An Investigation For Ionic Liquid Evaporation Process By Molecular Dynamics

Evaporation is a very common physical phenomenon which widely exists in mostaspects of people’s life and industry production. The liquid-vapor coexistence system atthermodynamic equilibrium state was investigated by molecular dynamics (MD)methods from the point of microscopic view in this paper. It was found that a largenumber of reflection phenomena appear in the process of molecular collision atliquid-vapor interface while molecular behavior at liquid-vapor interface was a researchemphasis. The reflection phenomena could be divided into molecular reflection processand molecular condensation then re-evaporation process by characteristic time methodand ion distribution in liquid phase region was controlled by periodic boundarycondition. On this basis, the evaporation model which can simulate ionic liquid wasbuilt and used to research the evaporation process of elemental liquid and ionic liquid.The research shows that vapor space size, film thickness and time step length havea greater impact on process of MD while the number of system slices just has a littleimpact. It is difficult to obtain accurate simulation values in low temperature range nearthe triple point of liquid as there is relatively large fluctuation of simulation values ineach molecular dynamics step.The results of research indicate that temperature has a big impact on molecularbehavior at liquid-vapor interface. Molecules motion enhances and the number ofcollision molecules increases with temperature increasing. Mocular reflectionphenomena also become frequent accordingly. These all lead to a decrease ofevaporation coefficient. The change of the number of collision molecules is strongerthan evaporation coefficient while temperature rises, so the phase transition rate behavesas an increasing function of temperature by calculation which can be gat from thenumber of and evaporation coefficient. Ions existing in liquid have a little impact on thenumber of collision molecules while lead to evaporation coefficient reduces with theincrease of ion concentration. It is obvious that phase transition rate of ionic liquid is adecreasing function of ion concentration.The evaporation process of elemental liquid and ionic liquid was also researched byexperiment from the point of macroscopic view in this paper. The experiment resultshows that the liquid evaporation rate will gradually increase as temperature rising. Ionsexisting in the liquid will reduce evaporation rate and liquid evaporation rate will decrease with increasing of ion concentration. The different species of ions in liquidlead to a significant difference of impact on evaporation process. Experiment result is inagreement with microscopic molecular dynamics simulation result.