Amide-induced phase separation of hexafluoroisopropanol-water mixtures depending on the hydrophobicity of amides.

Amide-induced phase separation of hexafluoro-2-propanol (HFIP)-water mixtures has been investigated to elucidate solvation properties of the mixtures by means of small-angle neutron scattering (SANS), (1)H and (13)C NMR, and molecular dynamics (MD) simulation. The amides included N-methylformamide (NMF), N-methylacetamide (NMA), and N-methylpropionamide (NMP). The phase diagrams of amide-HFIP-water ternary systems at 298 K showed that phase separation occurs in a closed-loop area of compositions as well as an N,N-dimethylformamide (DMF) system previously reported. The phase separation area becomes wider as the hydrophobicity of amides increases in the order of NMF < NMA < DMF < NMP. Thus, the evolution of HFIP clusters around amides due to the hydrophobic interaction gives rise to phase separation of the mixtures. In contrast, the disruption of HFIP clusters causes the recovery of the homogeneity of the ternary systems. The present results showed that HFIP clusters are evolved with increasing amide content to the lower phase separation concentration in the same mechanism among the four amide systems. However, the disruption of HFIP clusters in the NMP and DMF systems with further increasing amide content to the upper phase separation concentration occurs in a different way from those in the NMF and NMA systems.

Free-energy and structural analysis of ion solvation and contact ion-pair formation of Li(+) with BF4(-) and PF6(-) in water and carbonate solvents.

Free energy of contact ion-pair (CIP) formation of lithium ion with BF(4)(-) and PF(6)(-) in water, propylene carbonate (PC), dimethyl carbonate (DMC) are quantitatively analyzed using MD simulations combined with the energy representation method. The relative stabilities of the mono-, bi-, and tridentate coordination structures are assessed with and without solvent, and water, PC, and DMC are found to favor the CIP-solvent contact. The monodentate structure is typically most stable in these solvents, whereas the configuration is multidentate in vacuum. The free energy of CIP formation is not simply governed by the solvent dielectric constant, and microscopic analyses of solute-solvent interaction at a molecular level are then performed from energetic and structural viewpoints. Vacant sites of Li(+) cation in CIP are solvated with three carbonyl oxygen atoms of PC and DMC solvent molecules, and the solvation is stronger for the monodentate CIP than for the multidentate. Energetically favorable solute-solvent configurations are shown to be spatially more restricted for the multidentate CIP, leading to the observation that the solvent favors the monodentate coordination structure.

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.

Raman spectroscopic study, DFT calculations and MD simulations on the conformational isomerism of N-alkyl-N-methylpyrrolidinium bis-(trifluoromethanesulfonyl) amide ionic liquids.

The conformational behaviors of N-alkyl-N-methylpyrrolidinium bis-(trifluoromethanesulfonyl) amide ionic liquids (alkyl; propyl and butyl, [P(1n)][TFSA]; n = 3 and 4) were studied by Raman spectroscopy in the frequency range of 200-1700 cm(-1) at different temperatures. Observed Raman spectra in the frequency range 870-960 cm(-1) for [P(13)][TFSA] and at 860-950 cm(-1) for [P(14)][TFSA] depend on the temperature, indicating that pseudo rotational isomerization of the pyrrolidinium ring exists in the ionic liquids. DFT calculations revealed that the pseudo rotational potential energy surfaces for P(13)(+) and P(14)(+) ions were similar to each other, i.e., the e6 isomer is the global minimum, whereas the three other isomers e1, e4, and e5 are ca. 3 kJ mol(-1) higher in energy. Optimized geometries with no imaginary frequency were successfully obtained for the e6, e1, and e4 isomers. For both cations, the theoretical Raman spectra of the e6 isomers reproduce well the observed data. To explain their observed Raman spectra in a reasonable way, it is necessary to consider one or more species as predicted by DFT calculations, i.e., the e4 isomer of P(13)(+) rather than the e1, or the e1 isomer of P(14)(+) rather than the e4. In addition, the torsion energy potentials of the alkyl chains of the cations were scanned by DFT calculations. It turns out that the alkyl chains of the cations prefer all trans conformations. It should be emphasized that the alkyl chains of the pyrrolidinium cations show remarkably different conformational behaviors comparing with those of the imidazolium. The isomerization enthalpies Delta(iso)H degrees from the e6 to the e4 isomer of P(13)(+) and to e1 of P(14)(+) were reasonably estimated from the temperature dependence of Raman spectra based on our proposed assignments to be 2.9 kJ mol(-1) for P(13)(+) and 4.2 kJ mol(-1) for P(14)(+), respectively. Thus evaluated experimental Delta(iso)H degrees values, which may contain some uncertainties, are in agreement with those predicted by DFT calculations and MD simulations suggesting that pseudo rotational isomerization equilibria are established in the examined N-alkyl-N-methylpyrrolidinium ionic liquids. The conformational behavior of TFSA(-) was also investigated. The Delta(iso)H degrees from the trans (trifluoromethyl groups on opposite sides of the S-N-S plane) to the cis isomer were evaluated to be 4.2 kJ mol(-1) for [P(13)][TFSA] and 3.5 kJ mol(-1) for [P(14)][TFSA], respectively, which are similar to that for the 1-ethyl-3methylimidazolium ionic liquid.

Raman spectroscopic study on alkaline metal ion solvation in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ionic liquid.

The Raman spectra for 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide [BMI][TFSA] containing alkaline metal salts of TFSA(-), MTFSA (M = Li, Na, K and Cs), were recorded in the frequency range of 200-1800 cm(-1), with varying salt concentrations at 298 K. With Li(+) and Na(+) ions, at the frequency range of 730-760 cm(-1), new Raman bands ascribable to the anion bound to the ions appeared at higher frequency relative to that found in the neat ionic liquid. On the other hand, with K(+) and Cs(+) ions, single Raman bands were solely observed. According to the difference Raman spectra for the ionic liquids containing K(+) and Cs(+), evaluated by subtracting Raman spectra for the neat ionic liquid, it turned out that two-state approximation, i.e., bulk TFSA(-) and TFSA(-) bound to K(+) and Cs(+) ions, could hold, as Li(+) and Na(+) ions. By careful analyses of Raman band intensity arising from bulk TFSA(-) as a function of the salt concentration, the solvation numbers for the respective ions were successfully evaluated to be 1.95 for Li(+), 2.88 for Na(+), 3.2 for K(+) and 3.9 for Cs(+), respectively. By taking into account that TFSA(-) acts as a bidentate ligand, the atomic coordination numbers are proposed to be 4, 6, 6 and 8 for Li(+), Na(+), K(+) and Cs(+), respectively. Raman shifts for the TFSA(-) bound to the metal ions relative to that of the bulk TFSA(-) were plotted against the ionic radii for the solvated alkaline metal ions estimated via Shannon's ionic radii, to yield a straight line with a slope of almost unity, suggesting that the electrostatic interaction predominantly operates in the ion-ion interaction between the alkaline metal ions and TFSA(-), as expected. Moreover, the Raman spectra in the frequency range of 370-450 cm(-1) strongly depend on the alkaline metal ions, indicating that cis TFSA(-) is favored in the first solvation sphere of the Li(+) ion of a relatively small ionic radius, and that such a preferred conformational isomerism of TFSA(-) diminishes with an increase of the ionic radii of the central metal ions.

Lithium ion solvation in room-temperature ionic liquids involving bis(trifluoromethanesulfonyl) imide anion studied by Raman spectroscopy and DFT calculations.

The solvation structure of the lithium ion in room-temperature ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EMI(+)TFSI(-)) and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (BMP(+0TFSI(-)) has been studied by Raman spectroscopy and DFT calculations. Raman spectra of EMI(+)TFSI(-) and BMP(+)TFSI(-) containing Li(+)TFSI(-) over the range 0.144-0.589 and 0.076-0.633 mol dm(-3), respectively, were measured at 298 K. A strong 744 cm-1 band of the free TFSI(-) ion in the bulk weakens with increasing concentration of the lithium ion, and it revealed by analyzing the intensity decrease that the two TFSI(-) ions bind to the metal ion. The lithium ion may be four-coordinated through the O atoms of two bidentate TFSI(-) ions. It has been established in our previous work that the TFSI(-) ion involves two conformers of C(1) (cis) and C(2) (trans) symmetries in equilibrium, and the dipole moment of the C(1) conformer is significantly larger than that of the C(2) conformer. On the basis of these facts, the geometries and SCF energies of possible solvate ion clusters [Li(C(1)-TFSI(-))(2)](-), [Li(C(1)-TFSI(-))(C(2)-TFSI(-))](-), and [Li(C(2)-TFSI(-))(2)](-) were examined using the theoretical DFT calculations. It is concluded that the C(1) conformer is more preferred to the C(2) conformer in the vicinity of the lithium ion.

Solvation structure of Li+ in concentrated LiPF6-propylene carbonate solutions.

Time-of-flight neutron diffraction measurements were carried out for 6Li/7Li isotopically substituted 10 mol % LiPF6-propylene carbonate-d6 (PC-d6) solutions, in order to obtain structural information on the first solvation shell of Li+. Structural parameters concerning the nearest neighbor Li+...PC and Li+...PF6- interactions were determined through least-squares fitting analysis of the observed difference function, DeltaLi(Q). It has been revealed that the first solvation shell of Li+ consists in average of 4.5(1) PC molecules with an intermolecular Li+...O(PC) distance of 2.04(1) A. The angle Li+...O=C bond angle has been determined to be 138(2) degrees.