Vapor pressure of ionic liquids at low temperatures from AC-chip-calorimetry

Abstract

The very low vapor pressure of ionic liquids is challenging to measure. At elevated temperatures the liquids might start to decompose, and at relatively low temperatures the vapor pressure becomes too low to be measured by conventional methods. In this work we developed a highly sensitive method for mass loss determination at temperatures starting from 350 K. This technique is based on an alternating current calorimeter equipped with a chip sensor that consists of a free-standing SiNx-membrane (thickness <1 μm) and a measuring area with lateral dimensions of the order of 1 mm. A small droplet (diameter ca. 600 μm) of an ionic liquid is vaporized isothermally from the chip sensor in a vacuum-chamber. The surface-to-volume-ratio of such a droplet is large and the relative mass loss due to evaporation is therefore easy to monitor by the changing heat capacity (J K(-1)) of the remaining liquid. The vapor pressure is determined from the measured mass loss rates using the Langmuir equation. The method was successfully tested for the determination of the vapor pressure and the vaporization enthalpy of an archetypical ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][NTf2]). The data set created in this way in an extremely broad temperature range from 358 K to 780 K has allowed the estimation of the boiling temperature of [EMIm][NTf2]. The value (1120 ± 50) K should be considered as the first reliable boiling point of the archetypical ionic liquid obtained from experimental vapor pressures measured in the most possible close proximity to the normal boiling temperature.

In Situ Preparation of Crosslinked Polymer Electrolytes for Lithium Ion Batteries: A Comparison of Monomer Systems

“…The current density reached a value of 0.01 mA⋅cm −2 at 5.0 V and a value of 8.6 × 10 −4 mA⋅cm −2 at 4.3 V. In the anodic region, a strong decrease of the current density was measured, reaching the minimum (8.3 × 10 −3 mA⋅cm −2 ) at 0.35 V. At -1.0 V vs. Li/Li + , the current density reached -6.0 × 10 −3 mA⋅cm −2 . Although the curve in the negative region showed a steep increase and reaction peak, the values obtained were far below these of the polymer electrolyte (PEO + 25 %mol LiTFSI) prepared according to the literature [24]. PEO electrolytes are described as unstable in voltage ranges above 4 V, except special crosslinked samples with short PEO segments [90].…”

“…Therefore, the development of safer electrolytes has been intensively pursued following different concepts that are summarized in a number of excellent reviews [ 3 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ]. The concepts included (i) replacement of moisture-sensitive salts with less sensitive conducting salts, e.g., salts with non-coordinating anions with extensive charge delocalization, such as lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) [ 9 , 15 , 16 ]; (ii) substitution of the flammable organic liquids by non-flammable ionic liquids [ 8 , 17 , 18 , 19 , 20 , 21 , 22 ] with negligible vapor pressure [ 23 , 24 ]; (iii) incorporation of conducting salts into a swollen polymer (gel electrolytes) [ 20 , 25 , 26 , 27 ]; (iv) incorporation of conducting salts into dry polymers to yield solid polymer electrolytes, often reported with poly(ethylene oxide) (PEO) as the matrix to yield solid state electrolytes (SSE) [ 28 , 29 ]; (v) substitution of salts with polymers with ionic sites (often polymeric ionic liquids, PIL) [ 7 , 8 , 12 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 ]; (vi) complete replacement of organics by Garnet-type ceramics [ 38 , 39 ]; and recently, vii) preparation of organic/inorganic hybrids with inorganic nanoparticles such as TiO 2 to boost ionic conductivity and lithium transference numbers [ 31 , …”

Key Applications and Potential Limitations of Ionic Liquid Membranes in the Gas Separation Process of CO2, CH4, N2, H2 or Mixtures of These Gases from Various Gas Streams

“…Several studies focus on finding the vapour pressure of ionic liquids. Ahrenberg, et al [ 60 ] and his research team have focused on determining the vapor pressure of ionic liquids at low temperatures using AC-chip-calorimetry. A highly sensitive method for mass loss determination at temperatures starting from 350 K was successfully developed in their study.…”

“…They found the vapour pressure from the measured rates of mass loss using the Langmuir equation. The method that they have used has successfully determined the vapour pressure and the vaporization enthalpy of an archetypical ionic liquid 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide ([EMIM][NTf 2 ]) [ 60 ]. Maton, et al [ 61 ] have mentioned in their research that the thermal stability of an IL depends on the specific combination of the cation-anion [ 61 ].…”

Insight into the Liquid–Liquid Extraction System AuCl4−/HCl/A327H+Cl− Ionic Liquid/Toluene

“…The organic extractants used in liquid-liquid extraction include ionic liquids, which have attracted further interest due to their inherent properties [13][14][15][16], including high selectivity and conductivity [17], negligible vapor pressure and low volatility [18], inflammability or low flammability [19], strong thermal stability [20], high refractive index [21], and solvation power of organic and inorganic compounds [22,23]. Due to these properties, these liquids are considered to be green solvents.…”