Connecting Structural and Transport Properties of Ionic Liquids with Cationic Oligoether Chains

Abstract

X-ray diffraction and molecular dynamics simulations were used to probe the structures of two families of ionic liquids containing oligoether tails on the cations. Imidazolium and pyrrolidinium bis(trifluoromethylsulfonyl)amide ILs with side chains ranging from 4 to 10 atoms in length, including both linear alkyl and oligo-ethylene oxide tails, were prepared. Their physical properties, such as viscosity, conductivity and thermal profile, were measured and compared for systematic trends. Consistent with earlier literature, a single ether substituent substantially decreases the viscosity of pyrrolidinium and imidazolium ILs compared to their alkyl congeners. Remarkably, as the number of ether units in the pyrrolidinium ILs increases there is hardly any increase in the viscosity, in contrast to alkylpyrrolidinium ILs where the viscosity increases steadily with chain length. Viscosities of imidazolium ether ILs increase with chain length but always remain well below their alkyl congeners. To complement the experimentally determined properties, molecular dynamics simulations were run on the two ILs with the longest ether chains. The results point to specific aspects that could be useful for researchers designing ILs for specific applications. The focus of the ionic liquid (IL) community is shifting beyond the mere measurement of physical properties and identification of trends arising from particular structural moieties. Researchers are delving deeper into the nanostructural interactions between the ions to determine the topographical landscape of the ions within the liquids that influence IL properties.1-3 Such information is valuable when tuning the properties of ILs for particular applications. The ability to tune the properties of ionic liquids for specific applications is a major factor in their allure as remarkable solvents that make it possible to do extreme chemistry without extreme conditions. It has been acknowledged that although ionic liquids have a combination of physical properties that make them attractive alternatives to traditional solvents, they have relatively high viscosities that hamper their practical application in large-scale processes. For example, in the area of electrochemical energy storage devices there is still an urgent need for improved electrolytes exhibiting properties of combustion resistance, high conductivity, and wide electrochemical windows. ILs with improved transport properties (viscosity, conductivity and diffusivity) would be perfect candidates to address this need. Structural modification of the IL cation and anion is a proven tool to dramatically alter IL properties. In particular, substituting ether functionalities for alkyl functionalities on IL cations has been shown to reduce the viscosity of ionic liquids significantly.2,4-6 In this work we examine the effect of incorporating oligoether side chains of varying lengths (1-3 repeating ethoxy units, Figure 1) on the physical properties and structural characteristics of imidazolium and pyrrolidinium NTf 2 ionic liquids in ...

Ether‐Functionalized Pyrrolidinium‐Based Room Temperature Ionic Liquids: Physicochemical Properties, Molecular Dynamics, and the Lithium Ion Coordination Environment

“…Comparing Pyr 1,4O1 TFSA and Pyr 1,1O4 TFSA, which have the same molecular weight and are structural isomers, the density was higher when the ether oxygen atom was located further from the pyrrolidinium ring. This may reflect the differences in the possible stable cation structures depending on the position of the ether oxygen atom in the side chain, [20,22,36] as will be discussed in detail later based on the results of MD calculations.…”

“…For example, in the case of ammonium cations, it has been reported that the physicochemical properties of RTILs change depending on how the two ether oxygen species are introduced [17] . In pyrrolidinium‐based RTILs, many RTILs containing ether oxygen atoms have been reported, and their resulting physicochemical properties based on the introduction of ether oxygen atoms into the side chains have been investigated using both experimental and computational approaches [16,18–23] . However, few systematic studies have been carried out into the effects of the oxygen atom position on the physicochemical properties of these RTILs [20–22] .…”

“…In pyrrolidinium‐based RTILs, many RTILs containing ether oxygen atoms have been reported, and their resulting physicochemical properties based on the introduction of ether oxygen atoms into the side chains have been investigated using both experimental and computational approaches [16,18–23] . However, few systematic studies have been carried out into the effects of the oxygen atom position on the physicochemical properties of these RTILs [20–22] . In this context, our group recently clarified the correlation between the physicochemical properties and the number of methylene groups between the ether oxygen atom in the side chain and the nitrogen atom in the pyrrolidinium ring by a combination of experiments and molecular dynamics (MD) simulations [22] .…”

Curled cation structures accelerate the dynamics of ionic liquids

“…The rigidity of this adopted nanostructure is directly linked to the measured properties, leading to slower dynamics for more persistent structures. 28 Despite the dispute in the literature about the interrelations of different transport properties in ILs, their quantification and optimisation are still highly important for further development of and technical implementation of ILs. Quantification of the inter-relationship of transport properties and the development of a better understanding of the underlying transport mechanisms require systematic investigation of ILs.…”

Mixing divalent ionic liquids: effects of charge and side-chains

“…Although many factors can influence the dynamics of ILs, the higher proportion of anions to cations in the DIL (2 to 1) seems to overcome the fluidifying effect of ether chains, common in monovalent ILs. 48 The more concentrated charges increase Coulombic interactions that often strongly affect the viscosity. As a result, interactions between ether chains and imidazolium rings 49 could be forced, thus additionally increasing the viscosity.…”