Experimental Densities and Calculated Fractional Free Volumes of Ionic Liquids with Tri- and Tetra-substituted Imidazolium Cations

Abstract

Although it has been estimated that there are at least 1 million ionic liquids (ILs) that are accessible using commercially available starting materials, a great portion of the ILs that have been experimentally synthesized, characterized, and studied in a variety of applications are built around the relatively simple 1-n-alkyl-3-methylimidazolium ([C n mim]) cation motif. Yet, there is no fundamental limitation or reason as to why tri-or tetra-functionalized imidazolium cations have received far less attention. Scant physical property data exist for just a few trifunctionalized imidazolium-based ILs and there is virtually no data on tetra-functionalized ILs. Thus, there are a broad experimental spaces on the "map" of ILs that are largely unexplored. We have sought to make an initial expedition into these "uncharted waters" and have synthesized imidazolium-based ILs with one more functional group(s) at the C(2), C(4), and/or C(5) positions of the imidazolium ring (as well as N(1) and N( 3)). This manuscript reports the synthesis and experimental densities of these tri-and tetra-functionalized ILs as well as calculated densities and fractional free volumes from COSMOTherm. To the best of our knowledge, this is the first report of any detailed experimental measurements or computational studies relating to ILs with substitutions at the C(4) and C(5) positions. Article pubs.acs.org/jced

Analysis of the Frequency and Diversity of 1,3-Dialkylimidazolium Ionic Liquids Appearing in the Literature

“…How many times is a new IL indexed 1, 2, 3, or n years later? What does the IL landscape look like beyond n -alkyl substituents? ILs containing ethers, ,, fluoroalkyl, − and aryl , substituents can yield very different properties and molecular-level behaviors. What is the overall use of tri-, tetra-, and penta-substituted imidazolium cations , and the underlying reasons as to why there may be limited studies related to these ILs? Is IL synthetic and property data easily accessible for researchers such that literature searches lead to judicious design of ILs and IL application? If not, what compilations of data, file formats, and metadata does the community need? What are common IL co-occurrences in the literature?…”

“…What is the overall use of tri-, tetra-, and penta-substituted imidazolium cations , and the underlying reasons as to why there may be limited studies related to these ILs?…”

“…yl, 77−79 and aryl 80,81 substituents can yield very different properties and molecular-level behaviors. • What is the overall use of tri-, tetra-, and pentasubstituted imidazolium cations 82,83 and the underlying reasons as to why there may be limited studies related to these ILs? • Is IL synthetic and property data easily accessible for researchers such that literature searches lead to judicious design of ILs and IL application?…”

Substituted Azolium Disposition: Examining the Effects of Alkyl Placement on Thermal Properties

“…For comparison, 1,3-dimethylimidazolium n-dodecylsulfonate self-organizes into an enantiotropic SmA phase upon heating and displays a considerably higher clearing point (Cr • 90 • SmA • 177 • Iso ( • C)) whereas the corresponding 1,2,3-trimethylimidazolium (Cr • 202 • Iso ( • C)) and 1,3,4,5-tetramethylimidazolium (Cr • 109 • Iso ( • C)) salts are not LC [32]. Even in the realm of ILs, limited physical property data are available for tri-, tetra-, and pentasubstituted imidazolium salts (with the additional substituents often being limited to methyl groups) [29,30,[41][42][43][44][45][46][47][48]. Beyond differences in thermal properties, highly substituted imidazolium salts may be expected to show an increased stability toward base, particularly if they are to be used in, for instance, alkaline fuel cell membranes [49].…”