Effects of Ether Bonds on Liquid–Liquid Transitions in Quaternary Ammonium and Phosphonium Ionic Liquids under High Pressure

Synchrotron small- and wide-angle X-ray scattering and Raman spectroscopy were used to investigate the high-pressure (HP) liquid–liquid transitions (LLTs) in quaternary ammonium and phosphonium ionic liquids (ILs) at room temperature. Bis(trifluoromethylsulfonyl)imide ([TFSI] − ) was used as the anion, and triethyloctylammonium, (2-ethoxyethoxy)ethyltriethylammonium ([N 222(2O2O2) ] + ), triethyloctylphosphonium, and (2-ethoxyethoxy)ethyltriethylphosphonium ([P 222(2O2O2 )] + ) were used as cations. In diether-substituted [N 222(2O2O2) ][TFSI] and [P 222(2O2O2) ][TFSI], the polar/nonpolar nanodomains were not formed due to the flexible and curled alkyl chains. Despite no nanodomain, HP LLT occurred with changing the local structures. A series of quaternary ammonium and phosphonium ILs exhibited entirely different HP behavior, which implies that the ether bonds of the cations have an intrinsic property.