Effects of self-hydrogen bonding among formamide molecules on the UCST-type liquid-liquid phase separation of binary solutions with imidazolium-based ionic liquid, [C n mim][TFSI], studied by NMR, IR, MD simulations, and SANS.

The upper critical solution temperature (UCST)-type liquid-liquid phase separation of imidazolium-based ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C n mim][TFSI], where n represents the alkyl chain length of the cation, n = 6, 8, 10, and 12) binary solutions with formamide (FA) was examined as a function of temperature and the FA mole fraction x FA . The two-phase region (immiscible region) of the solutions is much larger and expands more with the increase in n , in comparison with the previous [C n mim][TFSI]-1,4-dioxane (1,4-DIO) systems. An array of spectroscopic techniques, including 1 H and 13 C NMR and IR combined with molecular dynamics (MD) simulations, was conducted on the present binary systems to clarify the microscopic interactions that contribute to the phase-separation mechanism. The hydrogen-bonding interactions of the imidazolium ring H atoms are more favorable with the O atoms of the FA molecules than with 1,4-DIO molecules, whereas the latter interact more favorably with the alkyl chain of the cation. Upon lowering the temperature, the FA molecules gradually self-aggregate through self-hydrogen bonding to form FA clusters. Concomitantly, clusters of ILs are formed via the electrostatic interaction between the counter ions and the dispersion force among the IL alkyl chains. Small-angle neutron scattering (SANS) experiments on the [C 6 mim][TFSI]-FA- d 2 and [C 8 mim][TFSI]-FA- d 2 systems revealed, similarly to [C n mim][TFSI]-1,4-DIO systems, the crossover of the mechanism from the 3D-Ising mechanism around the UCST x FA to the mean-field mechanism at both sides of the mole fraction. Interestingly, the x FA range of the 3D-Ising mechanism for the FA systems is wider compared with the range of the 1,4-DIO systems. In this way, the self-hydrogen bonding among FA molecules most significantly governs the phase equilibria of the [C n mim][TFSI]-FA systems.