Abstract
Tunable properties prompt the development of different “tailor-made” functional ionic liquids (FILs) for specific tasks. FILs with an ether group are good solvents for many organic compounds and enzymatic reactions. However, ionic composition influences the solubility by affecting the physiochemical properties of these FILs. To address the structure effect, a series of novel FILs with a mono-ether group (ME) based on imidazole were prepared through cationic functionalization and anionic exchange reactions, and characterized by NMR, mass spectroscopy, and Thermogravimetric analysis (TGA). The effect of ionic composition (cationic structure and anions) on density, viscosity, ionic conductivity, electrochemical window, and thermal properties of these ME-FILs were systematically investigated. In general, the viscosity and heat capacity increases with the bigger cationic volume of ME-FILs; in particular, the 2-alkyl substitution of imidazolium enhances the viscosity remarkably, whereas the density and conductivity decrease on the condition of the same [NTf2]− anion; For these ME-FILs with the same cations, the density follows the order of [NTf2]− > [PF6]− > [BF4]−. The viscosity follows the order of [PF6]− > [BF4]− > [NTf2]−. Ion conductivity follows the order of [NTf2]− ≈ [BF4]− > [PF6]−. It is noted that the dynamic density has a good linear relationship with the temperature, and the slopes are the same for all ME-FILs. Furthermore, these ME-FILs have broad electrochemical windows and glass transition temperatures in addition to a cold crystallization and a melt temperature for ME-FIL7. Therefore, the cationic structure and counter anion affect the physicochemical properties of these ME-FILs together.
Highlights
During the last 20 years, ionic liquids (ILs), broadly defined as organic salts with a melting point lower than 100 ◦ C, have gained recognition as environmentally benign alternatives to volatile organic solvents and have been applied in catalysis [1,2], separation [3], material synthesis [4,5], and electrochemistry [6,7] because they possess advantageous physicochemical properties including negligible vapour pressure, non-flammability, wide liquid range, and good solvating ability for both organic and inorganic substrates [8,9]
The results show that these mono-ether group (ME)-functional ionic liquids (FILs) are all stable to at least 250 °C, while ME-FIL1 only shows the onset of decomposition at 441 °C
The imidazolium-based mono-ether functional ionic liquids (ME-FILs) were prepared according to the previously reported procedure [30,31] with little modification and their abbreviations are summarized in Scheme 1
Summary
During the last 20 years, ionic liquids (ILs), broadly defined as organic salts with a melting point lower than 100 ◦ C, have gained recognition as environmentally benign alternatives to volatile organic solvents and have been applied in catalysis [1,2], separation [3], material synthesis [4,5], and electrochemistry [6,7] because they possess advantageous physicochemical properties including negligible vapour pressure, non-flammability, wide liquid range, and good solvating ability for both organic and inorganic substrates [8,9]. The properties of ILs can be modulated by changing the cationic structure or the combination of cation and anion. This tunability has prompted the development of different “tailor-made” ILs for specific tasks, and these functionalized ILs display many often-praised properties [10,11]. Davis et al first reported a new task-specific IL consisting of an imidazolium cation to which a primary amine moiety is covalently tethered This novel IL readily and reversibly sequesters CO2 at a molar adsorption ratio of 1:2 (mol CO2 :mol IL) [12,13].
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