Drying methods for [emim]⁺ based ionic liquid electrospray propellants

Ionic liquids (ILs) have become popular propellants for electrospray propulsion applications over the last two decades. ILs consisting of the imidazolium-based cation [Emim]⁺ (1-ethyl-3-methylimidazolium) paired with various anions are especially popular, despite their tendency to absorb water from the ambient atmosphere. Absorbed water can lead to poor electrospray performance and can reduce shelf life in some ionic liquids; thus, quantifying and reducing water content is important. Here we quantify the drying rate of three [Emim]⁺-based IL electrospray propellants by vacuum, and by direct contact with molecular sieves. We study [Emim]⁺ paired with [Im]^-, [BF₄]^-, and [EtSO₄]^- anions and find that the final water content of dried [Emim][Im] is approximately 5-30 times lower than that of [Emim][EtSO₄] under the same drying conditions. Our results show that water content after drying ranks [Emim][Im] < [Emim][BF₄] < [Emim][EtSO₄], matching trends in the literature. We find that vacuum drying at 1.5 Torr is more effective than direct contact with 3Å molecular sieves for [Emim][Im] and [Emim][BF₄], resulting in a final water content 2-3 times lower. For [Emim][EtSO₄], vacuum drying and drying by molecular sieves were equally effective.