Effects of Tetramethyl- and Tetraethylammonium Chloride on H₂O: Calorimetric and Near-Infrared Spectroscopic Study

The effect of tetraethylammonium chloride (TEAC) on H₂O was investigated by the 1-propanol (1P) probing thermodynamic methodology developed by us earlier. It was found that TEAC is an amphiphile with a small hydrophobic and a dominant hydrophilic contribution. An earlier application of the same 1P-probing methodology to tetramethylammonium chloride (TMAC) indicated that the latter is as hydrophilic as urea without any hydrophobic contribution. The hydrophilic effect of TEAC was found to be about twice stronger than that of TMAC. To investigate further these surprising findings, we applied a new analysis method using the concept of the excess partial molar absorptivity of the solute on the ν₂ + ν₃ combination band of H₂O in the near-infrared (NIR) range of their aqueous solutions. The results confirmed that both salts are indeed strongly hydrophilic toward H₂O which manifests itself in the 5123 cm^–1 chromophore of the NIR band of H₂O. Furthermore, we suggest from the behavior of the 5263 cm^–1 band that both solutes might form small aggregates in the H₂O-rich region of the respective aqueous solutions

Effects of Tetramethyl- and Tetraethylammonium Chloride on H₂O: Calorimetric and Near-Infrared Spectroscopic Study

The effect of tetraethylammonium chloride (TEAC) on H₂O was investigated by the 1-propanol (1P) probing thermodynamic methodology developed by us earlier. It was found that TEAC is an amphiphile with a small hydrophobic and a dominant hydrophilic contribution. An earlier application of the same 1P-probing methodology to tetramethylammonium chloride (TMAC) indicated that the latter is as hydrophilic as urea without any hydrophobic contribution. The hydrophilic effect of TEAC was found to be about twice stronger than that of TMAC. To investigate further these surprising findings, we applied a new analysis method using the concept of the excess partial molar absorptivity of the solute on the ν₂ + ν₃ combination band of H₂O in the near-infrared (NIR) range of their aqueous solutions. The results confirmed that both salts are indeed strongly hydrophilic toward H₂O which manifests itself in the 5123 cm^–1 chromophore of the NIR band of H₂O. Furthermore, we suggest from the behavior of the 5263 cm^–1 band that both solutes might form small aggregates in the H₂O-rich region of the respective aqueous solutions

Effects of Tetramethyl- and Tetraethylammonium Chloride on H₂O: Calorimetric and Near-Infrared Spectroscopic Study

The effect of tetraethylammonium chloride (TEAC) on H₂O was investigated by the 1-propanol (1P) probing thermodynamic methodology developed by us earlier. It was found that TEAC is an amphiphile with a small hydrophobic and a dominant hydrophilic contribution. An earlier application of the same 1P-probing methodology to tetramethylammonium chloride (TMAC) indicated that the latter is as hydrophilic as urea without any hydrophobic contribution. The hydrophilic effect of TEAC was found to be about twice stronger than that of TMAC. To investigate further these surprising findings, we applied a new analysis method using the concept of the excess partial molar absorptivity of the solute on the ν₂ + ν₃ combination band of H₂O in the near-infrared (NIR) range of their aqueous solutions. The results confirmed that both salts are indeed strongly hydrophilic toward H₂O which manifests itself in the 5123 cm^–1 chromophore of the NIR band of H₂O. Furthermore, we suggest from the behavior of the 5263 cm^–1 band that both solutes might form small aggregates in the H₂O-rich region of the respective aqueous solutions

Effects of Ethanol and Dimethyl Sulfoxide on the Molecular Organization of H₂O as Probed by 1-Propanol

We characterized the effects of ethanol (ET) and dimethyl sulfoxide (DMSO) on H₂O within a limited H₂O-rich region by the 1-propanol (1P)-probing methodology developed by us earlier. The results are displayed on a two-dimensional map with twin coordinates: one pertaining to hydrophobicity and the other to hydrophilicity. The locus of ET on this map was at a point in between methanol (ME) and 2-propanol (2P) as expected from our earlier findings by thermodynamic studies. That for DMSO, however, was surprisingly more hydrophilic than ME. Similar to N-methyl groups discussed recently (<i>J. Phys. Chem. B</i> <b>2011</b>, <i>115</i>, 2995), it was argued that the methyl groups attached to the S atom are made susceptible for direct hydrogen bonding to the surrounding H₂O molecules due possibly to the electronegativity of the S atom. In view of these findings, we suggest caution to be exercised for the conventional general trend of taking any methyl groups to be “hydrophobic.

Effects of Cyclic-Hydrocarbon Substituents and Linker Length on Physicochemical Properties and Reorientational Dynamics of Imidazolium-Based Ionic Liquids

We synthesized a series of alkylimidazolium-based ionic liquids (ILs) incorporating cyclopentyl, cyclohexyl, or phenyl groups as nonionic units substituted on an acyclic alkyl linker and characterized them with respect to physicochemical properties and reorientational dynamics. The effects of the nonionic substituents and linker length on the properties of these ILs were carefully examined. The physicochemical properties of the ILs are found to partially reflect the properties of the nonionic substituents. While the liquid densities showed a similar trend in linker-length dependence of each series of ILs, a distinct trend was observed for the shear viscosities of them. By comparison of correlation times obtained by ¹³C NMR spectroscopy, it is revealed that elongation of the linkers influences the characteristic effects of the nonionic substituents on the reorientational dynamics of the system

How Much Weaker Are the Effects of Cations than Those of Anions? The Effects of K⁺ and Cs⁺ on the Molecular Organization of Liquid H₂O

We characterized the effects of K⁺ and Cs⁺ ions on the molecular organization of H₂O by the 1-propanol probing methodology, previously developed by us (<i>Phys. Chem. Chem. Phys.</i> <b>2013</b>, <i>15</i>, 14548). The results indicated that both ions belong to the class of “hydration center”, which is hydrated by 4.6 ± 0.8 and 1.1 ± 0.5 H₂O molecules, respectively, and leave the bulk H₂O away from hydration shells unperturbed. Together with our previous results for the total of 7 cations and 11 anions, we display resulting characterization on a 2-D map and show a quantitative difference in their strength of the effects on H₂O between anions and cations

Comparison between Cycloalkyl- and <i>n</i>-Alkyl-Substituted Imidazolium-Based Ionic Liquids in Physicochemical Properties and Reorientational Dynamics

We synthesized three series of imidazolium-based ionic liquids (ILs) containing cycloalkyl groups such as cyclopentyl, cyclohexyl, or cycloheptyl groups incorporating bis­(trifluoromethanesulfonyl)­amide anions and characterized them with respect to physicochemical properties and molecular reorientational dynamics. A comparison of the physicochemical properties revealed that cycloalkyl-substituted imidazolium ILs have higher densities, viscosities, and glass transition temperatures than the respective <i>n</i>-alkyl-substituted imidazolium ILs. Among three series, the cyclopentyl-substituted IL exhibits exceptionally lower viscosity. Observation of correlation times by ¹³C NMR spectroscopy revealed that a remarkably lower viscosity for the cyclopentyl-substituted IL and a considerably higher viscosity for the cyclohexyl- and cycloheptyl-substituted ones are closely related to the respective reorientational motion of the cations. The cause of these distinctions is suggested to be attributed to the difference of activation energy for the conformational interconversion of their substituents

Comparison between Cycloalkyl- and <i>n</i>-Alkyl-Substituted Imidazolium-Based Ionic Liquids in Physicochemical Properties and Reorientational Dynamics

We synthesized three series of imidazolium-based ionic liquids (ILs) containing cycloalkyl groups such as cyclopentyl, cyclohexyl, or cycloheptyl groups incorporating bis­(trifluoromethanesulfonyl)­amide anions and characterized them with respect to physicochemical properties and molecular reorientational dynamics. A comparison of the physicochemical properties revealed that cycloalkyl-substituted imidazolium ILs have higher densities, viscosities, and glass transition temperatures than the respective <i>n</i>-alkyl-substituted imidazolium ILs. Among three series, the cyclopentyl-substituted IL exhibits exceptionally lower viscosity. Observation of correlation times by ¹³C NMR spectroscopy revealed that a remarkably lower viscosity for the cyclopentyl-substituted IL and a considerably higher viscosity for the cyclohexyl- and cycloheptyl-substituted ones are closely related to the respective reorientational motion of the cations. The cause of these distinctions is suggested to be attributed to the difference of activation energy for the conformational interconversion of their substituents

Phase Behavior of a Piperidinium-Based Room-Temperature Ionic Liquid Exhibiting Scanning Rate Dependence

The structural flexibility and conformational variety of the ions in room-temperature ionic liquids (RTILs) have significant effects on their physicochemical properties. To begin a systematic study of the thermodynamic properties of nonaromatic RTILs, 1-methyl-1-butylpiperidinium bis­(fluorosulfonyl)­amide ([Pip_1,4]­[FSA]) was selected as the first sample. In addition to the rotational flexibility of the alkyl group, the [Pip_1,4]⁺ cation has characteristic ring-flipping flexibility, which is very different from the behavior of the well-studied imidazolium-based cations. Calorimetry investigations using laboratory-made high-sensitivity calorimeters and Raman spectroscopy revealed that [Pip_1,4]­[FSA] has two crystalline phases, Cryst-α and Cryst-β, and that every phase change is linked to conformational changes of both the cation and anion. Each phase change is also governed by very slow dynamics. The phase changes from supercooled liquid to Cryst-α and from Cryst-α to Cryst-β, which were observed only during heating, are not in fact phase transitions but structural relaxations. Notably, the temperatures of these structural relaxations exhibited heating rate dependences, from which the activation energy of the ring-flipping was estimated to be 38.8 kJ/mol. It is thought that this phenomenon is due to the associated conformational changes of the constituent ions in viscous surroundings

Microscopic Structure of Naked Au Nanoparticles Synthesized in Typical Ionic Liquids by Sputter Deposition

Au nanoparticles (AuNPs) of varied sizes were synthesized in typical ionic liquids using the sputter deposition technique. Because AuNPs are dispersed freely in each ionic liquid without bonding to particular stabilizing agents, they are sometimes called “naked AuNPs”. To characterize the structure of naked AuNPs, we obtained a size distribution curve from small-angle X-ray scattering (SAXS) experiments and extracted the value of the peak position as the size of the most abundant AuNPs in each ionic liquid. The size was controlled by the type of ionic liquid and preparation temperature. To elucidate the local structure of the AuNPs in relation to their particle size, we measured the X-ray absorption fine structures (XAFS) at the Au L₃-edges. The analyses of the extended XAFS revealed that the Au–Au bond lengths for AuNPs ∼1 nm in size are 2.76 to 2.81 Å, in contrast with the 2.88 Å bond lengths for bulk Au. Comparing our SAXS and XAFS results with previously reported theoretical studies, we conclude that the surface Au atoms of the naked AuNPs in ionic liquids have shorter bond lengths than the inner atoms