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 (Cmim/NTf) is amphiphilic whereas isoelectronic ether-functionalized 1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (COmim/NTf) 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 Cmim/NTf. Whereas both the density of Cmim/NTf and COmim/NTf change at elevated pressure, the change is more dramatic for Cmim/NTf. 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 COmim/NTf, and the intramolecular conformational structure of COmim in particular, appear to be more resilient to pressure changes than those in the isoelectronic amphiphilic analogue.