Comparison of the Structural Response to Pressure of Ionic Liquids with Ether and Alkyl Functionalities

The response of ionic liquids to external perturbations including elevated pressure is a topic of current interest for applications such as tribology. Ionic liquids come in many classes, including those that are amphiphilic and some that are mostly polar having either cationic or anionic tails that are functionalized. The current study compares the effect of elevated pressure on two ionic liquids with different types of cationic tail. 1-Decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (C₁₀mim⁺/NTf₂^-) is amphiphilic whereas isoelectronic ether-functionalized 1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (C₇O₃mim⁺/NTf₂^-) has cationic tails that are more polar and conformationally different. We find that the response to elevated pressure for these two systems is quite unsimilar. Intramolecular conformational changes as well as changes in the structure of liquid nanodomains appear to be significantly more prominent in the case of C₁₀mim⁺/NTf₂^-. Whereas both the density of C₁₀mim⁺/NTf₂^- and C₇O₃mim⁺/NTf₂^- change at elevated pressure, the change is more dramatic for C₁₀mim⁺/NTf₂^-. The very different response for each of these two types of ionic liquids can be clearly gleaned from distribution functions in real space and the partial subcomponents of the X-ray structure function, S(q), in reciprocal space. Liquid structure in the case of C₇O₃mim⁺/NTf₂^-, and the intramolecular conformational structure of C₇O₃mim⁺ in particular, appear to be more resilient to pressure changes than those in the isoelectronic amphiphilic analogue.