Poly(ionic liquid)‐Armored MXene Membrane: Interlayer Engineering for Facilitated Water Transport

Abstract Two‐dimensional (2D) MXene‐based lamellar membranes bearing interlayers of tunable hydrophilicity are promising for high‐performance water purification. The current challenge lies in how to engineer the pore wall's surface properties in the subnano‐confinement environment while ensuring its high selectivity. Herein, poly(ionic liquid)s, equipped with readily exchangeable counter anions, succeeded as a hydrophilicity modifier in addressing this issue. Lamellar membranes bearing nanochannels of tailorable hydrophilicity are constructed via assembly of poly(ionic liquid)‐armored MXene nanosheets. By shifting the interlayer galleries from being hydrophilic to more hydrophobic via simple anion exchange, the MXene membrane performs drastically better for both the permeance (by two‐fold improvement) and rejection (≈99 %). This facile method opens up a new avenue for building 2D material‐based membranes of enhancing molecular transport and sieving effect.

Nylon-66 commercial microfiltration membrane was purchased from Tianjing JinTeng Experiment Equipment Co., Ltd. Deionized (DI) water was used throughout the experiment unless particularly stated.

Synthesis of poly(ionic liquid)s Poly(1-carboxamide-3-vinylimidazolium bromide) (PIL CN Br)
1-Vinylimidazole (23.5 g, 0.25 mol) and bromoacetonitrile (30 g, 0.25 mol) were added sequentially into diethyl ether (55 mL), and the solution was stirred overnight at room temperature. The reaction mixture was then filtered off, washed three times with diethyl ether, and dried in a vacuum oven at 40 °C overnight.
1-Cyanomethyl-3-vinylimidazolium bromide (10 g), and AIBN (200 mg) were added to DMSO (100 mL) placed in a 250 mL Schlenk flask. The reaction mixture was deoxygenated three times by a freeze-pumpthaw procedure. The reaction mixture was heated at 70 °C overnight. The reaction mixture was then precipitated into a 2 L THF/acetone mixture (v/v = 3:1), filtered off, washed with THF for three times, and then dried at 70 °C using a vacuum oven. Poly(1-cyanomethyl-3-vinylimidazolium bromide) was obtained as a slightly yellow powder. The corresponding chemical structures were confirmed by 1 H-NMR spectrum in Figure S23.

Poly(1-carboxamide-3-vinylimidazolium bromide) (PIL CONH2 Br)
In a 100 mL flask, 1-vinylimidazolium (20.0 g, 0.21 mol), 2-bromoacetamide (34.86 g, 0.21 mol) and 2,6-di-tert-butyl-4-methylphenol (0.4 g, 1.815mmol) were added into 70 mL of acetone. After the mixture was stirred for 24 hours at 50 °C, the precipitate was filtered off and washed with diethyl ether and finally dried under vacuum at 40 °C overnight. 1-Carboxamide-3-vinylimidazolium bromide was obtained as a white powder. For the polymerization, 10 g of 1-carboxamide-3-vinylimidazolium bromide monomer, 200 mg of AIBN, and 100 mL of DMSO were loaded into a 250 mL flask. The mixture was deoxygenated three times by a freeze-pump-thaw procedure and finally charged with nitrogen. The reaction mixture was then placed in an oil bath at 75 °C for 24 h. When cooling down to room temperature, the reaction mixture was dropwise added to excess THF. The precipitate was filtered off, washed with excess of THF and dried at 60 °C under vacuum. Poly(1-carboxamide-3-vinylimidazolium bromide) was obtained as a white powder. The product was then characterized via 1 H-NMR spectroscopy ( Figure S24).

Poly(1-carboxymethyl-3-vinylimidazolium bromide) (PIL COOH Br)
In a 100 mL flask, 1-vinylimidazole (20.0 g, 0.21 mol) and bromoacetic acid (29.2 g, 0.21 mol) were added into 70.0 mL of acetone. After the mixture was stirred for 24 hours at room temperature, the precipitate was filtered off and washed with diethyl ether and finally dried under vacuum at 40 °C overnight. 1-Carboxymethyl-3-vinylimidazolium bromide was obtained as a white powder. For the polymerization, 10 g of 1-carboxymethyl-3-vinylimidazolium bromide monomer, 200 mg of AIBN, and 100 mL of DMSO were loaded into a 250 mL flask. The mixture was deoxygenated three times by a freeze-pump-thaw procedure and finally charged with nitrogen. The reaction mixture was then placed in an oil bath at 75 °C for 24 h. When cooling down to room temperature, the reaction mixture was dropwise added to excess THF. The precipitate was filtered off, washed with excess of THF and dried at 80 °C under vacuum. Poly(1-carboxymethyl-3-vinylimidazolium bromide) was obtained as a white powder.
The product was then characterized via 1 H-NMR spectroscopy ( Figure S25).

Poly(1-benzyl-3-vinylimidazolium bromide) (PIL Ph Br)
1-Vinylilimidazole (18.82 g, 0.20 mol) and benzyl bromide (34.2 g, 0.22 mol) were dissolved into 40 mL methanol. The reaction was conducted at room temperature for three hours and then temperature increased to 50 °C for overnight. After that, the solution was added to diethyl ether dropwise. The reaction mixture was then filtered off, washed three times with diethyl ether, and dried in a vacuum oven at 40 °C overnight.
1-Benzyl-3-vinylimidazolium bromide was obtained as a white powder. For the polymerization, 10 g of 1-benzyl-3-vinylimidazolium bromide and 200 mg of AIBN were dissolved into 100 mL of DMF and were placed into a 250 mL Schlenk flask. The reaction mixture was deoxygenated three times by a freezepump-thaw procedure. The reaction mixture was heated at 70 °C overnight. Poly(1-benzyl-3vinylimidazolium bromide) as a yellowish power was obtained via precipitating the reaction mixture into THF. The product was then characterized via 1 H-NMR spectroscopy ( Figure S26).

Synthesis of MXene
MXene dispersion was synthesized following the minimally intensive layer delamination (MILD) method as reported previously, [1] with minor modification. In detail, 1.5 g of high-purity MAX power (equivalent

PDA coating of nylon-66 substrate
Dopamine hydrochloride (2 mg/mL) was dissolved in an aqueous solution (pH = 8.5, 5 mM) containing tris(hydroxymethyl) aminomethane (THAM) and CuSO4 (5 mM). A nylon-66 substrate was immersed in the dopamine THAM buffer solution for 3 h at 40 °C . Subsequently, it was washed by DI water to remove the residual PDA solution and then stored in DI water prior to use.

Fabrication of MXene/PILs membranes
First, 140 mg of PILCNBr power was dissolved in 60 mL DI water solution, 40 mL 0.875 mg/mL MXene suspension was then added dropwise into the PILCNBr solution for a 12-h mixing at room temperature to allow the assembly of MXene nanosheets. Next, the MXene/PILCNBr suspension was obtained after removing the residential PILCNBr by three-times centrifugation (10000 rpm). Afterwards, the pristine and PIL-assembled MXene membranes with different loadings were prepared by filtering the corresponding suspensions through a polydopamine-coated porous nylon-6 substrate. In this process, a vacuum filtration (0.1 MPa) treatment was applied to assist the filtering of various MXene suspensions, and the diameter of the used Buchner funnel for filtration was 40 mm. The resultant membranes were denoted as MXene and MXene/PILCNBr. In the anion exchange step, 100 mL of aqueous solutions of LiTFSI, LiOB, or NaTPB (chemical structure in Figure S4  MXene/PILCNBr suspension after washing was freeze-dried into power, the thermo gravimetric analysis (TGA, TA Instruments Discovery TG) was then applied to detect the grafting degree of PILs to the MXene nanosheets. X-ray diffraction (XRD, Bruker, D8 Advance, America) patterns of the membrane were collected using Cu-Kα radiation at 40 kV and 40 mA in the 2θ angle range of 5° to 90°, with the scan step of 0.02°. Zeta potential and dynamic light scattering (DLS) measurements of MXene/PILCNBr suspension after washing were performed on a Zetasizer (Malvern, England). X-ray photoelectron spectroscopy (XPS, Thermo Fisher, Escalab 250XI, America) analysis was carried out using a monochromatic Al-Kα X-ray source (hυ = 1486.6 eV) operated at 150 W. Inductively coupled plasma optical emission spectrometry (ICP-OES, iCAP7200plus, China) was applied for the determination of S content. The chemical digestion of a free-standing MXene/PILCNTFSI membrane was completed in the mixture of 40 wt% HNO3 and 30 wt% H2O2 (v/v = 10/1) at 80 ºC for 1 h. Contact angle to both water and methylene iodide was measured on a Geniometer (Krüss, DSA25, Germany) at ambient conditions. The surface energy of membranes studied here was evaluated by the Owens−Wendt method, [2] based on the experimentally determined intrinsic contact angles of water (water with water = 72.8 mN/m) and methylene iodide (MI with MI = 50.8 mN/m), where the dispersive ( = 21.8 mN/m) and polar components ( = 21.8 mN/m) of water, and those of methylene iodide ( = 49.5 mN/m and = 1.3 mN/m), were taken from literature. [3]

Details of molecular dynamics simulation
Ti3C2Tx MXene nanosheet models comprised of PILCNBr or PILCNTFSI and H2O molecules stuck in MXene slabs were displayed in Figure 1a. The size of the MXene nanosheet is 6.4 × 3.1 nm 2 , and the detailed number of PIL and water molecules are summarized in Table S4. Periodic boundary condition (PBC) was applied along with the MXene slab, while an open boundary condition was employed through the slab. Nonpolarizable all-atom optimized potentials for the liquid simulation (OPLS-AA) force field [4] was applied to describe IL interactions, which have been extensively utilized in studying the structure and property of ionic liquids. The TIP3P model [5] was implemented for water. Atomic charges and force field parameters for Ti3C2Tx MXene are listed in Table S5. The short-range electrostatic interactions are calculated using Coulomb pair interactions, while long-range terms use particle-particle particle-mesh (PPPM) solver. The vdW term is computed using 12-6 Lennard-Jones potential, and the geometric mixing rule was used to model the parameters. The cut-off distance was set to 1.2 nm for both electrostatic and vdW terms.
All modeling and simulations were operated at 300 K using the large-scale atomic/molecular massively parallel simulator (LAMMPS). [6] The timestep is 2 fs. After a 10 ns simulation, the MXene/PILs system will reach the equilibrated state, 5-ns-long simulations are conducted continuously, where the mass density distribution and coordination analysis will be performed. The molecular diffusion coefficient D was calculated from the molecular trajectories of water using Einstein's definition, where at least 5 independent simulation runs were implemented to obtain the averaging results.

NF performance tests of MXene-based membranes
The separation performance of the membrane was evaluated using a lab-scale cross-flow NF filtration for the tests. The permeability of the membrane was calculated using the following equation: where V is the permeate volume (L); A is the effective membrane area (m 2 ); t is the filtration time (h). P is the operating pressure, fixed at 1 bar.
The dye rejection (R) was calculated as the following equation: where and are the dye concentrations in the feed and permeance, respectively. A UV-Vis spectrophotometer (Thermo Fisher, GENESYS 150, America) was employed to measure the dye concentration. Figure S1. HRTEM image (inset shows the SAED pattern) of pristine MXene nanosheets.