Ionic liquid polymeric gel membranes containing from 20 wt % to 80 wt % of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) in poly(vinylidene fluorideco-hexafluoropropylene) (p(VDF-HFP)) were prepared by solvent casting from a solution in acetone. The effect of the ionic liquid on the performance and properties of the membranes was discussed and compared with the neat polymer. In the presence of an excess of ionic liquid, p(VDF-HFP) membranes swell in a significant way, especially above 70°C, becoming completely soluble above 90°C. DSC analysis shows a gradual decrease of the melting point of the gel and a decrease in the overall melting enthalpy with increasing IL content, whereas the melting enthalpy normalized for the polymer fraction shows an initial drop and then a gradual increase. In the presence of the ionic liquid, the elastic modulus and break strength decrease dramatically, while the maximum deformation first increases due to higher flexibility of the plasticized polymer and then rapidly decreases above 40 wt % of IL as a consequence of the progressive decrease of the number of entanglements. X-ray studies demonstrate a reduction in the overall crystal content. The position of the strongest diffraction peak remains unaltered in all samples, suggesting that only the polymer chains crystallize and that no cocrystallization of ionic liquid and polymer takes place. Preliminary gas permeation measurements show a significant increase of the permeability in the presence of [EMIM][TFSI], especially for carbon dioxide. This suggests a potential application in gas separation membranes, for instance for natural gas treatment or for CO 2 sequestration from flue gas.

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We present molecular dynamics simulations of the air-liquid interface for three room temperature ionic liquids with a common anion: bis(trifluoromethylsulfonyl) imide ([Tf(2)N]), and imidazolium-based cations that differ in the alkyl tail length: 1-butyl-3-methylimidazolium ([C(4)mim]), 1-hexyl-3-methylimidazolium ([C(6)mim]), and 1-octyl-3-methylimidazolium ([C(8)mim]). The CHARMM type force field is used with the partial charges based on quantum calculations for isolated ion pairs. The total charge on cations and anions is around 0.9e and -0.9e, respectively, which somewhat mimics the anion to cation charge transfer and many-body effects. The surface tension at 300 K is computed using the mechanical route and its value slightly overpredicts experimental values. The air-liquid interface is analyzed using the intrinsic method of Identification of the Truly Interfacial Molecules. Structural and dynamic properties of the interfacial, sub-interfacial and central layers are determined. To describe the structure of the interface, we compute the surface roughness, number density and charge density profiles, and orientation ordering of the ions. We further determine the survival probability, normal and lateral self-diffusion coefficients, and re-orientation correlation functions to characterize the dynamics of the cations and anions in the layers. We found a significant enhancement of the cation density and preferential orientation ordering of both the cations and anions at the interface. Overall, the surface of the interfacial layer is smoother than the surface of the sub-interfacial layer and the roughness of both the interfacial and sub-interfacial layers increases with the increase of the length of the cation alkyl tail. Finally, the ions stay considerably longer in the interfacial layer than in the sub-interfacial layer and dynamics of exchange of the ions between the consecutive layers is related to the distinct diffusion and re-orientation dynamics behavior of the ions within the layers.

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Ionic liquids have attracted the attention of the industry and research community as versatile solvents with unique properties, such as ionic conductivity, low volatility, high solubility of gases and vapors, thermal stability, and the possibility to combine anions and cations to yield an almost endless list of different structures. These features open perspectives for numerous applications, such as the reaction medium for chemical synthesis, electrolytes for batteries, solvent for gas sorption processes, and also membranes for gas separation. In the search for better-performing membrane materials and membranes for gas and vapor separation, ionic liquids have been investigated extensively in the last decade and a half. This review gives a complete overview of the main developments in the field of ionic liquid membranes since their first introduction. It covers all different materials, membrane types, their preparation, pure and mixed gas transport properties, and examples of potential gas separation applications. Special systems will also be discussed, including facilitated transport membranes and mixed matrix membranes. The main strengths and weaknesses of the different membrane types will be discussed, subdividing them into supported ionic liquid membranes (SILMs), poly(ionic liquids) or polymerized ionic liquids (PILs), polymer/ionic liquid blends (physically or chemically cross-linked ‘ion-gels’), and PIL/IL blends. Since membrane processes are advancing as an energy-efficient alternative to traditional separation processes, having shown promising results for complex new separation challenges like carbon capture as well, they may be the key to developing a more sustainable future society. In this light, this review presents the state-of-the-art of ionic liquid membranes, to analyze their potential in the gas separation processes of the future.

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