Takuya Shimomura

Effect of water on structure of hydrophilic imidazolium-based ionic liquid.

The state of water in room-temperature ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI(+)BF(4)(-)), has been investigated by measurements of absorption and desorption isotherms, attenuated total reflectance infrared (ATR-IR) spectroscopy, and (2)H NMR relaxation method. The absorption enthalpies of water for the ionic liquid were estimated from the absorption isotherms. The enthalpies in the water mole fraction range of x(w) <or= approximately 0.5 are lower than the enthalpy of vaporization for bulk water, but become larger than the value for bulk with increasing mole fraction of absorbed water. The ATR-IR spectra for EMI(+)BF(4)(-)-water solutions in the range of 0.09 <or= x(w) <or= 0.34 have revealed that individual water molecules hydrogen-bonded to the anions predominate in the solutions at x(w) <or= approximately 0.2, while approximately 30% of water molecules are hydrogen-bonded among them in the solutions at x(w) > approximately 0.3. In addition, the activation energies for the rotational motion of a water molecule estimated from the (2)H NMR relaxation rates have indicated that the motion of water molecules in EMI(+)BF(4)(-)-D(2)O solutions gradually becomes freer with increasing water content from x(w) = 0.10 to 0.30, but is retarded again at x(w) = 0.33. Therefore, all the present findings have suggested that the state of water molecules in EMI(+)BF(4)(-) significantly changes at x(w) approximately 0.3. On the other hand, to directly observe the effect of water on structure of EMI(+)BF(4)(-), LAXS experiments have been made on EMI(+)BF(4)(-)-water solutions. It has been suggested that the interactions between the C(2) atom within the imidazolium ring of EMI(+) and BF(4)(-) are strengthened with increasing water content, while those at the C(4) and C(5) atoms weaken. Thus, the present LAXS experiments have clarified the beginning of formation of ion pair in EMI(+)BF(4)(-) by adding water at the molecular level.

Clusters of imidazolium-based ionic liquid in benzene solutions.

Cluster formation of 1-dodecyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (C(12)mim(+)TFSA(-)) in benzene solutions was investigated using small-angle neutron scattering (SANS), NMR, attenuated total reflectance infrared (ATR-IR), and large-angle X-ray scattering (LAXS) techniques. The SANS measurements revealed that C(12)mim(+)TFSA(-) is heterogeneously mixed with benzene in the narrow range of benzene mole fraction 0.9 ≤ x(C6D6) ≤ 0.995 with a maximum heterogeneity at x(C6D6) ≈ 0.99. The NMR results suggested that the imidazolium ring is sandwiched between benzene molecules through the cation-π interaction. Moreover, TFSA(-) probably interacts with the imidazolium ring even in the range of x(C6H6) ≥ 0.9. Thus, the imidazolium rings, benzene molecules, and TFSA(-) would form clusters in the C(12)mim(+)TFSA(-)-benzene solutions. The LAXS measurements showed that the distance between the imidazolium ring and benzene is ∼3.8 Å with that between the benzene molecules of ∼7.5 Å. On the basis of these results, we discussed a plausible reason for the liquid-liquid equilibrium of the C(12)mim(+)TFSA(-)-benzene system.

Solvation of the Amphiphilic Diol Molecule in Aliphatic Alcohol-Water and Fluorinated Alcohol-Water Solutions

We investigated the solvation properties of aqueous solutions of aliphatic alcohols and fluorinated alcohols. These included ethanol (EtOH), 2-propanol (2-PrOH), 2,2,2-trifluoroethanol (TFE), and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). The amphiphilic diol, 1,4-pentanediol (1,4-PD), was used as the solute to probe solvation properties at the molecular level. Small-angle neutron scattering (SANS) experiments revealed that the inherent microheterogeneity of HFIP-water binary solutions was significantly enhanced by addition of 1,4-PD. In contrast, the addition of 1,4-PD to EtOH-, 2-PrOH-, and TFE-water solutions hardly changed the mixing state. Molecular dynamics simulations were used to obtain the spatial distribution functions for the oxygen atom of water molecules and the carbon and fluorine atoms of alcohol molecules around 1,4-PD. Of the alcohols studied, these spatial distributions illustrated that HFIP molecules formed the strongest hydrophobic solvation shell around the hydrocarbons of 1,4-PD. This preferential solvation of 1,4-PD by HFIP leads to enhancement of HFIP clusters in the solutions. (13)C NMR and infrared spectroscopic measurements on 1,4-PD in the different alcohol-water solutions suggested that the number of water molecules around the hydrocarbons of 1,4-PD decreased in aliphatic alcohol-water solutions. Additionally, HFIP molecules are thought to strongly interact with the hydrocarbons of 1,4-PD in HFIP-water solutions.

SANS, ATR-IR, and 1D- and 2D-NMR studies of mixing states of imidazolium-based ionic liquid and aryl solvents

The mixing states of imidazolium-based ionic liquid, 1-dodecyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (C12mim(+)TFSA(-)), and two aryl solvents toluene and α,α,α-trifluorotoluene (TFT) have been clarified on both meso- and microscopic scales using small-angle neutron scattering (SANS) and ATR-IR techniques. To elucidate the interactions between C12mim(+)TFSA(-) and aryl solvent molecules from the change in the electron densities of C12mim(+) and TFSA(-), 1D-NMR measurements for (1)H and (13)C atoms have been conducted on C12mim(+)TFSA(-)-aryl solvent solutions as a function of the aryl solvent mole fraction. In addition, the interactions between the dodecyl chain of C12mim(+) and aryl solvent molecules have been observed using 2D-NMR techniques of (1)H{(1)H} ROESY and (19)F{(1)H} HOESY. These results have been compared with those of benzene solutions previously investigated. The SANS measurements have shown that toluene is heterogeneously mixed with C12mim(+)TFSA(-) as well as benzene. However, the heterogeneity of the toluene solutions is slightly lower than that of the benzene solutions. In contrast, TFT is homogeneously mixed with the ionic liquid at least on the present SANS scale. The substituent effects of the three aryl solvent molecules of benzene, toluene, and TFT on the mixing states of the solutions have been discussed in terms of the cation-π interaction between the imidazolium and phenyl rings and the interaction between the dodecyl group and aryl solvent molecules.

Solvation Structure of Imidazolium Cation in Mixtures of [C4mim][TFSA] Ionic Liquid and Diglyme by NMR Measurements and MD Simulations

Interactions of 1-butyl-3-methylimidazolium cation ([C4mim]+) with bis(trifluoromethanesulfonyl)amide anion ([TFSA]-) and diethyleneglycol dimethyl ether (diglyme) in mixtures of [C4mim][TFSA] ionic liquid and diglyme have been investigated using 1H and 13C NMR spectroscopy and molecular dynamics (MD) simulations. The results of NMR chemical shift measurements and MD simulations showed that the diglyme oxygen atoms have contact with the imidazolium hydrogen atoms of [C4mim]+ in the mixtures. The contact between the hydrogen atoms of imidazolium and the oxygen atoms of [TFSA]- remains even when the diglyme mole fraction (xdiglyme) increases up to 0.9. However, the coordination numbers of the hydrogen atoms of [C4mim]+ with oxygen atoms of diglyme increase with xdiglyme. The [TFSA]- anions around [C4mim]+ are not completely replaced by diglyme even at xdiglyme > 0.9. The MD simulations revealed that the diglymes also have contact with the butyl group of [C4mim]+. The methyl groups of diglyme prefer to have contact with the terminal methyl group of the butyl group, whereas the diglyme oxygen atoms prefer to have contact with the methylene group connected to the imidazolium ring of [C4mim]+.

SANS, infrared, and 7Li and 23Na NMR studies on phase separation of alkali halide-acetonitrile-water mixtures by cooling.

Phase separation of alkali halide (MX) (M = Li+, Na+, and K+ and X = Cl– and Br–)–acetonitrile (AN)–water mixtures by cooling has been investigated at the molecular level. The phase diagram obtained for the MX–AN–H2O ternary systems showed that the temperatures of phase separation for the mixtures with MCl are higher than those with MBr. The phase-separation temperatures of the mixtures with MCl and MBr are higher in the sequence of NaX > KX > LiX, although the magnitude of the hydration enthalpies for the alkali metal ions is larger in the sequence of Li+ > Na+ > K+. To elucidate the reasons for the sequence of phase separation on the meso- and microscopic scales, small-angle neutron scattering (SANS), infrared (IR), and 7Li and 23Na NMR measurements have been conducted on MX–AN–water mixtures with lowering temperature. The results of SANS and IR experiments showed that the mechanism of phase separation of the mixtures by cooling is the same among all of the mixtures but did not clearly reveal the reasons for the phase separation sequence. In contrast, the spin–lattice relaxation rates and the chemical shifts of 7Li and 23Na NMR for the mixtures suggested the different solvation structure of Li+ and Na+ in the mixtures. In conclusion, the solvation of acetonitrile molecules for Li+ and the formation of Li+–X– contact ion pairs in the mixtures cause the weakest effect of LiX on phase separation of the mixtures by cooling among the alkali metal ions.