Electric-field-induced ion evaporation from the ionic liquid-vacuum interface

We studied ion evaporation from a planar interface between the room-temperature ionic liquid (RTIL) and vacuum under external electric fields by using molecular dynamics (MD) simulations. We calculated the ion evaporation rate (j(e)) as a function of the electric field normal to the RTIL surface (E-n), and results under Langevin, dissipative particle dynamics (DPD), and Berendsen thermostats were compared. It was found that DPD thermostat is more suitable for simulating the ion evaporation phenomenon. We also compared results under different methods for applying the electric field (constant potential, constant charge, and constant field methods) and found that j(e)-E-n curves from the three methods agree with each other. Temporal evolution of electric fields in vacuum was further analyzed, and variations of electric fields over time were traced to the induced electric field between the evaporated ion and RTIL film. This work could guide the selection of proper methods for MD simulations of electrospray in the pure ion regime and lays the foundation to study more complex electrospray phenomena using MD simulations.