## 1 Introduction  
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, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19], there is no comprehensive theory or a detailed framework for these properties. Nevertheless, some general picture slowly emerges [8, 20, 21]. At extreme low water concentrations [9], all water molecules are isolated in the IL and do not establish bounds. With increasing H\({}_{2}\)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; 24; 25; 26]. According to this model, the small quantity of water in ILs gives rise to separated solvent molecules, while for increasing H\({}_{2}\)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 BF\({}_{4}\) and PF\({}_{6}\) anions [27]. Studies of mixtures of ILs with solvents were also extended to alcohols [28; 29; 30] 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.