Ionic Liquids (ILs) are employed in various fields, for example, reaction engineering (reactions with gases, such as hydroformylation (CO, H2), hydrogenation (H2), oxidation (O2)) or separation technology (separations of gases, reactants, and high-volatility reaction products). For the basic engineering of such processes, knowledge of phase equilibria, particularly of mixtures, over a broad (p,T) range is mandatory. This contribution reports on recent experimental results from our laboratory: – for the simultaneous solubility of a binary gas mixture in a pure ionic liquid – for the solubility of a single gas in a binary liquid solvent mixture.
The term „Alternative Solvents“ often refers to ionic liquids (ILs), mainly to distinguish these substances from traditional (i.e., molecular) solvents.
In applications, any „alternative“ is required to perform better than something that is proven and tested.
To bring ILs into practical use, availability and costs are the fundamental points to be addressed.
There are two ways to proceed:
▬ tailoring a molecule, implanting the expected characteristics
advantage: optimization of the targeted performance possible
risks: outcome not always foreseeable, costly and time-intensive (“trial and error“)
▬ to target on mixtures of well-characterized ILs with molecular solvents
advantage: substance characteristics are known, less costly, less time-consuming,
mixture composition another tunable parameter
risks: the same but with less impact
For the basic engineering of technical processes, knowledge of phase equilibria, particularly of
mixtures, over a broad (p,T) range is mandatory.
This contribution reports on recent experimental results from our laboratory:
▬ the solubility of CO2 in a mixture of water and [bmim][CH3SO4]
▬ the solubility of CO2 in a mixture of methanol and [bmim][PF6]
In the last two decades, the solubility of gases in ionic liquids has attracted a lot of attention. While at the beginning most publications dealt with the solubility of a single gas in a pure ionic liquid, recently the interest starts to shift to the – from an application-oriented point of view – more challenging area of the simultaneous solubility of several gases in an ionic liquid and/or the solubility of a single gas in solvent mixtures with ionic liquids. The current state of information on experimental data (and the phenomena encountered) in such systems is summarized and discussed.
Untersuchungen zur gasinduzierten Phasentrennung bzw. zum Aussalzen mit nahekritischen Gasen wurden an verschiedenen wässrigen Lösungen (auch mit ionischem Fluid) durchgeführt. Ziel war der Nachweis und die Charakterisierung von Hochdruckmehrphasengleichgewichten, die sich für eine technische Anwendung eignen. Kriterien hierfür sind Auftreten des Flüssig/flüssig-Phasenzerfalls bei relativ niedrigem Druck, seine Existenz über einen weiten Druck- und Zusammensetzungsbereich und damit verbunden eine druckinduzierte Änderung der Zusammensetzung der koexistierenden Flüssigphasen. Die eingesetzten ionischen Fluide zeigen zwar das gewünschte Phasenverhalten, erfüllen die Kriterien aber nur teilweise. Im Vorgriff auf eine mögliche Anwendung wurde die Verteilung eines hochmolekularen, amphiprotischen Naturstoffs auf die koexistierenden Flüssigphasen untersucht und durch Einstellung des pH-Werts gezielt verändert. -------------------------------------------------------------------------------------------------------------------------------------
Investigations on gas-induced phase split, i.e., salting out by a nearcritical gas, were carried out for several aqueous solutions (including an ionic liquid). The research aimed to verify and characterize high-pressure multiphase equilibria that might be suitable for technical applications. The respective criteria are occurrence at relatively low pressures and the existence of the phase equilibrium phenomenon over a significant region of both composition and pressure, which is related to a pressure-induced significant alteration of the compositions of the coexisting liquid phases. The employed ionic liquids revealed the expected phase behavior but without full compliance of all criteria. In anticipation of an application, the partitioning of an amphiprotic, high-molecular biomolecule to the coexisting liquid phases was successfully administered via a pH variation.
Adding some salt to a homogeneous aqueous liquid solution of an organic solvent often results in a liquid–liquid phase split. However, such a phase split can also be achieved by charging such a liquid with a gas, in particular when the temperature is close to the critical temperature of that specific gas. This phenomenon is called 'salting out by a near-critical gas'. It might be applied in a high-pressure extraction process, for example, to separate and recover valuable biomolecules from aqueous phases. Using a neutral gas like, for example, ethene for pressurizing additionally allows to adjust the pH of the coexisting liquid phases and to influence the partitioning of biomolecules when they change their electric net charge with the pH of the solution. The design of such separation processes requires not only reliable information on the phase forming system, that is, the ternary system (near-critical gas + water + organic solvent), but also on the partitioning of typical solutes to the coexisting phases. The present publication reports data (from an experimental study with a static-analytical device) for the partitioning of four biomolecules, that is, L-histidine, Aspirin, cimetidine, and 4-dimethylaminoantipyrine (at nearly infinite dilution) to coexisting liquid phases of the high-pressure three-phase liquid–liquid–vapor (L1L2V) equilibrium of the ternary system (ethene + water + 2-propanol) at (293 and 333) K and pressures from about (5.5 to 17) MPa. The coexisting liquid phases are characterized by distinctly different compositions, the aqueous phase being more hydrophilic than the alkanol-rich phase. Moreover, electrolytes were additionally added to adjust the pH conditions in the liquid phases. The pH-dependent dissociation equilibrium and the related net charge of the biomolecules primarily determine the partitioning behavior: The pH effect is stronger than the impact of varying pressure or temperature. For example, a switch from basic to acidic conditions can invert the partitioning, if that switch at the same time effects a change in the net charge of the solute, for example, from an ionic to a neutral molecule (or vice versa). The ionic solute is more hydrophilic (and thus prefers the aqueous phase), whereas the neutral or zwitterionic solute is less hydrophilic (i.e., more lipophilic) and consequently prefers the propanol-rich liquid phase.
Experimental results are reported for the solubility of carbon dioxide in liquid mixtures of water and the ionic liquid 1-n-butyl-3-methylimidazolium methylsulfate ([bmim][CH3SO4]). Three (gas-free) solvent compositions were considered with mass fractions of [bmim][CH3SO4] of w' ≈ 0.15, 0.50, and 0.87 (mole fractions x' ≈ 0.013, 0.066, and 0.33). In the isothermal experimental series, the temperature was about (293, 333, and 373) K. The total pressure ranged up to about 10 MPa. The molality of carbon dioxide in the solvent mixture of (water + [bmim][CH3SO4]) (the mole fraction of carbon dioxide in the liquid) ranged up to approximately 1.64 mol·kg1 (about 0.0332). The experimental results are used to determine Henry's constant of carbon dioxide in the liquid mixtures of (water + [bmim][CH3SO4]) as well as the partial molar volume of that gas at infinite dilution in those solvent mixtures. The phase equilibrium is described by applying an extension (to solvent mixtures) of Pitzer's molality scale-based equation for the Gibbs excess energy.
The solubility of a single gas as well as the simultaneous solubility of several gases in an ionic liquid is of great interest in many areas of chemical engineering, particularly in heterogeneous catalysis. One of the most interesting features involved in the study of the simultaneous solubility of several gases in a pure ionic liquid is the difference between experimental data and predictions for the simultaneous solubility (based on experimental data for the solubility of the single gases). The well soluble gas might act as a cosolvent or as an antisolvent for a sparsely soluble second gas. New experimental results and a correlation are presented for the simultaneous solubility of hydrogen (i.e., a sparsely soluble gas) and carbon dioxide (a well soluble gas) in the ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([hmim][Tf2N]) at temperatures between 293 K and 373 K and pressures up to about 9.2 MPa. The experimental results reveal that carbon dioxide acts as a cosolvent for hydrogen in that particular ionic liquid. A correlation shows that the cosolvency effect can be modeled by considering interaction parameters between hydrogen and carbon dioxide in liquid [hmim][Tf2N]. The correlation can also be used to quantify that cosolvency effect: one mole of carbon dioxide in 1000 g of [hmim][Tf2N] increases the solubility of (small amounts of) hydrogen by about 20%.
Some homogeneous liquid mixtures of an ionic liquid and a lower alkanol (e.g., methanol) can be forced to undergo a liquidliquid phase split (resulting in a three-phase liquid–liquid–vapor (L1L2V) equilibrium) by pressurization with a gas. Such systems exhibit the phenomenon of "salting out by a nearcritical gas". That phenomenon is often observed at temperatures around the critical temperature of the gas in liquid mixtures where at least one of the liquid components is a good solvent for that gas. New experimental results for both the L1L2V equilibrium and the corresponding critical endpoint lines of the two ternary systems (carbon dioxide + methanol + 1-n-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6]) and (carbon dioxide + 1-butanol + 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [hmim][Tf2N]) at temperatures between 293 and 333 K are reported. The experiments were performed with an "analytical method". The liquid–liquid phase split exists only at pressures between a lower critical endpoint line (where both liquid phases become critical and coexist with a vapor phase) and an upper critical endpoint line (where one of the phases becomes critical with the vapor phase while the critical phases coexist with another liquid phase). The compositions of the coexisting three phases L1, L2, and V differ considerably; a high-density, ionic liquid-rich liquid phase (L1) coexists with an alkanol-rich liquid phase (L2) of lower density and a vapor phase (V) that is virtually ionic liquid-free. For both systems, it was observed that, at constant temperature, increasing pressure shifts the L1 phase towards higher contents of ionic liquid and gas and less alkanol, whereas in the L2 phase the mole fractions of ionic liquid and alkanol decrease. Higher temperatures result in a smaller difference between the pressures of both critical endpoint lines. Experimental results for both critical endpoint lines and for the compositions of the coexisting liquid phases are reported.