Ammonium electrolytes quench ion evaporation in colloidal propulsion

The desire to improve the specific impulse of colloidal thrusters calls for use of propellants with increasing electrical conductivities, whose electrosprays produce drops of higher and higher charge-to-mass ratio q/m. This favorable trend, however, is limited by the onset of ion evaporation from the tip of the Taylor cone, whereby the combined ejection of ions and drops reduces the propulsion efficiency. This paper studies positively charged electrosprays of formamide seeded with ammonium salts, where the relatively high solvation energy of NH4+ eliminates ion evaporation, even at the largest conductivities reached. The effects of solvent and solute volatility are studied by contrasting the behavior of volatile ammonium acetate and involatile ammonium nitrate solutes. The high conductivity and low ionic volatility of the latter lead to record mass-to-charge ratios (m/q = 0.16 g/C) and propulsion efficiencies in the pure colloidal regime.