Structure and dynamics of the protic ionic liquid monomethylammonium nitrate ([CH3NH3][NO3]) from ab initio molecular dynamics simulations

Liquid Structure and Hydrogen Bonding in Aqueous Hydroxylammonium Nitrate

Hydroxylammonium nitrate (HAN) has emerged as a promising component in ionic liquid-based spacecraft propellants. However, the physicochemical and structural properties of aqueous HAN have been largely overlooked. The purpose of this study is to investigate the hydrogen bonding in aqueous HAN and understand its implications on these properties and the proton transfer mechanism as a function of concentration. Classical polarizable molecular dynamics simulations have been employed with the APPLE&P force field to analyze the geometry of individual hydrogen bonds and the overall hydrogen-bonding network in various concentrations of aqueous HAN. Radial distribution functions (RDFs) and spatial distribution functions (SDFs) indicate the structural arrangement of the species and their hydrogen bonds. Projections of water density and the orientation of its electric dipole moment near the ions provide insight into the hydrogen-bonding network. The incorporation of water into the hydrogen-bonding network at high ion concentrations occurs via interstitial accommodation around the ions immediately outside the first solvation shell. While ion pairs are observed at all concentrations considered, the frequency of Ha···On hydrogen bonds increases substantially with the ion concentration. The findings contribute to a better fundamental understanding of HAN and the precursors of reactivity, crucial to the development of "green" spacecraft propellants.

Highlighting the difference in nanostructure between domain-forming and domainless protic ionic liquids

Nanoheterogeneity in some ionic liquids is a known phenomenon, but quantifying or sometimes even identifying it is not a straightforward task. We compared several known and suggested some novel approaches to identify and characterize domain segregation using the results of atomistic simulations. 10 ammonium-based protic ionic liquids with different propensity to form segregated polar and apolar domains as suggested by experimental studies were considered. They include butyl-, propyl-, 2-methoxyethylammonium nitrate, butyl- and propylammonium hydrogen sulfate, butylammonium thiocyanate (domain-forming liquids), ethylammonium and pyrrolidinium nitrate (weakly pronounced segregation), methylammonium and 2-hydroxyethylammonium nitrate (domainless liquids). Molecular dynamics simulations were performed using models based on the OPLS-AA force field with scaled ion charges. Results show that domains can be recognized and the characteristic domain length scale can be determined from peaks of Ripley's functions, peaks and large-period oscillations of finite-volume radial distribution function integral, or difference of such integrals for polar and apolar atoms, and peaks of local atom density variance. These peaks disappear with increasing temperature due to the disruption of segregated domains. In domain-forming liquids, apolar atoms are more homogeneously distributed in space than polar atoms. In addition, the probability of molecular-sized cavity formation is significantly higher in apolar domains, which determines better solubility of apolar species in domain-forming ILs. The suggested approaches can be applied to various nanostructured liquids including both ionic and molecular solvents and mixtures, as well as other systems with mesoscale ordering.

Spectroscopic Signatures of Hydrogen-Bonding Motifs in Protonic Ionic Liquid Systems: Insights from Diethylammonium Nitrate in the Solid State

Diethylammonium nitrate, [N0 0 2 2][NO3], and its perdeuterated analogue, [N D D 2 2] [NO3], were structurally characterized and studied by infrared, Raman, and inelastic neutron scattering (INS) spectroscopy. Using these experimental data along with state-of-the-art computational materials modeling, we report unambiguous spectroscopic signatures of hydrogen-bonding interactions between the two counterions. An exhaustive assignment of the spectral features observed with each technique has been provided, and a number of distinct modes related to NH···O dynamics have been identified. We put a particular emphasis on a detailed interpretation of the high-resolution, broadband INS experiments. In particular, the INS data highlight the importance of conformational degrees of freedom within the alkyl chains, a ubiquitous feature of ionic liquid (IL) systems. These findings also enable an in-depth physicochemical understanding of protonic IL systems, a first and necessary step to the tailoring of hydrogen-bonding networks in this important class of materials.

Hydrogen Bond Kinetics, Ionic Dynamics, and Voids in the Binary Mixtures of Protic Ionic Liquids with Alkanolamines

Classical molecular dynamics simulations were used to investigate the structural and dynamical properties of the mixtures of ionic liquids (ILs) with the conjugate forms of the cation in a 1:1 molar ratio. The experimental studies suggested the combination of ethanolamines and ILs as novel absorbents for acidic gases such as CO₂ and H₂S, which provide the advantage of efficient absorption of gases at low pressures. However, the microscopic properties of the ionic mixtures are not studied. From our computational investigations, the densities of mixtures are reported and compared with the experimental results. The structural evolution of mixtures is reported by radial distribution functions, coordination numbers, void analysis, and spatial distribution functions. The mixtures' dynamic properties were studied by analyzing the hydrogen bond, ion-pair, and ion-cage lifetimes of the system. Monoethanolammonium and triethanolammonium ILs show different types of spatial distribution functions. The cations have lesser effect on dynamics compared with anions. The charge on the anion greatly affects the dynamics of mixtures. The dianion mixtures show slower dynamics than the monoanionic mixtures. The hydrogen bonding between cations and anions is stronger than that between cations and neutral molecules due to strong coulombic attractive forces. The cations spend more time around the dianions as compared to monoanions. The distributions of voids show that the void sizes are smaller in triethanolamine-based mixtures. The sulfobenzoate-based mixtures show voids smaller than those of pyridine-3-carboxylate-based mixtures due to more available free space between the entities, which facilitates the overall dynamics.

Extension of the CL&Pol Polarizable Force Field to Electrolytes, Protic Ionic Liquids, and Deep Eutectic Solvents

The polarizable CL&Pol force field presented in our previous study, Transferable, Polarizable Force Field for Ionic Liquids (J. Chem. Theory Comput. 2019, 15, 5858, DOI: http://doi.org/10.1021/acs.jctc.9b0068910.1021/acs.jctc.9b00689), is extended to electrolytes, protic ionic liquids (PIL), deep eutectic solvents (DES), and glycols. These systems are problematic in polarizable simulations because they contain either small, highly charged ions or strong hydrogen bonds, which cause trajectory instabilities due to the pull exerted on the induced dipoles. We use a Tang-Toennies (TT) function to dampen, or smear, the interactions between charges and induced dipole at a short range involving small, highly charged atoms (such as hydrogen or lithium), thus preventing the "polarization catastrophe". The new force field gives stable trajectories and is validated through comparison with experimental data on density, viscosity, and ion diffusion coefficients of liquid systems of the above-mentioned classes. The results also shed light on the hydrogen-bonding pattern in ethylammonium nitrate, a PIL, for which the literature contains conflicting views. We describe the implementation of the TT damping function, of the temperature-grouped Nosé-Hoover thermostat for polarizable molecular dynamics (MD) and of the periodic perturbation method for viscosity evaluation from non-equilibrium trajectories in the LAMMPS MD code. The main result of this work is the wider applicability of the CL&Pol polarizable force field to new, important classes of fluids, achieving robust trajectories and a good description of equilibrium and transport properties in challenging systems. The fragment-based approach of CL&Pol will allow ready extension to a wide variety of PILs, DES, and electrolytes.

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.

Microstructural and Dynamical Heterogeneities in Ionic Liquids

Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of ILs lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

The effect of structural heterogeneity upon the microviscosity of ionic liquids

The behaviour of two molecular rotors, one charged - 3,3'-diethylthiacarbocyanine iodide (Cy3) and one neutral - 8-[4-decyloxyphenyl]-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY-C10), have been studied in various ionic liquids. The fluorescent decay lifetime has been used to elucidate the structure of the immediate region around the rotor. The neutral BODIPY-C10 was found to prefer the non-polar alkyl chain environment, leading to two trends in the lifetime of the dye: one when it was fully partitioned into the non-polar domain, and one when it also sampled polar moieties. The positively charged Cy3 dye showed a complex relationship between the bulk viscosity of the ionic liquid and lifetime of the molecular rotor. This was attributed to a combination of polarity related spectral changes, changes in anion cages around the dye, and temperature dependent fluorescent lifetimes alongside the dependence of the rotor upon the viscosity.

Heterogeneity in the microstructure and dynamics of tetraalkylammonium hydroxide ionic liquids: insight from classical molecular dynamics simulations and Voronoi tessellation analysis

Microscopic structural and dynamic heterogeneities were investigated for three ionic liquids (ILs), tetraethylammonium hydroxide, tetrapropylammonium hydroxide, and tetrabutylammonium hydroxide employing classical molecular dynamics (MD) simulations.

Equilibrium Structure, Hydrogen Bonding, and Proton Conductivity in Half-Neutralized Diamine Ionic Liquids

Recent experiments on proton conducting ionic liquids point to half-neutralized diamine-triflate salts as promising candidates for applications in power generation and energy conversion electrochemical devices. Structural and dynamical properties of the simplest among these compounds are investigated by a combination of density functional theory (DFT) and molecular dynamics (MD) simulations based on an empirical force field. Three different cations have been considered, consisting of a pair of amine-ammonium terminations joined by a short aliphatic segment -(CH₂) _n- with n = 2, 3, and 4. First, the ground state structure, vibrational eigenstates, and hydrogen-bonding properties of single ions, neutral ion pairs, small neutral aggregates of up to eight ions, and molecularly thin hydrogen bonded wires have been investigated by DFT computations. Second, structural and dynamical properties of homogeneous liquid and amorphous phases are investigated by MD simulations over the temperature range of 200 ≤ T ≤ 440 K. Structure factors, radial distribution functions, diffusion coefficient, and electrical conductivity are computed and discussed, highlighting the inherent structural heterogeneity of these compounds. The core investigation, however, is the characterization of connected paths consisting of cation chains that could support proton transport via a Grotthuss-type mechanism. Since simulations are carried out using a force field of fixed bonding topology, this analysis is based on the equilibrium structure only, using geometrical criteria to identify potential paths for proton conduction. Paths of connected cations can reach a length of 80 cations and 30 Å, provided that bridging oxygen atoms from triflate anions are taken into account. The effects of water contamination at 1% weight concentration on the structure, dynamics, and paths for proton transport are discussed.

Probing spatial locality in ionic liquids with the grand canonical adaptive resolution molecular dynamics technique

We employ the Grand Canonical Adaptive Resolution Simulation (GC-AdResS) molecular dynamics technique to test the spatial locality of the 1-ethyl 3-methyl imidazolium chloride liquid. In GC-AdResS, atomistic details are kept only in an open sub-region of the system while the environment is treated at coarse-grained level; thus, if spatial quantities calculated in such a sub-region agree with the equivalent quantities calculated in a full atomistic simulation, then the atomistic degrees of freedom outside the sub-region play a negligible role. The size of the sub-region fixes the degree of spatial locality of a certain quantity. We show that even for sub-regions whose radius corresponds to the size of a few molecules, spatial properties are reasonably reproduced thus suggesting a higher degree of spatial locality, a hypothesis put forward also by other researchers and that seems to play an important role for the characterization of fundamental properties of a large class of ionic liquids.

Finding the best density functional approximation to describe interaction energies and structures of ionic liquids in molecular dynamics studies

Ionic liquids raise interesting but complicated questions for theoretical investigations due to the fact that a number of different inter-molecular interactions, e.g., hydrogen bonding, long-range Coulomb interactions, and dispersion interactions, need to be described properly. Here, we present a detailed study on the ionic liquids ethylammonium nitrate and 1-ethyl-3-methylimidazolium acetate, in which we compare different dispersion corrected density functional approximations to accurate local coupled cluster data in static calculations on ionic liquid clusters. The efficient new composite method B97-3c is tested and has been implemented in CP2K for future studies. Furthermore, tight-binding based approaches which may be used in large scale simulations are assessed. Subsequently, ab initio as well as classical molecular dynamics simulations are conducted and structural analyses are presented in order to shed light on the different short- and long-range structural patterns depending on the method and the system size considered in the simulation. Our results indicate the presence of strong hydrogen bonds in ionic liquids as well as the aggregation of alkyl side chains due to dispersion interactions.

How to Harvest Grotthuss Diffusion in Protic Ionic Liquid Electrolyte Systems

Hydrogen is often regarded as fuel of the future, and there is an increasing demand for the development of anhydrous proton-conducting electrolytes to enable fuel-cell operation at elevated temperatures exceeding 120 °C. Much attention has been directed at protic ionic liquids as promising candidates, but in the search for highly conductive systems the possibility of designing Grotthuss diffusion-enabled protic ionic liquids has been widely overlooked. Herein, the mechanics of proton-transfer mechanism in the equimolar mixture of N-methylimidazole and acetic acid was explored using ab initio molecular dynamics simulations. The ionicity of the system is approximated with good agreement to experiments. This system consists mostly of neutral species but exhibits a high ionic conductivity through Grotthuss-like proton conduction. Chains of acetic-acid molecules and other species participating in the proton-transfer mechanisms resembling Grotthuss diffusion could be directly observed. Furthermore, based on these findings, a series of static quantum chemical calculations was conducted to investigate the effect of substituting the anion and cation with different functional groups. We predict whether a given combination of cation and anion will be a true ionic liquid or a molecular mixture and propose some systems as candidates for Grotthuss diffusion-enabled protic ionic liquids.

Evidence of C--F-P and aromatic π--F-P weak interactions in imidazolium ionic liquids and its consequences

A simple change from alkyl group to alkene in side chain of imidazolium cation with same anion resulted in a drastic impact on physical properties (e.g., melting point) from bmimPF₆ IL to cmimPF₆ IL. The underlying reasons have been elucidated by structural and interaction studies with the help of DSC, SCXRD, vibrational and multi-nuclear NMR spectroscopic techniques. Experiments reveal existence of new weak interactions involving the carbon and π cloud of the imidazolium aromatic ring with fluoride of PF₆ anion (i.e., C₂--F-P and π--F-P) in cmimPF₆ but are absent in structurally similar prototype IL, bmimPF₆. Though weak, these interactions helped to form ladder type supramolecular arrangement, resulting in quite high melting point for cmimPF₆ IL compared to bmimPF₆ IL. These findings emphasize that an IL system can behave uniquely because of the existence of uncommon weak interactions.

Towards open boundary molecular dynamics simulation of ionic liquids

We extend the use of the adaptive resolution (AdResS) method in its grand canonical-like version (GC-AdResS) to the molecular dynamics simulation of 1,3-dimethylimidazolium chloride. We show that the partitioning of the total system in a subsystem of interest with atomistic details and a reservoir of coarse-grained particles leads to satisfactory results. The challenging aspect of this study, compared to previous AdResS simulations, is the presence of charged particles and the necessity of addressing the question about the minimal physical input needed to model the coarse-grained particles in the reservoir. We propose two different approaches and show that in both cases they are sufficient to capture the decisive physical characteristics that allow a valid system-reservoir coupling. The technically satisfactory results pave the way for the multiscale analysis of ionic liquids and truly open boundary molecular simulations.

Mapping the Free Energy of Lithium Solvation in the Protic Ionic Liquid Ethylammonuim Nitrate: A Metadynamics Study

Understanding lithium solvation and transport in ionic liquids is important due to their possible application in electrochemical devices. Using first-principles simulations aided by a metadynamics approach we study the free-energy landscape for lithium ions at infinite dilution in ethylammonium nitrate, a protic ionic liquid. We analyze the local structure of the liquid around the lithium cation and obtain a quantitative picture in agreement with experimental findings. Our simulations show that the lowest two free energy minima correspond to conformations with the lithium ion being solvated either by three or four nitrate ions with a transition barrier between them of 0.2 eV. Other less probable conformations having different solvation pattern are also investigated.

Quantum Chemical Methods for the Prediction of Energetic, Physical, and Spectroscopic Properties of Ionic Liquids

The accurate prediction of physicochemical properties of condensed systems is a longstanding goal of theoretical (quantum) chemistry. Ionic liquids comprising entirely of ions provide a unique challenge in this respect due to the diverse chemical nature of available ions and the complex interplay of intermolecular interactions among them, thus resulting in the wide variability of physicochemical properties, such as thermodynamic, transport, and spectroscopic properties. It is well understood that intermolecular forces are directly linked to physicochemical properties of condensed systems, and therefore, an understanding of this relationship would greatly aid in the design and synthesis of functionalized materials with tailored properties for an application at hand. This review aims to give an overview of how electronic structure properties obtained from quantum chemical methods such as interaction/binding energy and its fundamental components, dipole moment, polarizability, and orbital energies, can help shed light on the energetic, physical, and spectroscopic properties of semi-Coulomb systems such as ionic liquids. Particular emphasis is given to the prediction of their thermodynamic, transport, spectroscopic, and solubilizing properties.

Trends in Two- and Three-Body Effects in Multiscale Clusters of Ionic Liquids

Applications of higher correlated levels of ab initio theory to condensed systems require a significant amount of computational resources. The recent development of the fragment molecular orbital (FMO) approach alleviates this issue by splitting the system into individual fragments and achieves the accuracy of the method by accounting for all possible two-body and three-body interactions. In this work a comprehensive application of the FMO approach in combination with a second order of Møller-Plesset perturbation theory method, MP2, is presented for multiscale clusters of ionic liquids such as [C₁mim]X, [C₁mpyr]X, [C₂py]X, and [NMe₄]X, where X = chloride and tetrafluoroborates, BF₄^-, with the clusters varying in size from 4, 8, 16, to 32 ion pairs. Reliable cutoff criteria for the inclusion of two-body and three-body interactions are identified for both HF energy and MP2 correlation energy to achieve the desired accuracy of 1 kJ mol^-1. The importance of two-body and three-body interactions in ionic liquids is also discussed.

Accurate prediction of energetic properties of ionic liquid clusters using a fragment-based quantum mechanical method

Accurate prediction of physicochemical properties of ionic liquids (ILs) is of great significance to understand and design novel ILs with unique properties.

Noncovalent interactions underlying binary mixtures of amino acid based ionic liquids: insights from theory

Mixtures of ionic liquids formed by blending a common 1-methyl-3-butylimidazolium [Bmim] cation with the dicarboxylic amino acid anions viz., aspartic acid [Asp], asparagine [Asn], glutamic acid [Glu], and glutamine [Gln], have been investigated by employing dispersion corrected density functional theory.

Deep eutectic solvents: similia similibus solvuntur?

Multiple similar interactions in reline facilitates a large entropy and low melting point of the liquid state.

Modern ab initio valence bond theory calculations reveal charge shift bonding in protic ionic liquids

The nature of bonding interactions between the cation and the anion of an ionic liquid is at the heart of understanding ionic liquid properties. A particularly interesting case is a special class of ionic liquids known as protic ionic liquids. The extent of proton transfer in protic ionic liquids has been observed to vary according to the interacting species. Back proton transfer renders protic ionic liquids volatile and to be considered as inferior ionic liquids. We try to address this issue by employing modern ab initio valence bond theory calculations. The results indicate that the bonding in the cation and the anion of a prototypical ionic liquid, ethylammonium nitrate, is fundamentally different. It is neither characteristic of covalent/polar covalent bonding nor ionic bonding but rather charge shift bonding as a resonance hybrid of two competing ionic molecular electronic structure configurations. An investigation of other analogous protic ionic liquids reveals that this charge shift bonding seems to be a typical characteristic of protic ionic liquids while the ionic solid analogue compound ammonium nitrate has less charge shift bonding character as compared to protic ionic liquids. Further the extent of charge shift bonding character has been found to be congruent with the trends in many physicochemical properties such as melting point, conductivity, viscosity, and ionicity of the studied ionic liquids indicating that percentage charge shift character may serve as a key descriptor for large scale computational screening of ionic liquids with desired properties.

Dual Ionic and Organic Nature of Ionic Liquids

Inherited the advantages of inorganic salts and organic solvents, ionic liquids (ILs) exhibit many superior properties allowing them promising green solvents for the future. Although it has been widely acknowledged that the unique features of ILs originate from their dual ionic and organic nature, its microscopic physical origin still remains blurry. In this work, by comparing the ion/molecule cage structures obtained from molecular dynamics simulations for seven prototypic liquids--a molten inorganic salt, four ILs, a strongly polar organic solvent, and a weakly polar organic solvent, we have revealed that the depth of the cage energy landscape characterizes the ionic nature of ILs, whereas the slope and curvature of its mimimum determine the organic nature of ILs. This finding advances our understanding of ILs and thus will help their efficient utilization as well as the systematic design of novel functionalized ILs.

Brønsted acidity of protic ionic liquids: a modern ab initio valence bond theory perspective

Modern ab initio valence bond theory calculations of representative primary and tertiary ammonium protic ionic liquids indicate that modern ab initio valence bond theory can be employed to assess the acidity and ionicity of protic ionic liquids a priori.

Significance of weak interactions in imidazolium picrate ionic liquids: spectroscopic and theoretical studies for molecular level understanding

The effects of interionic hydrogen bonding and π-π stacking interactions on the physical properties of a new series of picrate anion based ionic liquids (ILs) have been investigated experimentally and theoretically. The existence of aromatic (C2-HO) and aliphatic (C7-HO-N22 and C6-HO-N20) hydrogen bonding and π-π stacking interactions in these ILs has been observed using various spectroscopic techniques. The aromatic and aliphatic C-HO hydrogen bonding interactions are found to have a crucial role in binding the imidazolium cation and picrate anion together. However, the π-π stacking interactions between two successive layers are found to play a decisive role in tight packing in ILs leading to differences in physical properties. The drastic difference in the melting points of the methyl and propyl derivatives (mmimPic and pmimPic respectively) have been found to be primarily due to the difference in the strength and varieties of π-π stacking interactions. While in mmimPic, several different types of π-π stacking interactions between the aromatic rings (such as picrate-picrate, picrate-imidazole and imidazolium-imidazolium cation rings) are observed, only one type of π-π stacking interaction (picrate-picrate rings) is found to exist in the pmimPic IL. NMR spectroscopic studies reveal that the interaction of these ILs with solvent molecules is different and depends on the dielectric constant of the solvent. While an ion solvation model explains the solvation in high dielectric solvents, an ion-pair solvation model is found to be more appropriate for low dielectric constant solvents. The enhanced stability of these investigated picrate ILs compared with that of inorganic picrate salts under high doses of γ radiation clearly indicates the importance of weak interionic interactions in ILs, and also opens up the possibility of the application of picrate ILs as prospective diluents in nuclear separation for advanced fuel cycling process.