Elucidating the hydration state and driving force for LCST-type phase transitions in thermoresponsive ionic liquids and thermoresponsive ionic liquid-derived polyelectrolyte gels

Abstract

Thermoresponsive materials that exhibit a lower critical solution temperature (LCST)-type phase transition behaviour when mixed with water have been extensively studied for various applications. Understanding the mechanism of these phase transitions is crucial for optimising their functionalities. For poly(N-isopropylacrylamide) (PNIPAM), a representative thermoresponsive polymer, the phase transition mechanism is increasingly becoming understood through experimental and theoretical evaluations of its hydration state before and after the phase transition. In contrast, thermoresponsive ionic liquids (TR-ILs), which are low-formula-weight salts, have been reported to exhibit an LCST-type phase transition behaviour, separating into a water-rich phase and an IL-rich phase upon heating. Thermoresponsive ionic liquid-derived polyelectrolyte (TR-poly(IL)) gels, obtained by polymerizing TR-IL monomers, also exhibit LCST-type phase transitions, absorbing water upon cooling and deswelling upon heating. While research is progressing to elucidate the phase transition mechanisms of these TR-IL-derived materials, they are not as well understood as those of PNIPAM. In this study, we evaluated the hydration state of TR-ILs, N,N-dibutyl-N-(4-vinylbenzyl)butan-1-aminium 1-hexanesulfonate ([N4][C6]), which are low-formula-weight salts, before and after their phase transition using nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). The results suggested that dehydration occurs near the alkyl chains of both the cation and the anion of the TR-IL upon the phase transition, driven by the motion of the cation. Furthermore, DSC analysis of the polymeric TR-poly(IL) gels revealed a sharp decrease in the number of water molecules in the secondary hydration shell of the TR-IL units within the gel during the phase transition. These findings clarify that the LCST-type phase transition of ILs involves the dehydration of water molecules in the secondary hydration shell of the IL and that this process is driven by cation motion.