Correlated/non-correlated ion dynamics of charge-neutral ion couples: the origin of ionicity in ionic liquids

Bridging Database and Experimental Analysis to Reveal Super-hydrodynamic Conductivity Scaling Regimes in Ionic Liquids

Ion transport through electrolytes critically impacts the performance of batteries and other devices. Many frameworks used to model ion transport assume hydrodynamic mechanisms and focus on maximizing conductivity by minimizing viscosity. However, solid-state electrolytes illustrate that non-hydrodynamic ion transport can define device performance. Increasingly, selective transport mechanisms, such as hopping, are proposed for concentrated electrolytes. However, viscosity-conductivity scaling relationships in ionic liquids are often analyzed with hydrodynamic models. We report data-centric analyses of hydrodynamic transport models of viscosity-conductivity scaling in ionic liquids by merging three databases to bridge physical properties and computational descriptors. With this expansive database, we constrained scaling analyses using ion sizes defined from simulated volumes, as opposed to estimating sizes from activity coefficients. Remarkably, we find that many ionic liquids exhibit positive deviations from the Nernst-Einstein model, implying ions move faster than hydrodynamics should allow. We verify these findings using microrheology and conductivity experiments. We further show that machine learning tools can improve predictions of conductivity from molecular properties, including predictions from solely computational features. Our findings reveal that many ionic liquids exhibit super-hydrodynamic viscosity-conductivity scaling, suggesting mechanisms of correlated ion motion, which could be harnessed to enhance electrochemical device performance.

Recent advances in NMR spectroscopy of ionic liquids

This review presents recent developments in the application of NMR spectroscopic techniques in the study of ionic liquids. NMR has been the primary tool not only for the structural characterization of ionic liquids, but also for the study of dynamics. The presence of a host of NMR active nuclei in ionic liquids permits widespread use of multinuclear NMR experiments. Chemical shifts and multinuclear coupling constants are used routinely for the structure elucidation of ionic liquids and of products formed by their covalent interactions with other materials. Also, the availability of a multitude of NMR techniques has facilitated the study of dynamical processes in them. These include the use of NOESY to study inter-ionic interactions, pulsed-field gradient techniques for probing transport properties, and relaxation measurements to elucidate rotational dynamics. This review will focus on the application of each of these techniques to investigate ionic liquids.

Charge transfer and polarisability in ionic liquids: a case study

The practical use of ionic liquids (ILs) is benefiting from a growing understanding of the underpinning structural and dynamic properties, facilitated through classical molecular dynamics (MD) simulations. The predictive and explanatory power of a classical MD simulation is inextricably linked to the underlying force field. A key aspect of the forcefield for ILs is the ability to recover charge based interactions. Our focus in this paper is on the description and recovery of charge transfer and polarisability effects, demonstrated through MD simulations of the widely used 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C₄C₁im][NTf₂] IL. We study the charge distributions generated by a range of ab initio methods, and present an interpolation method for determining atom-wise scaled partial charges. Two novel methods for determining the mean field (total) charge transfer from anion to cation are presented. The impact of using different charge models and different partial charge scaling (unscaled, uniformly scaled, atom-wise scaled) are compared to fully polarisable simulations. We study a range of Drude particle explicitly polarisable potentials and shed light on the performance of current approaches to counter known problems. It is demonstrated that small changes in the charge description and MD methodology can have a significant impact; biasing some properties, while leaving others unaffected within the structural and dynamic domains.

Curled cation structures accelerate the dynamics of ionic liquids

Ionic liquids are modern liquid materials with potential and actual implementation in many advanced technologies. They combine many favourable and modifiable properties but have a major inherent drawback compared to molecular liquids - slower dynamics. In previous studies we found that the dynamics of ionic liquids are significantly accelerated by the introduction of multiple ether side chains into the cations. However, the origin of the improved transport properties, whether as a result of the altered cation conformation or due to the absence of nanostructuring within the liquid as a result of the higher polarity of the ether chains, remained to be clarified. Therefore, we prepared two novel sets of methylammonium based ionic liquids; one set with three ether substituents and another set with three butyl side chains, in order to compare their dynamic properties and liquid structures. Using a range of anions, we show that the dynamics of the ether-substituted cations are systematically and distinctly accelerated. Liquefaction temperatures are lowered and fragilities increased, while at the same time cation-anion distances are slightly larger for the alkylated samples. Furthermore, pronounced liquid nanostructures were not observed. Molecular dynamics simulations demonstrate that the origin of the altered properties of the ether substituted ionic liquids is primarily due to a curled ether chain conformation, in contrast to the alkylated cations where the alkyl chains retain a linear conformation. Thus, the observed structure-property relations can be explained by changes in the geometric shape of the cations, rather than by the absence of a liquid nanostructure. Application of quantum chemical calculations to a simplified model system revealed that intramolecular hydrogen-bonding is responsible for approximately half of the stabilisation of the curled ether-cations, whereas the other half stems from non-specific long-range interactions. These findings give more detailed insights into the structure-property relations of ionic liquids and will guide the development of ionic liquids that do not suffer from slow dynamics.

Correlated Dynamics in Ionic Liquids by Means of NMR Relaxometry: Butyltriethylammonium bis(Trifluoromethanesulfonyl)imide as an Example

¹H and ¹⁹F spin-lattice relaxation experiments have been performed for butyltriethylammonium bis(trifluoromethanesulfonyl)imide in the temperature range from 258 to 298 K and the frequency range from 10 kHz to 10 MHz. The results have thoroughly been analysed in terms of a relaxation model taking into account relaxation pathways associated with ¹H–¹H, ¹⁹F–¹⁹F and ¹H–¹⁹F dipole–dipole interactions, rendering relative translational diffusion coefficients for the pairs of ions: cation–cation, anion–anion and cation–anion, as well as the rotational correlation time of the cation. The relevance of the ¹H–¹⁹F relaxation contribution to the ¹H and ¹⁹F relaxation has been demonstrated. A comparison of the diffusion coefficients has revealed correlation effects in the relative cation–anion translational movement. It has also turned out that the translational movement of the anions is faster than of cations, especially at high temperatures. Moreover, the relative cation–cation diffusion coefficients have been compared with self-diffusion coefficients obtained by means of NMR (Nuclear Magnetic Resonance) gradient diffusometry. The comparison indicates correlation effects in the relative cation–cation translational dynamics—the effects become more pronounced with decreasing temperature.

Targeted modifications in ionic liquids - from understanding to design

Ionic liquids are extremely versatile and continue to find new applications in academia as well as industry. This versatility is rooted in the manifold of possible ion types, ion combinations, and ion variations. However, to fully exploit this versatility, it is imperative to understand how the properties of ionic liquids arise from their constituents. In this work, we discuss targeted modifications as a powerful tool to provide understanding and to enable design. A 'targeted modification' is a deliberate change in the structure of an ionic liquid. This includes chemical changes in an experiment as well as changes to the parameterisation in a computer simulation. In any case, such a change must be purposeful to isolate what is of interest, studying, as far as is possible, only one concept at a time. The concepts can then be used as design elements. However, it is often found that several design elements interact with each other - sometimes synergistically, and other times antagonistically. Targeted modifications are a systematic way of navigating these overlaps. We hope this paper shows that understanding ionic liquids requires experimentalists and theoreticians to join forces and provides a tool to tackle the difficult transition from understanding to design.

Relaxometry models compared with Bayesian techniques: ganglioside micelle example

In this work a methodology to perform Bayesian model-comparison is developed and exemplified in the analysis of nuclear magnetic relaxation dispersion (NMRD) experiments of water in a ganglioside micelle system. NMRD is a powerful tool to probe slow dynamics in complex liquids. There are many interesting systems that can be studied with NMRD, such as ionic and lyotropic liquids or electrolytes. However, to progress in the understanding of the studied systems, relatively detailed theoretical NMRD-models are required. To improve the models, they need to be carefully compared, in addition to physico-chemical considerations of molecular and spin dynamics. The comparison is performed by computing the Bayesian evidence in terms of a thermodynamic integral solved with Markov chain Monte Carlo. The result leads to a conclusion of two micelle water-pools, and rules out both less and more parameters, i.e., one and three pools. On the other hand, if only the quality of the fits is considered (i.e., mean square deviation or χ2) a three water-pool model is the best. The latter can be expected since with more adjustable parameters a better fit is more likely. Bayesian evidence is needed to rank the uncertainty of the models, and in order to explain the outcome a notation of Ockham-entropy is defined. The two approximate selection tools, Akaike and Baysian information criterions, may lead to wrong conclusions compared to the full integration. This methodology is expected to be one of several important tools in NMRD model development; however, it is completely general and should find awakened use in many research areas.

Insights into the translational and rotational dynamics of cations and anions in protic ionic liquids by means of NMR fast-field-cycling relaxometry

Understanding the translational and rotational dynamics of cations and anions in hydrogen bonded protic ionic liquids (PIls) is still a challenge. In this study, we determine self-diffusion coefficients and rotational correlation times of both ions in triethylammonium based PILs by means of NMR Fast-Field-Cycling (FFC) relaxometry. Global fits of 1H and 19F nuclear magnetic relaxation dispersion (NMRD) curves allowed proper separation into intra and inter molecular relaxation rates for both NMR sensitive nuclei and thus a reliable description of translational and rotational motion for both ions individually. The diffusion coefficients of the cations are in the order of 6 × 10-11 m2 s-1 at room temperature and about 50 per cent larger than those of the anions. The diffusion coefficients of cations and anions in both PILs were compared with those we derived from applying an universal dispersion power law and those known from pulsed field gradient (PFG) NMR studies. Considering the Nernst-Einstein equation, molar conductivities were calculated from cationic and anionic diffusion coefficients and related to directly measured molar conductivities, allowing the determination of the degree of dissociation. The rotational correlation times τR ranging from 50 ps up to 2 ns as a function of temperature were compared with those obtained from high-field NMR quadrupolar relaxation time measurements addressing explicitly the rotation of the NH vector and giving insights into the acidic proton mobility. The Stokes-Einstein and Stokes-Einstein-Debye relations were applied to relate the diffusion coefficients and rotational correlation times to the macroscopic bulk viscosity. The results were also discussed with respect to the archetypical PIL ethylammonium nitrate.

Ion Dissociation in Ionic Liquids and Ionic Liquid Solutions

The extent to which cations and anions in ionic liquids (ILs) and ionic liquid solutions are dissociated is of both fundamental scientific interest and practical importance because ion dissociation has been shown to impact viscosity, density, surface tension, volatility, solubility, chemical reactivity, and many other important chemical and physical properties. When mixed with solvents, ionic liquids provide the unique opportunity to investigate ion dissociation from infinite dilution in the solvent to a completely solvent-free state, even at ambient conditions. The most common way to estimate ion dissociation in ILs and IL solutions is by comparing the molar conductivity determined from ionic conductivity measurements such as electrochemical impedance spectroscopy (EIS) (which measure the movement of only the charged, i.e., dissociated, ions) with the molar conductivity calculated from ion diffusivities measured by pulse field gradient nuclear magnetic resonance spectroscopy (PFG-NMR, which gives movement of all of the ions). Because the NMR measurements are time-consuming, the number of ILs and IL solutions investigated by this method is relatively limited. We have shown that use of the Stokes-Einstein equation with estimates of the effective ion Stokes radii allows ion dissociation to be calculated from easily measured density, viscosity, and ionic conductivity data (ρ, η, λ), which is readily available in the literature for a much larger number of pure ILs and IL solutions. Therefore, in this review, we present values of ion dissociation for ILs and IL solutions (aqueous and nonaqueous) determined by both the traditional molar conductivity/PFG-NMR method and the ρ, η, λ method. We explore the effect of cation and anion alkyl chain length, structure, and interaction motifs of the cation and anion, temperature, and the strength of the solvent in IL solutions.

Molecular Dynamics Insights into the Nanoscale Structural Organization and Local Interaction of Aqueous Solutions of Ionic Liquid 1-Butyl-3-methylimidazolium Nitrate

Considering the growing number of applications of the aqueous ionic liquids (ILs), atomistic molecular dynamics (MD) simulations were used to probe the effect of water molar fraction, x_w, ranging from 0.00 to 0.90, on the nanoscale local structure of 1-butyl-3-methylimidazolium nitrate, [bmim][NO₃], IL. The results prove that, with water addition, the cation-anion, cation-cation, and anion-anion structural correlations are weakened, while strong anion-water and unconventional cation-water hydrogen bonds are formed in the solutions. Water molecules were detected as bridges between nitrate anions, and the water cluster size distribution at different x_w's was investigated. Simulation shows a similar pattern of probability densities for water and anion around the acidic hydrogen atoms of the reference cation ring, while both species move away from the cation butyl chain. Increasing the water concentration to x_w = 0.90 causes decreasing of the local arrangement of the nearest-neighboring cations, because of the weakening of cation-cation π-π stacking. In addition, this dilution reduces the probability of the in-plane cation-anion conformation, disrupts both the polar ionic network and nonpolar domains, and diminishes the nanoaggregation of the cation butyl chains compared to those of the neat IL. These results can rationalize the origins of the fluidity enhancements and transport property trends upon adding water to the imidazolium-based ILs. The current study proposes a deep atomistic-level insight into the complex coupling between water concentration, microscopic structure, and local interactions of aqueous imidazolium-based ILs with hydrophilic anions.

Ordered Solvents and Ionic Liquids Can Be Harnessed for Electrostatic Catalysis

Herein, we employ classical molecular dynamics simulations using the Drude oscillator-based polarizable force field, quantum chemical calculations, and ONIOM multiscale calculations to study (a) how an external field orders the solvent environment in a chemical reaction and then (b) whether in the absence of this same applied field the ordered solvent environment alone can electrostatically catalyze a chemical reaction when compared with the corresponding disordered solvent. Our results show that a 0.2 V/Å external electric field, which is below the threshold for bond breaking of solvent molecules, leads to significant ordering of bulk methanol solvent and the ionic liquid [EMIM][BF₄]. Importantly, in the absence of this same field, the ordered solvent lowers the activation energy of the hydrogen-transfer reaction of o-alkylphenyl ketones in excess of 20 kcal/mol when the solvent is methanol and by over 30 kcal/mol for [EMIM][BF₄]. Even a 0.1 V/Å external field has effects of ca. 10 and 20 kcal/mol, respectively. This work suggests a possible strategy for scaling electrostatic catalysis by applying a pulsed external field to the reaction medium to maintain solvent ordering while allowing the reaction to proceed largely in the absence of an external field.

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.

Irregular solution thermodynamics of wood pulp in the superbase ionic liquid [m-TBDH][AcO]

Knowledge of solution thermodynamics is fundamental for solution control and solvent selection processes. Herein, experimentally determined thermodynamic quantities for solutions of wood pulp (hardwood dissolving pulp, i.e. cellulose) in [m-TBDH][AcO] are presented. Model-free activities (a_i,j) and associated mass fraction (w_i,j) activity coefficients (Ω_i,j), are determined to quantify inherent solution non-ideality. Access to the Gibbs energy of mixing, G_mix, in combination with associated partial molar thermodynamic quantities, reveal strong enthalpically favourable (exothermic) interactions due to solvent-j and solute-i contact-encounters. Onset of an entropy driven phase instability appears at increased temperatures as excess entropic contributions dominate solvation character of the irregular solutions formed.

Thermochemical analysis of cellulose dissolution character in the superbase containing protic ionic liquid [m-TBDH][AcO] reveals lower critical solution temperature (LCST) behaviour.

Reorientation dynamics and ion diffusivity of neat dimethylimidazolium dimethylphosphate probed by NMR spectroscopy

NMR spectroscopy at two magnetic field strengths was employed to investigate the dynamics of dimethylimidazolium dimethylphosphate ([C₁C₁IM][(CH₃)₂PO₄]). [C₁C₁IM][(CH₃)₂PO₄] is a low-melting, halogen-free ionic liquid comprising of only methyl groups. ¹³C spin–lattice relaxation rates as well as self-diffusion coefficients were measured for [C₁C₁IM][(CH₃)₂PO₄] as a function of temperature. The rotational correlation times, τ_c, for the cation and the anion were obtained from the ¹³C spin–lattice relaxation rates. Although from a theoretical point of view cations and anions are similar in size, they show different reorientation mobilities and diffusivities. The self-diffusion coefficients and the rotational correlation times were related to the radii of the diffusing spheres. The analysis reveals that the radii of the cation and the anion, respectively, are different from each other but constant at temperatures ranging from 293 to 353 K. The experimental results are rationalised by a discrete and individual cation and anion diffusion. The [(CH₃)₂PO₄]⁻ anion reorients faster compared to the cation but diffuses significantly slower indicating the formation of anionic aggregates. Relaxation data were acquired with standard liquid and magic-angle-spinning NMR probes to estimate residual dipolar interactions, chemical shift anisotropy or differences in magnetic susceptibility within the sample.

Liquid and HR-MAS NMR spectroscopy at two magnetic field strengths was employed to investigate the dynamics of ([C₁C₁IM][(CH₃)₂PO₄]).

Structure and dynamics elucidation of ionic liquids using multidimensional Laplace NMR

We demonstrate the ability of multidimensional Laplace NMR (LNMR), comprising relaxation and diffusion experiments, to reveal essential information about microscopic phase structures and dynamics of ionic liquids that is not observable using conventional NMR spectroscopy or other techniques.

Capturing the effect of [PF3(C2F5)3]−vs. [PF6]−, flexible anion vs. rigid, and scaled charge vs. unit on the transport properties of [bmim]+-based ionic liquids: a comparative MD study

Effect of [PF₃(C₂F₅)₃]⁻vs. [PF₆]⁻, flexible anion vs. rigid, and scaled charge vs. unit on the transport properties of ILs.

Transport properties of protic and aprotic guanidinium ionic liquids

Ionic liquids (ILs) are a promising class of solvents, functional fluids and electrolytes that are of high interest for both basic as well as applied research. For further fundamental understanding of ILs and a successful implementation in technical processes, a deeper insight into transport properties and their interrelations is of particular importance. In this contribution we synthesised a series of mostly novel protic and aprotic ILs based on the tetramethylguanidinium (TMG) cation that is a derivative of the superbase guanidine. Different substitution patterns and anions from acids with broadly varied pK_a values were investigated. We measured general properties, such as thermal transitions and densities of these ILs, as well as their transport quantities by means of rheology, impedance spectroscopy and NMR diffusometry. Different models for the correlation of the transport properties, namely the Nernst–Einstein, Walden and Stokes–Einstein–Sutherland relations were applied. The deviation from ideal behaviour of fully dissociated electrolytes, often termed as ionicity, was quantified by the reciprocal Haven ratio, fractional Walden rule and ionicity obtained from the Walden plot. Velocity cross-correlation coefficients were calculated to gain further insight into the correlation between ion movements. Both protic and aprotic TMG ILs show transport properties comparable to other ILs with similar molecular weight and high ionicity values especially in contrast to other protic ILs. Lowest ionicity values were found for the protic ILs with smallest ΔpK_a values between constituting acid and base. This can either be explained by stronger hydrogen bonding between cation and anion or lower anti-correlations between the oppositely charged ions. These results aim to provide insight into the properties of this interesting cations class and a deeper understanding of the transport properties of ILs and their interrelations in general.

New protic and aprotic ionic liquids based on superbase cations show promising properties and enrich the field of cation classes

NMR and Rheological Study of Anion Size Influence on the Properties of Two Imidazolium-based Ionic Liquids

NMR self-diffusion and relaxation, coupled with viscosity, were used to study the properties and structure of two imidazolium-based ionic liquids, 1-ethyl-3-methylimidazolium acetate [C₂MIM][OAc] and 1-ethyl-3-methylimidazolium octanoate [C₂MIM][OOct]. The experimental results point to the formation of different types of aggregates in each ionic liquid. These aggregates are small and stable under flow and temperature in [C₂MIM][OAc], whereas the aggregates are large and sensitive to flow and temperature in [C₂MIM][OOct]. In the latter case the size of aggregates decreases both under flow and temperature increase.

Cation Dynamics in Supercooled and Solid Alkyl Methylimidazolium Bromide Ionic Liquids

The molecular dynamics of alkyl methylimidazolium bromide ionic liquids with different side groups of the cation are studied over a wide range of temperatures, covering the supercooled and crystalline states. Nuclear magnetic resonance relaxation dispersion (NMRD) at different magnetic field strengths was combined with NMR pulsed field gradient (PFG) diffusion measurements in order to obtain a description of the temperature dependence of the cationic mobility. While an Arrhenius dependence of the correlation times was found at high temperatures, a deviation is observed below a critical temperature of T_dyn ∼ 275 K which corresponds to about 1.25 T_g for two of the substances. The macroscopic diffusion coefficient, on the other hand, is best described by a VFT dependence down to a similar temperature, and a much weaker temperature dependence below. Measurements carried out in the crystalline state of 1-butyl-3-methylimidazolium bromide (Bmim Br) exhibit a dramatically increased self-diffusion coefficient in agreement with earlier reports of strong dynamic heterogeneity in the presence of minute amounts of water.

Transport properties and ionicity of phosphonium ionic liquids

Novel phosphonium-based ionic liquids with different anions as well as side chain composition and length in the cations exhibit transport properties deviating from ideal behaviour. This is attributed to the ionicity and is evident from the Haven ratio and the fractional Walden rule.