Michał Zorębski

Acoustics as a Tool for Better Characterization of Ionic Liquids: A Comparative Study of 1-Alkyl-3-methylimidazolium Bis[(trifluoromethyl)sulfonyl]imide Room-Temperature Ionic Liquids

Acoustic properties of three (1-ethyl-, 1-butyl-, and 1-octyl-) 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] imide room-temperature ionic liquids are reported and discussed. The speeds of sound in RTILs were measured as a function of temperature in the range 288-323 K by means of a sing around method. The densities and isobaric heat capacities were determined from 288.15 to 363.15 K and from 293.15 to 323.15 K, respectively. The related properties, like isentropic and isothermal compressibilities, isobaric coefficients of thermal expansion, molar isochoric heat capacities, and internal pressures, were calculated. It was found that for some ionic liquids, temperature dependence of isobaric coefficients of thermal expansion is small and negative. All investigations were completed by the ultrasound absorption coefficient measurements in 1-ethyl- and 1-octyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] imide as a function of frequency from 10 to 300 MHz at temperatures 293.15-298.15 K. The ultrasound absorption spectra indicate relaxation frequencies in the megahertz range.

Structure and thermal properties of salicylate-based-protic ionic liquids as new heat storage media. COSMO-RS structure characterization and modeling of heat capacities

During this research, we present a study on the thermal properties, such as the melting, cold crystallization, and glass transition temperatures as well as heat capacities from 293.15 K to 323.15 K of nine in-house synthesized protic ionic liquids based on the 3-(alkoxymethyl)-1H-imidazol-3-ium salicylate ([H-Im-C1OC(n)][Sal]) with n = 3-11. The 3D structures, surface charge distributions and COSMO volumes of all investigated ions are obtained by combining DFT calculations and the COSMO-RS methodology. The heat capacity data sets as a function of temperature of the 3-(alkoxymethyl)-1H-imidazol-3-ium salicylate are then predicted using the methodology originally proposed in the case of ionic liquids by Ge et al. 3-(Alkoxymethyl)-1H-imidazol-3-ium salicylate based ionic liquids present specific heat capacities higher in many cases than other ionic liquids that make them suitable as heat storage media and in heat transfer processes. It was found experimentally that the heat capacity increases linearly with increasing alkyl chain length of the alkoxymethyl group of 3-(alkoxymethyl)-1H-imidazol-3-ium salicylate as was expected and predicted using the Ge et al. method with an overall relative absolute deviation close to 3.2% for temperatures up to 323.15 K.

Speed of Sound and Ultrasound Absorption in Ionic Liquids

A complete review of the literature data on the speed of sound and ultrasound absorption in pure ionic liquids (ILs) is presented. Apart of the analysis of data published to date, the significance of the speed of sound in ILs is regarded. An analysis of experimental methods described in the literature to determine the speed of sound in ILs as a function of temperature and pressure is reported, and the relevance of ultrasound absorption in acoustic investigations is discussed. Careful attention was paid to highlight possible artifacts, and side phenomena related to the absorption and relaxation present in such measurements. Then, an overview of existing data is depicted to describe the temperature and pressure dependences on the speed of sound in ILs, as well as the impact of impurities in ILs on this property. A relation between ions structure and speeds of sound is presented by highlighting existing correlation and evaluative methods described in the literature. Importantly, a critical analysis of speeds of sound in ILs vs those in classical molecular solvents is presented to compare these two classes of compounds. The last part presents the importance of acoustic investigations for chemical engineering design and possible industrial applications of ILs.

A Comparative Ultrasonic Relaxation Study of Lower Vicinal and Terminal Alkanediols at 298.15 K in Relation to Their Molecular Structure and Hydrogen Bonding

Ultrasonic relaxation spectra were determined for lower vicinal and terminal alkanediols at ambient pressure and a temperature of 298.15 K. The ultrasound absorption measurements were made by means of the standard pulse technique for 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,4-butanediol, 2,3-butanediol, and 1,5-pentanediol within the frequency range of 5-300 or 10-300 MHz. Relaxation processes were observed for all compounds except 1,2-ethanediol. The relaxation regions were dependent on both the carbon chain length and the position of hydroxyl groups. In addition, the terminal diols showed lower absorption than the adequate vicinal diols did. The results are discussed in terms of molecular structure and molecular interactions, as well as compared with the behavior of adequate lower 1-alkanols. A comparison with classical absorption is also made. The results are discussed in term of shear viscosity relaxation.

Ultrasonic Relaxation Study of 1-Alkyl-3-methylimidazolium-Based Room-Temperature Ionic Liquids: Probing the Role of Alkyl Chain Length in the Cation

Ultrasound absorption spectra of four 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides were determined as a function of the alkyl chain length on the cation from 1-propyl to 1-hexyl from 293.15 to 323.15 K at ambient pressure. Herein, the ultrasound absorption measurements were carried out using a standard pulse technique within a frequency range from 10 to 300 MHz. Additionally, the speed of sound, density, and viscosity have been measured. The presence of strong dissipative processes during the ultrasound wave propagation was found experimentally, i.e., relaxation processes in the megahertz range were observed for all compounds over the whole temperature range. The relaxation spectra (both relaxation amplitude and relaxation frequency) were shown to be dependent on the alkyl side chain length of the 1-alkyl-3-methylimidazolium ring. In most cases, a single-Debye model described the absorption spectra very well. However, a comparison of the determined spectra with the spectra of a few other imidazolium-based ionic liquids reported in the literature (in part recalculated in this work) shows that the complexity of the spectra increases rapidly with the elongation of the alkyl chain length on the cation. This complexity indicates that both the volume viscosity and the shear viscosity are involved in relaxation processes even in relatively low frequency ranges. As a consequence, the sound velocity dispersion is present at relatively low megahertz frequencies.

Water-induced aggregation and hydrophobic hydration in aqueous solutions of N-methylpiperidine

Liquid system N-methylpiperidine–water shows a miscibility gap with a lower critical solution temperature of 316.7 K. The phase separation is most likely due to the aggregation of N-methylpiperidine–water complexes, evident in the intensity of the small-angle neutron scattering at temperatures much lower than the LCST. Such complexes arise because the attraction forces between unlike molecules are stronger than the water–water ones, and aggregation is possible through the O–H⋯O bonds involving the hydration water molecules. The aggregates are dynamic structures with nanoseconds-order relaxation times, as it was estimated by the ultrasonic absorption experiment. While hydrophilic aggregation prevails at relatively high concentrations of the amine, the hydrophobic hydration is possible at low concentrations, likely consisting of the formation of structures resembling those in the sH clathrates observed in the solid state. The hydrophobic hydration of N-methylpiperidine is manifested in the minima of the partial volume isotherms at the amine mole fraction close to 0.01 and in the limiting partial molar compression approximately equal to zero. Essential similarity of the N-methylpiperidine–water system to aqueous solutions of pyridine and its methyl derivatives studied previously, suggests that those amines are potential clathrate hydrate promoters.

Supramolecular structure fluctuations of an imidazolium-based protic ionic liquid

At 20, 25, 30, and 40 °C, the ultrasonic absorption spectra of the protic ionic liquid 3-(butoxymethyl)-1H-imidazol-3-ium salicylate have been measured between 0.6 and 900 MHz. Below 250 MHz, the absorption coefficient decreases with temperature, potentially indicating a major effect of the viscosity and/or a relaxation time. Essentially the broad spectra can be favorably represented by two relaxation terms in addition to an asymptotic high-frequency contribution. One term reflects an asymmetric relaxation time distribution. It is described by a model of noncritical fluctuations in the structure and thermodynamic parameters of the liquid in order to yield the fluctuation correlation length and the mutual diffusion coefficient. Applying the Stokes-Einstein-Kawasaki-Ferrell relation, these quantities can be used to show that the effective shear viscosity controlling the fluctuations is substantially smaller than the steady-state shear viscosity. This result is consistent with dispersion in the shear viscosity as revealed by viscosity measurements at 25, 55, and 81 MHz. The other term can be well described by a Debye-type relaxation function. It has been tentatively assigned to a structural isomerization of the butoxymethyl chain of the imidazole molecule. However, it cannot be completely excluded that this term reflects, at least in parts, a Brønstedt acid-base equilibrium or a specific association process.

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.

Ultrasonic velocity and absorption in the binary mixtures of 1‐butanol with isomeric butanediols at 298.15 K

From the early days of ultrasonic technology, ultrasonic velocity and ultrasonic absorption have been a rich source of information on the structure and condition of the materials trough in which the ultrasonic waves are propagated. Here an investigation into binary mixtures (in the whole concentration range) of 1‐butanol with 1,2‐butanediol and 2,3‐butanediol at 298.15 K is reported. For the ultrasonic velocity measurements at 2.15 MHz, the pulse‐echo‐overlap method was used and the ultrasonic absorption in the frequency range 10–200 MHz was determined by the standard pulse technique, where the amplitude of the first transmitted pulse was registered as a function of distance. The results were compared with the previous data for 1‐butanol with 1,3‐butanediol and 1,4‐butanediol and discussed in terms of molecular interactions in highly associated liquids.