亚/超临界环境下气液界面性质的分子动力学模拟

Molecular Dynamics Simulation of Liquid-Vapor Interface in Sub/Supercritical Surroundings

采用OPLS-AA力场对正庚烷在亚/超临界环境下的气液界面性质进行分子动力学模拟。真空环境下,得到气液相密度、界面厚度及界面张力等性质随模拟分子数、截断半径及模拟温度的变化规律,并与实验值进行对比,验证了力场模型的合理性;在真空以及由氮气形成的亚/超临界环境下,给出流体由亚临界过渡到超临界状态时的温度分布情况,总结了气液界面性质随模拟环境的变化规律:随模拟环境由亚临界过渡到超临界,气、液相密度差及界面张力减小,界面厚度增大。在低超临界状态下,界面性质与亚临界状态尚无本质区别,仅当达到较高超临界状态时,流体才表现出明显的超临界特征。

The liquid-vapor interfacial properties of n-heptane in sub/supercritical surroundings are investigated using molecular dynamics method and based on OPLS-AA. For vacuum surroundings, the variation trend of interfacial properties changing with molecular number, cut-off radius and temperature are obtained, then compared with experimental results to verify the accuracy of OPLS-AA potential function in this simulation.For sub/supereritical surroundings,the temperature distribution and interfacial properties are investigated while ambient condition changes from vacuum to supercritical one.It is found that the density difference between the liquid and gas phases as well as the surface tension decrease when the ambient conditions transition from subcritical to supercritical, but on the contrary, the interface thickness increases in the same process. However, the fluids could not become supercritical in lower supercritical surroundings, only in a sufficiently high supercritical ambient could fluids transition to supercritical state.