Molecular Dynamics Simulation Study Of The Hydrodynamic Behavior And Salt Cleavage Mechanism Of Ultrathin Self-supporting Polyamide Membranes

Aromatic polyamide(PA)reverse osmosis(RO)membranes fabricated from interfacial polymerization are widely used for desalination and water treatment.The traditional PA membrane is thick,which is about a few hundred nanometers,the chemical composition of its positive and negative surface is uneven,and there are indefinite shape protrusions.The thickness of the new free-standing PA membrane is less than 10 nm,and the positive and negative surfaces are uniform and the structure is dense,which makes it an ideal membrane material.The preparation of high performance PA membranes requires a deep understanding of the molecular mechanism and salt interception mechanism of hydrodynamics in PA,but due to the limitations of experimental methods,the understanding of this aspect is not deep enough.To solve this problem,the atomic model of PA membrane was constructed by computer simulations,the influence of PA structure parameters on its performance was quantitatively and qualitatively studied,and the intrinsic water transport and salt interception mechanism of the PA membrane from the microscopic level were explored.The key research contents and results are as follows:(1)Unveiling the molecular mechanisms of thickness-dependent water dynamics in an ultrathin free-standing polyamide membraneMolecular 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?similar as 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 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.(2)Quantitively unveiling the activity-structure relationship of polyamide membrane:a molecular dynamics simulation studyThe water permeation and salt rejection of PA membranes with various thicknesses and different pore size distribution were simulated and characterized by non-equilibrium molecular dynamics(NEMD)simulations.The effect of a series of different pressures on pure water permeation was investigated.Results show that the number of permeated molecules(related to water flux)increases linearly with time at pressures of 300?1500 MPa.For water desalination,the membrane exhibits water permeability of 1.48×10^-20m²s^-1pa^-1,of the same orders of magnitude as that of the experimental PA,and completely blocks sodium and chloride ions.The separation properties for saline reveal that water permeance is negatively correlated with PA thickness.Inversely,larger pore size favors water transport due to a substantial increase of free water inside PA.Further,the formula for the influence of membrane thickness and pore size on membrane flux was summarized.Based on the relationship between PA membrane structure and performance,different molecular dynamics simulation methods were used in this paper to explore and provide some conclusions about the thickness-dependent water dynamic and structure-activity relationship of PA membrane at the molecular level,which is helpful for the design of the next generation of high-throughput PA membrane.