Breaking the Coulombic Ordering: Water-Induced Structural Reorganization and Enhanced Electric-Driven Transport of Ionic Liquids Propellant

Imidazolium-based ionic liquid (IL) propellants for chemical-electric dual-mode propulsion incorporate water to balance the performance of the chemical mode by moderating catalytic combustion temperatures and preventing catalyst deactivation. However, the potential impact of water on the transport behavior of ILs within porous media under the electric mode has been largely overlooked. The microscopic structural evolution and dynamic response of [EMIM][EtSO4] and its water mixtures in alumina nanochannels are investigated by molecular dynamics simulations. The results indicate that the long-range Coulombic ordering is disrupted by water molecules and hydrophilic walls, as part of a restructuring of ILs density and charge distribution. Meanwhile, enhanced interfacial electrostatic interactions and a dense hydrogen-bond (HB) network alter the interfacial conformation of ions, leading to a significant deviation of the [EMIM]+ imidazolium ring from the parallel-to-wall orientation. Kinetic analysis reveals that ion diffusion capability enhances with increasing pore size, approaching the bulk diffusion level at a pore size of 12 nm. Under an external electric field at this scale, cations and anions exhibit plug-like and concave plug-like velocity profiles, respectively. In addition, the presence of water is associated with an increase in the electric field-driven velocity, suggesting that water may facilitate transport under confinement as a possible contributing factor. Notably, at a water content of 50 mol% (7.08 wt%), the fluid velocity surges from 0.2474 Å/ps to 0.3314 Å/ps, with [EMIM]+ showing the most significant improvement in mobility. This work provides molecular-level guidance for optimizing dual-mode propellant formulations and porous emitter transport in electric mode operation.