Scaled water sigma potential as a descriptor of LCST and UCST behavior in aqueous ionic liquid solutions

This study introduces the scaled sigma potential of water (σr), a new metric using the COSMO-RS (Conductor like Screening Model for Real Solvents) framework, as a descriptor of upper and lower critical solution temperature (UCST and LCST) governed phase separation in aqueous ionic liquid (IL) solutions. The maximum and minimum values of σr correspond to the homogenous and phase separated states, at a given composition, for both UCST and LCST cases. Furthermore, the value of σr changes from positive to negative as the temperature increases in LCST exhibiting solutions, while as the temperature increases in UCST exhibiting solutions, the value of σr changes from negative to positive. Therefore, σr provides a less computationally intensive alternate to Gibbs free energy calculations and to the generation of phase diagrams for assessing UCST/LCST behavior in aqueous IL solutions.