Anion Specific Effects Drive the Formation of Li-Salt Based Aqueous Biphasic Systems

Abstract

Aqueous biphasic systems (ABS) can form when mixing water with two compounds such as polymers, ionicliquids or simple salts. While this phenomenon has been known for decades and found applications in various fields such as biology, recycling or even more recently electrochemistry, the physics behind the formation of ABSs remains ill-understood. It was recently demonstrated that ABSs can be composed of two salts sharing the same cation (Li + ) but different anions (sulfonamide and halide). Interestingly, their formation could not be explained by the position of the anions within the chaotropic/kosmotropic series and was rather proposed to originate from an anion size mismatch, albeit the size for these anions was never measured yet owing to the lack of a proper experimental methodology. Here, we combine experimental techniques and molecular simulations to assess the specific effects (size, shape, hydrophobic/hydrophilic character) of a series of anions and correlate them with the formation of ABSs. We demonstrate that while the anion size mismatch is a prerequisite for the formation of Li-salts based ABSs, their shape can also play an important role, providing general guidelines for forming new ABSs with potential future applications.

Separation of Rare Earths and Transition Metals Using Ionic-Liquid-Based Aqueous Biphasic Systems

“…An alternative type of extraction system is the aqueous biphasic system (ABS), in which water is used as a solvent in both phases. , A conventional ABS consists of two immiscible aqueous phases that are based on polymer–polymer or polymer–salt combinations, with each aqueous phase rich in one solute. Recently, salt–salt-induced ABSs have been reported by mixing different lithium salts, such as lithium chloride and lithium bis­(trifluoromethanesulfonyl)­imide. , The use of ILs as components of ABSs can mitigate the drawbacks of conventional ABSs (e.g., low selectivity) due to the tuneability of ILs. , IL-based ABSs were first reported in 2003 by adding an IL to an aqueous salt solution, forming an upper IL-rich phase and a lower salt-rich phase . Since then, a number of IL-based ABSs composed of an IL and inorganic/organic salts, polymers, carbohydrates, or amino acids have been reported. − Several IL-based ABSs have been applied to the extraction of metal ions, such as the separation of Ni/Co and Pt/Co metal pairs. − However, only a few of these IL-based ABSs were used for the separation of the rare-earth elements (REEs). , REEs are economically important and are widely used in cutting-edge technologies, such as samarium-cobalt (SmCo) magnets, neodymium-iron-boron (NdFeB) magnets, REE phosphors, and nickel-metal hydride (NiMH) batteries. , …”

Two-Step Mechanism for Halogen Intercalation in Graphite Enabled by Aqueous Biphasic Systems

“…Phase diagrams were measured using the cloud point titration method previously described. − Briefly, starting from a known mass of a concentrated solution of one component (LiBr 3 or LiTFSI), a concentrated solution of the second component is weighed while being added dropwise and vortexed until the solution turns cloudy. Then, Milli-Q water is weighed while being added dropwise and vortexed until the solution turns clear again (even though still showing the red color).…”

“…Our recent studies on ABS show that the halide-rich (top) phase contains a small amount of LiTFSI, while the TFSI-rich (bottom) phase contains a small amount of halogens. − To evaluate the potential impact of LiTFSI and LiCl on Br – oxidation, solutions containing both lithium halides (LiBr and LiCl) or LiBr with LiTFSI were studied. Addition of a large amount of LiCl (5m to 8m) or a small amount (0.1m) of LiTFSI in concentrated LiBr solutions does not lead to substantial changes in Br – oxidation (Figure S5).…”

Fine tuning of electrosynthesis pathways by modulation of the electrolyte solvation structure

“…Knowing that some supporting salts are known to induce demixing of ACN and water 64 while others maintain the nanoheterogeneity without inducing phase separation, fundamental work is currently devoted to understanding salt-induced liquid–liquid phase separation phenomena. 65,66 Such phenomena are ubiquitous, with implications in various fields such as biology, chemical extraction or batteries. However, not all synthetically relevant mixtures of solvents exhibit structuring, like mixtures of N , N -dimethylformamide (DMF) and water.…”