title: Temperature-dependent formation and transformation of mesostructures in water-ionic liquid mixtures.
abstract: The temperature-dependent formation and transformation of mesostructures in binary mixtures of the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim(+)][BF(4)(-)]) and water are characterized. Through addition of nitroxide radicals as paramagnetic spin probes, the temperature dependence of the solution structure can be assessed by using electron paramagnetic resonance (EPR) spectroscopy from typical solutes' points of view. Additionally, the phase behavior on cooling and reheating is probed by differential scanning calorimetry (DSC). Thermal hysteresis and memory effects are observed, and DSC is used to identify the crystallization and thawing of ice as the pertinent phase transition. The EPR data of the nitroxide radicals before and after freezing and thawing reveal a transformation of the mesostructures, probably triggered by the crystallization of water pools to ice. A more polar state results after thawing, thereby suggesting a rupture and dissolution of the ordered IL-rich mesostructures. If the thawed solutions are not agitated, the system relaxes very slowly, that is, at room temperature with a time constant of approximately 90 h, to its equilibrium state of mesophase-separated IL-rich and bulk-like water regions.
authors.affiliation: Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Kattnig
authors.forename: Dariush
authors.initials: D
authors.lastname: Hinderberger