Unraveling the Roles of Ionic Size and Hydrogen Bonding in Electric Field-Driven Ion Emission from Hydroxylamine Nitrate-Based Ionic Liquids

Ionic size and hydrogen bonding (HB) may play significant roles in controlling ion emission from HAN (hydroxylamine nitrate)-based ionic liquids (ILs) but have received little attention. In this paper, the ion emission behavior and mechanism in an external electric field are meticulously investigated using the molecular dynamics (MD) method and density functional theory. We find that the higher the proportion of ionic HAN in the blend of ILs, the longer the delay time of the ion start-up emission. In the positive mode, cations can evaporate directly from the surface of the studied ILs and manifest exclusively as the [EMIM]⁺ monomers within the simulation time scale, whereas in the negative mode, a variety of complicated anion clusters are emitted. As a result, the average charge-to-mass ratio of the positively charged species remarkably exceeds that of the negatively charged species. This large difference is attributed to the relatively larger size of the [EMIM]⁺ ion and the absence of substantial HB interactions between the [EMIM]⁺ ion and any other monomer, leading to diminished binding energies. Conversely, the strong HB interactions, primarily constituted by N-H--O and O-H--O hydrogen bonds, are clearly found in the [EtSO₄]^--based and HAN-based clusters. In addition, the [NO₃]^- and [EtSO₄]^- ions tend to combine with the small-sized [HA]⁺ ions to form large anion clusters rather than with the [EMIM]⁺ ions. The energy decomposition results further elucidate that the orbital interaction plays a pivotal role in the [NO₃]^- and [EtSO₄]^--based clusters. The findings clearly elucidate the experimental phenomena observed in previous studies and have implications for the formulation of multimode IL propellants.