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Aqueous Biphasic Systems (ABS), in which two aqueous
phases with different compositions coexist as separate liquids, have first been
reported over a century ago with polymer solutions. Recent observations of ABS forming
from concentrated mixtures of inorganic salts and ionic liquids raise the
fundamental question of how "different" the components of such
mixtures should be for a liquid-liquid phase separation to occur. Here we show that even two monovalent salts
sharing a common cation (lithium) but with different anions, namely LiCl and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), may result in the formation of ABSs
over a wide range of compositions at room temperature. Using a combination of
experimental techniques and molecular simulations, we analyze the coexistence diagram
and the mechanism driving the phase separation, arising from the different
anion sizes. The understanding and control of ABS may provide new avenues for aqueous-based