Critical phenomena of {1-butanol + 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide} binary solution

Effects of self-hydrogen bonding among formamide molecules on the UCST-type liquid-liquid phase separation of binary solutions with imidazolium-based ionic liquid, [Cnmim][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_nmim][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_nmim][TFSI]-1,4-dioxane (1,4-DIO) systems. An array of spectroscopic techniques, including ¹H and ¹³C 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 [C₆mim][TFSI]-FA-d₂ and [C₈mim][TFSI]-FA-d₂ systems revealed, similarly to [C_nmim][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_nmim][TFSI]-FA systems.

Assessment of the UCST-type liquid-liquid phase separation mechanism of imidazolium-based ionic liquid, [C8mim][TFSI], and 1,4-dioxane by SANS, NMR, IR, and MD simulations

Liquid-liquid phase separation of binary systems for imidazolium-based ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C_nmim][TFSI], where n represents the alkyl chain length of the cation), with 1,4-dioxane (1,4-DIO) was observed as a function of temperature and 1,4-DIO mole fraction, x_1,4-DIO. The phase diagrams obtained for [C_nmim][TFSI]-1,4-DIO systems showed that the miscible region becomes wider with an increase in the alkyl chain length, n. For n = 6 and 8, an upper critical solution temperature (UCST) was found. To clarify the mechanism of the UCST-type phase separation, small-angle neutron scattering (SANS) experiments were conducted on the [C₈mim][TFSI]-1,4-DIO-d₈ system at several x_1,4-DIO. The critical exponents of γ and ν determined from the SANS experiments showed that phase separation of the system at the UCST mole fraction occurs via the 3D-Ising mechanism, while that on both sides of UCST occurs via the mean field mechanism. Thus, the crossover of mechanism was observed for this system. The microscopic interactions among the cation, anion, and 1,4-DIO were elucidated using ¹H and ¹³C NMR and IR spectroscopic techniques, together with the theoretical method of molecular dynamics (MD) simulations. The results on the microscopic interactions suggest that 1,4-DIO molecules cannot strongly interact with H atoms on the imidazolium ring, while they interact with the octyl chain of the cation through dispersion force. With a decrease in temperature, 1,4-DIO molecules gradually aggregate to form 1,4-DIO clusters in the binary solutions. The strengthening of the C-H⋯O interaction between 1,4-DIO molecules by cooling is the key to the phase separation. Of course, the electrostatic interaction between the cations and anions results in the formation of IL clusters. When IL clusters are excluded from 1,4-DIO clusters, liquid-liquid phase separation occurs. Accordingly, the balance between the electrostatic force between the cations and anions and the C-H⋯O interaction between the 1,4-DIO determines the 3D-Ising or the mean field mechanism of phase separation.

The heat capacities and critical behaviors of binary ionic solutions

The heat capacities of nine binary room temperature ionic solutions {[C4mim][BF4] + 1,2-butandiol}, {[C8mim][BF4] + 1-pentanol}, {[C8mim][BF4] + 2-pentanol}, {[C8mim][BF4] + 1-hexanol}, {[C8mim][BF4] + 1-heptanol}, {[C8mim][PF6] + 1-propanol}, {[C8mim][PF6] + 1-butanol}, {[C8mim][PF6] + 2-butanol} and {[C8mim][PF6] + tert-butanol} are reported herein. The combination of the data obtained with the corresponding measured coexistence curves infers that the critical asymmetry parameter of the coexistence curves linearly varies with the molar volume ratio of the two components for each of the studied binary ionic solutions after the heat capacity contribution is considered. This indicates the importance of the heat capacity contribution to the critical asymmetry. For further analysis of the critical characteristics of the ionic solutions, a large amount of experimental data was collected and systematically discussed in detail regarding which critical character is important in the binary ionic solutions. A general increasing tendency for the RPM (restricted primitive model)-rescaled critical parameters with the relative permittivity εr,c of the solvent at the critical temperature of the corresponding system was found. This is attributed not only to the screening effect of the solvent medium, but also to solvophobic interactions, which both increase with εr,c. This study also demonstrates that the critical amplitudes increased, while the relative contribution of the heat capacity to the asymmetry of the coexistence curve decreased with an increase in εr,c.

Critical universality and asymmetry of ionic solution {iodobenzene + 1-decyl-3-methyl-imidazolium bis(trifluorosulfonyl)imide}

The liquid-liquid coexistence curve, heat capacity in the critical and noncritical regions, and the turbidity in critical one-phase and two-phase regions of the binary solution {iodobenzene + 1-decyl-3-methylimidazolium bis(trifluorosulfonyl)imide ([C₁₀mim][NTf₂])} have been precisely measured. From the data collected in the critical region, the critical exponents α, β, γ and ν, as well as the universal critical amplitude ratios R_B and X were obtained and were shown to agree well with their theoretical values for the 3D-Ising universality class, which further confirmed the 3D-Ising criticality of ionic solutions even in a very low relative permittivity solvent. The coulombic character of the studied system was suggested by the small values of the RPM reduced critical temperature and density. Furthermore, it was found that both the asymmetric behaviors of the diameter of the coexistence curve and the osmotic compressibility could be well described using the complete scaling theory.