5 research outputs found
Local Structure in Terms of Nearest-Neighbor Approach in 1-Butyl-3-methylimidazolium-Based Ionic Liquids: MD Simulations
Description of the local microscopic
structure in ionic liquids
(ILs) is a prerequisite to obtain a comprehensive understanding of
the influence of the nature of ions on the properties of ILs. The
local structure is mainly determined by the spatial arrangement of
the nearest neighboring ions. Therefore, the main interaction patterns
in ILs, such as cationâanion H-bond-like motifs, cationâcation
alkyl tail aggregation, and ring stacking, were considered within
the framework of the nearest-neighbor approach with respect to each
particular interaction site. We employed classical molecular dynamics
(MD) simulations to study in detail the spatial, radial, and orientational
relative distribution of ions in a set of imidazolium-based ILs, in
which the 1-butyl-3-methylimidazolium (C<sub>4</sub>mim<sup>+</sup>) cation is coupled with the acetate (OAc<sup>â</sup>), chloride
(Cl<sup>â</sup>), tetrafluoroborate (BF<sub>4</sub><sup>â</sup>), hexafluorophosphate (PF<sub>6</sub><sup>â</sup>), trifluoromethanesulfonate
(TfO<sup>â</sup>), or bisÂ(trifluoromethanesulfonyl)Âamide (TFSA<sup>â</sup>) anion. It was established that several structural
properties are strongly anion-specific, while some can be treated
as universally applicable to ILs, regardless of the nature of the
anion. Namely, strongly basic anions, such as OAc<sup>â</sup> and Cl<sup>â</sup>, prefer to be located in the imidazolium
ring plane next to the CâH<sup>2/4â5</sup> sites. By
contrast, the other four bulky and weakly coordinating anions tend
to occupy positions above/below the plane. Similarly, the H-bond-like
interactions involving the H<sup>2</sup> site are found to be particularly
enhanced in comparison with the ones at H<sup>4â5</sup> in
the case of asymmetric and/or more basic anions (C<sub>4</sub>mimOAc,
C<sub>4</sub>mimCl, C<sub>4</sub>mimTfO, and C<sub>4</sub>mimTFSA),
in accordance with recent spectroscopic and theoretical findings.
Other IL-specific details related to the multiple H-bond-like binding
and cation stacking issues are also discussed in this paper. The secondary
H-bonding of anions with the alkyl hydrogen atoms of cations as well
as the cationâcation alkyl chain aggregation turned out to
be poorly sensitive to the nature of the anion
SANS, Infrared, and <sup>7</sup>Li and <sup>23</sup>Na NMR Studies on Phase Separation of Alkali HalideâAcetonitrileâWater Mixtures by Cooling
Phase separation of alkali halide (MX) (M = Li<sup>+</sup>, Na<sup>+</sup>, and K<sup>+</sup> and X = Cl<sup>â</sup> and Br<sup>â</sup>)âacetonitrile (AN)âwater
mixtures by
cooling has been investigated at the molecular level. The phase diagram
obtained for the MXâANâH<sub>2</sub>O 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<sup>+</sup> >
Na<sup>+</sup> > K<sup>+</sup>. To elucidate the reasons for the
sequence
of phase separation on the meso- and microscopic scales, small-angle
neutron scattering (SANS), infrared (IR), and <sup>7</sup>Li and <sup>23</sup>Na NMR measurements have been conducted on MXâANâwater
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 spinâlattice
relaxation rates and the chemical shifts of <sup>7</sup>Li and <sup>23</sup>Na NMR for the mixtures suggested the different solvation
structure of Li<sup>+</sup> and Na<sup>+</sup> in the mixtures. In
conclusion, the solvation of acetonitrile molecules for Li<sup>+</sup> and the formation of Li<sup>+</sup>âX<sup>â</sup> 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
Effects of Tetrafluoroborate and Bis(trifluoromethylsulfonyl)amide Anions on the Microscopic Structures of 1âMethyl-3-octylimidazolium-Based Ionic Liquids and Benzene Mixtures: A Multiple Approach by ATR-IR, NMR, and Femtosecond Raman-Induced Kerr Effect Spectroscopy
The
microscopic aspects of the two series of mixtures of 1-methyl-3-octylimidazolium
tetrafluoroborate ([MOIm]Â[BF<sub>4</sub>])âbenzene and 1-methyl-3-octylimidazolium
bisÂ(trifluoromethylsulfonyl)Âamide ([MOIm]Â[NTf<sub>2</sub>])âbenzene
were investigated by several spectroscopic techniques such as attenuated
total reflectance IR (ATR-IR), NMR, and fs-Raman-induced Kerr effect
spectroscopy (fs-RIKES). All three different spectroscopic results
indicate that the anions more strongly interact with the cations in
the [MOIm]Â[BF<sub>4</sub>]âbenzene mixtures than in the [MOIm]Â[NTf<sub>2</sub>]âbenzene mixtures. This also explains the
different miscibility features between the two mixture systems well. The <i>x</i><sub>C<sub>6</sub>H<sub>6</sub></sub> dependences of the
chemical shifts and the CâH out-of-plane bending mode of benzene
are similar: the changes are large in the high benzene concentration
(<i>x</i><sub>C<sub>6</sub>H<sub>6</sub></sub> > âŒ0.6)
compared to the low benzene concentration. In contrast, the linear <i>x</i><sub>C<sub>6</sub>H<sub>6</sub></sub> dependences of the
first moments of the low-frequency spectra less than 200 cm<sup>â1</sup> were observed in both the [MOIm]Â[BF<sub>4</sub>]âbenzene
and [MOIm]Â[NTf<sub>2</sub>]âbenzene systems. The difference
in the <i>x</i><sub>C<sub>6</sub>H<sub>6</sub></sub> dependent
features between the chemical shifts and intramolecular vibrational
mode and the intermolecular/interionic vibrational bands might come
from the different probing space scales. The traces of the parallel
aromatic ring structure and the T-shape structure were found in the
ATR-IR and NMR experiments, but fs-RIKES did not observe a clear trace
of the local structure. This might imply that the interactions between
the imidazolium and benzene rings are not strong enough to librate
the imidazolium and benzene rings together. The bulk properties, such
as miscibility, density, viscosity, and surface tension, of the two
ionic liquid-benzene mixture series were also compared to the microscopic aspects
Solvation Structure of 1,3-Butanediol in Aqueous Binary Solvents with Acetonitrile, 1,4-Dioxane, and Dimethyl Sulfoxide Studied by IR, NMR, and Molecular Dynamics Simulation
The
solvation structure of 1,3-butanediol (1,3-BD) in aqueous binary
solvents of acetonitrile (AN), 1,4-dioxane (DIO), and dimethyl sulfoxide
(DMSO) at various mole fractions of organic solvent <i>x</i><sub>OS</sub> has been clarified by means of infrared (IR) and <sup>1</sup>H and <sup>13</sup>C NMR. The change in the wavenumber of
OâH stretching vibration of 1,3-BD in the three systems suggested
that water molecules which are initially hydrogen-bonded with the
1,3-BD hydroxyl groups in the water solvent (<i>x</i><sub>OS</sub> = 0) are more significantly replaced by organic solvent
molecules in the order of DMSO â« DIO > AN. This agrees with
the order of the electron donicities of the organic solvents. The <sup>1</sup>H and <sup>13</sup>C chemical shifts of 1,3-BD also revealed
the most remarkable replacement of water molecules on the hydroxyl
groups by DMSO. In contrast to the DMSO system, the OâH vibration
band of 1,3-BD in the AN and DIO systems suggested the formation of
the intramolecular hydrogen bond between the two hydroxyl groups of
1,3-BD above <i>x</i><sub>OS</sub> = âŒ0.9. To further
evaluate the intramolecular hydrogen bonding of 1,3-BD in ANâwater
binary solvents, molecular dynamics (MD) simulations and NMR experiments
for spinâlattice relaxation times <i>T</i><sub>1</sub> and <sup>1</sup>Hâ<sup>1</sup>H nuclear Overhauser effect
(NOE) were conducted on 1,3-BD in the AN system. These results showed
the intramolecular hydrogen bond within 1,3-BD in the ANâwater
binary solvents in the high AN mole fraction range of <i>x</i><sub>AN</sub> > 0.9. Especially, the pair correlation functions <i>g</i>(<i>r</i>) of the OHâO interactions of
1,3-BD obtained from the MD simulations indicated that the intramolecular
hydrogen bond remarkably increases in the AN solvent as the <i>x</i><sub>AN</sub> rises to the unity
Solvent-Dependent Properties and Higher-Order Structures of Aryl Alcohol + Surfactant Molecular Gels
Molecular organogels,
comprising small organic gelators in solvents,
can be applied for dispersal of optical devices, such as emitters.
Phenolic compounds and the surfactant bisÂ(2-ethylhexyl) sulfosuccinate
(AOT) are known examples of self-assembly organogels. However, conventional
phenol + AOT gels in aromatic and acyclic alkane solvents are optically
turbid, which is an obstacle for use as host materials in optical
devices. In this study, a variety of aryl alcoholâAOTâsolvent
sets have been investigated systematically, and the correlation between
the molecular architecture and optical transparency of the gels was
considered. Accordingly, <i>p</i>-chlorophenol + AOT gels
in cyclic alkane solvents were shown to form optically transparent
gels. In contrast, aromatic and acyclic alkane solvents gave rise
to turbid or opaque gels, even when utilizing the same gelators. AFM,
NMR, SAXS, and FTIR were employed to determine the organogel structures.
Consequently, we found that the gel transparency strongly depends
on the size of the fibrous network of the gel, the structure of which
is attributed to higher-order aggregates of the gelators. The average
contour length and diameter of the fibrous network, <i>l</i><sub>av</sub> and <i>d</i><sub>av</sub>, respectively,
were determined from AFM images. The transparent gels were shown to
have <i>l</i><sub>av</sub> = 4â9 ÎŒm and <i>d</i><sub>av</sub> †0.3 ÎŒm, whereas the turbid
gels had <i>l</i><sub>av</sub> = 15 ÎŒm and <i>d</i><sub>av</sub> = 0.4â0.6 ÎŒm. Such differences
in the size of the fibrous network significantly affected the mechanical
response of the gels, as shown by stressâstrain measurements