this way, the diffusion footage is encoded in successive     Fig. 1d. However, the Hahn SED follows neither an exponential τ 3 decay (i.e. according to equation (1)), nor a SFD according to equation (2), as clearly seen in Fig. 4a. Notably, an excellent fit is acquired by inverting the Hahn SED data according to equation (1)   To validate this result a 2D 1 H NMR experiment was performed at 400K. In a experiment a sequence of CPMG SEDs is acquired with the pulse sequence described in Supplementary Fig. 4, and subsequently inverted with a 2D inversion algorithm 14,15,27,28 , where a distribution of diffusion processes is considered following the τ 3 exponential decay described in equation (1). The relevant contour plot at 400K is shown in Fig. 4b. To comprehend the theoretical background of the above experimental results, free cluster static and molecular dynamic DFT calculations were performed with the ORCA software package 29,30 . Geometry optimizations showed that adsorption of the RTIL is governed by hydrogen bonding with the silanol groups of the silica surface. The non-polar butyl-"tail" of [BMIM] + appears to be repulsed from the polar surface of MCM-41, aligning almost vertically to the pore walls, as shown in Fig. 1a, and Supplementary Fig. 5. Furthermore, interaction energies and enthalpies showed that it is favorable for the IL to detach as ion pairs from the silica surface (Supplementary Table 2); this motion in pairs of ions restricts further free-tumbling in the limited pore space, as schematically shown in the upper right panel of Fig. 1a, thus favoring SFD. 12 Notably, with increasing temperature, adsorbed [BMIM] + cations gradually detach, and for T > 360K a single phase is observed as acquainted by the distributions in Fig. 5b.

Materials
The ionic liquid (IL)  Supplementary Fig. 1) confirmed the complete blockage of the MCM and SBA pores by the IL.

NMR experiments
A broadband coherent pulse NMR spectrometer was used, operating in the frequency range 5 MHz -1 GHz. The π/2 pulse length was set to 2 μs; with this setup, a slice of the sample with a thickness of 0.7 mm along the z-axis was possible to excite. The 1 H NMR effective spin-spin relaxation ( ) and self-diffusion coefficient D measurements were accomplished by combining the Hahn echo and the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences, as described in the Supplementary Fig. 4 and the relevant text.
For the vs. temperature measurements, in the temperature range 100 K to 400 K, an Oxford 1200CF continuous-flow cryostat was employed, with an accuracy of . The samples were initially cooled down to 100 K, and the NMR measurements were obtained on heating to rule out any thermal hysteresis effects.

Computational Methods
In order to qualitatively examine the Silica -RTIL system and validate the experimental results, a series of free cluster DFT simulations were conducted. Geometry optimizations and frequency analysis were performed using the ORCA suite of programs and the PBEh-3c parametrized functional 29,30,32 . For the simulation of the silica surface, a ring of six silanol tetrahedra was cropped from the cif file in ref. 33 . The cif file was also used to simulate the MCM-41 pore structure in Fig. 1a and inset in Fig. 4b. Molecular structures processing was conducted with Chemcraft software 34 .
The desorption of the IL from the silica surface was examined with the help of Ab Initio Molecular Dynamics (AIMD) simulations, using the ORCA suite of programs. The PBEh-3c parametrized functional was implemented, along with the Beredsen thermostat, and the time step was fixed at 0.5fs 32 . Geometry constraints were applied on silicon atoms in order to simulate the solid surface of the silica better.