Search

Li-bis(trifluoromethylsulfonyl)imide based ionic liquids with either butyl-trimethylammonium or N,N-dimethyl-N-(2-(propionyloxy)-ethyl)butan-1-ammonium as the anion were studied using proton and fluorine relaxometry as well as using field-gradient diffusometry to gain separate access to cation and anion dynamics in these compounds. The transport parameters obtained for these ionic liquids are compared with the estimates based on the conductivity data from literature and from the present work. The impact of cation size on correlation effects, the latter parameterized in terms of various Haven ratios, is mapped out.

The glass transitions of amorphous ices as well as of aqueous phosphoric acid solutions were reported to display very large 1H/2H isotope effects. Using dielectric spectroscopy, in both types of glassformers for equimolar protonated/deuterated mixtures an almost ideal isotope-mixing behavior rather than a bimodal relaxation is found. For the amorphous ices this finding is interpreted in terms of a glass-to-liquid rather than an orientational glass transition scenario. Based on calorimetric results revealing that major 16O/18O isotope effects are missing, the latter scenario was previously favored for the amorphous ices. Considering the dielectric results on 18O substituted amorphous ices and by comparison with corresponding results for the aqueous phosphoric acid solutions, it is argued that the present findings are compatible with the glass-to-liquid scenario. To provide additional information regarding the deeply supercooled state of 1H/2H isotopically mixed and 18O substituted glassformers, the aqueous phosphoric acid solutions are studied using shear mechanical spectroscopy as well, a technique which so far could not successfully be applied to characterize the glass transitions of the amorphous ices., Phys. Chem. Chem. Phys.;24(24)

We performed rheological measurements of the typical deep eutectic solvents (DESs) glyceline, ethaline, and reline in a very broad temperature and dynamic range, extending from the low-viscosity to the high-viscosity supercooled-liquid regime. We find that the mechanical compliance spectra can be well described by the random free-energy barrier hopping model, while the dielectric spectra on the same materials involve significant contributions arising from reorientational dynamics. The temperature-dependent viscosity and structural relaxation time, revealing non-Arrhenius behavior typical for glassy freezing, are compared to the ionic dc conductivity and relaxation times determined by broadband dielectric spectroscopy. For glyceline and ethaline we find essentially identical temperature dependences for all dynamic quantities. These findings point to a close coupling of the ionic and molecular translational and reorientational motions in these systems. However, for reline the ionic charge transport appears decoupled from the structural and reorientational dynamics, following a fractional Walden rule. Especially, at low temperatures the ionic conductivity in this DES is enhanced by about one decade compared to expectations based on the temperature dependence of the viscosity. The results for all three DESs can be understood without invoking a revolving-door mechanism previously considered as a possible charge-transport mechanism in DESs., final accepted version of the manuscript

Self- and cross-correlation dynamics of deeply supercooled liquids were recently identified using photon correlation spectroscopy on the one hand and dielectric investigations on the other. These results fueled a controversial discussion whether the "generic" response identified by photon correlation spectroscopy, or rather the nonuniversal dielectric response, reflect the liquid's structural relaxation. The present study employs physical aging and oscillatory shear rheology to directly access the structural relaxation of a nonassociating glass-forming liquid and reveals that collective equilibrium fluctuations of simple liquids and not single-particle dynamics govern their structural relaxation. The present results thus challenge recent views that the glassy response of polar supercooled liquids can generically be decomposed into a Debye-type, supramolecular response and a single-particle dynamics with the latter reflecting the "true" structural relaxation. Furthermore, the current findings underscore the pivotal role dielectric spectroscopy plays in glass science as one of the rare molecular-level reorientation techniques that senses dynamical cooperativity directly.

International audience; The understanding of the microstructure of associated liquids promoted by hydrogen-bonding andconstrained by steric hindrance is highly relevant in chemistry physics, biology and for many aspects ofdaily life. In this study we use a combination of X-ray diffraction, dielectric spectroscopy and moleculardynamics simulations to reveal temperature induced changes in the microstructure of different octanolisomers, i.e., linear 1-octanol and branched 2-, 3- and 4-octanol. In all octanols, the hydroxyl groupsform the bases of chain-, cyclic- or loop-like bonded structures that are separated by outwardlydirected alkyl chains. This clustering is analyzed through the scattering pre-peaks observed from X-rayscattering and simulations. The charge ordering which pilots OH aggregation can be linked to thestrength of the Debye process observed in dielectric spectroscopy. Interestingly, all methods used hereconverge to the same interpretation: as one moves from 1-octanol to the branched octanols, thecluster structure evolves from loose large aggregates to a larger number of smaller, tighter aggregates.All alcohols exhibit a peculiar temperature dependence of both the pre-peak and Debye process, whichcan be understood as a change in microstructure promoted by chain association with increased chainlength possibly assisted by ring-opening effects. All these results tend to support the intuitive picture ofthe entropic constraint provided by branching through the alkyl tails and highlight its capital entropicrole in supramolecular assembly.

Combining results from impedance spectroscopy and oscillatory shear rheology, the present work focuses on the relation between the mass and charge flows and on how these are affected by the H-bonding in viscous ionic liquids (ILs). In particular, we compare the relaxational behaviors of the paradigmatic IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) and its OH-functionalized counterpart 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (OHEMIM-TFSI). Our results and their analysis demonstrate that the presence of cationic OH-groups bears a strong impact on the overall dynamics of OHEMIM-TFSI, although no signatures of suprastructural relaxation modes could be identified in their dielectric and mechanical responses. To check whether at the origin of this strong variation is the H-bonding or merely the difference between the corresponding cation sizes (controlling both the hydrodynamic volume and the inter-charge distance), the present study includes 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (PMIM-TFSI), mixtures of EMIM-TFSI and PMIM-TFSI with lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI), and mixtures of OHEMIM-TFSI with PMIM-TFSI. Their investigation clearly reveals that the dynamical changes induced by H-bonding are significantly larger than those that can be attributed to the change in the ion size. Moreover, in the mixtures of OHEMIM-TFSI with PMIM-TFSI, a dilution of the OH-groups leads to strong deviations from ideal mixing behavior, thus highlighting the common phenomenological ground of hydroxy-functionalized ILs and other H-bonded liquids.

Employing dielectric spectroscopy, oscillatory shear rheology, and calorimetry, the present work explores the molecular dynamics of the widely used insecticide imidacloprid above and below its glass transition temperature. In its supercooled liquid regime, the applied techniques yield good agreement regarding the characteristic structural (alpha) relaxation times of this material. In addition, the generalized Gemant-DiMarzio-Bishop model provides a good conversion between the frequency-dependent dielectric and shear mechanical responses in its viscous state, allowing for an assessment of imidacloprid's molecular hydrodynamic radius. In order to characterize the molecular dynamics in its glassy regime, we employ several approaches. These include the application of frequency-temperature superposition (FTS) to its isostructural dielectric and rheological responses as well as use of dielectric and calorimetric physical aging and the Adam-Gibbs-Vogel model. While the latter approach and dielectric FTS provide relaxation times that are close to each other, the other methods predict notably longer times that are closer to those reflecting a complete recovery of ergodicity. This seemingly conflicting dissimilarity demonstrates that the molecular dynamics of glassy imidacloprid strongly depends on its thermal history, with high relevance for the use of this insecticide as an active ingredient in technological applications.

Oscillatory shear rheology has been employed to access the structural rearrangements of deeply supercooled sulfuric acid tetrahydrate (SA4H) and phosphoric acid monohydrate, the latter in protonated (PA1H) and deuterated (PA1D) forms. Their viscoelastic responses are analyzed in relation to their previously investigated electric conductivity. The comparison of the also presently reported dielectric response of deuterated sulfuric acid tetrahydrate (SA4D) and that of its protonated analog SA4H reveals an absence of isotope effects for the charge transport in this hydrate. This finding clearly contrasts with the situation known for PA1H and PA1D. Our analyses also demonstrate that the conductivity relaxation profiles of acid hydrides closely resemble those exhibited by classical ionic electrolytes, even though the charge transport in phosphoric acid hydrates is dominated by proton transfer processes. At variance with this dielectric simplicity, the viscoelastic responses of these materials depend on their structural compositions. While SA4H displays a "simple liquid"-like viscoelastic behavior, the mechanical responses of PA1H and PA1D are more complex, revealing relaxation modes, which are faster than their ubiquitous structural rearrangements. Interestingly, the characteristic rates of these fast mechanical relaxations agree well with the characteristic frequencies of the charge rearrangements probed in the dielectric investigations, suggesting appearance of a proton transfer in mechanical relaxation of phosphoric acid hydrates. These findings open the exciting perspective of exploiting shear rheology to access not only the dynamics of the matrix but also that of the charge carriers in highly viscous decoupled conductors.

Monohydroxy alcohols have been in the focus of scientific research for a long time and their Debye (-like) process was studied predominantly using dielectric spectroscopy. However, a number of other techniques are useful to unravel the dynamics of these supramolecular liquids. For a recent review on neat monohydroxy alcohols, see R. Bohmer, C. Gainaru, R. Richert, Phys. Rep. (accepted). On this background the present article deals mostly with mixtures involving monohydroxy alcohols using various experimental methods. Examples given include dielectric spectroscopy on a mixture which shows a small Debye process well separated from the structural relaxation. For another mixture it is demonstrated that the time scales of the dielectric and the rheological signatures of the Debye process coincide. Isotope labeling is exploited to map out the rotational dynamics of both components in binary mixtures of 1-butanol (BuOH) and 1-bromobutane (BuBr) using spin–lattice relaxation nuclear magnetic resonance (NMR) spectroscopy. While the hydroxyl motion in BuOH becomes faster upon admixture of BuBr, the alkyl bromide dynamics is virtually independent of composition. Two-time and four-time stimulated-echo NMR experiments show that the dynamic exchange in a BuOH–BuBr mixture is similar to that of supercooled liquids devoid of a Debye process and hence it does not provide a rationale to understand the symmetric broadening of the structural dielectric loss peak in this and related mixtures. Finally, a wavelength dependent derivative analysis of near-infrared spectra recorded for pure and mixed monohydroxy alcohols over wide temperature ranges shows that the rearrangement of the hydrogen network differs below and above about 250 K.

To characterize the structural relaxation of an aqueous solution of LiCl, frequency-dependent shear rheological experiments are carried out near its glass transition. Analyzed within the fluidity representation, the generic spectral shape that was previously found for a range of different kinds of glass formers is confirmed for the currently studied hydrogen-bonded fluid as well. Furthermore, the validity of the rheological equivalent of the Barton-Nakajima-Namikawa relation is demonstrated for the aqueous LiCl solution. Its mechanical response is compared with that obtained using dielectric spectroscopy, a technique which is sensitive to both the reorientational dynamics of the water molecules and the translational dynamics of the ionic species. The extent to which these electrical polarization processes are coupled to those governing the viscoelastic response is discussed, also in comparison with the behavior of other ion conducting liquids.

Highly unusual linear-response spectra involving contributions from hydrogen-bonded supramolecular processes and from structural relaxations are found in 4-methyl-3-heptanol mixed with 2-ethyl-1-hexylbromide. Although the mean time scales of the underlying relaxations are separated by more than 3 decades, the overall spectra cannot be decomposed into a sum of these processes. This finding challenges the ubiquitous practice of disentangling susceptibility spectra of Debye liquids by adding suitable subspectra. The spectral shape of the studied viscous mixtures is excellently described using the Williams ansatz, here a necessary approach and not as previously considered merely an alternative to additive analyses.