Comparative Study of the High Pressure Thermophysical Properties of 1-Ethyl-3-methylimidazolium and 1,3-Diethylimidazolium Ethyl Sulfates for Use as Sustainable and Efficient Hydraulic Fluids

In this work, we discussed and compared new acoustic (speed of sound), transport (viscosity), surface (surface tension and contact angle on stainless steel and glass), and thermophysical properties with special regard for compressibilities and isobaric thermal expansion of 1,3-diethylimidazolium ethyl sulfate ([C2C2im][EtSO4]) with those of 1-ethyl-3-methylimidazolium ethyl sulfate ([C2C1im][EtSO4]), the reference mineral, synthetic, and biodegradable oils as well as hydraulic oils for use as hydraulic fluids. The refractive indices, NMR spectra, and cytotoxicities of [C2C1im][EtSO4] and [C2C2im][EtSO4] were also investigated. [C2C1im][EtSO4] and [C2C2im][EtSO4] have low isothermal compressibility and isobaric thermal expansion values, which are weakly dependent on the pressure and temperature. The viscosity, surface tension, and contact angle of [C2C2im][EtSO4] are more similar to that of commercial hydraulic oils than [C2C1im][EtSO4]. The investigated ILs have a 20 times lower toxicity on normal fibroblast from human new-born skin than that of bis(trifluoromethylsulfonyl)imide-based ILs. The presented results show that [C2C2im][EtSO4] can potentially be used as a hydraulic fluid in the same way as [C2C1im][EtSO4].