During the past decade, ionic-liquid-based Aqueous Biphasic Systems (ABS) have been the focus of a significant amount of research. Based on a compilation and analysis of the data hitherto reported, this critical review provides a judicious assessment of the available literature on the subject. We evaluate the quality of the data and establish the main drawbacks found in the literature. We discuss the main issues which govern the phase behaviour of ionic-liquid-based ABS, and we highlight future challenges to the field. In particular, the effect of the ionic liquid structure and the various types of salting-out agents (inorganic or organic salts, amino acids and carbohydrates) on the phase equilibria of ABS is discussed, as well as the influence of secondary parameters such as temperature and pH. More recent approaches using ionic liquids as additives or as replacements for common salts in polymer-based ABS are also presented and discussed to emphasize the expanding number of aqueous two-phase systems that can actually be obtained. Finally, we address two of the main applications of ionic liquid-based ABS: extraction of biomolecules and other added-value compounds, and their use as alternative approaches for removing and recovering ionic liquids from aqueous media.

Recently, many studies concerning the environmental impact of ionic liquids (ILs) have shown that despite their unique properties and clear advantages in an ever wide range of applications and processes, ILs are not intrinsically green. In a search for biodegradable and low toxicity, a new type of ILs has been developed, the deep eutectic solvents (DESs). In this context, the aim of this work is to provide accurate densities, viscosities, and refractive indices for DESs prepared using cholinium chloride as the hydrogen bond acceptor and several carboxylic acids (levulinic, glutaric, malonic, oxalic, and glycolic) as the hydrogen bond donors. The impact of two different synthetic methodologies, heating and grinding, in the thermophysical properties of the prepared DESs was assessed. The obtained DESs were analyzed using NMR spectroscopy, FTIR, and electrospray ionization mass spectrometry in order to check their structures and purities. Thermophysical properties, densities, viscosities, and refractive indices were rationalized in terms of the chemical structure of the prepared DESs. The effect of the presence of water in the thermophysical properties of the compounds was also evaluated. Finally, comparisons between the DESs and the corresponding ILs are presented.

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The ionic nature of ionic liquids (ILs) results in a unique combination of intrinsic properties that produces increasing interest in the research of these fluids as environmentally friendly "neoteric" solvents. One of the main research fields is their exploitation as solvents for liquid-liquid extractions, but although ILs cannot vaporize leading to air pollution, they present non-negligible miscibility with water that may be the cause of some environmental aquatic risks. It is thus important to know the mutual solubilities between ILs and water before their industrial applications. In this work, the mutual solubilities of hydrophobic yet hygroscopic imidazolium-, pyridinium-, pyrrolidinium-, and piperidinium-based ILs in combination with the anions bis-(trifluoromethylsulfonyl)imide, hexafluorophosphate, and tricyanomethane with water were measured between 288.15 and 318.15 K. The effect of the ILs structural combinations, as well as the influence of several factors, namely cation side alkyl chain length, the number of cation substitutions, the cation family, and the anion identity in these mutual solubilities are analyzed and discussed. The hydrophobicity of the anions increases in the order [C(CN) 3 ] < [PF 6 ] < [Tf 2 N] while the hydrophobicity of the cations increases from [C n mim] < [C n mpy] e [C n mpyr] < [C n mpip] and with the alkyl chain length increase. From experimental measurements of the temperature dependence of ionic liquid solubilities in water, the thermodynamic molar functions of solution, such as Gibbs energy, enthalpy, and entropy at infinite dilution were determined, showing that the solubility of these ILs in water is entropically driven and that the anion solvation at the IL-rich phase controls their solubilities in water. The COSMO-RS, a predictive method based on unimolecular quantum chemistry calculations, was also evaluated for the description of the water-IL binary systems studied, where it showed to be capable of providing an acceptable qualitative agreement with the experimental data.