Thermophysical properties of 1-ethyl-3-methylimidazolium ethylsulfate + water binary mixtures between 283 and 343K: A feasible fluid for absorption refrigeration

Abstract

Absorption-refrigeration cycles have been used for more than a century, and the most frequent refrigerant-absorbent working pairs are H₂O-LiBr and NH₃-H₂O. The high coefficient of performance (COP) and affordability of these fluids still justifies their use, despite major drawbacks like the corrosion and crystallization risks, as well as limitations in terms of operating temperature and pressure for H₂O-LiBr solutions, NH₃ flammability and toxicity and use under high pressure, corrosiveness and alkalinity of its aqueous solutions, and the need of distillation solutions for NH₃-H₂O separation. Societal demands to reduce the environmental and health impact of chemicals used in industry, has led to the search for new systems for industrial applications. Recently, ILs emerged as absorbent candidates for absorption systems, due to their thermal stability, very low vapour pressure, and solvent capacity, which adds the possible use of their aqueous mixtures with low viscosity and toxicity, and significant COP values, making them excellent alternatives for new working absorbing pairs. From several ionic liquids proposed, 1-ethyl-3-methylimidazolium ethylsulfate, [C₂mim] [EtSO₄], is a good candidate, namely mixed with water. This low-toxicity ionic liquid was chosen to screen the reliability of available data on aqueous mixtures for the estimate of coefficient of performance (COP), targeting a potential application in absorption refrigeration cycles (ARC). The aqueous-rich mixtures (x_IL < 0.1) present solution viscosities compatible with pumping devices used nowadays in refrigeration systems, and ensure enhanced wetting, affording improved heat transfer by conduction.

Several mixtures of pure ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate ([C₂mim] [EtSO₄]) with water were studied over the whole composition range, between 283.15 K and 343.15 K, at 0.1 MPa. Density, speed of sound, refractive index, viscosity, surface tension, and electrical conductivity were measured experimentally and compared with literature data. The experimental results are consistent with the evolvement of solution molecular structure with increasing water content previously predicted by MD simulations, evidencing a remarkable agreement of the notable transition points, and supporting previous studies in this laboratory with other ionic liquid aqueous systems, namely [C₂mim] [CH₃SO₃] and [C₂mim] [N(CN)₂].