Effect of Water on Structure of Hydrophilic Imidazolium-Based Ionic Liquid

High-pressure phase behavior of 1-ethyl-3-methylimidazolium tetrafluoroborate and carbon dioxide system

The phase behavior of a 1-ethyl-3-methylimidazolium tetrafluoroborate and carbon dioxide system from atmospheric to supercritical state was investigated.

Physical properties of high Li-ion content N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide based ionic liquid electrolytes

Electrolytes based on bis(fluorosulfonyl)imide (FSI) with a range of LiFSI salt concentrations were characterized using physical property measurements, as well as NMR, FT-IR and Raman spectroscopy.

A new way to interpret perturbation-correlation moving-window two-dimensional correlation spectroscopy: probing the dynamic interaction of ionic liquid 1-ethyl-3-methylimidazolium acetate to absorb atmospheric water

A rule to interpret perturbation-correlation moving-window two-dimensional correlation spectroscopy (PCMW2D) was developed. Compared with Morita's rule, this proposed rule retains the ability to obtain interval features (i.e., monotonicity, concavity, and convexity) and adds the function to quickly and accurately determine both tipping points (i.e., local extrema and inflection points). It could be described as follows: the local extrema and inflection point could be determined by the zero point with an opposite sign on its left and right side in ΠΦ (synchronous PCMW2D) and ΠΨ (asynchronous PCMW2D), respectively. Specifically, a negative left (right) side and a positive right (left) side of point indicates a local minimal (maximal) value. By using the rule to interpret ΠIR (PCMW2D infrared spectroscopy) of 1-ethyl-3-methyl-imidazolium acetate [EMIM][Ac]-atmospheric water (H2O) as a function of time, we found that the atmospheric water was absorbed only into the bulk of [EMIM][Ac] before 150 min by hydrogen-bonding interaction, only onto the surface of [EMIM][Ac] after 330 min by van der Waals force, and both to the bulk and surface of [EMIM][Ac] between 150 and 330 min by hydrogen-boding and van der Waals force simultaneously. The proportion of bulk water sorption and surface water sorption to [EMIM][Ac] was about 4 and 96%, respectively.

Nanoparticles in ionic liquids: interactions and organization

Interactions between nanoparticles and ionic liquids can lead to a variety of organized structures.

The dynamic process of atmospheric water sorption in [BMIM][Ac]: quantifying bulk versus surface sorption and utilizing atmospheric water as a structure probe

The dynamic process of the atmospheric water absorbed in acetate-based ionic liquid 1-butyl-3-methyl-imidazolium acetate ([BMIM][Ac]) within 360 min could be described with three steps by using two-dimensional correlation infrared (IR) spectroscopy technique. In Step 1 (0-120 min), only bulk sorption via hydrogen bonding interaction occurs. In Step 2 (120-320 min), bulk and surface sorption takes place simultaneously via both hydrogen bonding interaction and van der Waals force. In Step 3, from 320 min to steady state, only surface sorption via van der Waals force occurs. Specifically, Step 2 could be divided into three substeps. Most bulk sorption with little surface sorption takes place in Step 2a (120-180 min), comparative bulk and surface sorption happens in Step 2b (180-260 min), and most surface sorption while little bulk sorption occurs in Step 2c (260-320 min). Interestingly, atmospheric water is found for the first time to be able to be used as a probe to detect the chemical structure of [BMIM][Ac]. Results show that one anion is surrounded by three C4,5H molecules and two anions are surrounded by five C2H molecules via hydrogen bonds, which are very susceptible to moisture water especially for the former one. The remaining five anions form a multimer (equilibrating with one dimer and one trimer) via a strong hydrogen bonding interaction, which is not easily affected by the introduction of atmospheric water. The alkyl of the [BMIM][Ac] cation aggregates to some extent by van der Walls force, which is moderately susceptible to the water attack. Furthermore, the proportion of bulk sorption vs surface sorption is quantified as about 70% and 30% within 320 min, 63% and 37% within 360 min, and 11% and 89% until steady-state, respectively.

Structures of ionic liquid–water mixtures investigated by IR and NMR spectroscopy

Imidazolium-based ionic liquids having different anions 1-butyl-3-methylimidazolium ([BMIM]X: X = Cl⁻, Br⁻, I⁻, and BF₄⁻) and their aqueous mixtures were investigated by IR absorption and proton NMR spectroscopy.

State of hydrophobic and hydrophilic ionic liquids in aqueous solutions: are the ions fully dissociated?

Molecular dynamics simulations were performed for aqueous solutions of five ionic liquids (ILs): 1-ethyl-3-methylimidazolium ([C2mim]) bis(trifluoromethanesulfonyl) imide ([NTf2]), 1-n-butyl-3-methylimidazolium ([C4mim]) [NTf2], 1-n-hexyl-3-methylimidazolium ([C6mim]) [NTf2], [C2mim] ethylsulfate ([C2H5SO4]), and [C2mim] chloride (Cl) to determine whether the ions of these ILs are associated at relatively high dilutions and whether the association is governed by hydrophobicity/hydrophilicity of the ILs. The adaptive biasing force technique was applied to calculate the potential of mean force (PMF) for each IL ion pair. For all of the ILs, the PMF is characterized by two distinct contact minima in which the ions have different relative conformations. The hydrophobic ILs bearing the anion [NTf2](-) exist predominantly in the associative state; the strength of the association of these ILs increases with increase in the alkyl chain length. The most hydrophilic IL [C2mim] Cl was determined to be almost fully dissociated at the concentration examined in the study. [C2mim] [C2H5SO4] showed hydration behavior that was intermediate between that exhibited by the ILs in which the anion is substituted with either Cl(-) or [NTf2](-) paired with [C2mim](+). Association constants for these ILs were also computed. Radial distribution functions calculated by constraining the ions at the contact minima showed that hydration of the anion plays the dominant role in determining the microscopic behavior of these ILs in aqueous solutions.

Interaction of levitated ionic liquid droplets with water

The influence of a humid or dry atmosphere on acoustically levitated ionic liquid droplets was studied by volumetric analysis and vibrational spectroscopy. Imidazolium-based ionic liquids with two types of anions, fluorinated (BF(4) and PF(6)) and alkylsulfate anions, were investigated. The amount of absorbed water was correlated with structural differences of the ionic liquids and analyzed in terms of equilibrium mole fraction as well as absorption rate. The type of anion had the most significant influence on the amount of adsorbed water from the atmosphere. Furthermore, spectral changes in the in situ Raman spectra due to absorbed water were studied for all investigated ionic liquids. For 1-ethyl-3-methylimidazolium ethylsulfate, an exemplary detailed analysis of the intermolecular interactions between cations, anions and water was carried out based on the spectroscopic data. The observed band shifts were explained with a hydrogen bond between the anion and water.

Investigation on the effect of water on the spectroscopic behaviors of TPPS in acidic imidazolium-based ionic liquids

The effect of water on the spectroscopic behaviors of meso-tetrakis-(4-sulfonatophenyl) porphyrin (TPPS) in acidic 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) is investigated by spectroscopic methods. In acidic ionic liquids without water, the Soret bands of the protonated form (H4TPPS2-) and J-aggregates of TPPS are centered at 449 and 469 nm, respectively, and the Q-band is centered at 679 nm. When increasing the content of water, the Soret band of H4TPPS2- is blue-shifted to 430 nm. However, the Soret and Q-bands of J-aggregates are red-shifted to 487 and 701 nm, respectively. The NMR and pyrene-scale polarity experiments reveal that the change of the bulk environment is responsible for the red-shift of J-aggregates and the blue-shift of H4TPPS2-. According to the analysis of the fluorescence spectra, the content of H4TPPS2- increases gradually with the addition of water, while the content of H2TPPS4- takes the opposite trend. Simultaneously, the content of J-aggregates decreases substantially when the addition of water is 0 ~ ca. 12 μL (or 13 μL) but it decreases slowly with further addition of water. Remarkably, the fluorescence intensity of J-aggregates is relatively strong. In addition, the fluorescence of J-aggregates can be detected when λex is set at both λex for TPPS monomers and λex for J-aggregates. While the fluorescence of TPPS monomers can be detected only when λex is set at λex for monomers. Moreover, with the addition of water, the fluorescence lifetimes of H2TPPS4- and H4TPPS2- increase from 3.202 and 0.830 ns to 9.623 and 2.964 ns, respectively.

High-Field EPR Spectroscopic Characterization of Spin Probes in Aqueous Ionic Liquid Mixtures

Binary mixtures of the hydrophilic room temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim+][BF4 -]) and water have been probed using high-field/high-frequency electron paramagnetic resonance (EPR) spectroscopy. By adding nitroxide radicals as paramagnetic tracers, the solution structure could be assessed from typical solutes' points of view. Three probe molecules, differing in charge and hydrogen bonding affinity, have been employed: the dianionic Fremy's salt, amphiphilic TEMPO, and hydrophilic TEMPOL. High-field pulse EPR spectroscopy has been used to determine hyperfine interaction tensors and g-matrices under cryogenic conditions. Polarity-proticity plots reveal the interaction of the nitroxide moiety with apolar (TEMPO) and ionic (FS) nano-domains, depending on the probe structure. Only small variations are observed on changing the water content suggesting that the probes are well shielded from aqueous subdomains. The results indicate that addition of water in fact stabilizes IL-rich mesostructures by embedding them in pools of water.

Multinuclear NMR studies on translational and rotational motion for two ionic liquids composed of BF4 anion

Two ionic liquids (ILs) based on the BF(4)(-) anion are studied by (1)H, (11)B, and (19)F NMR spectroscopy by measuring self-diffusion coefficients (D) and spin-lattice relaxation times (T(1)). The cations are 1-ethyl-3-methylimidazolium (EMIm) and 1-butyl-3-methylimidazolium (BMIm). Since two NMR nuclei ((11)B and (19)F) of BF(4)(-) exhibit narrow lines and high sensitivity, the (11)B and (19)F NMR measurements of D(BF4) and T(1)(BF(4)) were performed in a wide temperature range. The temperature-dependent behaviors of T(1)((19)F) and T(1)((11)B) were remarkably different, although the values of D(BF4)((19)F) and D(BF4)((11)B) almost agreed. Since the Arrhenius plots of T(1)'s for (1)H, (19)F, and (11)B exhibited T(1) minima, the correlation times τ(c)((1)H), τ(c)((19)F), and τ(c)((11)B) were evaluated. The D(cation) and D(BF(4)) were plotted against 1/τ(c)((1)H) and 1/τ(c)((19)F), respectively, and the relationships between translational and rotational motion are discussed. The translational diffusion of the cations is related to molecular librational motion and that of BF(4) is coupled with reorientational motion. The τ(c)((11)B) derived from (11)B T(1) can be attributed to a local jump. From the plots of the classical Stokes-Einstein (SE) equation, the empirical c values, which were originally derived by theoretical boundary conditions, were estimated for each ion. The empirical c(BF(4)) was about 4.4(5), while the c values of the cations were smaller than 4.

Acetonitrile boosts conductivity of imidazolium ionic liquids

We apply a new methodology in the force field generation (Phys. Chem. Chem. Phys.2011, 13, 7910) to study binary mixtures of five imidazolium-based room-temperature ionic liquids (RTILs) with acetonitrile (ACN). Each RTIL is composed of tetrafluoroborate (BF(4)) anion and dialkylimidazolium (MMIM) cations. The first alkyl group of MIM is methyl, and the other group is ethyl (EMIM), butyl (BMIM), hexyl (HMIM), octyl (OMIM), and decyl (DMIM). Upon addition of ACN, the ionic conductivity of RTILs increases by more than 50 times. It significantly exceeds an impact of most known solvents. Unexpectedly, long-tailed imidazolium cations demonstrate the sharpest conductivity boost. This finding motivates us to revisit an application of RTIL/ACN binary systems as advanced electrolyte solutions. The conductivity correlates with a composition of ion aggregates simplifying its predictability. Addition of ACN exponentially increases diffusion and decreases viscosity of the RTIL/ACN mixtures. Large amounts of ACN stabilize ion pairs, although they ruin greater ion aggregates.

Proton transfer and polarity changes in ionic liquid-water mixtures: a perspective on hydrogen bonds from ab initio molecular dynamics at the example of 1-ethyl-3-methylimidazolium acetate-water mixtures--part 1

The ionic liquid 1-ethyl-3-methylimidazolium acetate [C(2)C(1)Im][OAc] shows a great potential to dissolve strongly hydrogen bonded materials, related with the presence of a strong hydrogen bond network in the pure liquid. A first step towards understanding the solvation process is characterising the hydrogen bonding ability of the ionic liquid. The description of hydrogen bonds in ionic liquids is a question under debate, given the complex nature of this media. The purpose of the present article is to rationalise not only the existence of hydrogen bonds in ionic liquids, but also to analyse their influence on the structure of the pure liquid and how the presence of water, an impurity inherent to ionic liquids, affects this type of interaction. We perform an extensive study using ab initio molecular dynamics on the structure of mixtures of the ionic liquid 1-ethyl-3-methylimidazolium acetate with water, at different water contents. Hydrogen bonds are present in the pure liquid, and the presence of water modifies and largely disturbs the hydrogen bond network of the ionic liquid, and also affects the formation of other impurities (carbenes) and the dipole moment of the ions. The use of ab initio molecular dynamics is the recommended tool to explore hydrogen bonding in ionic liquids, as an explicit electronic structure calculation is combined with the study of the condensed phase.