Anomalous Freezing of Nano-Confined Water in Room-Temperature Ionic Liquid 1-Butyl-3-Methylimidazolium Nitrate

Abstract

Non-crystal formation of ice is investigated by simultaneous X-ray diffraction and differential scanning calorimetry measurements upon cooling to -100 °C. At room temperature, size-tunable water confinement (≈20 Å size) in a room-temperature ionic liquid (RTIL, 1-butyl-3-methylimidazolium nitrate, [C4 mim][NO3 ]) exists in a water-rich region (70-90 mol % D2 O). The confined water (water pocket) is characterized by almost monodispersive size distribution. In [C4 mim][NO3 ]-x mol % D2 O (70<x<94), the absence of sharp Bragg reflections and a distinct exothermal peak indicate that crystallization/cold crystallization both of [C4 mim][NO3 ] and D2 O is suppressed, even upon slow cooling and heating.

Keywords: anomalous freezing; crystal polymorphs; ionic liquids; nano-confined amorphous ice; nano-confinement in liquids.