Liquid–liquid
phase separation occurs at room temperature
when mixing an excess of benzene with solid viologen bistriflimide
salts with various alkyl side-chain lengths. A liquid phase composed
of (almost) pure benzene is above the other sponge-like liquid phase
with salt absorbed in benzene. Nuclear magnetic resonance experiments
indicate that the mole ratio of benzene/salt in the sponge-like phase
remains unchanged upon varying the amounts of (nonexcessive) salt
or benzene. Moreover, the benzene/viologen salt mole ratio in the
sponge-like phase increases linearly with respect to the side-chain
length of the cation. Similarly, when an excess of viologen salt is
added in benzene, a sponge-like liquid phase composed of salt absorbed
by benzene is observed in equilibrium with some solid viologen salt
neither dissolved nor absorbed by the solvent. The mole ratio of the
sponge-like liquid phase again increases linearly with side-chain
length, while it remains independent of the relative amount of benzene
and viologen salt as long as the latter is in excess. Finally, when
appropriate amounts of benzene and viologen salt are mixed, a single
sponge-like liquid phase is observed at an intermediate composition
between the lower and upper limits. Molecular dynamics simulations
reveal that because of their dual ionic and organic nature, when absorbed
in benzene, the studied salts form nanoscale segregated liquid structures,
akin ionic liquids, with a continuous polar network composed of anions
and cationic charged groups, along with nonpolar domains composed
of alkyl cationic side chains. Benzene molecules are preferentially
absorbed inside the nonpolar region, which effectively expands the
nonpolar region to be sponge-like and consequently liquidizes the
viologen salt. The linearity of the benzene/salt ratio in the upper
and lower phase boundaries comes from the fact that the effective
volume of the nonpolar region for accommodating benzene molecules
grows linearly with cationic alkyl side-chain length. The occurrence
of the above phenomena is attributed to the nonpolar feature of benzene
molecules, and there is no evidence of π–π or ion−π
interaction between the ions and benzene molecules. Moreover, the
diffusion of benzene in the sponge-like phase is found to be close
to that in n-alkanes, supporting the idea of nanoscale
segregation of polar and nonpolar regions in the sponge-like phase.
The revealed mechanism is anticipated to be general for understanding
liquid–liquid phase separation observed in mixtures of organic
salts (ionic liquids) having relatively long alkyl chains with small
organic molecules.