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 (${\sigma }_r$), 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 ${\sigma }_r$ correspond to the homogenous and phase separated states, at a given composition, for both UCST and LCST cases. Furthermore, the value of ${\sigma }_r$ 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 ${\sigma }_r$ changes from negative to positive. Therefore, ${\sigma }_r$ 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.