For many applications of ionic liquids (ILs), mixing them with solvents—in particular, water—is required. Among such possible utilizations one can cite the dissolution of cellulose [1
], otherwise insoluble in pure water, specific treatments to recover organic pollutants [2
], the storage and modulations of properties of biomolecules [3
], the use as electrolytes in electrochemical devices [4
] such as fuel cells [5
], the production of microemulsions for synthesis, (bio-)catalysis, polymerization, (nano-)material preparation, drug delivery and separations [6
]. Hence, a large amount of research was devoted to the investigation of the phase equilibria of ILs with various solvents and some predictive models were proposed [7
]. However, compared to the extensive studies of the macroscopic properties, such as excess volume, viscosity or mixing enthalpy, little is known concerning the microscopic interaction of ILs and water [8
Due to the limited amount of experimental data on the interactions of ILs and water at the molecular level [9
], there is no comprehensive theory or a detailed framework for these properties. Nevertheless, some general picture slowly emerges [8
]. At extreme low water concentrations [9
], all water molecules are isolated in the IL and do not establish bounds. With increasing H2
O concentrations, water tends to be bound more strongly to the anions than to the cations [22
]. The interaction between anion and cation in ionic couples are reduced, due to the competing interaction with water. The main steps are as follows: (1) the continuum network structure of ILs is gradually transformed into ion-clusters, (2) the ion clusters are dissociated into ion-pairs surrounded by water and (3) the ion-pairs are dissociated into hydrated and separated ions [8
]. Notice that the hydration is not always completed, even at extremely high concentrations of water, as the separation of ionic couples is more effective in aprotic ionic liquid than in protic ones where strong interactions between anion and cation remain.
A valuable contribution to the development of this general description has been provided by computational investigations of the mixtures of ionic liquids and water [23
]. According to this model, the small quantity of water in ILs gives rise to separated solvent molecules, while for increasing H2
O concentration a water percolation network is formed and the hydrophobicity of the anion controls the water pocket size [23
]. There is a large consensus on the fact that water molecules solvated in aprotic ILs assemble in cavities inside the polar nanoregions of ILs, that progressively grow until they form a percolating network [23
]. On the other hand, water more homogeneously mixes with protic ILs, that are able to form hydrogen bonds.
It must be noted that all the described properties are valid at sufficient low temperatures so that the decomposition of anions due to the interaction with water molecules is avoided. Such an effect was observed for example for the BF4
Studies of mixtures of ILs with solvents were also extended to alcohols [28
] and acetone [31
]. In some cases, phase diagrams of these mixtures have been constructed, as in the case of choline bis(trifluoromethylsulfonyl)imide and N,N,N-trimethylpropylammonium bis(trifluoromethylsulfonyl)imide with propanol [30
]. These mixtures can crystallize at low temperature in the whole concentration range, but for concentrations of propanol higher than 35 mol%, one observes also a clouding point.
Concerning the water mixtures of ILs and water, little is known about the temperature dependence of the interactions between IL and solvent or about the occurrence of crystallization or glass transition at low temperatures. Quite recently the infrared spectra of two mixtures of 1-butyl-3-methylimidazolium dicyanamide ionic liquid and water were investigated down to 140 K, with a special attention to the changes of the environment of water molecules and the interface between water and ionic liquid as a function of temperature [32
]. A cold crystallization only occurred for small water concentrations absorbed from the atmosphere, while for higher concentrations (0.024 H2
O molecules per ionic couple) the crystallization was not observed. The analysis of the O-H stretching bands indicated the existence of two different “liquid like” water environments. When cold crystallization took place, the water molecules, which seemed less coordinated with the other H2
O molecules and mostly linked to the anions, become part of the crystallized sample. For all water concentrations, it seems that at the microscopic level the samples were not homogeneous, but more likely they were composed of separated clusters or regions of bulk water confined in the ionic liquid.
In the present paper, we extend this low temperature investigation of the infrared spectra of water-IL mixtures to the hydrophilic 1-butyl-1-methylpyrrolidinium dicyanamide ionic liquid at larger concentrations of water, in order to explore the water-rich region of the phase diagram.
2. Materials and Methods
The 1-Butyl-1-methylpyrrolidinium dicyanamide (PYR14
-DCA) was purchased from Solvionic. The structure of the ions compositing this IL are displayed in Figure 1
. As the purity of the IL was 99.5%, the sample was investigated as received without further purification. It must be noted that despite that the initial purity of the sample was very high and the initial water content was extremely low, during the charging of the cell, it absorbed a large quantity of water due to its hygroscopic nature, as it will be more deeply commented in the following Section “Results”.
Infrared absorbance measurements were conducted at the AILES beamline of Soleil Synchrotron, with a resolution of 1 cm−1
, using a Bruker IFS125 HR spectrometer. A thin layer of liquid was placed in a vacuum sealed cell for liquids, equipped with two diamond optical windows and a 6 μm thick spacer. The temperature was varied in the range 300–160 K by means of the Cryomech cryostat available at AILES with a temperature rate of 5 K∙min−1
. The spectra were recorded in the mid-infrared range by combining a KBr beamsplitter and a wide range low noise MCT [33
Measurements were performed both on the as received sample (in the following called PYR14-DCA) and on an intentionally hydrated sample (in the following indicated as PYR14-DCA + H2O), obtained by mixing the starting materials with 57 wt% of bidistilled water, corresponding to the addition of ~6.6 water molecules for each ionic couple.
The dicyanamide anion and the 1-butyl-1-methylpyrrolidinium cation were also investigated computationally in vacuum by means of DFT calculations at the B3LYP level of theory, using the 6–31G** basis set, as reported in Reference [32
]. After optimization of the geometry, the infrared vibrational frequencies and intensities were calculated. A simulated absorption spectrum was constructed by summing Gaussian curves centered at each calculated vibration frequency, with a 10 cm−1
linewidth and an intensity proportional to the calculated one.