Nanoscale Organization in Aqueous Dicationic Ionic Liquid Solutions

DFT and COSMO-RS studies on dicationic ionic liquids (DILs) as potential candidates for CO2 capture: the effects of alkyl side chain length and symmetry in cations

The weaker interaction energy between anions and cations, the stronger interaction of a CO2 molecule with the cation. Also, the selectivity of CO2 from H2, CO and CH4 gases decreases slightly with increasing the length of side alkyl chains.

Effect of Lithium-Ion on the Structural Organization of Monocationic and Dicationic Ionic Liquids

In recent times, ionic liquid-based (ILs) electrolytic system has emerged as suitable alternative to the conventional organic solvent-based electrolytic system. However, since, anion of ILs is known to form aggregates in the presence of lithium-ions (Li⁺), and this can influence the transport properties of Li⁺ ion in a significant manner, it is, therefore, important to understand how lithium-ions influence the structure and dynamics of ILs. With this objective, in the present study, intermolecular interaction, structural organization, and dynamics of monocationic ILs (MILs) and dicationic IL (DIL) have been studied in the absence and presence of lithium salt. Specifically, for this purpose, two MILs, 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([C₃C₁im][NTf₂]), 1-hexyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([C₆C₁im][NTf₂]), and a DIL, 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide ([C₆(mim)₂][NTf₂]₂) have been chosen in such a way that either the alkyl chain of MILs becomes equal or half of the spacer chain length of DIL. To understand the effect of the addition of lithium-ion on the structural organization of MILs and DIL, steady-state absorption and fluorescence spectroscopies, time-resolved fluorescence anisotropy and nuclear magnetic resonance (NMR) techniques have been used. Structural organization in the apolar and polar domains of ILs has been probed by following the rotational diffusion of suitably chosen solute in the concerned media through time-resolved fluorescence anisotropy (TRFA) measurements. TRFA studies have revealed that with the addition of Li⁺ ion, coordination between the Li⁺ ions and anions of MILs and DILs takes place in the ionic region leading to a change in the structural organization of the apolar regions of the respective medium. In fact, upon adding lithium-ions, a reduction in the packing of alkyl chains has also been observed for the MILs. However, not much change in the structural organization of the apolar region of the DIL has been observed when Li⁺ ion is added to it. In the presence of Li⁺ ions, a similar trend in the change of structural organization of polar regions for both MILs and DIL has been observed. Further, measurements of the self-diffusion coefficient through NMR have also supported the observation that Li⁺ ion also perturbs the nanostructural organization of the MIL in a significant manner than that it does for the DIL. The behavior of DIL in the presence of Li⁺ ion, as revealed by the present study, has been rationalized by considering the folded arrangement of DIL in the fluid-structure. Essentially, all of these investigations have suggested that the addition of lithium-ion significantly alters the microscopic behavior of MILs in comparison to that of DIL. The outcome of this study is expected to be helpful in realizing the potentials of these media as electrolytes in battery applications.

Understanding the Behavior of Monocationic and Dicationic Room-Temperature Ionic Liquids through Resonance Energy-Transfer Studies

The present work has been undertaken with an objective to understand the differences in the local structural organization of imidazolium-based monocationic ionic liquids (MILs) and dicationic ionic liquids (DILs) through resonance energy-transfer (RET) studies. In this study, a neat IL is used as a donor and a charged species rhodamine 6G (R6G) is used as an acceptor unit because of the fact that they satisfy the spectroscopic criteria that are needed for an RET event to take place. Additionally, R6G, being a charged species, is expected to facilitate the electrostatic interactions with the ILs which are also charged. Specifically, two imidazolium-based germinal DILs and their monocationic counterparts are used for the present investigations. Additionally, the studies are carried out in some selected MILs where the lengths of the alkyl side chains are kept unchanged for MILs and DILs. Interestingly, the present data reveal that the RET interaction is more favorable for DILs than for MILs, even though the DILs are relatively bulkier than their monocationic counterparts. More interestingly, the RET interaction is also found to be more favorable for DILs than for MILs, where the length of the alkyl group is kept fixed for MILs and DILs. The result of the present study delineates that the alkyl chain length on the cation is not the sole factor contributing to the RET outcomes for DILs and MILs but the local structure of DILs also contributes significantly to the same. The current investigation clearly indicates that DILs have a more compact local structure than that of MILs. Essentially, the current study highlights that a cost-effective, noninvasive technique such as RET is quite effective in capturing the differences in the nanostructural organization of MILs and DILs.

Molecular Dynamics Study of the Effect of Water on Hydrophilic and Hydrophobic Ionic Liquids

We performed molecular dynamics (MD) simulations of ionic liquid (IL)-water mixtures to investigate the effects of water addition. The IL cation 1-butyl-3-methylimidazolium ([C₄mim]) and the four anions, nitrate (NO₃), tetrafluoroborate (BF₄), hexafluorophosphate (PF₆), and bis(trifluoromethanesulfonyl)imide (NTf₂), were used to examine the effects of differences in hydrophobicity and anion size. The radial distribution functions of water-water have two different water content dependences. In NO₃ and BF₄ systems, the effect of water-anion-water structures connected by hydrogen bonds due to the strong interaction of anion-water is large. The growth of water clusters changes the peak heights of the radial distribution functions in PF₆ and NTf₂ systems. The increase in the diffusion constant is small in the NO₃ system but is not small in the other systems. The relaxation time of the anion-water hydrogen bonds in the NO₃ system is much longer than those in the other systems. It is the reason for the low water content dependency of the diffusion constants of the NO₃ system. The water constant dependences of structures in NO₃ and BF₄ systems are similar, but that in the diffusion constant is not. The BF₄ system shows hydrophilic features in the structural change and hydrophobic features in the water content dependences of the diffusion constant. The addition of water molecules provides various hydrophobicity/hydrophilicity of anions.

Differences in the behavior of dicationic and monocationic ionic liquids as revealed by time resolved-fluorescence, NMR and fluorescence correlation spectroscopy

With an aim to understand the behavior in terms of the intermolecular interactions, structure and dynamics of dicationic and monocationic ionic liquids (ILs), two imidazolium-based dicationic ionic liquids (DILs), 1,8-bis-(3-methylimidazolium-1-yl)octane bis-(trifluoromethylsulfonyl)amide ([C₈(mim)₂][NTf₂]₂), 1,9-bis-(3-methylimidazolium-1-yl)nonane bis-(trifluoromethylsulfonyl)amide ([C₉(mim)₂][NTf₂]₂), and one monocationic ionic liquid (MIL), 1-butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide ([C₄(mim)][NTf₂]), have been investigated through combined fluorescence, electron paramagnetic resonance (EPR), NMR and fluorescence correlation spectroscopy (FCS). The DILs were synthesized by following a standard synthetic protocol and subsequently characterized by different analytical techniques. Steady state absorption, emission and EPR spectroscopic data reveal that DILs are less polar compared to MIL. The polarities of the DILs and MIL were found to be close to those of acetonitrile and short chain alcohols, respectively. The excitation wavelength dependent emission data reveals that DILs are more micro-heterogeneous in nature than MIL. The rotational diffusion of two organic solutes, perylene and 8-methoxypyrene-1,3,6-sulfonate (MPTS), were examined in the DILs and MIL. The rotational diffusion data for perylene and MPTS were analyzed in light of the Stokes-Einstein-Debye (SED) hydrodynamic theory. The rotation of perylene in the DILs was observed to be relatively faster to that in the MIL, and it goes beyond the limit predicted by the SED theory. In order to explain the rotational motion of perylene in DILs, the data was analyzed further by invoking quasi-hydrodynamic theory. The observed rotational behavior of perylene has been explained by considering the fact that perylene is located in the nonpolar region of ILs, and larger solvent molecules (DILs) induce a lower friction to the rotating solute. Interestingly, unlike perylene, rotations of MPTS in both of the ILs were observed to be much hindered indicating a relatively stronger MPTS-IL interaction than perylene-IL interaction. More interestingly, rotation of MPTS was observed to be faster in the DILs than that in the MIL despite the fact that DILs are more viscous than MILs. Relatively faster rotation of MPTS in DILs has been explained by resorting to NMR and FCS studies. The outcomes of the NMR and FCS studies revealed that DILs in the experimental condition exist in their folded form and because of this structural restriction of DILs it becomes difficult for the bulky MPTS to make stronger hydrogen bonding interactions with DILs, which eventually makes the rotation of MPTS in DILs faster. Essentially, the outcomes of all of these studies have demonstrated that the behavior of DILs is quite different to that of the usual MILs.

Molecular dynamics simulation of geminal dicationic ionic liquids [Cn(mim)2][NTf2]2 – structural and dynamical properties

The structural and dynamical properties of two dicationic ionic liquids, i.e. [C_n(mim)₂][NTf₂]₂ with n = 3 and 5, have been studied to obtain a fundamental understanding of the molecular basis of the macroscopic and microscopic properties of the bulk liquid phase.

Structural properties of geminal dicationic ionic liquid/water mixtures: a theoretical and experimental insight

The structural properties of geminal dicationic ionic liquid/water mixtures have been investigated using EXAFS spectroscopy and molecular dynamics simulations.

Ionic Liquid Coatings for Titanium Surfaces: Effect of IL Structure on Coating Profile

Dicationic imidazolium-based ionic liquids (ILs) having bis(trifluoromethylsulfonyl)imide (NTf2) and amino acid-based (methionine and phenylalanine) anionic moieties were synthesized and used to coat titanium surfaces using a dip-coating technique. Dicationic moieties with varying alkyl chains (8 and 10 carbons) and anions with distinct characteristics were selected to understand the influence of IL structural features on deposition profile. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used in this study to help elucidate intermolecular interactions within ILs as well as between ILs and TiO2 surfaces and to investigate IL coating morphology. Charge concentration on IL moieties, as well as the presence of functional groups that can interact via hydrogen bond, such as carboxylate and amino groups, were observed to influence the deposition profile. ILs containing amino acids as the anionic moiety were observed to interact strongly with TiO2, which resulted in more pronounced changes in Ti 2p binding energy. The higher hydrophobicity of the IL having NTf2 as the anionic moiety resulted in higher adhesion strength between the IL coating and TiO2.

Effect of spacer chain length on the liquid structure of aqueous dicationic ionic liquid solutions: molecular dynamics studies

Cations in an aqueous [C₁₆(MIm)₂][2Br] solution aggregate spontaneously to form micelles, with a hydrophobic core composed of spacer alkyl chains and a hydrophilic surface composed of imidazolium head groups.

[Et3NH][HSO4]-mediated functionalization of hippuric acid: an unprecedented approach to 4-arylidene-2-phenyl-5(4H)-oxazolones

Facile, sustainable and economical synthesis of Erlenmeyer azlactones.

Ionic and Molecular Liquids: Working Together for Robust Engineering

Because of their outstanding versatility, room-temperature ionic liquids (RTILs) are utilized in an ever increasing number of novel and fascinating applications, making them the Holy Grail of modern materials science. In this Perspective, we address the fundamental research and prospective applications of RTILs in combination with molecular liquids, concentrating on three significant areas: (1) the use of molecular liquids to decrease the viscosity of RTILs; (2) the role of RTIL micelle formation in water and organic solvents; and (3) the ability of RTILs to adsorb pollutant gases. Current achievements are examined, and future directions for the potential uses of RTILs are outlined.

Molecular dynamic simulation of dicationic ionic liquids: effects of anions and alkyl chain length on liquid structure and diffusion

Structures and dynamics of nine geminal dicationic ionic liquids (DILs) Cn(mim)2X2, where n = 3, 6, and 9 and X = PF6(-), BF4(-), and Br(-), were studied by molecular dynamic simulations (J. Phys. Chem.B2004, 108, 2038-2047). A force field with a minor modification for C3(mim)2 × 2 was adopted for the simulations. Densities, detailed microscopic structures, mean-square displacements (MSD), and self-diffusivities for various ion pairs from MD simulations have been presented. The calculated densities for C9(mim)2X2 (X = Br(-) and BF4(-)) agreed well with the experimental values. The calculated RDFs show that anions are well organized around the imidazolium rings. The calculated RDFs indicate that, unlike the mono cationic ILs, the anions and cations in DILs distribute homogeneously. Enthalpies of vaporization were calculated and correlated with the structural features of DILs. The local structure of C9(mim)2X2 (X = Br, PF6) was examined by the spatial distribution function (SDF). The calculated SDFs show that similar trends were found by other groups for mono cationic ionic liquids (ILs). The highest probability densities are located around the imidazolium ring hydrogens. The calculated diffusion coefficients show that the ion diffusivities are 1 order of magnitude smaller than that of the mono cationic ionic liquids. The effects of alkyl chain length and anion type on the diffusion coefficient were also studied. The dynamics of the imidazolium rings and the alkyl chain in different time scales have also discussed. The calculated transference numbers show that the anions have the major role in carrying the electric current in a DIL.