Thermophysical Properties of Imidazolium-Based Ionic Liquids

Abstract

Ionic liquids (ILs) are salts that are liquid at low temperatures, usually including the region around room temperature. They are under intense investigation, especially as replacement solvents for reactions and separations, since they exhibit negligible vapor pressure and would not, therefore, contribute to air pollution. Clearly, basic thermophysical properties are vital for design and evaluation for these applications. We present density as a function of temperature, melting temperatures, glass-transition temperatures, decomposition temperatures, and heat capacities as a function of temperature for a series of 13 of the popular imidazolium-based ILs. The ionic liquids investigated here are 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium bromide, 1-butyl-3-methylimidazolium dicyanamide, 1-butyl-3-methylimidazolium trifluoromethanesulfonate, 1-butyl-3-methylimidazolium tris(trifluoromethylsulfonyl)methide, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide, 2,3-dimethyl-1-ethylimidazolium bis(trifluoromethylsulfonyl)imide, 2,3dimethyl-1-propylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-2,3-dimethylimidazolium tetrafluoroborate, and 1-butyl-2,3-dimethylimidazolium hexafluorophosphate. The properties follow quite reasonable trends. For instance, density decreases as the length of the alkyl chain on the cation increases. For a given cation, the density increases as the molecular weight of the anion increases for the anions studied here. Many of the ILs tend to subcool easily, forming glasses at very low temperatures rather than exhibiting crystallization or melting transitions. The thermal stability increases with increasing anion size, and heat capacities increase with temperature and increasing number of atoms in the IL.

Ionic liquids: solvents and sorbents in sample preparation

“…Very recently, Liu et al investigated the 1-butyl-3-benzylimidazolium bromide ([C 4 BnIM + ][Br − ]) IL for the extraction of Pt(IV) and Pd(II) using a LLE technique [11]. In this study, the IL was dissolved in chloroform to lower the viscosity of the extraction phase and subsequently mixed with an aqueous solution containing metal ions.…”

“…The authors discovered that an insoluble complex was formed when the bromide ions of the IL exchanged with [PtCl 6 -2 ]. For Pd(II), IR spectroscopy and single crystal X-ray diffraction studies revealed a strong dependence on the cation of the [C 4 BnIM + ][Br − ] IL for the extraction of the IL-Pd(II) complex from aqueous solution.…”

“…Following cellulose dissolution, up to 97% of the tetraoctylphosphonium acetate [P 8888 + ][OAc − ] IL phase could be recovered by a combination of phase separation and washing the swollen cellulose materials with ethanol, demonstrating the potential reusability of these ILs. Kassaye et al treated biomass with an alkaline solution and the 1-butyl-3-methylimidazolium chloride ([C 4 MIM + ][Cl − ]) IL in an effort to improve the hydrolysis of cellulose [18]. The authors found that cellulose hydrolysis was more efficient due to the removal of lignin and hemicellulose by the pretreatment step, which significantly reduced the complexity of the remaining biomass and improved glucose hydrolysis.…”

“…Fried pork sausage samples were pretreated with NaOH and a filtration step through diatomaceous earth and then subjected to DI-SDME. Mixing the [C 4 MIM + ][PF 6 − ] IL, MWCNTs, and NC increased the viscosity and ultimately the stability of the extraction solvent droplet. However, the nanoparticle concentration required optimization to prevent saturating the IL with particles and decreasing the extraction efficiency.…”

“…Zhang and Zha utilized the [C 4 MIM + ][PF 6 − ] IL as an extraction solvent in the SAW-SDME of acid green-25 from an aqueous solution [26]. Plastic sample tubing containing acid green-25 and polydimethylsiloxane (PDMS) was inserted into a droplet of the [C 4 MIM + ][PF 6 − ] IL. Application of a SAW at 30.6 dBm enabled the equilibration of the IL and the organic dye in just 2 min and resulted in nearly quantitative extraction.…”