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

Abstract

Membranes with sub-nano channels formed by self-organization of ionic liquid-crystalline (LC) compounds have great potential as water treatment membranes. In this study, the transport mechanisms of water molecules and ions in the sub-nano channels of the LC membranes are investigated by molecular dynamics simulations for NaCl and NaNO 3 solutions. The simulation results suggest that there are different transport mechanisms for water molecules and ions; the transport of water molecules occurs by Brownian diffusion, whereas that of ions occurs by jump diffusion between particular sites in the sub-nano channel. A free-energy landscape of an ion in the channel is analyzed using a metadynamics method, which indicates distinct local minima at particular sites and supports the jump diffusion mechanism. The effects of the LC compounds' structural flexibility and the electrostatic interaction with the wall of the sub-nano channels on the permeability of water molecules and ions are also investigated by molecular dynamics simulations. Simulation results suggest both the structural flexibility and the electrostatic interaction, which are characteristics of the LC membranes, are important factors in determining the water treatment performance. The structural flexibility affects the permeability of the membrane to water molecules and the electrostatic interaction reduces the permeability to ions. 604 | Environ. Sci.: Water Res. Technol., 2020, 6, 604-611This journal is † Electronic supplementary information (ESI) available: Details of the simulation method, MD simulations of the bulk solutions, an example of the jump diffusion mechanism for Na + observed during the simulation, and the mobility of water molecules in a nonequilibrium MD simulation. See Recently, we developed liquid crystalline (LC) membranes with sub-nano channels formed by self-organization of thermotropic ionic LC compounds. The LC membranes have great potential as excellent water treat membranes. In this work, we elucidated the transport mechanisms of water molecules and ions in the LC membranes by molecular dynamics simulations. We also elucidated the effects of the structural flexibility and the electrostatic interaction, which are characteristics of the LC membranes, on the water treatment performance. The knowledge obtained in this work has great potential to realize the development of future LC membranes of which the performances to water treatment are significantly improved.

Molecular insights on confined water in the nanochannels of self-assembled ionic liquid crystal

“…1C . The initial molecular structure of subnanochannel was prepared by following the early study of similar compounds by Nada et al ( 35 ). One of the layers consists of four ion pairs.…”

“…The ammonium groups within the cation are located as a square shape in the monolayer. The distances between the N atoms of the neighboring ammonium groups are set to be 1 nm, which is slightly larger than the distance of Nada et al by taking into consideration the full-atomistic model used in this study ( 35 ). The BF 4 anions are placed by setting its center of mass at the midpoint of each adjacent pair of N atoms, and the monolayer forms a parallelogram with an angle of 60°.…”

“…Then, let us move on to the case of ILC nanochannels. Nada et al (35) computed the diffusivity and stability of water and ions inside the subnanochannel divided from thermotropic ILC. Shirts and co-workers (36)(37)(38) recently reported the molecular transport mechanism of water and organic solutes inside the 1D nanochannel of the columnar nanostructures of lyotropic ILC.…”

Liquid Crystal‐Based Water Treatment Membranes

“…[16,17] Additionally, materials such as carbon nanotubes and polymerized liquid crystals have attracted a great deal of interest for efficient water treatment membranes, and the pore alignments in these materials is generally a vital factor for the high permeability of water, thus the pore alignment can be easily achieved for polymerized liquid crystals than for carbon nanotubes. [18] Moreover, liquid crystal-based membranes exhibited strong electrostatic interactions with the water molecules because of the flexible sub-nano-channels. However, these features were not exhibited by the carbon nanotube-based membranes, which reveals that each of these features influences the water permeability and contributes significantly to water treatment performance.…”

Pathways for the formation of ice polymorphs from water predicted by a metadynamics method

“…Metadynamics (MTD) is an enhanced sampling method that increases the probability of reaching high free-energy states by adding a Gaussian bias potential to the Hamiltonian of a state 6,7 . The MTD method has been used to analyze reaction pathways [8][9][10][11][12][13][14][15] , the conformations of additives at crystal surfaces [16][17][18][19] , crystal structures [20][21][22][23][24][25] , crystal nucleation [26][27][28][29][30][31][32][33] , drug design [34][35][36][37][38] , and the transport mechanism of an ion in a sub-nanopore 39 . The MTD method affords a free-energy landscape in a space of collective variables (CVs) for a system of interest.…”