Polymorphs in room-temperature ionic liquids: Hierarchical structure, confined water and pressure-induced frustration

Room-temperature ionic liquids (RTILs) can extract hidden information about water. Various types of hydrogen bonds of water are detected clearly in the liquid, glass, and crystal states. In addition to hydrogen bonding in water, balancing among charge (scalar), dipole (vector), and coordination number (topology) contribute to heterogeneous structure on the multiple scales in RTILs-water systems. The water-mediated hierarchical structure in the liquid is connected to macroscopic properties such as AC impedance, pH oscillations, density, and differential scanning calorimetry trace as a function of water concentration. Nanoscale water confinement was observed inside the RTILs, and the size and distribution of confined water were tuned by water concentration and temperature. The loosely packed confinement plays an important role in the engineering of next generation novel nanoheterogeneous materials. High-pressure crystal polymorphs were identified in pure RTILs by X-ray diffraction and Raman spectroscopy. By further compression, amorphous appeared partially in the deformed crystalline.