Interplay of Phase Separation, Tail Aggregation, and Micelle Formation in the Nanostructured Organization of Hydrated Imidazolium Ionic Liquid

Ultralow Friction and High Robustness of Monolayer Ionic Liquids

Ultralow friction between interacting surfaces in relative motion is of vital importance in many pure and applied sciences. We found that surfaces bearing ordered monolayer ionic liquids (ILs) can have friction coefficient μ values as low as 0.001 at pressures up to 78 MPa and exhibit good structure recoverability. This extreme lubrication is attributed primarily to the ordered striped structure driven by the "atomic-locking" effect between carbon atoms on the alkyl chain of ILs and graphite. The longer alkyl chain has lower μ values, and the stripe periodicity is decisive in reducing energy dissipation during the sliding process. In combination with simulation, the alternate atomic-scale ordered and disordered ionic regions were recognized, whose ratio fundamentally determines the μ values and lubrication mechanism. This finding is an important step toward the practical utilization of ILs with negligible vapor pressure as superlubricating materials in future technological applications operating under extreme conditions.

Multicomponent Phase Separation in Ternary Mixture Ionic Liquid Electrolytes

We investigate the phase behavior of ternary mixtures of ionic liquid, organic solvent, and lithium salt by molecular dynamics simulations. We find that at room temperature, the electrolyte separates into distinct phases with specific compositions; an ion-rich domain that contains a fraction of solvent molecules and a second domain of pure solvent. The phase separation is shown to be entropy-driven and is independent of lithium salt concentration. Phase separation is only observed at microsecond time scales and greatly affects the transport properties of the electrolyte.

Molecular simulation of osmometry in aqueous solutions of the BMIMCl ionic liquid: a potential route to force field parameterization of liquid mixtures

Despite widespread development and use of ionic liquids (ILs) in both academic and industrial research, computational force fields (FFs) for most of those are not available for a precise description of inter-species interactions in aqueous environments. In the scope of this study, by means of molecular simulations, the osmotic coefficient of an aqueous solution of an IL is calculated and used as a basis to reparameterize popular IL-FFs existing in the literature. We first calculate the osmotic coefficients (at 298.15 K and 1 atm pressure) of aqueous solutions of 1-butyl-3-methylimidazolium chloride (BMIMCl), a generic IL, popularly used in biomass processing and the subsequent conversion to value-added intermediates. The performance of two popular atomic, nonpolarizable FFs developed for BMIMCl, one by Lopes, Pádua, and coworkers (FF-LP) and the other by Sambasivarao, Acevedo, and coworkers (FF-SA), when mixed with the SPC/E water model, is tested with respect to their ability to reproduce the experimental osmotic coefficient data. Interestingly, the osmotic coefficient is found to be increasing with a gradual increase in IL molality within the concentration range of our investigation, which is contrary to the experimental trend reported in the literature for the same IL-water mixture. Henceforth, necessary corrections to the nonbonded ion-ion and ion-water interactions are made to match the experimental osmotic coefficient. To further assess the reliability of the new FF, we extensively explore the thermodynamic (density, isothermal compressibility, and thermal expansion coefficient), dynamic (diffusivity and viscosity), and association/dissociation properties (rationalized with the help of radial distribution functions) with both the original and reparameterized FF for a wider range of concentrations up to a molality of 18.50 mol kg-1. The calculated quantities are compared against experimental data wherever available. The modified FF parameters exhibit significant improvements in terms of its ability to match experimental solution properties, such as density, viscosity, association/dissociation, etc. We report that excessive dissociation of BMIMCl in water is responsible for the shortcomings observed in the original FFs and improved prediction of physicochemical properties could be achieved using the modified FFs.

The wetting behavior of aqueous imidazolium based ionic liquids: a molecular dynamics study

Aqueous ionic liquids are of particular interest due to their tunability of physical and chemical properties and a deeper understanding of their structure-property relationship is desired. Molecular dynamics (MD) simulations were conducted to study the wetting behavior of aqueous imidazolium-based ionic liquids (ILs), consisting of a 1-ethyl-3-methylimidazolium [EMIM]+ cation and either a hydrophilic boron tetrafluoride [BF4]- or a hydrophobic bis(trifluoromethylsulfonyl)imide [NTF2]- anion mixed in water. To understand the effect of anion and concentration of ILs at the graphite solid-liquid interface, wettability studies were performed with IL concentrations from 0-50 wt%. The contact angle of aqueous IL droplets decreases with increasing IL concentration. Droplet characteristics near the surface were investigated by profiling the density perpendicular (z-direction) and horizontal (r-direction) to the graphite sheet; this was further quantified by an orientation order parameter. Due to the preferred adsorption of ILs, water depletes near the surface as IL concentration increases. The hydrophobic [NTF2]- anion forces the IL toward the interface from the bulk, whereas the hydrophilic [BF4]- anion causes the IL to remain in the bulk of the droplet. Differences in water-anion hydrogen bonding, the nature of the anions, and their interfacial tensions are crucial factors in the wetting behavior of aqueous ionic liquids.

Tuning the Electrocatalytic Performance of Ionic Liquid Modified Pt Catalysts for the Oxygen Reduction Reaction via Cationic Chain Engineering

Modifying Pt catalysts using hydrophobic ionic liquids (ILs) has been demonstrated to be a facile approach for boosting the performance of Pt catalysts for the oxygen reduction reaction (ORR). This work aims to deepen the understanding and initiate a rational molecular tuning of ILs for improved activity and stability. To this end, Pt/C catalysts were modified using a variety of 1-methyl-3-alkylimidazolium bis(trifluoromethanesulfonyl)imide ([C _n C₁im][NTf₂], n = 2-10) ILs with varying alkyl chain lengths in imidazolium cations, and the electrocatalytic properties (e.g., electrochemically active surface area, catalytic activity, and stability) of the resultant catalysts were systematically investigated. We found that ILs with long cationic chains (C6, C10) efficiently suppressed the formation of nonreactive oxygenated species on Pt; however, at the same time they blocked active Pt sites and led to a lower electrochemically active surface area. It is also disclosed that the catalytic activity strongly correlates with the alkyl chain length of cations, and a distinct dependence of intrinsic activity on the alkyl chain length was identified, with the maximum activity obtained on Pt/C-[C₄C₁im][NTf₂]. The optimum arises from the counterbalance between more efficient suppression of oxygenated species formation on Pt surfaces and more severe passivation of Pt surfaces with elongation of the alkyl chain length in imidazolium cations. Moreover, the presence of an IL can also improve the electrochemical stability of Pt catalysts by suppressing the Pt dissolution, as revealed by combined identical-location transmission electron microscopy (TEM) and in situ inductively coupled plasma mass spectrometry (ICP-MS) analyses.

Probing translational and rotational dynamics in hydrophilic/hydrophobic anion based imidazolium ionic liquid-water mixtures

In this investigation, we examine the effect of water concentration and temperature on the dynamical properties of [Hmim][Cl] and [Hmim][NTf2] ionic liquids (ILs). The dynamical properties such as translational diffusion coefficients, ion-pair lifetimes, and rotational correlation times are calculated using molecular dynamics simulations. The simulations predict that water concentration also significantly impacts the magnitude of dynamical properties. At low, intermediate and high water concentrations, the following trend in diffusion coefficients is seen: Cl- > Hmim+; Cl- > NTf2-; Hmim+ ([Hmim][Cl]) > Hmim+ ([Hmim] [NTf2]). At ultra-low water concentrations of [Hmim][Cl] IL, several bridge like configurations form between water molecules and Cl- anions, which are supported by a complex distribution of water clusters. The effect of an increase in the water concentration leads to a decrease in ion-pair lifetimes between the Hmim+ cations and Cl-/NTf2- anions, which strongly correlates with the trends observed from the diffusion coefficients. A biexponential function was found to be the best fit for the RACF at neat/ultra-low water concentrations of [Hmim][Cl] and [Hmim][NTf2] ILs, whereas a single exponential function was sufficient to fit the RACF at low, intermediate and high water concentrations. The rotational relaxation time of the Hmim+ cations is larger in neat [Hmim][Cl] compared to that in neat [Hmim][NTf2] with an opposite trend seen with hydration. The rotational correlation time of water molecules is larger in [Hmim][Cl] compared to that in [Hmim][NTf2] at low and intermediate water concentrations, with similar correlation times observed at high water concentrations.

Effect of molecular solvents of varying polarity on the self-assembly of 1-n-dodecyl-3-methylimidazolium octylsulfate ionic liquid

Large-scale molecular dynamics simulations consisting of more than 88,000–106,000 atoms for approximately 250 ns (including equilibration and production) were conducted to assess the effect of polar, nonpolar and amphiphilic molecular solvents on the nanoscale structuring of 1-[Formula: see text]-dodecyl-3-methylimidazolium [C[Formula: see text]mim] octylsulfate [C8SO4] ionic liquid (IL). Water [H2O], [Formula: see text]-octane [C8H[Formula: see text]] and 1-octanol [C8H[Formula: see text]OH] are employed as examples of polar, nonpolar, and amphiphilic molecules, respectively. The results indicate that each of these molecular solvents modify the nanosegregation behavior of the ionic liquid in a unique way. Water induces a high order of structuring of the ionic liquid as indicated by extremely high nematic order parameter for the system. In addition, the morphology of the neat ionic liquid is transformed from layer-like to that of bilayer-like in which the polar and nonpolar domains alternate. The presence of water also causes the stretching of the nonpolar domain, thus, increasing its size. At the concentration examined in this work, [Formula: see text]-octane is found to be only partially miscible with the ionic liquid. The polar network is maintained; however, the continuous cationic nonpolar domain is split into multiple domains. [Formula: see text]-octane is accommodated in the ionic liquid nonpolar domain. Similarly, the amphiphilicity of 1-octanol leads to an increase in the number of cationic as well as anionic domains. The overall nonpolar domain length, however, remains nearly identical to that found for the pure ionic liquid. Additional characterization of structural features of the three systems is discussed in terms of one-dimensional number densities, nematic order parameters for the overall systems and their components and structure factors.

"Solvent-in-salt" systems for design of new materials in chemistry, biology and energy research

Inorganic and organic "solvent-in-salt" (SIS) systems have been known for decades but have attracted significant attention only recently. Molten salt hydrates/solvates have been successfully employed as non-flammable, benign electrolytes in rechargeable lithium-ion batteries leading to a revolution in battery development and design. SIS with organic components (for example, ionic liquids containing small amounts of water) demonstrate remarkable thermal stability and tunability, and present a class of admittedly safer electrolytes, in comparison with traditional organic solvents. Water molecules tend to form nano- and microstructures (droplets and channel networks) in ionic media impacting their heterogeneity. Such microscale domains can be employed as microreactors for chemical and enzymatic synthesis. In this review, we address known SIS systems and discuss their composition, structure, properties and dynamics. Special attention is paid to the current and potential applications of inorganic and organic SIS systems in energy research, chemistry and biochemistry. A separate section of this review is dedicated to experimental methods of SIS investigation, which is crucial for the development of this field.

Mobility and association of ions in aqueous solutions: the case of imidazolium based ionic liquids

Combining MD simulations and RISM calculations with experiments, we demonstrated that the interionic interactions of investigated ILs as model 1,1 electrolytes in water solutions are weak but evidently dependent on the molecular structure.