Solution And Diffusion Process Of Water And Salts In Reverse Osmosis Membrane By Molecular Simulation

COMPASS force field was adopted and Grand Canonical Monte Carlo （GCMC）and Molecular dynamics （MD） simulation were used to discuss the solution anddiffusion process of water and salts in reverse osmosis composite membrane.Selected eight kinds of typical polymer composite reverse osmosis membrane（TMC/MPD, TMC/MMPD, TMC-HT/MMPD, HT/MMPD, CFIC/MMPD,CFIC/MPD, ICIC-HT/MPD, ICIC-IPC/MPD）, the highest concentrations inseawater the four salt molecules （NaCl, MgCl₂, CaSO₄, K₂SO₄） for the mock object.First, the paper choose molecular dynamics simulation to get the movementtrajectory of water and salts in reverse osmosis composite membrane. And theclustering analysis is used to analyze the MSD curve to obtain the diffusioncoefficient value. The simulation results indicated that the diffusion coefficient ofH2O molecules is far larger than the diffusion coefficient of salt ion;In those eightkinds of membrane, the diffusion coefficient of monovalent ions was larger thandivalent ions; The structure monomer of the membrane material also affected thediffusion coefficient of water and salts,like TMC/MMPD and CFIC/MMPD,there isone more Methyl in the structure monomer of them than TMC/MPD andCFIC/MPD,which increases the effect of a resistance, and impede the movement ofparticles; and due to the increasing, the free volume fraction change small, thusmakes its diffusion coefficient decreases.Water molecules and salt ion in polymermembrane system is constantly moving, and in most cases do small vibration, onlyin the right time, location and speed,the particles jump from a free volume hole toanother. This shows the size and distribution of the free volume of the system havean effect on the permeability of molecules.The greater the free volume within thesystem,the more hole within the system,the permeate molecules will jump a smaller distance and the diffusion will be faster.The diffusion of one component in thesolution is influenced by other components. Under the same conditions, the diffusioncoefficient of particles in simple system is larger than in complex system, for thesimultaneous existence of multiple ions in complex system hindered the diffusion ofeach ion. In addition, for the the same anion, its diffusion coefficient differedbecause of the existed different cations.Grand Canonical Monte Carlo （GCMC） is adopted to simulate the solutionprocess of H2O in eight kinds of reverse osmosis composite membrane. In theperiodic boundary condition, we can get the adsorption isotherm of H2O inmembrane,and then fit it by double adsorption model,at last the solubilitycoefficient and osmotic coefficient can be obtain. The simulation results indicatedthat double adsorption model can fit the adsorption isotherm of H2O in membranewell; When the pressure is low, the water molecules are adsorped in microporousarea of polymer quickly; As the pressure rises, microporous area tends to saturated,water molecules are adsorbed in dense area of polymer, until achieve adsorptionequilibrium gradually with a lower speed. Water molecules in different compositemembrane have different solubility coefficient and osmotic coefficient;Watermolecules occupy more position in the system when its solubility coefficient islarger.