Molecular Simulation Of Diffusion And Solution Of Gas Molecules In Polysulfone Membranes

Molecular dynamics and Grand Canonical Monte Carlo simulation were used to discussthe process of diffusion and solution of hydrogen， ethylene，methane，oxygen and nitrogenin polysulfone membrane using Materials Studio software.So we can， from a microscopicpoint，understand the mechanism of solution-diffusion of gas molecules in membrane.Andwe can also obtain the diffusion coefficient and solubility coefficient of the gas molecules，through which the permeation coefficient and selectivity coefficient can be calculated.Fist，the process of diffusion of gas molecules was simulated through the moleculardynamics method.In the whole simulation procedure，periodic boundary condition andCOMPASS force filed were used to get the curve of mean-square displacement of gasmolecules in polysulfone. And the diffusion coefficient of gas was obtained from the MSDcurve by means of Einstein equation.The results showed that the diffusion coefficient ofhydrogen was far larger than the other gases in the same temperature， and the diffusioncoefficients of the gas molecules in polysulfone membrane decreased with the increasingdegree of polymerization. Synergistic effect existed when gas molecules diffuse inpolysulfone membrane.The diffusion coefficient of gas molecules in complex system is largerthan in simple system.In addition，the diffusion of gas molecules in polysulfone membrane isnot continuous， but small vibration combining large jump.In most cases， the gas moleculesdo small vibration in a hole of polysulfone.And only in the right time，the gas moleculesjump to another hole.Then，the process of solution of gas molecules was simulated through Grand CanonicalMonte Carlo method in298K.And in the whole simulation procedure，periodic boundarycondition and COMPASS force filed were used to get the adsorption isotherm of gasmolecules in polysolfone membrane. Then fit the adsorption isotherm by Langmuir-mode toobtain the solubility coefficient of gas molecules.The results showed that the number of gasadsorbate in polysolfone membrane increased with the increasing pressure until achievedadsorption equilibrium.In the same simulation condition，gas molecules competed in theprocess of adsorption in polysolfone membrane.The more the component of gas molecule in asimulative system，the less the solubility coefficient.