We combine direct surface force measurements with thermodynamic arguments to demonstrate that pure ionic liquids are expected to behave as dilute weak electrolyte solutions, with typical effective dissociated ion concentrations of less than 0.1% at room temperature. We performed equilibrium force-distance measurements across the common ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C 4 mim][NTf 2 ]) using a surface forces apparatus with in situ electrochemical control and quantitatively modeled these measurements using the van der Waals and electrostatic double-layer forces of the Derjaguin-LandauVerwey-Overbeek theory with an additive repulsive steric (entropic) ion-surface binding force. Our results indicate that ionic liquids screen charged surfaces through the formation of both bound (Stern) and diffuse electric double layers, where the diffuse double layer is comprised of effectively dissociated ionic liquid ions. Additionally, we used the energetics of thermally dissociating ions in a dielectric medium to quantitatively predict the equilibrium for the effective dissociation reaction of [C 4 are not effectively dissociated and thus do not contribute to electrostatic screening. We also provide a general, molecular-scale framework for designing ionic liquids with significantly increased dissociated charge densities via judiciously balancing ion pair interactions with bulk dielectric properties. Our results clear up several inconsistencies that have hampered scientific progress in this important area and guide the rational design of unique, high-free-ion density ionic liquids and ionic liquid blends.Boltzmann distribution | electrostatic interaction | interfacial phenomena I onic liquids are fluids composed solely of ions (1, 2). Much of the recent scientific interest surrounding ionic liquids derives from the fact that ionic liquids have been demonstrated for numerous applications, such as safe, high-efficiency electrochemical storage devices (3, 4), self-assembly media (5), and lubrication (6). A key paradigm within ionic liquids research is that the physical properties of ionic liquids can be controlled to an unprecedented degree through the judicious design of cation-anion pairs (1-9). Thus, ionic liquids are known as "designer" solvents/materials (1). However, fully realizing this advantage requires the development of a comprehensive framework that can be used to rationalize the relationship between an ionic liquid's molecular structure and its bulk and interfacial behavior and properties, which are governed by a complex interplay of coulomb, van der Waals, dipole, hydrogen bonding, and steric interactions (10, 11).Recent work has greatly progressed a fundamental understanding of ionic liquids at charged interfaces, but there are currently inconsistencies between experiment and theory (4, 7-9, 12-23); this is particularly true for the ranges of surface-induced ordering and electrostatic screening. For example, a comparison of the values obtained from ionic conductivity and...
The properties of mixtures of ionic liquids (ILs) with a variety of different aprotic solvents have been examined in detail. The ILs selected bis(trifluoromethanesulfonyl)imide (TFSI − ) salts with N-methyl-N-pentylpyrrolidinium (PY 15 + ), -piperidinium (PI 15 + ), or -morpholinium (MO 15 + ) cationsenabled the investigation of how cation structure influences the mixture properties. This study includes the characterization of the thermal phase behavior of the mixtures and volatility of the solvents, density and excess molar volume, and transport properties (viscosity and conductivity). The mixtures with ethylene carbonate form a simple eutectic, whereas those with ethyl butyrate appear to form a new IL−solvent crystalline phase. Significant differences in the viscosity of the mixtures are found for different solvents, especially for the IL-rich concentrations. In contrast, only minor differences are noted for the conductivity with different solvents for the IL-rich concentrations. For the solvent-rich concentrations, however, substantial differences are noted in the conductivity, especially for the mixtures with acetonitrile.
The use of mixed salts to generate new composite ionic liquids (ILs) provides a facile means of readily tuning or tailoring the desired properties of ionic media. Despite this, very little information is available about how the structure of the selected ions and composition impacts the properties of salt mixtures. To explore this, six binary IL1–IL2 mixtures based on N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide salts have been characterized. The physicochemical properties (density, viscosity, and ionic conductivity) and phase behavior of these mixtures are reported. The variation of the alkyl chains lengths on the cations plays a significant role in determining both the phase behavior and the physicochemical properties of the mixtures. Notably, the “tunability” of the properties of the IL mixtures is much easier to control than is found by simply making small structural changes to the ions in a given salt.
Molecular dynamics (MD) simulations using a many-body polarizable APPLE&P force field have been performed on mixtures of the N-methyl-N-pentylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PY15TFSI) ionic liquid (IL) with three molecular solvents: propylene carbonate (PC), dimethyl carbonate (DMC), and acetonitrile (AN). The MD simulations predict density, viscosity, and ionic conductivity values that agree well with the experimental results. In the solvent-rich regime, the ionic conductivity of the PY15TFSI-AN mixtures was found to be significantly higher than the conductivity of the corresponding -PC and -DMC mixtures, despite the similar viscosity values obtained from both the MD simulations and experiments for the -DMC and -AN mixtures. The significantly lower conductivity of the PY15TFSI-DMC mixtures, as compared to those for PY15TFSI-AN, in the solvent-rich regime was attributed to the more extensive ion aggregation observed for the -DMC mixtures. The PY15TFSI-DMC mixtures present an interesting case where the addition of the organic solvent to the IL results in an increase in the cation-anion correlations, in contrast to what is found for the mixtures with PC and AN, where ion motion became increasingly uncorrelated with addition of solvent. A combination of pfg-NMR and conductivity measurements confirmed the MD simulation predictions. Further insight into the molecular interactions and properties was also obtained using the MD simulations by examining the solvent distribution in the IL-solvent mixtures and the mixture excess properties.
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