## 3 Results
### Temperature Dependence of the As-Received Sample  
Figure 4 reports the temperature dependence of the absorption spectrum of the PYR\({}_{14}\)-DCA sample. The spectrum recorded at the lowest temperature of 160 K looks similar to that measured at the highest \(T\) (320 K), i.e., the same bands are present in the two spectra, they are just narrower and better defined at the lower temperature, as expected for a sample that does not undergo phase transitions on cooling. The main difference is observed in the region between 3200 and 3600 cm\({}^{-1}\), corresponding to the OH stretching vibrations of water: while at room temperature there are two bands centered around 3430 and 3490 cm\({}^{-1}\), at low \(T\), one observes an increase of the intensity of the band at -3420 cm\({}^{-1}\) and a third band, centered around 3280 cm\({}^{-1}\), develops. On increasing temperature, no major changes are visible up to 220 K. On the contrary, at 240 K all the absorption lines get narrower and some of them split, the same features are visible also at 250 K. On further heating, starting from 260 K the bands return to the shape they had at 220 K and they maintain that same shape up to the maximum temperature of 320 K. The evolution of the absorption bands as a function of temperature are typical of a lack of crystallization on cooling; the sample likely transforms into a glass at low \(T\). On heating, the sample undergoes a cold crystallization in a restricted temperature range including 240 and 250 K. Starting from 260 K, the specimen liquefies and recovers the original liquid state. It is remarkable that this ionic liquid is able to undergo a crystallization process despite the presence of a large amount of water (~3.3 molecules of H\({}_{2}\)O per ionic couple). In hydrophilic 1-butyl-3-methylimidazolium dicyanamide  
Figure 3: Comparison of the infrared absorption spectra measured at 300K of PYR\({}_{14}\)-DCA and PYR\({}_{14}\)-DCA + H\({}_{2}\)O. (bmim DCA) the crystallization was previously suppressed by adding a much smaller concentration of bidistilled water (2.45 wt%, i.e., 0.024 molecules of water per ionic couple) [32]. In order to study quantitatively the microstructure evolution of the sample as a function of T, we performed a detailed investigation of the dependence on \(T\) of the peak position, width and intensity of some significant bands: (1) the absorption around 1310 cm\({}^{-1}\), that was attributed to the asymmetric stretching mode of the bridging N and C atoms of the anion [35; 36; 37; 38]; (2) the absorptions around 2230 cm\({}^{-1}\), attributed to the triple bond between the C and the terminal N atoms of the anion [35; 36; 37; 38]; (3) the OH stretching bands between 3200 and 3600 cm\({}^{-1}\), coming from undesired water. For the OH bands, three contributions were considered, centered around 3500, 3440 and 3270 cm\({}^{-1}\) at the lowest temperature. These three contributions correspond to different coordination of water by means of hydrogen bonds [32]: while the bands between 3400 and 3500 cm\({}^{-1}\) are due to an intermediate coordination 2 or 3 of water molecules, the band centered around 3270 cm\({}^{-1}\) is typical of 4-fold coordination and was largely observed in pure water in the solid state [38]. All these absorption bands were fitted by Gaussian curves as a function of increasing \(T\) and the best fit parameters (peak frequency, intensity and full width at half maximum) are reported in Figure 5.  
Figure 4: Temperature dependence of the spectra of PYR\({}_{14}\)-DCA measured on heating from 160 up to 320 K in different frequency regions (700–1550 and 2050–2300 cm\({}^{-1}\) in the left panel and 2800–3700 cm\({}^{-1}\) in the right panel).