Design of a teaching experiment for the nanofiltration mechanism of porous organic cage membrane based on molecular simulation
[Objective]Computer molecular simulation is an indispensable method widely used in scientific research.The teaching experiment designed in this work aims to simulate the nanofiltration mechanism of porous organic cage(POC)membranes based on nonequilibrium molecular dynamics simulation.Both solvent permeability and solute rejection through the POC membrane are predicted.The key factors that dominate the permeation of solvents are discussed.Importing molecular simulation studies into teaching experiments can help students understand the experimental phenomena theoretically and can broaden students'views of scientific research methods.[Methods]In this course,students are guided to understand the process of molecular simulation through five teaching sections:background investigation,model construction,algorithm setting,data processing,and theoretical analysis.For model construction,CC3 was constructed as an example of POC.The CC3 membrane was located in the middle of a box and sandwiched by two solvent boxes,named the feed side and permeate side.Three solvents,namely,water,methanol,and acetonitrile,were considered.Methylene blue(MB)molecules were added as solutes.Two grapheme plates were set on each side of the box,respectively,and acted as pistons.The pressure difference between the feed side and the permeate side was set as 600 bar.A nonequilibrium molecular dynamics simulation was performed to calculate the nanofiltration process.The solvent permeabilities and solute rejections through the POC membrane were calculated by tracing the net flows of molecules.[Results]The results revealed that the permeabilities of the solvents through the CC3 membrane followed the trend water＞methanol＞acetonitrile.As the size of MB(1.40 nm)was distinctly larger than the aperture size of CC3(0.58 nm),the rejections of MB were observed as 100％.To theoretically illustrate the solvent permeation,the interaction energies between the solvents and the CC3 membrane were discussed.The trend of interaction energies followed water＜methanol＜acetonitrile,which was in contrast with that of solvent permeabilities.This indicated that the strong interaction between solvent and membrane could slow down the diffusion of solvents in membranes.To quantitatively understand the pathway of the solvents through the CC3 membrane,the radial distribution functions of the solvents around specific atoms in CC3 were analyzed.Results showed that the solvents accumulated around the atoms in the CC3 cavity,and no solvent was observed around the atoms outside the cavity.On this basis,we could infer that the channels for solvents to pass through were formed by the pore structures of the CC3 cage and the window-to-window structures between CC3 cages.[Conclusions]On the basis of the simulation results,we could summarize that all three solvents could pass through the CC3 membrane,with water having the highest permeability.The interactions between the solvents and the CC3 membrane played a dominant role in determining solvent permeation.A stronger interaction energy resulted in a slower permeation.The window-to-window structure between CC3 cages helped solvents move from one cage to another.The rejection of MB molecules was mainly governed by the size sieving.Overall,this teaching experiment introduced advanced materials into the class on the separation process,resulting in a good combination of traditional courses and academic foreland and improving the theoretical knowledge and scientific literacy of students.
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