Unveiling the Molecular Mechanisms of Thickness-Dependent Water Dynamics in an Ultrathin Free-Standing Polyamide Membrane

Abstract

Aromatic polyamide (PA) membranes fabricated from interfacial polymerization are widely used for desalination and water treatment. The fabrication of the high-flux PA membrane requires a fundamental understanding of the molecular mechanisms of water dynamics in the PA, which is still obscure due to the limited experimental methods. Herein, molecular dynamics (MD) simulations were employed to establish an atomic model of ultrathin free-standing PA membranes with various thickness and to explore the thickness-dependent dynamics of water molecules in the PA membrane. Simulation results illustrate that the simulated PA membrane has an average pore radius of 3 Å similar to the free volume size of the experimental PA membrane measured by PALS. The PA could be identified as the swelling layer (SL) and the confined layer (CL) based on their water diffusion rates. The diffusivity of water in the confined layer of PA membrane was much lower than that in the swelling layer and thus determined the water flux of the PA membrane. The water diffusivity in the sub-8 nm PA membrane is greatly enhanced due to a very thin confined layer thickness, illustrating the mechanism of the experimentally fabricated sub-8 nm PA membrane having the dramatically enhanced water permeability. Furthermore, results show that water molecules tend to transport rapidly in the free space inside the PA membrane. Our results provide some insights into the thickness-dependent water dynamics in the PA on a molecular level and may help to design the next generation of high-flux PA membranes.

Elucidating the Roles of Polyamide Layer Structural Properties in the Permeability–Selectivity Tradeoff Governing Aqueous Separations

“…Overall, the analysis using the hindered transport model shows that the experimental R r −J w trends as the membranes become more permeable can be explained by a slight increase in d p and a substantial decrease in L. Because the marginal enlargement of the free volume sizes alone is not able to fully account for the markedly enhanced water permeabilities, the membrane structural property L is identified as a more significant factor governing A: shortening the effective transport pathway of the selective layer substantially elevates the membrane water permeability. This is consistent with the related studies that employed computational simulations and experimental characterizations of layer-by-layer polyamide membranes to show that water permeability is significantly influenced by the membrane thickness, 79,80 suggesting that L can be decreased by tuning the constituent elements, specifically thickness, fractional free volume, and tortuosity.…”

Planting Anion Channels in a Negatively Charged Polyamide Layer for Highly Selective Nanofiltration Separation

“…A cross-linked PA molecule was created using 12 PIP and eight TMC molecules based on energy minimization . Next, the PA molecules, TA molecules, and Fe 3+ were originally placed inside a simulation box of 8.5 × 8.5 × 6 nm 3 , followed by amplification in the canonical ensemble (NVT) at 300 K and the homogenization via six heating–annealing cycles . The resulting cross-linked pristine network was solvated to mimic the swelling of the network in an alkaline solution.…”

“…36 Next, the PA molecules, TA molecules, and Fe 3+ were originally placed inside a simulation box of 8.5 × 8.5 × 6 nm 3 , followed by amplification in the canonical ensemble (NVT) at 300 K and the homogenization via six heating− annealing cycles. 37 The resulting cross-linked pristine network was solvated to mimic the swelling of the network in an alkaline solution. Alkaline solution with or without PEI molecules was first equilibrated at the normal pressure (NPT, 298.15 K, 1 bar) in an orthogonal box of 4 nm for 2 ns to obtain a suitable density of small cells.…”

Quantitatively Unveiling the Structure–Activity Relationship of Polyamide Nanofiltration Membranes with Complex Structures

“…28 Shi et al studied the swelling behavior of PA membranes and discussed the thickness-dependent dynamics behavior of water molecules. 29,30 Moreover, the nonequilibrium molecular dynamics (NEMD) simulations were performed to study the water transport process. For example, Gao et al studied the effects of pressure on the transport behavior of water molecules and found that the pressure accelerated the transport of water molecules.…”

“…In MD simulation, the construction methods of the PA model are similar to our former work, as shown in Figure 1. 29,30 The point charge (Mulliken charge) of piperazine (PIP) and 1,3,5-benzenetricarbonyl trichloride (TMC) molecules were calculated using B3LYP functional and the 6-31+g(d,p) basis set in the density functional theory calculations by the Gaussian 09 code. 32−34 After that, the structural parameters of all molecules, such as atom type, bond length, bond angles, and dihedral angles, were represented by the GROMOS force field and PRODRG.…”