Jin Yunhe, Shi Yuantong, Qi Xiujuan, Huang Shi, Zhang Qinghua
Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), Mianyang 621900, P. R. China.
Sichuan Co-Innovation Center for Energetic Materials, Mianyang 621900, China.
J Phys Chem A. 2020 Apr 16;124(15):2942-2950. doi: 10.1021/acs.jpca.9b10994. Epub 2020 Apr 6.
Hypergolic ionic liquids (HILs) are a new kind of green rocket fuels, which are used as potential replacements for toxic hydrazine derivatives in liquid bipropellants. These functional HILs can react with oxidizers and release a large amount of heat in a very short time, finally leading to ignition of the propellant system. Among them, most borohydride-rich HILs were very sensitive to water, but a few special examples displayed good hydrophobicity and remained very stable in air even after a month or more. However, the reasons behind their hydrolytic stability are unclear. In this study, several calculation methods including electrostatic potentials (ESPs), molecular orbital energy gaps, and interaction energy were used to explore the water stability of eight typical borohydride-rich HILs. The obtained results demonstrated that negatively charged anions with high absolute ESP values usually reacted more easily with positively charged water. The large molecular orbital energy gap with BPB, BCNBCN, CTB, and BTB indicates the high degree of difficulty of interactions between anions and water, leading to a better hydrolytic stability of borohydride-rich anions. During the analyses of interaction energy, the relatively water-sensitive borohydride-rich anions (BH, BHCN, etc.) generally had lower interaction energy with water than stable anions such as BPB and BCNBCN. Studies on their stepwise hydrolysis mechanism demonstrate that, in the case of all the reactions, the first step is the rate-determining step and high energy barrier values of anions correspond to good hydrophobicity. This study will help us understand the hydrolysis of borohydride-rich HILs and provide a guide for the development of new HILs with promising properties.
自燃离子液体（HILs）是一种新型绿色火箭燃料，用作液体双组元推进剂中有毒肼衍生物的潜在替代品。这些功能性HILs能与氧化剂反应，并在极短时间内释放大量热量，最终导致推进剂系统点火。其中，大多数富含硼氢化物的HILs对水非常敏感，但有几个特殊例子表现出良好的疏水性，即使在一个月或更长时间后在空气中仍非常稳定。然而，它们水解稳定性背后的原因尚不清楚。在本研究中，采用了几种计算方法，包括静电势（ESPs）、分子轨道能隙和相互作用能，来探究八种典型富含硼氢化物的HILs的水稳定性。所得结果表明，具有高绝对ESP值的带负电阴离子通常更容易与带正电的水发生反应。BPB、BCNBCN、CTB和BTB的大分子轨道能隙表明阴离子与水之间相互作用的难度较大，导致富含硼氢化物的阴离子具有更好的水解稳定性。在相互作用能分析过程中，相对对水敏感的富含硼氢化物的阴离子（BH、BHCN等）与水的相互作用能通常比BPB和BCNBCN等稳定阴离子低。对其逐步水解机理的研究表明，在所有反应中，第一步是速率决定步骤，阴离子的高能垒值对应良好的疏水性。本研究将有助于我们理解富含硼氢化物HILs的水解，并为开发具有良好性能的新型HILs提供指导。
