Molecular Dynamics Study Of The Effect Of CNT Dispersion On Desalination By Polyamide-Blended Membrane

Membrane based reverse osmosis is currently the mainstream seawater desalination technology.The development of superior membrane materials is one of the major tasks of reverse osmosis.Polyamide composite membrane(PA)is one of the most widely used membrane materials,which has the advantages of large water flux,high salt rejection and good chemical stability,but it also has some disadvantages such as low antifouling and existing trade-off restrictions between permeability and ion selectivity,etc.People usually modify the membrane to improve these problems.The development of nanotechnology has provided a new breakthrough for the solution of these problems,in which the incorporation of CNTs to polyamide active layers(CNTs/PA)has received extensive attention.However,in the process of preparation,there may be unhomogenous dispersion of CNTs,resulting in defects in the membrane,which in turn reduce the performance of the membrane.The current experimental methods can only relatively improve this situation,but cannot specifically regulate the specific dispersion morphology in the membrane,thus it is impossible to know which morphology is favorable to the improvement of membrane performance.However,the molecular dynamics(MD)method as a "virtual experiment" can be done,so MD simulation was used in this paper to explore the desalination performance of plain PA membranes,and the microstructure and desalination performance of CNTs/PA blended membranes,and to analyze the effect of the dispersion morphology of CNTs on blended membranes.Firstly,the plain PA membrane system was studied in this paper.The microstructure and dynamics of the water molecules inside and outside the membrane,and the desalination performance of the PA membrane were investigated.The results show that there exists local aggregation of water molecules inside and outside the membrane,and the location of hydrophilic functional groups in the membrane have an important effect on water transportation inside the membrane.At the same time,due to the restriction of membrane channels,water molecules mainly form two hydrogen bonds in the membrane to constitute a chain-like hydrogen bonding structure,which in turn facilitates the diffusion of water molecules in the membrane.The water molecules form a cluster of water in the membrane by hydrogen bonding.When the size of the largest cluster of water grows to 300,the channel of the membrane has been penetrated by water clusters.That is,there are water channels in the membrane that run through the entire membrane area,which would be advantageous to the transport of water molecules.In this dissertation,the salt rejection of the membrane is 100%,while water flux is consistant to the literature value.The increase of temperature increases the water flux.Then,the CNTs/PA blended membranes were simulated to investigate the effect of the dispersion morphology of CNTs on the desalination proformance of the blend membranes.In all blended membranes,the salt rejection is 100%.For water flux,the dispersion morphology of CNTs in membrane has a great influence on it.Among them,the blended membrane with the dispersion morphology in the parallel state has the highest water flux compared with pure PA membrane,which increases by 31.8%.This is mainly due to the fact that the CNTs provide a straight and fast transport channel for the water molecules,and thus the water flux of the membrane is greatly improved.By investigating the hydrogen bond characteristics inside CNTs of blended membranes,it was found that water molecules mainly form 2 and 3 hydrogen bonds,and form a quadrilateral water molecule structure.The formation of this structure is beneficial to the high-speed transport of water molecules in the CNTs,and further contribute to water flow.The results from MD simulations provide molecular insight into the water transport mechanism in PA membrane and the effect of dispersion morphology of CNTs on the desalination proformace of CNTs/PA blended membranes,which present some theoretical guidance for the preparation of PA membranes with high desalination proformance.