Smart ionic liquid/water mixture system with dual stimuli-response to temperature and CO₂

Stimuli-responsive materials have important applications in chemistry and chemical engineering. Here, we synthesized five different polyetheramine-fatty acids (PEFA) ionic liquids (ILs), possessing the dual stimuli-responsive ability to temperature and CO₂. These PEFA ILs have reversible lower critical solution temperature (LCST) phase behavior over a wide temperature range of 37–91 °C, and reversible heterogeneous-homogeneous phase transition towards the addition and removal of CO₂. Furthermore, the droplet size of the IL–water mixture system increased from 6.5 to 21.0 nm as the temperature increased from 25 to 56 °C, and then recovered to 6.5 nm when the temperature decreased to 25 °C. The addition and removal of CO₂ also reversibly modulated the droplet size of the system. Results from nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectra further showed that the temperature-dependent conformation of polyether amine chain in the cation dominates the temperature response, while the reversible formation of bicarbonate and fatty acids (FA) from CO₂ and anion controls the CO₂-based reversible phase transition. Molecular simulations revealed a microscopic response mechanism of the IL–water system to temperature and CO₂, and a synergistic effect between the dual stimuli of temperature and CO₂. These findings may provide a basis for the rational design and understanding of ILs-based stimuli-responsive materials and nanoreactors.