Ionic liquids were initially proposed as replacements for conventional organic solvents; however, their chemistry has developed remarkably and offers unexpected opportunities in numerous fields, ranging from electrochemistry to biology. As a consequence of ionic liquids advancing towards potential and actual applications, a comprehensive determination of their environmental, health and safety impact is now required. This critical review aims to present an overview of the current understanding of the toxicity and environmental impact of the principal ionic liquid groups, and highlights some emerging concerns. Each cation type is considered separately, examining the significance of the biological data, and identifying the most critical questions, some yet unresolved. The need for more, and more detailed, studies is highlighted (176 references).

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.

Interfacial tension (using a drop-shape analysis technique), fluorescence (of a widely used spectroscopic molecular probe, pyrene), and ¹H NMR measurements were used to monitor the adsorption at the aqueous solution–air interface and self-aggregation behaviour (critical micelle concentration, CMC) of room-temperature ionic liquids (ionic liquids) of the 1-alkyl-3-methylimidazolium family of cations, [C_nmim]⁺, with different linear alkyl chain lengths, C_nH_2n+1 (½n = 1–7), and different counter-ions, namely [C_nmim]Cl (n = 2–14), [C_nmim][PF₆] (n = 4 or 10), and [C₁₀mim][NTf₂]. Only [C_nmim]Cl with n > 8 unambiguously form aggregates in solution and the nature of this self-aggregation is discussed in terms of the electrostatic vs. hydrophobic contributions of the isolated cation. In contrast, the shortest chains behave, as anticipated, as simple salts. In turn, the transitional ionic liquid, [C₆mim]Cl is able to develop a monolayer at the aqueous solution–air interface but shows no noticeable self-aggregation in the bulk fluid. Moreover, the micellar characteristics of the well-studied sodium dodecyl sulfate (SDS) aqueous solutions as a function of the total concentration of [C_nmim]Cl (½n = 1–7) showed a clear change in the behaviour of the mixtures [C_nmim]Cl + SDS for n ≈ 6–8, with a characteristic mixed-micelle formation for the longer and a pure salt effect for the shorter chain lengths of [C_nmim]Cl.

The synthesis, characterisation and toxicological assessment of a new group of environmentally friendly ionic liquids are presented. Focussing on the toxic effect of the anion, the ionic liquids were designed by combining the benign cholinium cation, [NMe₃(CH₂CH₂OH)]⁺, with a range of linear alkanoate anions ([C_nH_2n+1CO₂]⁻, n = 1-9), as well as two structural isomers (n = 3 or 4). The toxicity of these ionic liquids was evaluated using filamentous fungi as model eukaryotic organisms. Surprisingly, most of the tested species showed active growth in media containing extremely high ionic liquid concentrations, up to molar ranges in some cases. The biodegradability of these ionic liquids was assessed, and new biotechnological applications for them are proposed, e.g. as solvents for biopolymers. This study leads to the better understanding of the anion influence on the ionic liquid toxicity, but its core is the recognition that conscious design of ionic liquids can be used to deliver truly biocompatible salts without adversely affecting one of the most striking of their properties—their outstanding solvent ability.