Calculation Of The Potential Of Mean Force Of Ionic Liquids Separating Gas And Microstructure

Ionic liquids?ILs?have served as a novel class of green solvents for gas separation due to their unique properties and extremely versatile tunability.Molecular dynamical?MD?simulation as a powerful approach can bridge the molecular design and practical application.The free energy calculation by equilibrium MD simulation is key micro indicator of thermodynamics for gas absorption and desorption process.In this work,the microscopic structure of [Emim][Tf₂N],[Emim][BF4],[Bmim][Tf₂N],[Bmim][BF4] around gas molecules CO₂/CH₄ was obtained by analyzing radial distribution functions from MD simulation.In addition,the potential of mean force?PMF?of CO₂ and CH₄ molecular pairs in the four systems were calculated respectively by weighted histogram analysis method under different umbrella sampling with MD simulation.By analyzing the difference of PMF curves,the potential well of CO₂ in [Bmim][Tf₂N] is the deepest in the four types of ILs.The indicator from PMF curves in molecular level is consistent with the solubility in experiment.The PMF indicator plays an important role in choosing proper cations and anions to design the specific ILs for gas separation.