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Phase Equilibria of Imidazolium-based Ionic Liquids and Water
Rocha, Alejandra
Rocha, Alejandra
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Abstract
Ionic liquids (ILs) are widely investigated materials due to their green properties including negligible vapor pressure, chemical tunability, and high temperature stability. The presence of water in ionic liquids has been observed to affect certain IL properties. Due to the ubiquitous nature of water and the fact that all ILs are hygroscopic to an extent, studies on the sorption of water into ILs are necessary. This research describes the application of the gravimetric microbalance technique for water sorption measurements in ILs and presents reproducible and accurate solubility data for water absorption and desorption in five ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate [C2C1im][BF4], 1-ethyl-3-methylimidazolium 1,1,2,2-tetrafluoroethane-1-sulfonate [C2C1im][TFES], 1-ethyl-3-methylimidazolium acetate [C2C1im][OAc], 1-butyl-3-methylimidazolium acetate [C4C1im][OAc], and 1-butyl-3-methylimidazolium chloride [C4C1im][Cl] at 283 K – 315 K and 0 – 70 % relative humidity (RH). The water solubility in order of highest to lowest at 303.15 K and 25 % RH was: [C2C1im][OAc] (78.4 mol%) [C4C1im][OAc] (77.5 mol%) [C4C1im][Cl] (68.6 mol%) [C2C1im][TFES] (26.5 mol%) [C2C1im][BF4] (19.5 mol%). The solubility data were well correlated with the Non-Random Two Liquid (NRTL) activity coefficient model and time dependent concentration data were used to determine the binary diffusion coefficients of water in the water-IL systems. The diffusion coefficients in order of increasing relative humidity ranged from: 1.3 x 10-10 to 2.7 x 10-11 m2/s for [C2C1im][BF4], 4.4 x 10-11 to 1.2 x 10-10 m2/s for [C2C1im][TFES], 4.6 x 10-12 to 2.8 x 10-11 m2/s for [C2C1im][OAc], 8.8 x 10-12 m2/s to 3.9 x 10-11 m2/s for [C4C1im][OAc], and 4.5 x 10-12 m2/s to 2.8 x 10-11 m2/s for [C4C1im][Cl]. To analyze the difference in diffusion behavior, the diffusing radii were calculated for each water-IL system using the Stokes-Einstein relationship, and the heats of absorption were calculated for water sorption in the five ionic liquid systems using the Clausius-Clapeyron equation. The analysis concluded that as water concentration increases in the IL systems, the water-water interactions increase as more water molecules begin to hydrogen bond with each other. The analysis suggests that a few water molecules may form clusters with the [OAc] and [Cl] anions and that much larger water/BF4¯ clusters/networks may form in the [C2C1im][BF4] system which increase in size with increasing water concentration. Therefore, even though viscosity of ILs decrease with increasing water concentration, the diffusion of water in [C2C1im][OAc], [C4C1im][OAc], and [C4C1im][Cl], increase, while the diffusion in [C2C1im][BF4] decreases with increasing water concentration. Studies are also emerging on water-IL mixtures and binary IL mixtures as a functional ionic liquid design to achieve certain properties. However, ternary mixtures of ILs and water are not commonly studied. This work presents an experimental study inspired by molecular simulation predictions for the phase behavior of water addition to equimolar mixtures of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2C1im][NTf2]) + 1-ethyl-3-methylimidazolium acetate ([C2C1im][OAc]), [C2C1im][NTf2] + 1-ethyl-3-methylimidazolium chloride ([C2C1im][Cl]), and [C2C1im][OAc]+[C2C1im][Cl]. The experiments verified the liquid-liquid phase separation which occurred when water was added to [C2C1im][NTf2]:[C2C1im][OAc] and to [C2C1im][NTf2]:[C2C1im][Cl] and verified that water addition to the miscible equimolar mixture of [C2C1im][OAc]:[C2C1im][Cl] does not induce a phase change.
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Date
2018-12-31
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University of Kansas
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Keywords
Thermodynamics, gravimetric, ionic liquid, liquid-liquid equilibria, NMR, vapor-liquid equilibria, water