posted on 2021-05-14, 19:15authored byNicolas Dubouis, Arthur France-Lanord, Amandine Brige, Mathieu Salanne, Alexis Grimaud
Aqueous
biphasic systems (ABSs) 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+) 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.