Experimental investigation of mixtures of 1-ethyl-3-methylimidazolium ethyl sulfate and ethylammonium nitrate with electrospray propulsion applications

Physical properties, performance metrics, and mass spectra of mixtures of two ionic liquids (1-ethyl-3-methylimidazolium ethyl-sulfate aka [Emim][EtSO₄] and ethylammonium nitrate aka EAN) are investigated from the standpoint of electrospray propulsive characteristics. Spectra suggest that as the [Emim][EtSO₄] mixture fraction increases, the intensity of [Emim][EtSO₄] related species in the plume also increases, while the presence of EAN decreases. Spectra also show presence of both EAN proton transfer species and species related to chemical reaction of [EtSO₄] and water. Despite the volatility of EAN, it emits stably into vacuum at low flow rates when mixed with 25% [Emim][EtSO₄]. Spectra also show species swapping between [Emim][EtSO₄] and EAN, suggesting microscopic reorganization and non-linear mixing of physical properties. Experimental measurements of surface tension, conductivity, and density for various mixture mass percentages also show non-linear mixing characteristics. Conductivity differs by over 40% from linear predicted values. Analytic predictions of propulsion performance are presented based on the nonlinear experimental property results, and compared to predictions based on linear mixing models. Results indicate non-linear mixing is important when making performance predictions, with differences of at least 40% possible. Curve fits of experimentally determined properties of the mixtures versus mixture ratio can be used to determine maximum or minimum performances based on different performance metrics (current, thrust, and specific impulse); for these ionic liquids the metrics are shown to optimize at different mixture mass percentages, over a 2% range; this range will increase when mixing liquids with more disparate densities.