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.
Highlights
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
Structure of EOEOEOMmim NTf2: Radial distribution functions At the outset of the discussion of radial distribution functions (RDFs) it is important to make a distinction between the elegant simulation treatment of Shimizu et al.[1] based on the X-ray data of Russina and Triolo,[40,41] where the ether groups are connected by a methylene (CH2) group between the imidazolium nitrogen and the first ether oxygen, and our ionic liquids where an ethylene (CH2CH2) group connects the cation nitrogen and the first ether oxygen
The consequences of ether-functionality substitutions on the physical and structural properties of ionic liquids have been the objects of study for many years, this investigation has broken new ground in terms of fully characterizing the physical properties of longer oligoether ILs in both the imidazolium and pyrrolidinium families, and in X-ray scattering and molecular dynamics studies of oligoether pyrrolidinium ILs
Summary
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. Substituting ether functionalities for alkyl functionalities on IL cations has been shown to reduce the viscosity of ionic liquids significantly.[2,4,5,6] 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 NTf2 ionic liquids in comparison to the homologous series bearing alkyl side chains. X-ray scattering results Transport properties of ionic liquids are intimately linked to their nanoscale structure, and previous structural and molecular dynamics studies of imidazolium, ammonium and phosphonium ILs with pendant ether groups[1,2,28,39,40,41] have shown that their organization on the molecular scale is different than their normal alkyl chain congeners. Note that it is easy to observe aggregation of the charged red anionic and blue cationic head-group moieties, as spatially distinct from the grey EOEOEOM tails
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