Room-temperature ionic liquids (RTILs) constitute a new class of liquids that attract much attention because of their characteristic properties and potential utilities as functional liquids. Unique properties are remarkably manifested in their thermal behaviors such as low melting point despite being salts, hardness in crystallization, premelting over a wide temperature range, excessive supercooling, and the existence of complex thermal histories. In addition, we found peculiar thermal behaviours at the phase changes of imidazolium-based RTILs, namely ‘rhythmic melting and crystallization’, ‘intermittent crystallization’ in the premelting regions and ultraslow phase changes. These unique thermal behaviours of imidazolium-based RTILs are attributed to conformational changes and flexibility of alkyl chains bonding to the imidazolium ring.
Room temperature ionic liquids (RTILs) are very unique substances which may form interfaces with polar and/or non-polar liquids. Though their macroscopic and/or phenomenological physico-chemical properties have been investigated so far, microscopic or local structural knowledge at the interface is very limited and the overall problems related to the interface still remain unsolved. In this paper, we have reviewed our series of experimental results on surfaces or buried interfaces (liquid/liquid and liquid/solid interfaces) by using X-ray reflectivity and diffraction measurements as well as IR-Vis sum-frequency generation spectroscopy. Heterogeneous and complex systems of such interfaces may explain various functions of ionic liquids occurring at the interfaces.
This article shows our recent study on electronic structure of several typical ionic liquids. Ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy and soft x-ray emission spectroscopy (SXES) were used for elucidating the electronic structure of ionic liquids based on methylbutylimidazolium ([Cn mim]+) cation. Non-resonant SXES spectra measured above N, O, and F 1s edges selectively probed the partial density of states of the cation and anion. They give a clear evidence that the highest occupied molecular orbital of the [C4 mim]+ cation contributes to the topmost occupied states of the ionic liquids [C4 mim]+PF6−. Resonant soft x-ray emission spectra at the N 1s edge of these ionic liquids revealed that the energy gap of [C4 mim]+PF6− is solely determined by the [C4 mim]+ cation, in contrast to usual ionic crystals like NaCl and NaBr. The ionic liquids composed of fluoride anions PF6− and BF4− have anomalous electronic structure. Such basic knowledge about the electronic structure of ionic liquid must be helpful for new molecular design of ionic liquids.
Ionic liquids are designable and fine-tunable in terms of their properites, which allows them to be task-specific. This paper focuses on electrochemical devices using ionic liquid electrolytes, which exhibit wide liquid temperature ranges, negligible vapor pressure and high ionic conductivity. Here, we review the relationship between physicochemical and structural properties of ionic liquids. Based on the unique physicochemical properties, we also introduce the novel electrochemical devices such as polymer actuators, lithium secondary batteries and nonhumidified intermediate temperature fuel cells.