CO_2/离子液体体系热力学性质的分子动力学模拟

Molecular Dynamics Simulation of Thermodynamic Properties for CO_2/Ionic Liquid Systems

超临界CO2和离子液体(ILs)是两种绿色溶剂.离子液体可以溶解超临界CO2,而超临界CO2不能溶解离子液体.由此设计构成的CO2/IL二元系统,同时具备了超临界CO2和离子液体的许多优点:既可以降低离子液体的粘度,还便于相分离,是新型的耦合绿色溶剂.其物理化学性质对于设计反应、分离等过程非常重要.因此,本文以CO2/IL二元系统为研究对象,通过选择合适的分子力场和系综,运用分子动力学(MD)模拟方法研究了CO2/[bmim][PF6]、CO2/[bmim][NO3]等体系的热力学性质.结果表明,CO2对ILs膨胀度的影响非常小,当CO2摩尔分数为0.5时,ILs膨胀仅为15%.CO2/ILs的扩散系数远小于CO2膨胀甲醇、乙醇溶液的扩散系数.随着CO2含量的增加,ILs的扩散系数提高,粘度显著下降,表明CO2能有效地改善ILs扩散性,减小其粘度.因此CO2可用以改善离子液体溶剂体系的传递特性,增强反应分离过程在其中的进行.

Supercritical carbon dioxide and ionic liquids（ILs） are two kinds of green solvents.Supercritical carbon dioxide can be dissolved in ionic liquids,but ionic liquids cannot be dissolved in supercritical carbon dioxide.CO2/IL binary systems,therefore,have many advantages for supercritical carbon dioxide and ILs such as decreasing the viscosity of ILs and easy phase separations.It is a new kind of coupled green solvent.Its physical chemical properties are very important for the design of reaction and separation processes.The CO2/IL binary systems（CO2/[bmim][PF6] and CO2/[bmim][NO3]） were selected as model compounds and the thermodynamic properties of these systems were simulated by molecular dynamics simulation method with available molecular force field parameters and ensembles.Results show that the ILs expanded only 15% at a CO2 molar fraction of 0.5.The diffusion coefficients of CO2/ILs are much smaller than those of CO2/methanol and CO2/ethanol systems.With the content of CO2 increasing,the diffusion coefficients of the ILs increased while their viscosities decreased significantly.These results indicate that CO2 can effectively overcome the shortcomings of ILs that have poor diffusion coefficients and high viscosities.We conclude that CO2 can improve the transport properties of ionic liquid solvent systems and enhanced the reaction and separation processes in these systems.