Although the “like-dissolves-like”
rule is often
invoked to explain why sodium chloride dissolves in water, hidden
behind this explanation is the delicate balance between the very large
cohesive energy of the ionic crystal and large solvation energies
of the ions. Room-temperature ionic liquids (ILs) are liquid analogues
of ionic crystals and, as dictated by a similar energetic balance,
may either fully mix with water or be immiscible with water depending
on ion type and cation/anion combination. In this work, we study three
hydrophobic and three hydrophilic ILs to examine whether a priori
prediction of water miscibility is possible based on analysis of bulk
properties alone. We find that hydrophilic and hydrophobic ILs exhibit
distinct signatures in their (reciprocal space) Coulomb interactions
that indicate predisposition to water mixing. Hydrophilic ILs exhibit
a prominent peak in their electrostatic interactions at ∼5–8
Å length scale, largely due to repulsion between neighboring
anion shells. When mixed with water, this peak is significantly reduced
in magnitude, indicating that electrostatic screening by water molecules
is an important driving force for mixing. In contrast, hydrophobic
ILs show no such peak, indicating no predisposition to mixing. In
addition to this analysis, we compute and compare solvation free energies
of the six different anions in water, ion-pairing free energies at
“infinitely” dilute concentration, and water absorption
free energies in the different ILs. Analyzed within the context of
empirical data, our calculations suggest that hydrophobicity trends
of different ILs are very sensitive to precise water content at dilute
conditions. For example, we predict that bis(fluorosulfonyl)imide-based
ILs exhibit anomalously large water absorption free energies at zero
water content, with increasing hydrophobicity as preferential absorption
sites within the IL become saturated.