Electric-Field-Driven Ion Emission from the Free Surface of Room Temperature Ionic Liquids.

Electric-field-driven ion emission from the free surface of a planar room-temperature ionic liquid (RTIL) film was studied by using molecular dynamics simulations. We calculated ion emission rate (je) as a function of the electric field normal to the RTIL/vacuum surface (En) and found that the logarithm of je over the charge density on the surface (σ) is proportional to En1/2, in agreement with classical ion evaporation theories. The composition of emitted ions includes monomers and dimers. It was found that the monomer has to move across two barriers before emission. The fraction of dimers was found to depend on the external field and ion-ion interactions. We further performed replica exchange molecular dynamics simulations and identified different metastable states of the emitting ion near the liquid film. Our results showed that En and molecular details of ion/surface determine the rate and composition of ion emission from RTIL/vacuum surfaces. Fundamental insights revealed in this study form the basis to improve ion evaporation theories and performance of electrospray applications ranging from space propulsion to nanomanufacturing.