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Novel deep eutectic solvents (DES) based on three different hydrogen-bond donors (HBD), namely phenol, o-cresol, and 2,3-xylenol, and choline chloride (ChCl) were successfully synthesized with different mole ratios of HBD to ChCl. Melting temperature of these DES were measured. Compared with an ideal mixture of the two components, the freezing temperature of the DES depresses greatly from (120 to 127) K. The physical properties, such as density, viscosity, and conductivity of phenol-based and o-cresol-based DES were determined at atmospheric pressure and temperatures from (293.2 to 318.2) K at an interval of 5 K. The results show that the type of HBD, the mole ratio of HBD to ChCl, and temperature have great influences on the physical properties of DES. Densities and viscosities of DES formed by phenol and ChCl decrease with increases of temperature and phenol content. The conductivities of the DES are from (1.40 to 7.06) mS·cm–1, similar to that of room temperature ionic liquids. The conductivities of the DES increase with an increase of temperature, and reach the highest values at phenol to ChCl mole ratios of 4.00 to 5.00. The temperature dependence of densities and conductivities for these DES were correlated by an empirical second-order polynomial with relative deviations less than 0.91 %, and the viscosities were fitted to the VTF equation with relative deviations less than 0.52 %.

Task-specific ionic liquids (TSILs) have been experimentally demonstrated to absorb more sulfur dioxide (SO(2)) than normal ILs from gas mixtures with low SO(2) concentrations; however, the differences of SO(2) solubilities in the two kinds of ILs at given temperatures and pressures have not been studied systematically. Moreover, the mechanism of the interaction between SO(2) and ILs still remains unclear. In this work, the solubilities of SO(2) in TSILs (1,1,3,3-tetramethylguanidinium lactate and monoethanolaminium lactate) and normal ILs (1-butyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate) were determined. The solubilities of SO(2) are correlated by a modified Redlich-Kwong equation of state (RK EoS). The chemical absorption and physical absorption are differentiated, and the absorption mechanism has been proposed with the aid of the modified RK EoS. SO(2) absorption capacity in TSILs is contributed from both chemical interaction and physical interaction. Two TSIL molecules chemically absorb one SO(2) molecule, and the chemical absorption amount follows the chemical equilibrium. Normal ILs only physically absorb SO(2) following Henry's law. The chemical equilibrium constant, reaction enthalpy, Gibbs energy of reaction, reaction entropy, and Henry's law constant of SO(2) absorbed in ILs have been calculated. The present model can predict SO(2) absorption capacity for capture and SO(2) equilibrium concentration in IL for recovery.

As a kind of novel and efficient material, ionic liquids (ILs) are used for capture of acidic gases including SO2 and CO2 from flue gas. Due to very low content of acidic gases in flue gas, it is important to find functional ILs to absorb the acidic gases. However, up to now, there is no criterion to distinguish if the ILs are functional or not before use, which greatly influences the design of functional ILs. In this work, a series of ILs were synthesized and used to determine functional or normal ILs for the capture of acidic gases. It has been found that the pKa of organic acids forming the anion of ILs can be used to differentiate functional ILs from normal ILs for the capture of acidic gases from flue gas. If the pKa of an organic acid is larger than that of sulfurous acid (or carbonic acid), the ILs formed by the organic acid can be called functional ILs for SO2 (or CO2) capture, and it can have a high absorption capacity of SO2 (or CO2) with low SO2 (or CO2) concentrations. If not, the IL is just a normal IL. The pKa of organic acids can also be used to explain the absorption mechanism and guide the synthesis of functional ILs.

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