Controlling phase separation behavior of thermo-responsive ionic liquids through the directed distribution of anionic charge

•H-bond donor/acceptor functionality alters ionic liquid/water bonding.•Cation localization influences the magnitude of anion charge distribution for thermoresponsive ILs.•The directionality and magnitude of H-bonding are not defining characteristics of LCST or UCST separation behavior.•Cation-anion stoichiometry leads to consistent phase separation behavior while lowering viscosity and increasing solubility. Thermoresponsive ionic liquids (TR-ILs) are room temperature liquid salt electrolytes with dynamic physical properties which have been hailed as potential solutions to inefficiencies in energy storage and material separations. That potential is hindered by the sensitivity of TR-IL phase separation to chemical structure. An accurate assessment of the effect of ion structure on molecular bonding is required for rational design as bonding changes translate to bulk material behavior. We systematically modify the structure of TR-ILs which exhibit either lower critical solution temperature (LCST) or upper critical solution temperature (UCST) phase separation to isolate the effect of specific types of bonding on eight tetrabutylphosphonium benzoate derived ILs through COSMO-RS sigma profile analysis and variable temperature (VT) 1H NMR. Our results reveal that in addition to Hydrogen bonding, cation conformational flexibility, functional group availability, cation–anion coordination strength and directionality of hydrogen bonds play key roles in governing the IL phase separation behavior. © 2017 Elsevier Inc. All rights reserved.