Collision-induced fragmentation of the EMI-BF4 propellant in electrospray thrusters: <i>Ab initio</i> molecular dynamics simulations

Abstract

Fragmentation of ionic liquid propellants is a major obstacle to achieving the designed performance for electrospray thrusters. This paper investigates the basic electronic properties and fragmentation processes of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4) and its constituents by performing quantum-chemistry calculations using the Q-Chem software package, which then informed collision simulations. The calculations characterized optimized molecular structures for monomers, dimers, and the neutral pair, both with and without applied electric fields. Ab initio molecular dynamics simulations of ions in the acceleration region, focusing on collisions, were then performed. Thousands of simulations were carried out to investigate the collision dynamics under electrospray-relevant field strengths, velocities, and impact parameters, emphasizing dissociation and subsequent post-collision dynamics. Across impact parameters b = 0 to 6 Å and relative velocities of 5 and 20 km/s, deflection distribution functions show a monotonic shift to higher deflection angles as b decreases. Investigations into final velocity magnitude resolve distinct incident and target populations that converge by b ≤ 2 Å, indicating strong momentum exchange in central impacts. In negative mode, peak intensities decrease approximately exponentially along BF4−→BF3→BF2→BF due to sequential fluorine loss. In positive mode, proton availability and cationic bond energetics favor EMI+→C2H5++NMI and elevate H+. Ionic fragmentation dominates at 5 km/s, whereas covalent bond cleavage is most prevalent at 20 km/s. The results clarify the fragmentation pathways and the composition of reaction products, which provide necessary inputs for multiscale assessments of thruster performance.