# Elucidating the Microscale Behavior and Phase Separation Kinetics of Thermally Responsive Ionic Liquid–Water Mixtures

## TL;DR

This paper studies how thermally responsive ionic liquid-water mixtures separate into two phases when heated, focusing on the microscale behavior and how it affects the overall phase separation process.

## Contribution

The paper introduces a novel investigation into the microscale colloidal behavior and its correlation with macroscale phase separation kinetics in LCST ionic liquid-water mixtures.

## Key findings

- Discontinuous microscale size distributions were observed for four different ionic liquid materials.
- Theoretical settling velocities predicted using Stokes’ law align reasonably well with experimental phase separation times.
- The study establishes a foundation for understanding phase separation behavior in LCST ionic liquids for water-energy applications.

## Abstract

Thermally responsive ionic liquids (ILs) exhibit liquid–liquid
phase separation into a water-rich (WR) and ionic-liquid-rich (ILR)
phase when heated above a lower critical solution temperature (LCST).
This phase behavior has been leveraged for applications ranging from
forward osmosis (FO) desalination, where the IL acts as a draw solute,
to refrigeration and dehumidification cycles, where the IL acts as
a liquid desiccant. While significant effort has been devoted to characterizing
the thermodynamic and thermophysical properties of LCST ILs, their
phase separation kinetics have not been investigated. In this work,
we describe the macroscale phase separation kinetics (phase separation
time) by gleaning insight into the microscale colloidal behavior of
aqueous mixtures of four different materials, P4444TFA
(tetrabutylphosphonium-2,4-trifluoroacetate), P4444DMBS
(tetrabutylphosphonium-2,4-dimethyl-benzenesulfonate), N4444Sal (tetrabutylammonium salicylate), and P4444Sal (tetrabutylphosphonium
salicylate) as a function of IL concentration at a separation temperature
of 70 °C. We report the discontinuous microscale size distributions
for each material and correlate their theoretical settling velocities
to experimental phase separation times. The results indicate that
a simple Stokes’ law model can predict the phase separation
time within reasonable accuracy. Overall, this work lays the foundation
for understanding the micro- to macroscale phase separation behavior
and kinetics of LCST ILs for various water-energy applications.