## 4 Discussion  
### Temperature Evolution of Water  
The infrared absorbance due to OH stretching vibrations have been largely investigated in the past and it has been proved that their analysis is able to provide useful information about the coordination of water molecules. Water can form hydrogen bonds with itself or with other molecules. The existence of different number of coordinated molecules gives rise to OH stretching bands centered at different frequencies. The less coordinated water molecules are usually characterized by a doublet centered around 3600 and 3550 cm\({}^{-1}\)[9]. With increasing coordination, the absorption bands move toward lower frequencies and the two peaks tend to merge in a single structured absorption [9; 32]. In the case of four-fold coordination, a band centered at 3270 cm\({}^{-1}\) is clearly visible; this band has been largely investigated in the solid state [38]. In some cases, these three types of coordination are called multimer (MW), intermediate (IW) and network water (NW) [18; 41; 42]. In an unknown system, the investigation of the frequency position of the OH stretching bands can become a powerful mean to determine the coordination of water molecules.  
In the presently investigated samples, no band above 3600 cm\({}^{-1}\) is visible, in agreement with the high-water content and the expected high coordination of water molecules. It must be noted that in the present case, the concentration of water is much higher than in the previously investigated bmim-DCA [32], where the concentration was lower than 0.024 molecules of water per ionic couple. In this system, bands above 3550 cm\({}^{-1}\) were also absent. In the present study, even though the concentration of H\({}_{2}\)O is as high as \(\sim\)3.3 molecules per ionic couple, one can observe a cold crystallization in the PYR\({}_{14}\)-DCA sample. Part of the water molecules participate to the crystallization process, as witnessed by the narrowing of the OH band centered around 3500 cm\({}^{-1}\). In PYR\({}_{14}\)-DCA, at low temperatures there is an increase of the intensity of the bands due to H\({}_{2}\)O (see Figure 5) and even the presence of network water, centered around 3270 cm\({}^{-1}\), is visible well below the crystallization temperature. When cold crystallization takes place, the intensity of the band of the network water decreases and it is drastically reduced at 260 K, below the melting point of bulk water. In this framework, it seems that the ionic liquid and water behave in two almost independent ways, except for the limited amount of less coordinated H\({}_{2}\)O involved in the crystallization process (band centered around 3500 cm\({}^{-1}\)).  
In PYR\({}_{14}\)-DCA + H\({}_{2}\)O, all OH stretching bands are red shifted compared to PYR\({}_{14}\)-DCA, indicating a stronger interaction of water with itself and with the IL, forming hydrogen bonding. Moreover, in PYR\({}_{14}\)-DCA + H\({}_{2}\)O, network water is already present at room temperature, further suggesting stronger interaction in this sample. However, it must be noted that the intensity of this band has two plateau regions, one below and one above 240 K, the same temperature at which the network water band starts to disappear in PYR\({}_{14}\)-DCA. It seems plausible that water transforms into a glassy state below 240 K in both samples.