<p>Aqueous
biphasic systems (ABS) can form when mixing water with two compounds such as
polymers, ionic-liquids 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<sup>+</sup>) 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. </p>
Interfaces between Charged Surfaces and Ionic Liquids: Insights from Molecular Simulations
