Molecular Simulation Of Diffusion Process Of Boric Acid,Borate And Their Complexes In Reverse Osmosis Membranes

In this paper,molecular simulation is used to solve the key problems of transmembrane transport mechanism of trace small molecules/ions such as boric acid,borate and polyhydroxy complexes.Seven monomers,namely,TMC,MPD,HT,MMPD,CFIC,ICIC and IPC,were used to construct reverse osmosis membrane structure models,and the cross-linking optimization of these membrane structure models was carried out.The diffusion characteristics of water molecule,chloride ion,sodium ion,boric acid molecule,borate ion and sorbitol-borate complex in neutralized solution of TMC/MPD reverse osmosis polymer membrane were studied by molecular dynamics simulation.The results show that the diffusion coefficients of all particles in solution are larger than those in membrane;the diffusion coefficients of water molecules in solution and membrane are much larger than those of solute molecules and ions;the diffusion coefficients of single complex are larger than those of double complex;the larger the volume of non-charged particles and charged particles are,the smaller the diffusion coefficients are.From the trajectory diagram of each particle,it can be seen that the movement of particles in the membrane can be seen as jumping from one hole trapped in the polyamide film into another hole.The movement time of particles in the hole of the membrane is much longer than the jumping time,and the movement in the hole is more intensive.The diffusion characteristics of boric acid molecules,borate ions and their complexes with three polyhydroxy compounds(D-mannitol,N-methyl D-glucamine and D-sodium gluconate)in eight typical polymer reverse osmosis composite membranes(TMC/MPD,TMC/MMPD,TMC-HT/MMPD,HT/MMPD,CFIC/MMPD,CFIC/MPD,ICIC-HT/MPD,ICIC-IPC/MPD)were analyzed.The results show that the diffusion coefficient of boric acid molecule in ICIC-HT/MPD reverse osmosis membrane is the smallest,that of borate ion in TMC/MPD reverse osmosis membrane is the smallest,that of mono-D-mannitol-borate complex in TMC-HT/MMPD reverse osmosis membrane is the smallest,that of mono-N-methyl D-glucosamine-borate complex in HT/MMPD reverse osmosis membrane is the smallest,and that of mono-D-sodium gluconate-borate complex in ICIC-IPC/MPD reverse osmosis membrane is the smallest.According to the free volume fraction of eightmembrane models,the free volume fraction of composite membrane with CFIC monomer is larger than that of other monomer composite membranes,and the free volume fraction of composite membrane with ICIC/MPD monomer is smaller than that of other monomer composite membranes.