unified way [25,26]. Experimental studies of supercooled water have been extensively conducted using mesoporous materials such as MCM-41 because water confined in such materials does not freeze far below the melting point [26–31]. Meanwhile, there are few studies on the temperature dependence of confined water using reverse micelles in liquids [15–18]. However, there has been a serious issue regarding the interaction between confined water and the internal surface of the cage used for the confinement [26]. The surroundings of the confined water used for supercooled water studies should only weakly affect its properties.
Terahertz (THz) spectroscopy is a powerful method for studying cooperative intermolecular motions of water on timescales of picoseconds [32–34]. Rønne and co-workers have investigated the temperature behavior of the relaxation time of liquid water above the melting point, and demonstrated that it appears to diverge at ~228 K [32], which is also observed in the thermodynamic quantities of supercooled water. This indicates that THz spectroscopy of water can provide information on the microscopic mechanism underlying the thermodynamic anomaly of water at low temperatures. A reverse micellar solution is suitable for THz spectroscopy because the nonpolar solvent shows extremely weak absorption of THz electric waves compared with polar solvents such as water and because it does not hinder the data acquisition of the water or biomolecules in the reverse micelle [13,14,22]. Mittleman and co-workers have examined water in AOT reverse micelles in a wide w0 range at room temperature with THz spectroscopy, and demonstrated the existence of two relaxation processes in the THz range [13]; AOT [=bis(2-ethyl-hexyl)sulfosuccinate] is a surfactant. More