Development of Nanostructured Water Treatment Membranes Based on Thermotropic Liquid Crystals: Molecular Design of Sub‐Nanoporous Materials

Abstract

Supply of safe fresh water is currently one of the most important global issues. Membranes technologies are essential to treat water efficiently with low costs and energy consumption. Here, the development of self‐organized nanostructured water treatment membranes based on ionic liquid crystals composed of ammonium, imidazolium, and pyridinium moieties is reported. Membranes with preserved 1D or 3D self‐organized sub‐nanopores are obtained by photopolymerization of ionic columnar or bicontinuous cubic liquid crystals. These membranes show salt rejection ability, ion selectivity, and excellent water permeability. The relationships between the structures and the transport properties of water molecules and ionic solutes in the sub‐nanopores in the membranes are examined by molecular dynamics simulations. The results suggest that the volume of vacant space in the nanochannel greatly affects the water and ion permeability.

Transport mechanisms of water molecules and ions in sub-nano channels of nanostructured water treatment liquid-crystalline membranes: a molecular dynamics simulation study

“…We also investigated the mobility of water molecules and ions in the channel of the LC membranes by molecular dynamics (MD) simulations for a NaCl solution. 25 The simulation results qualitatively explained the experimental results for the permeability of water molecules and ions. Like our LC membranes, some LC membranes developed by other groups have also exhibited selectivity for ions and molecules.…”

“…1a). 25 Following the previous MD simulation study, 25 the simulation system including a sub-nano channel formed by the LC compounds was constructed using a simplified LC compound model from which the alkoxy chains of the nonionic moiety were eliminated (hereafter, the model is referred to as the LC monomer). For each LC compound, a rectangular parallelepiped system in which a sub-nano channel was created with 16 LC monomers and 16 counter Cl − ions (unit system, Fig.…”

“…However, these films contain a disordered structure and the nanopores or sub-nanopores formed in those membranes for the transportation of water molecules and ions are not uniform. [3][4][5][6][7][8][9][10][11][12] Hence, materials that contain nanopores or sub-nanopores with uniform sizes, such as carbon nanotubes (CNTs) [13][14][15][16][17][18][19][20] and polymerized liquid crystals, [21][22][23][24][25][26][27][28][29] have attracted a great deal of interest as candidates for efficient water treatment membranes. It is expected that the alignment of pores, which is required for high water permeability, will be easier for polymerized liquid crystals than for CNTs.…”

“…Previously, we reported the development of liquidcrystalline (LC) membranes with sub-nano channels formed by self-organization of thermotropic ionic LC compounds. 23,25 These membranes showed high performance for water treatment, including excellent water permeability, salt rejection, and ion selectivity. We also investigated the mobility of water molecules and ions in the channel of the LC membranes by molecular dynamics (MD) simulations for a NaCl solution.…”

“…Because it is difficult to analyze the transport mechanisms experimentally, MD simulations must be used. Although previous MD simulations have suggested that the mobility of water molecules and ions in sub-nano channels depends on the channel size, 25 more detailed studies are needed to elucidate the factors that determine the water treatment performance.…”