Modeling collision-induced fragmentation in commonly-used ionic liquid propellants for electrospray thrusters

The performance of electrospray thrusters is constrained by molecular fragmentation that broadens charge-to-mass distributions and reduces thrust per unit power. To extend understanding beyond the most studied ionic liquid ion source propellants, a comparative collision study was conducted across four ionic liquids: EMI– $$\textrm{BF}_4,$$ EMI–Im, EMI–DCA, and EMI–SCN. A high-throughput workflow using the Atomic Simulation Environment with the second-generation geometry, frequency, and noncovalent-interaction extended tight-binding (GFN2-xTB) calculator propagated 34,560 monomer impacts on neutral pair and dimer targets in two imposed speed regimes motivated by representative extraction and acceleration region conditions. The outcomes are quantified using deflection distribution functions, sina plots of the final center of mass speeds, simulated mass spectra, and opacity functions. We find that the deflection angle and product formation are strongly dependent on impact parameter $$b$$ and incident speed. Near-central encounters produce the largest deflections and the highest probability of fragment formation, while grazing interactions favor parent survival with minimal momentum exchange. The final velocity magnitude distributions transition from compact unimodal structures at small $$b$$ toward bimodal incident/target populations at large $$b$$ at low incident speeds, whereas high incident speeds yield pronounced tails attributable to light covalent fragments. Under the imposed conditions, EMI–DCA and EMI–SCN remain more parent-dominated in the simulated mass spectra, while EMI– $$\textrm{BF}_4$$ and EMI–Im exhibit richer fragmentation signatures. High speed cases also produce fast, light fragments that may be relevant to downstream plume divergence and surface interactions. These propellant trends should be interpreted within the imposed speed definitions rather than as strictly isoenergetic propellant rankings. The resulting propellant-resolved distributions provide a baseline for ionic liquid ion source design trade studies aimed at reducing collision-driven breakup through emitter geometry, emitter–extractor spacing, and operating conditions.