Multiscale Studies on Ionic Liquids

Revisiting Ionic Liquid Structure-Property Relationship: A Critical Analysis

In the last few years, ionic liquids (ILs) have been the focus of extensive studies concerning the relationship between structure and properties and how this impacts their application. Despite a large number of studies, several topics remain controversial or not fully answered, such as: the existence of ion pairs, the concept of free volume and the effect of water and its implications in the modulation of ILs physicochemical properties. In this paper, we present a critical review of state-of-the-art literature regarding structure–property relationship of ILs, we re-examine analytical theories on the structure–property correlations and present new perspectives based on the existing data. The interrelation between transport properties (viscosity, diffusion, conductivity) of IL structure and free volume are analysed and discussed at a molecular level. In addition, we demonstrate how the analysis of microscopic features (particularly using NMR-derived data) can be used to explain and predict macroscopic properties, reaching new perspectives on the properties and application of ILs.

A One-Pot Strategy to Synthesize Block Copolyesters from Monomer Mixtures Using a Hydroxy-Functionized Ionic Liquid

One-pot transformation of monomer mixtures into block copolymers remains a key challenge. Herein, a metal-free route to prepare block copolymers from monomer mixtures by a hydroxyl functionalized ionic liquid of 3-(2-hydroxyl-ethyl)-1-methylimidazolium bromide (HEMIMB) is described. HEMIMB can bridge two catalytic cycles including ring-opening alternating copolymerization (ROAC) of phthalic anhydride (PA) with epoxides and ring-opening polymerization (ROP) of L-lactide (LA), and enable a selective copolymerization from PA, LA, and epoxides. The selective copolymerization depends on the presence of PA in mixed feedstocks, exhibits the first ROAC of PA with epoxides and then ROP of LA to the formation of block polyesters in one-pot strategy. This work is beneficial to the development of metal-free catalysts for sequence-controlled polymerization that enable block architectures from mixtures of monomers.

Origin of a High Overpotential of Co Electrodeposition in a Room-Temperature Ionic Liquid

Metal electrodeposition in room-temperature ionic liquids (RTILs) often shows high overpotentials. Although this is often explained by the formation of a negatively charged metal complex due to the coordination of RTIL anions and the hindrance of its close approach onto the negatively charged electrode, we propose an alternative model based upon surface-enhanced infrared absorption spectroscopy measurements under Co electrodeposition. We found that the anionic first layer exists on the negatively charged electrode, and its replacement with a cationic one and Co electrodeposition both begin at an identical onset potential. The correlation between the interfacial structure and the electrodeposition reaction that can be modified by additives indicated that the high overpotential can be mainly attributed to the restructuring of the characteristic interfacial multilayer structure stabilized by its charge order, which is required for the reorganization of solvent ions after the reduction of Co²⁺.

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.

Macroscopic Differentiators for Microscopic Structural Nonideality in Binary Ionic Liquid Mixtures

Combining two ionic liquids to form a binary ionic liquid mixture is a simple yet effective strategy to not only expand the number of ionic liquids but also precisely control various physicochemical properties of resultant ionic liquid mixtures. From a fundamental thermodynamic point of view, it is not entirely clear whether such mixtures can be classified as ideal solutions. Given a large number of binary ionic liquid mixtures that emerge, the ability to predict the presence of nonideality in such mixtures a priori without the need for experimentation or molecular simulation-based calculations is immensely valuable for their rational design. In this research report, we demonstrate that the difference in the molar volumes (ΔV) of the pure ionic liquids and the difference in the hydrogen-bonding ability of anions (Δβ) are the primary determinants of nonideal behavior of binary ionic liquid mixtures containing a common cation and two anions. Our conclusion is derived from a comparison of microscopic structural properties expressed in terms of radial, spatial, and angular distributions for binary mixtures and those of the corresponding pure ionic liquids. Molecular dynamics simulations of 16 binary ionic liquid mixtures, containing a common cation 1-n-butyl-3-methylimidazolium [C₄mim]⁺ and combinations of (less basic) fluorinated {trifluoromethylacetate [TFA]^-, trifluoromethanesulfonate [TFS]^-, bis(trifluoromethanesulfonyl)imide [NTf₂]^-, and tris (pentafluoroethyl) trifluorophosphate [eFAP]^-} versus (more basic) nonfluorinated {chloride Cl^-, acetate [OAC]^-, methylsulfate [MeSO₄]^-, and dimethylphosphate [Me₂PO₄]^-} anions, were conducted. The large number of binary ionic liquid mixtures examined here enabled us to span a broad range of ΔV and Δβ values. The results indicate that binary mixtures of two ionic liquids for which ΔV > 60 cm³/mol and Δβ > 0.4 are expected to be microscopically nonideal. On the other hand, ΔV < 60 cm³/mol and Δβ < 0.4 will lead to molecular structures that are not differentiated from those of their pure ionic liquid counterparts.

The Impact of Reactive Ionic Liquids Addition on the Physicochemical and Sorption Properties of Poly(Vinyl Alcohol)-Based Films

A new type of hybrid polymeric-based film containing 1-(1,3-diethoxy-1,3-dioxopropan-2-ylo)-3-methylimidazolium bromide (RIL1_Br) and 1-(2-etoxy-2-oxoethyl)-3-methylimidazolium bromide (RIL2_Br) reactive ionic liquids was elaborated. Poly(vinyl alcohol) (PVA)-based films with 9–33 wt % of RILs were subsequently characterized using Fourier transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR), scanning electron microscopy (SEM), atomic force microscopy (AFM), thermogravimetric analysis (TGA) and TGA-FTIR. PVA-RIL films were also studied in tensile tests, contact angle and sorption measurements. RIL incorporation enhanced thermal and mechanical stability of PVA membranes due to the hydrogen bonds between RILs and polymer chains. Membrane swelling behavior in water (H₂O), ethanol (EtOH), and propan-2-ol (IPA) and the kinetics of water sorption process revealed that PVA-RILs membranes possess the highest affinity towards water. It was pointed out that both the RIL type and the RIL amount in the polymer matrix have significant influence on the membrane swelling behavior and the water sorption kinetics.

Modulation of Cation Diffusion by Reversible Supramolecular Assemblies in Ionic Liquid-Based Nanocomposites

Ionic liquid (IL) properties, such as high ionic conductivity under ambient conditions combined with nontoxicity and nonflammability, make them important materials for future technologies. Despite high ion conductivity desired for battery applications, cation transport numbers in ILs are not sufficient enough to attain high power density batteries. Thus, developing novel approaches directed toward improvement of cation transport properties is required for the application of ILs in energy-storing devices. In this effort, we used various experimental techniques to demonstrate that the strategy of mixing ILs with ultrasmall (1.8 nm) nanoparticles (NPs) resulted in melt-processable composites with improved transport numbers for cations at room temperature. This significant enhancement in the transport number was attributed to the specific chemistry of NPs exhibiting a weaker cation and stronger anion coordination at ambient temperature. At high temperature, significantly weakened NP-anion associations promoted a liquid-like behavior of composites, highlighting the melt-processability of these composites. These results show that designing a reversible dynamic noncovalent NP-anion association controlled by the temperature may constitute an effective strategy to control ion diffusion. Our studies provide fundamental insights into mechanisms driving the charge transport and offer practical guidance for the design of melt-processable composites with an improved cation transport number under ambient conditions.

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.

Conformational design concepts for anions in ionic liquids

The identification of specific design concepts for the in silico design of ionic liquids (ILs) has been accomplished using theoretical methods. Molecular building blocks, such as interchangeable functional groups, are used to design a priori new ILs which have subsequently been experimentally investigated. The conformational design concepts are developed by separately and systematically changing the central (imide), bridging (sulfonyl) and end (trifluoromethyl) group of the bis(trifluoromethanesulfonyl)imide [N(Tf)2]- anion and examining the resultant potential energy surfaces. It is shown that these design concepts can be used to tune separately the minimum energy geometry, transition state barrier height and relative stability of different conformers. The insights obtained have been used to design two novel anions for ILs, trifluoroacetyl(methylsulfonyl)imide [N(Ms)(TFA)]- and acetyl(trifluoromethanesulfonyl)imide [N(Tf)(Ac)]-. The computationally predicted structures show excellent agreement with experimental structures obtained from X-ray crystallography. [C4C1im][N(Tf)(Ac)] and [C4C1im][N(Ms)(TFA)] ILs have been synthesised and ion diffusion coefficients examined using pulsed field gradient stimulated echo NMR spectroscopy. Significantly increased diffusion was observed for the more flexible [N(Tf)(Ac)]- compared with the more rigid [N(Ms)(TFA)]- analogue. Furthermore, a pronounced impact on the fluidity was observed. The viscosity of the IL with the rigid anion was found to be twice as high as the viscosity of the IL with the flexible anion. The design concepts presented in this work will enable researchers in academia and industry to tailor anions to provide ILs with specific desired properties.

Novel recyclable deep eutectic solvent boost biomass pretreatment for enzymatic hydrolysis

Deep eutectic solvent (DES) with protonic acid shows the great potential for biomass valorization. However, the acid corrosion and recycling are still severe challenges in biorefinery. Herein, a novel DES by coordinating FeCl₃ in choline chloride/glycerol DES was designed for effective and recyclable pretreatment. As compared to DESs with FeCl₂, ZnCl₂, AlCl₃ and CuCl₂, DES with FeCl₃ approvingly retained most of cellulose in pretreated Hybrid Pennisetum (95.2%). Meanwhile, the cellulose saccharification significantly increased to 99.5%, which was six-fold higher than that of raw biomass. The excellent pretreatment performance was mainly attributed to the high removal of lignin (78.88 wt%) and hemicelluloses (93.63 wt%) under the synergistic effect of Lewis acid and proper hydrogen-bond interaction of DES with FeCl₃. Furthermore, almost all cellulose still can be converted into glucose after five recycling process. Overall, the process demonstrated designed pretreatment was great potential for the low-cost biorefinery and boost the biofuel development.

Crystal structure of 3-(2-ethoxy-2-oxoethyl)-1-vinyl-1H-imidazol-3-ium hexafluoridophosphate(V), C9H13F6N2O2P

[C9H13F6N2O2P], monoclinic, P21/c (no. 14), a = 6.5994(13) Å, b = 10.754(2) Å, c = 19.754(4) Å, β = 96.267(2)°, V = 1393.6(5) Å3, Z = 4, R gt(F) = 0.0674, wR ref(F 2) = 0.2124, T = 296(2) K.

SO2 absorption in pure ionic liquids: Solubility and functionalization

The SO₂ solubility in ionic liquids and absorption mechanisms with different functionalities, including ether, halide, carboxylate, dicarboxylate, thiocynate, phenol, amino, azole groups, etc., are presented in this review. Strategies of improving SO₂ capture with low binding energy and the separation performance from CO₂ are also concluded. Generally, moderate basicity is favourable for enhancing SO₂ capacity and the water (below 6 wt%) effect on absorption is indefinite but generally slight. Introducing electron-withdrawing substituents such as nitrile, halogen, aldehyde and carboxylic groups are proposed to decrease the chemical absorption enthalpy between ionic liquid and SO₂ in order to reduce regeneration power consumption. Although it is promising, the absorption enthalpy is still much higher than the physisorption performance especially of the ether-functionalized ones. The biocompatible choline-based, betaine-based, and amino acid ionic liquids have clear trends to be applied in SO₂ capture due to their biodegradability, nontoxicity and easy accessibility. Generally, comparing to the traditional solvents, ionic liquids have made great improvement in SO₂ capacity, however, the high viscosity and desorption energy are two main obstacles for SO₂ absorption and separation. Molecular simulations have been applied to reveal the absorption regimes involving the roles of basic functionalities and physical interactions especially the hydrogen bonds, which could be referred for structure designing of the available ionic liquids with readily fluid characteristics and absorption ability.

Chiral Ionic Liquids: Structural Diversity, Properties and Applications in Selected Separation Techniques

Ionic liquids (ILs) are chemical compounds composed of ions with melting points below 100 °C exhibiting a design feature. ILs are commonly used as the so-called green solvents, reagents or highly efficient catalysts in varied chemical processes. The huge application potential of ionic liquids (IL) justifies the growing interest in these compounds. In the last decade, increasing attention has been devoted to the development of new methods in the synthesis of stable chiral ionic liquids (CILs) and their application in various separation techniques. The beginnings of the successful use of CILs to separate enantiomers date back to the 1990 s. Most chiral ILs are based on chiral cations or chiral anions. There is also a limited number of CILs possessing both a chiral cation and a chiral anion. Due to the high molecular diversity of both ions, of which at least one has a chiral center, we have the possibility to design a large variety of optically active structures, thus expanding the range of CIL applications. Research utilizing chiral ionic liquids only recently has become more popular. However, it is the area that still has great potential for future development. This review aimed to describe the diversity of structures, properties and examples of applications of chiral ionic liquids as new chiral solid materials and chiral components of the anisotropic environment, providing chiral recognition of enantiomeric analytes, which is useful in liquid chromatography, countercurrent chromatography and other various CIL-based extraction techniques including aqueous biphasic (ABS) extraction systems, solid-liquid two-phase systems, liquid-liquid extraction systems with hydrophilic CILs, liquid-liquid extraction systems with hydrophobic CILs, solid-phase extraction and induced-precipitation techniques developed in the recent years. The growing demand for pure enantiomers in the pharmaceutical and food industries sparks further development in the field of extraction and separation systems modified with CILs highlighting them as affordable and environmentally friendly both chiral selectors and solvents.

Development of Poly(l-Lactic Acid)-Based Bending Actuators

This work reports on the development of bending actuators based on poly(l-lactic acid) (PLLA)/ionic liquid (IL) blends, through the incorporation of 40% wt. of the 1-ethyl-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim][TFSI]) IL. The films, obtained by solvent casting at room temperature and 50 °C, were subjected to several post-thermal treatments at 70, 90, 120 and 140 °C, in order to modify the crystallinity of the films. The influence of the drying temperature and of [Emim][TFSI] blending on the morphological, structural, mechanical and electrical properties of the composite materials were studied. The IL induced the formation of a porous surface independently of the processing conditions. Moreover, the [Emim][TFSI] dopant and the post-thermal treatments at 70 °C promoted an increase of the degree of crystallinity of the samples. No significant changes were observed in the degree of crystallinity and Young Modulus for samples with thermal treatment between 70 and 140 °C. The viability of the developed high ionic conductive blends for applications as soft actuators was evaluated. A maximum displacement of 1.7 mm was achieved with the PLLA/[Emim][TFSI] composite prepared at 50 °C and thermally treated at 140 °C, for an applied voltage of 10 Vpp, at a frequency of 100 mHz. This work highlights interesting avenues for the use of PLLA in the field of actuators.

Theoretical Insights into the Structures and Capacitive Performances of Confined Ionic Liquids

Room-temperature ionic liquids (RTILs) together with nano-porous electrodes are the most promising materials for supercapacitors and batteries. Many theoretical works have addressed the structures and performances of RTILs inside nanopores. However, only limited attention has been given to how the dispersion forces of RTILs influence the behavior of ions inside the slit pores. Toward this aim, we investigate the effects of various dispersion forces between ions on the macroscopic structures in nanoconfinement and the capacitance performance of supercapacitors by the classical density functional theory (CDFT). The results show that the dispersion force can significantly change the mechanism of the charging process and even the shape of differential capacitance curves. In addition, the voltage-dependent structures of RTILs with appropriate dispersion force appears in a given silt pore, which leads to extremely high capacitance and enhances the energy storage density. We hope that this work could further offer guidance for the optimizing of electrolytes for electrical double layer capacitors, like tuning the dispersion force between ions by adding/removing certain chemical groups on the cations and anions of RTILs.

Decoding biomass recalcitrance: Dispersion of ionic liquid in aqueous solution and efficient extraction of lignans with microwave magnetic field

Alkaline ionic liquid aqueous solutions were used to extract biphenyl cyclooctene lignans derivatives, and hydrolyze to the free-state biphenyl cyclooctene lignans simultaneously from Schisandra chinensis by microwave-assisted heating. The hydrogen bonds formatted between ionic liquid and water molecular attacks the amorphous region of cellulose. Selective heating by microwave produce the more polar regions, which results in swelling and fragmentation of raw materials near the hot spots. Therefore, ionic liquid-microwave-assisted extraction method of free-state biphenyl cyclooctene lignans was set up. The solid residue after treatment was characterized by infrared spectroscopy and scanning electron microscopy, which showed that cellulose, hemicellulose, and lignin were removed partially. The water content of ionic liquid solution affected its viscosity and diffusivity, and in turns the extraction efficiency of lignans. The IL solutions with different mole fractions of IL were detected by FTIR and Raman spectroscopy, the result shows that IL solutions with higher water contents (>0.6) won't form clusters. The optimum hydrolysis conditions were 0.2 g of ionic liquid catalyst per 5.0 g of S. chinensis fruits, a microwave irradiation power of 600 W, and heating time of 12 min, which gave a yield of free-state biphenyl cyclooctene lignans of 4.12±0.37 mg g-1. Besides, a hydrolysis mechanism of ester-bond biphenyl cyclooctene lignans and decreasing "biomass recalcitrance effect" by ionic liquid microwave-assisted method was proposed.

Accurate Diels-Alder Energies and Endo Selectivity in Ionic Liquids Using the OPLS-VSIL Force Field

Our recently developed optimized potentials for liquid simulations-virtual site ionic liquid (OPLS-VSIL) force field has been shown to provide accurate bulk phase properties and local ion-ion interactions for a wide variety of imidazolium-based ionic liquids. The force field features a virtual site that offloads negative charge to inside the plane of the ring with careful attention given to hydrogen bonding interactions. In this study, the Diels-Alder reaction between cyclopentadiene and methyl acrylate was computationally investigated in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF₆], as a basis for the validation of the OPLS-VSIL to properly reproduce a reaction medium environment. Mixed ab initio quantum mechanics and molecular mechanics (QM/MM) calculations coupled to free energy perturbation and Monte Carlo sampling (FEP/MC) that utilized M06-2X/6-31G(d) and OPLS-VSIL gave activation free energy barriers of 14.9 and 16.0 kcal/mol for the endo-cis and exo-cis Diels-Alder reaction pathways, respectively (exptl. ΔH^‡ of 14.6 kcal/mol). The endo selectivity trend was correctly predicted with a calculated 73% endo preference. The rate and selectivity enhancements present in the endo conformation were found to arise from preferential hydrogen bonding with the exposed C4 ring hydrogen on the BMIM cation. Weaker electronic stabilization of the exo transition state was predicted. For comparison, our earlier ±0.8 charge-scaled OPLS-2009IL force field also yielded a ΔG^‡ of 14.9 kcal/mol for the favorable endo reaction pathway but did not adequately capture the highly organized solvent interactions present between the cation and Diels-Alder transition state.

A space-confined strategy toward large-area two-dimensional crystals of ionic liquid

Understanding and manipulating nano-confined ionic liquids (ILs) has tremendous implications in nanotechnology and chemical engineering. Here, a peculiar growth phenomenon of a nano-confined [Bmim][NTFI] ionic liquid is revealed by utilizing two-dimensional channels in mica. The intercalated ILs underwent liquid-solid transition and self-assembled into a self-similar two-dimensional crystal in an epitaxial relation with the confining material. The terraced IL crystals, ranging from monolayer to bilayer to several dozen layers, are characterized by unexpectedly large areas extending to μm-scale and enhanced thermal stability with a melting temperature 73 K higher than that of the corresponding bulk IL. The notable asymmetric feature of the layered crystals hints at anisotropic growth under confinement, which produces a well-defined hexagonal geometric shape. Finally, a molecular scale growth mechanism of ordered ILs is qualitatively interpreted by a birth-and-spread model. Our findings have enabled new research on nanoconfined ILs and opened up an avenue to tailoring the structure of ILs for their applications under confinement.

Ionic liquids-modified cellulose coated magnetic nanoparticles for enzyme immobilization: Improvement of catalytic performance

In this work, ionic liquids-modified magnetic carboxymethyl cellulose nanoparticles (IL-MCMC) were prepared and used as supports for enzyme immobilization. The specific activity of immobilized lipase PPL-IL-MCMC was 1.43 and 2.81 folds higher than that of free PPL and PPL-MCMC, respectively. Water contact angle analysis indicated that the introduction of ionic liquids increased the hydrophobicity of supports, which in tune induced the lid-opening of lipase, allowing its active sites to become more accessible. In addition, the affinity between lipase and substrate immobilized on the prepared supports was enhanced. The same method was also applied to analyze immobilize penicillin G acylase (PGA) to further investigate the general applicability of the method. The results showed that the immobilized PGA exhibited higher stability than many other reported PGAs. The developed composites may be utilized as excellent supports for enzyme immobilization in industrial application.

Synthesis of chiral iron-based ionic liquids: modelling stable hybrid materials

A simple method to prepare asymmetric ionic liquids combining the optical, magnetic and Lewis acidic properties of [FeX₄]⁻ anions with the chirality of imidazolium cations.

Ionic liquid-stabilized vanadium oxo-clusters catalyzing alkane oxidation by regulating oligovanadates

The specific ionic liquid [TBA][Pic]-stabilized vanadium oxo-clusters exist in the form of a trimer and a dimer and are highly active for catalyzing C–H bond oxidation with H₂O₂ as an oxidant.

Ether functionalisation, ion conformation and the optimisation of macroscopic properties in ionic liquids

Ionic liquids are an attractive material class due to their wide liquid range, intrinsic ionic conductivity, and high chemical as well as electrochemical stability. However, the widespread use of ionic liquids is hindered by significantly higher viscosities compared to conventional molecular solvents. In this work, we show how the transport properties of ionic liquids can be altered significantly, even for isostructural ions that have the same backbone. To this end, structure-property relationships have been determined for a set of 16 systematically varied representative ionic liquids. Variations in molecular structure include ammonium vs. phosphonium, ether vs. alkyl side chains, and rigid vs. flexible anions. Ab initio calculations are used to relate molecular structures to the thermal, structural and transport properties of the ionic liquids. We find that the differences in properties of ether and alkyl functionalised ionic liquids are primarily dependent on minimum energy geometries, with the conformational flexibility of ether side chains appearing to be of secondary importance. We also show unprecedented correlations between anion conformational flexibility and transport properties. Critically, increasing fluidity upon consecutive introduction of ether side chains and phosphonium centres into the cation is found to be dependent on whether the anion is flexible or rigid. We demonstrate that targeted design of functional groups based on structure-property relationships can yield ionic liquids of exceptionally high fluidity.