Viscoelastic properties of room temperature ionic liquids

Dynamics in Quaternary Ionic Liquids with Non-Flexible Anions: Insights from Mechanical Spectroscopy

The present work investigates how mechanical properties and ion dynamics in ionic liquids (ILs) can be affected by ILs' design while considering possible relationships between different mechanical and transport properties. Specifically, we study mechanical properties of quaternary ionic liquids with rigid anions by means of Dynamical Mechanical Analysis (DMA). We are able to relate the DMA results to the rheological and transport properties provided by viscosity, conductivity, and diffusion coefficient measurements. A good agreement is found in the temperature dependence of different variables described by the Vogel-Fulcher-Tammann model. In particular, the mechanical spectra of all the measured liquids showed the occurrence of a relaxation, for which the analysis suggested its attribution to a diffusive process, which becomes evident when the ion dynamics are not affected by the fast structural reorganization of flexible anions on a local level.

Sustainable and Printable Nanocellulose-Based Ionogels as Gel Polymer Electrolytes for Supercapacitors

A new gel polymer electrolyte (GPE) based supercapacitor with an ionic conductivity up to 0.32–0.94 mS cm⁻² has been synthesized from a mixture of an ionic liquid (IL) with nanocellulose (NC). The new NC-ionogel was prepared by combining the IL 1-ethyl-3-methylimidazolium dimethyl phosphate (EMIMP) with carboxymethylated cellulose nanofibers (CNFc) at different ratios (CNFc ratio from 1 to 4). The addition of CNFc improved the ionogel properties to become easily printable onto the electrode surface. The new GPE based supercapacitor cell showed good electrochemical performance with specific capacitance of 160 F g⁻¹ and an equivalent series resistance (ESR) of 10.2 Ω cm⁻² at a current density of 1 mA cm⁻². The accessibility to the full capacitance of the device is demonstrated after the addition of CNFc in EMIMP compared to the pristine EMIMP (99 F g⁻¹ and 14.7 Ω cm⁻²).

Correlated Ion Transport and the Gel Phase in Room Temperature Ionic Liquids

Here we present a theory of ion aggregation and gelation of room temperature ionic liquids (RTILs). Based on it, we investigate the effect of ion aggregation on correlated ion transport-ionic conductivity and transference numbers-obtaining closed-form expressions for these quantities. The theory depends on the maximum number of associations a cation and anion can form and the strength of their association. To validate the presented theory, we perform molecular dynamics simulations on several RTILs and a range of temperatures for one RTIL. The simulations indicate the formation of large clusters, even percolating through the system under certain circumstances, thus forming a gel, with the theory accurately describing the obtained cluster distributions in all cases. However, based on the strength and lifetime of associations in the simulated RTILs, we expect free ions to dominate ionic conductivity despite the presence of clusters, and we do not expect the percolating cluster to trigger structural arrest in the RTIL.

Apparent Non-Newtonian Behavior of Ionic Liquids

A significant viscosity variation with the shear rate has been observed for several ionic liquids in rheometry experiments above a critical shear rate. Depending on the liquid and the rheological conditions, both viscosity increase and decrease have been reported. So far, these variations have been interpreted as a signature of a non-Newtonian behavior. However, the measured critical shear rates are orders of magnitude below the ones predicted by numerical simulations. In this work, we perform new rheometry experiments with both ionic liquids and Newtonian liquids to elucidate this discrepancy. For these two types of liquids, both a viscosity decrease and increase have been measured depending on the geometry of the rheometer and the zero-shear viscosity of the liquid. We interpret the viscosity decrease as resulting from viscous heating, since the viscosity of the investigated liquids is also highly temperature-dependent, and the viscosity increase as resulting from the development of instabilities at high shear rates.

Neutron scattering studies on short- and long-range layer structures and related dynamics in imidazolium-based ionic liquids

Alkyl-methyl-imidazolium ionic liquids CnmimX (n: alkyl-carbon number, X: anion) have short-range layer structures consisting of ionic and neutral (alkylchain) domains. To investigate the temperature dependences of the interlayer, interionic group, and inter-alkylchain correlations, we have measured the neutron diffraction (ND) of C16mimPF₆, C9.5mimPF₆, and C8mimPF₆ in the temperature region from 4 K to 470 K. The quasielastic neutron scattering (QENS) of C16mimPF₆ was also measured to study the dynamics of each correlation. C16mimPF₆ shows a first-order transition between the liquid (L) and liquid crystalline (LC) phases at T_c = 394 K. C8mimPF₆ exhibits a glass transition at T_g = 200 K. C9.5mimPF₆, which is a 1:3 mixture between C8mimPF₆ and C10mimPF₆, has both transitions at T_c = 225 K and T_g = 203 K. In the ND experiments, all samples exhibit three peaks corresponding to the correlations mentioned above. The widths of the interlayer peak at ca. 0.2 Å^-1 changed drastically at the L-LC transitions, while the interionic peaks at ca. 1 Å^-1 exhibited a small jump at T_c. The peak position and area of the three peaks did not change much at the transition. The structural changes were minimal at T_g. The QENS experiments demonstrated that the relaxation time of the interlayer motion increased tenfold at T_c, while those of other motions were monotonous in the whole temperature region. The structural and dynamical changes mentioned above are characteristic of the L-LC transition in imidazolium-based ionic liquids.

Ultrasonic Relaxation Spectra for Pyrrolidinium Bis(trifluoromethylsulfonyl)imides: A Comparison with Imidazolium Bis(trifluoromethylsulfonyl)imides

Ultrasound absorption spectra within the frequency range 10-300 MHz were determined for 1-propyl- and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imides at ambient pressure and at temperatures in the ranges 293.15-313.15 and 293.15-323.15 K, respectively. For both compounds, a single Debye model (relaxation times between 0.451 and 0.778 ns) thoroughly describes the observed ultrasound absorption spectra in the investigated ranges. The spectra resemble those observed for imidazolium-based ionic liquids with the same anion. The ultrasound relaxation is dependent on the alkyl chain length of pyrrolidinium ring. In comparison to adequate imidazolium-based bis(trifluoromethylsulfonyl)imides, the relaxation in pyrrolidinium-based bis(trifluoromethylsulfonyl)imides is stronger; the pyrrolidinium cation causes clearly greater absorption than the imidazolium cation. Also, estimated ultrasound velocity dispersion is stronger in the case of pyrrolidinium imides in comparison to imidazolium imides. In turn, comparison of the ultrasonic data and literature data for the dielectric spectra exemplified for the 1-butyl- side chain in the cation indicates strong coupling in the case of imidazolium ring and weak coupling in the case of pyrrolidinium ring. The effect of absorption on the speed of sound is also discussed.

Assessment of QCM array schemes for mixture identification: citrus scented odors

A comparative study of QCM based MSAs, VSAs, and V-MSAs for complex mixture analysis.

Atomic resolution imaging of gold nanoparticle generation and growth in ionic liquids

Recent advances in in situ transmission electron microscopy (TEM) techniques have provided unprecedented knowledge of chemical reactions from a microscopic viewpoint. To introduce volatile liquids, in which chemical reactions take place, use of sophisticated tailor-made fluid cells is a usual method. Herein, a very simple method is presented, which takes advantage of nonvolatile ionic liquids without any fluid cell. This method is successfully employed to investigate the essential steps in the generation of gold nanoparticles as well as the growth kinetics of individual particles. The ionic liquids that we select do not exhibit any anomalous effects on the reaction process as compared with recent in situ TEM studies using conventional solvents. Thus, obtained TEM movies largely support not only classical theory of nanoparticle generation but also some nonconventional phenomena that have been expected recently by some researchers. More noteworthy is the clear observation of lattice fringes by high-resolution TEM even in the ionic liquid media, providing intriguing information correlating coalescence with crystal states. The relaxation of nanoparticle shape and crystal structure after the coalescence is investigated in detail. The effect of crystal orientation upon coalescence is also analyzed and discussed.

Surface waves on non-Newtonian viscoelastic liquids

In the present work, we study the dynamics of thermally excited fluctuations on the surface of non-Newtonian viscoelastic liquids, which shows complex behaviors such as crossover between the capillary wave and the elastic wave, and the coexistence of several modes. We show that the power spectrum is separated into surface localized modes and the bulk shear modes, and they are decomposed further into several modes using the partial fraction expansion. The peak positions of these modes are characterized by roots of a polynomial equation. We calculate the decomposition of the surface wave spectra numerically, and discuss evolution of constituent peaks with liquid parameters.

Mode-coupling theoretical analysis of transport and relaxation properties of liquid dimethylimidazolium chloride

The mode-coupling theory for molecular liquids based on the interaction-site model is applied to a representative molecular ionic liquid, dimethylimidazolium chloride, and dynamic properties such as shear viscosity, self-diffusion coefficients, reorientational relaxation time, electric conductivity, and dielectric relaxation spectrum are analyzed. Molecular dynamics (MD) simulation is also performed on the same system for comparison. The theory captures the characteristics of the dynamics of the ionic liquid qualitatively, although theoretical relaxation times are several times larger than those from the MD simulation. Large relaxations are found in the 100 MHz region in the dispersion of the shear viscosity and the dielectric relaxation, in harmony with various experiments. The relaxations of the self-diffusion coefficients are also found in the same frequency region. The dielectric relaxation spectrum is divided into the contributions of the translational and reorientational modes, and it is demonstrated that the relaxation in the 100 MHz region mainly stems from the translational modes. The zero-frequency electric conductivity is close to the value predicted by the Nernst-Einstein equation in both MD simulation and theoretical calculation. However, the frequency dependence of the electric conductivity is different from those of self-diffusion coefficients in that the former is smaller than the latter in the gigahertz-terahertz region, which is compensated by the smaller dispersion of the former in the 100 MHz region. The analysis of the theoretical calculation shows that the difference in their frequency dependence is due to the different contribution of the short- and long-range liquid structures.

Bulk shear-mode contribution to thermally generated capillary waves on a room-temperature ionic-liquid surface

In the present work, we show that the autocorrelation function of the capillary-wave displacement is expressed by the sum of the ordinary oscillator and the bulk shear-mode terms. The former is expressed by a simple damped oscillator form or a sum of exponentially damping functions depending on the extent of damping. The latter is also written by superposition of exponentially damping modes, and an analytically exact formulation is obtained. We performed surface dynamic light-scattering experiment for the surface of an ionic liquid, 1-butyl- 3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, and compared the experimental autocorrelation function with the theoretical one. We observed for the first time the bulk shear-mode contribution, and confirmed that the experimental data is well explained by the theoretical autocorrelation function.

Dispersion relation of capillary waves on ionic liquids: observation of the fast overdamped mode

We measured the capillary wave spectra on the surface of ionic liquids using dynamic light scattering techniques. Empirical dispersion relations deduced from the time domain analysis clearly reveal the transition from oscillating to overdamped behavior with changing temperature and the wave vector. In particular, not only slow but also fast modes were observed in the overdamped regime. The empirical dispersion relations coincide with the theoretical dispersion relations deduced from the linearized Navier-Stokes equation when experimentally determined density, viscosity, and surface tension are assigned to the latter.