Su Lipeng, Yao Zhaopu, He Wenyong, Yan Hao, Tian Yi, Lei Xiaoyang, Li Shuiqing
Key Laboratory of Power Station Energy Transfer Conversion and System, Ministry of Education, North China Electric Power University, Beijing 102206, China.
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China.
J Phys Chem B. 2024 Aug 29;128(34):8183-8193. doi: 10.1021/acs.jpcb.4c02942. Epub 2024 Aug 20.
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 [EtSO]-based and HAN-based clusters. In addition, the [NO] and [EtSO] 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 [NO] and [EtSO]-based clusters. The findings clearly elucidate the experimental phenomena observed in previous studies and have implications for the formulation of multimode IL propellants.
离子大小和氢键作用（HB）在控制基于硝酸羟胺（HAN）的离子液体（ILs）的离子发射方面可能发挥重要作用，但却很少受到关注。本文采用分子动力学（MD）方法和密度泛函理论，对外部电场中的离子发射行为及机理进行了细致研究。我们发现，在离子液体混合物中，离子型HAN的比例越高，离子启动发射的延迟时间就越长。在正模式下，阳离子可直接从所研究的离子液体表面蒸发，并且在模拟时间尺度内仅表现为[EMIM]单体，而在负模式下，则会发射出各种复杂的阴离子簇。因此，带正电物质的平均荷质比显著超过带负电物质的平均荷质比。这种巨大差异归因于[EMIM]离子相对较大的尺寸以及[EMIM]离子与任何其他单体之间不存在大量的HB相互作用，导致结合能降低。相反，在基于[EtSO]和基于HAN的簇中，明显发现了主要由N-H···O和O-H···O氢键构成的强HB相互作用。此外，[NO]和[EtSO]离子倾向于与小尺寸的[HA]离子结合形成大的阴离子簇，而不是与[EMIM]离子结合。能量分解结果进一步表明，轨道相互作用在基于[NO]和[EtSO]的簇中起关键作用。这些发现清楚地阐明了先前研究中观察到的实验现象，并对多模式离子液体推进剂的配方具有启示意义。
