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Ionic liquids (ILs) are key materials for the development of a wide range of emerging technologies. Protic ionic liquids, an important class of ILs, have long been envisioned as promising anhydrous electrolytes for fuel cells. It is well known that in comparison to all other cations, protons exhibit abnormally high conductivity in water. Such superprotonic dynamics was expected in protic ionic conductors as well. However, many years of extensive studies led to the disappointing conclusion that this is not the case and most protic ionic liquids display subionic behavior. Therefore, the relatively low conductivity seems to be the main obstacle for the application of protic ionic liquids in fuel cells. Using dielectric spectroscopy, herein we report the observation of highly decoupled conductivity in a newly synthesized protic ionic conductor. We show that its proton transport is strongly decoupled from the structural relaxation, in terms of both temperature dependence and characteristic rates. This finding offers a fresh look on the charge transport mechanism in PILs and also provides new ideas for design of anhydrous materials with exceptionally high proton conductivity.

Liquid−liquid transition (LLT) between two disordered phases of single-component material remains one of the most intriguing physical phenomena. Here, we report a first-order LLT in a series of ionic liquids containing trihexyl(tetradecyl)phosphonium cation [P666,14]+ and anions of different sizes and shapes, providing an insight into the structure-property relationships governing LLT. In addition to calorimetric proof of LLT, we report that ion dynamics exhibit anomalous behavior during the LLT, i.e., the conductivity relaxation times (τσ) are dramatically elongated, and their distribution becomes broader. This peculiar behavior is induced by isobaric cooling and isothermal compression, with the τσ(TLL,PLL) constant for a given system. The latter observation proves that LLT, in analogy to liquid-glass transition, has an isochronal character. Finally, the magnitude of discontinuity in a specific volume at LLT was estimated using the Clausius-Clapeyron equation. Liquid−liquid transitions (LLTs) have been reported for some molecular systems but are difficult to observe under high pressure conditions. Here the authors report and characterize a first-order LLT in a series of ionic liquids containing the trihexyl(tetradecyl)phosphonium cation and anions of different sizes and shapes, using calorimetric and dielectric measurements. [ABSTRACT FROM AUTHOR]

Temperature and pressure dependent broadband dielectric measurements were performed on the protic ionic liquid C8HIM NTf2 over a frequency range from 0.1 Hz to 1 MHz. The temperature dependence of the inverse dc-conductivity exhibits the super-Arrhenius like behavior typical for glass forming materials. However, in the pressure dependence both slower and faster than exponential developments occur, resulting in an inflection in the corresponding curves. The experimental data was successfully fitted with a model incorporating both features. While similar transitions have been observed in the pressure dependent viscosity or structural relaxation times, this is the first time such a behavior is reported in the conductivity. [ABSTRACT FROM AUTHOR]

Temperature and pressure dependent broadband dielectric measurements were performed on the protic ionic liquid C8HIM NTf2 over a frequency range from 0.1 Hz to 1 MHz. The temperature dependence of the inverse dc-conductivity exhibits the super-Arrhenius like behavior typical for glass forming materials. However, in the pressure dependence both slower and faster than exponential developments occur, resulting in an inflection in the corresponding curves. The experimental data was successfully fitted with a model incorporating both features. While similar transitions have been observed in the pressure dependent viscosity or structural relaxation times, this is the first time such a behavior is reported in the conductivity. [ABSTRACT FROM AUTHOR]

Polymerized ionic liquids (polyILs), composed mostly of organic ions covalently bonded to the polymer backbone and free counterions, are considered as ideal electrolytes for various electrochemical devices, including fuel cells, supercapacitors, and batteries. Despite large structural diversity of these systems, all of them reveal a universal but poorly understood feature: a charge transport faster than the segmental dynamics. To address this issue, we studied three novel polymer electrolyte membranes for fuel cells as well as four single-ion conductors, including highly conductive siloxane-based polyIL. Our ambient and high pressure studies revealed fundamental differences in the conducting properties of the examined systems. We demonstrate that the proposed methodology is a powerful tool to identify the charge transport mechanism in polyILs in general and thereby contribute to unraveling the microscopic nature of the decoupling phenomenon in these materials. [ABSTRACT FROM AUTHOR]

Conductivity in polymer electrolytes has been generally discussed with the assumption that the segmental motions control charge transport. However, much less attention has been paid to the mechanism of ion conductivity where the motions of ions are less dependent (decoupled) on segmental dynamics. This phenomenon is observed in ionic materials as they approach their glass transition temperature and becomes essential for design and development of highly conducting solid polymer electrolytes. In this paper, we study the effect of chain rigidity on the decoupling of ion transport from segmental motion in three polymerized ionic liquids (polyILs) containing the same cation-anion pair but differing in flexibility of the polymer backbones and side groups. Analysis of dielectric and rheology data reveals that decoupling is strong in vinyl-based rigid polymers while almost negligible in novel siloxane-based flexible polyILs. To explain this behavior, we investigated ion and chain dynamics at ambient and elevated pressure. Our results suggest that decoupling has a direct relationship to the frustration in chain packing and free volume. These conclusions are also supported by coarse-grained molecular dynamics simulations. [ABSTRACT FROM AUTHOR]

[Display omitted] • First oligomeric ionic liquids made of lidocaine and a dicarboxylic acid was synthesised. • The composition of this ionic liquid was 2:3 lidocaine:azelaic acid. • A cocrystal phase was also isolated comprising 2:1 lidocaine:azelaic acid. Tuning of physicochemical properties of ionic liquids and crystalline materials is a challenge that opens up unlimited possibilities for expanding applications and controlling biological activity of pharmaceutical multicomponent phases incorporating active pharmaceutical ingredients (APIs) with counterions or coformers. In this work we have investigated the effect of changing lidocaine (LID) and azelaic acid (AZE) ratios on the physicochemical properties of their corresponding multicomponent systems using a thermodynamics-based approach. Microscopy, X-ray diffraction analysis, infrared spectroscopy, nuclear magnetic resonance, and thermogravimetric analysis provided complimentary characterisation. Mechanochemical synthesis of LID:AZE systems at a range of stoichiometries yielded at least two distinct liquid phases and two distinct crystalline phases, one of which involved a unique 2:3 LID:counterion composition not observed previously. Furthermore, to the best of our knowledge the formation of oligomeric ionic liquids involving dicarboxylic acids is also being reported for the first time. This work highlights the need for a careful characterisation of multicomponent systems, especially for pharmaceutical applications. [ABSTRACT FROM AUTHOR]

Over the last decade, the volume has been envisioned as a critical variable controlling the dynamics of glass-forming systems in general, and charge transport of ionic conductors in particular. Herein, we use theoretical and experimental approaches to investigate the volume effects in a specific 1,2,3-triazolium-based poly(ionic liquid) (PIL) at both the macro and nano scales. The size of the local free volume cavity in the studied system was determined by positron annihilation lifetime spectroscopy (PALS). On the other hand, the theoretical approach enabled us to estimate the hard-core volume (V hc) and thus free volume (V f) in the studied PIL. Since the experimental value determined for V f is relatively large it can be considered as the main factor controlling the superionic properties of this material. Furthermore, the analysis of specific volume and relaxation dynamics in T - P plane in terms of the locally correlated lattice (LCL) and cooperative free volume (CFV) models shows that none of them works for the studied system. Image 1 • Investigations into the size of local free volume cavity of polymerized ionic liquids by means of PALS. • Studies on the effect of free volume on ion dynamics in polymerized ionic liquids. • The failure of the LCL and CFV models in case of ion-conducting polymers. [ABSTRACT FROM AUTHOR]

Abstract The series of N -alkylpyridinium bis(trifluoromethylsulfonyl)imide, [C n py][NTf 2 ], where n = 2, 3, 4, 6, and 8 have been synthesized and the effect of alkyl chain length in cation on their thermophysical properties have been studied and compared with those observed in analogous imidazolium-based ILs. To drive this comparison, the speed of sound, density, isobaric heat capacity, electric conductivity, TGA for all [C n py][NTf 2 ], as well as, refractive index of [C n py][NTf 2 ] (n = 3, 6) and surface tension of [C n py][NTf 2 ] (n = 2, 4, 6, 8) were determined as a function of temperature at atmospheric pressure. From these experimental data, the isentropic compressibility, isobaric thermal expansion, isothermal compressibility, isochoric heat capacity, internal pressure, as well as, the surface entropy and surface enthalpy were calculated. The electric conductivity together with the literature viscosity data are used to obtain the Walden plot for the investigated ionic liquids. The minimum of the speed of sound dependence on the alkyl chain length in the cation is observed for [C 6 py][NTf 2 ]. The isentropic compressibility and the isothermal compressibility of [C n py][NTf 2 ] increase with increasing alkyl chain length in the cation and the slope of this dependence is changed around n = 6. These effects were more pronounced than that for imidazolium-based ionic liquids with [NTf 2 ]− anion. Finally, COSMOthermX software was used to further evaluate its predictive capability by comparing calculated density and isobaric heat capacity as a function of the IL structure and temperature with experimental data for the [C n py][NTf 2 ] (n = 2, 3, 4, 6, 8) series. This analysis shows a very good agreement in the case of the C p with an average absolute relative deviation close to 1.4% while a deviation close to 0.72% is observed in the case of the density of the [C n py][NTf 2 ]. Highlights • Molecular reorganization was confirmed for [C n py][NTf 2 ]. • Minimum of speed of sound in [C 6 py][NTf 2 ] was detected for [C n py][NTf 2 ]. • Density and isobaric heat capacity were evaluated by COSMOthermX. [ABSTRACT FROM AUTHOR]