Nanostructure–Thermal Conductivity Relationships in Protic Ionic Liquids

Experimental Study on the Transport Properties of 12 Novel Deep Eutectic Solvents

Deep eutectic solvents (DESs) are complex substances composed of two or three components, wherein hydrogen bond donors and acceptors engage in intricate interactions within a hydrogen bond network. They have attracted extensive attention from researchers due to their easy synthesis, cost-effectiveness, broad liquid range, good stability, and for being green and non-toxic. However, studies on the physical properties of DESs are still scarce and many theories are not perfect enough, which limits the application of DESs in engineering practice. In this study, twelve DESs were synthesized by using choline chloride and betaine as HBAs, and ethylene glycol, polyethylene glycol 600, o-cresol, glycerol, and lactic acid as HBDs. The variation rules of their thermal conductivity and viscosity with temperature at atmospheric pressure were systematically investigated. The experimental results showed that the thermal conductivity of the 1:4 choline chloride/glycerol solvent was the largest at 294 K, reaching 0.2456 W·m-1·K-1, which could satisfy the demand for high efficiency heat transfer by heat-transferring workpieces. The temperature-viscosity relationship of the DESs was fitted using the Arrhenius model, and the maximum average absolute deviation was 6.77%.

Linking Structure to Dynamics in Protic Ionic Liquids: A Neutron Scattering Study of Correlated and Single-Particle Motions

Coupling between dynamical heterogeneity of ionic liquids and their structural periodicity on different length-scales can be directly probed by quasielastic neutron scattering with polarization analysis. The technique provides the tools to investigate single-particle and cooperative ion motions separately and, thus, dynamics of ion associations affecting the net charge transport can be experimentally explored. The focus of this study is the structure-dynamic relationship in the protic ionic liquid, triethylammonium triflate, characterized by strong hydrogen bonds between cations and anions. The site-selective deuterium/hydrogen-isotope substitution was applied to modulate the relative contributions of different atom groups to the total coherent and incoherent scattering signal. This approach in combination with molecular dynamics simulations allowed us to obtain a sophisticated description of cation self-diffusion and confined ion pair dynamics from the incoherent spectral component by using the acidic proton as a tagged particle. The coherent contribution of the neutron spectra demonstrated substantial ion association leading to collective ion migration that preserves charge alteration on picosecond time scale, as well as correlation of the localized dynamics occurring between adjacent ions.

Sub-μL measurements of the thermal conductivity and heat capacity of liquids

We present the analysis of the thermal conductivity, κ, and heat capacity, C_p, of a wide variety of liquids, covering organic molecular solvents, ionic liquids and water-polymer mixtures. These data were obtained from ≈0.6 μL samples, using an experimental development based on the 3ω method, capable of the simultaneous measurement of κ and C_p. In spite of the different type and strength of interactions, expected in a priori so different systems, the ratio of κ to the sound velocity is approximately constant for all of them. This is the consequence of a similar atomic density for all these liquids, notwithstanding their different molecular structures. This was corroborated experimentally by the observation of a C_p/V ≈ 1.89 × 10⁶ J K^-1 m^-3 (≈3R/2 per atom), for all liquids studied in this work. Finally, the very small volume of the sample required in this experimental method is an important advantage for the characterization of systems like nanofluids, in which having a large amount of the dispersed phase is sometimes extremely challenging.

Dynamic Heterogeneity and Flexibility of the Alkyl Chain in Pyridinium-Based Ionic Liquids

Changing the number of carbon atoms in the substituents of ionic liquids (ILs) is a way to shift the balance between Coulomb and van der Waals forces and, thus, to tune physicochemical properties. Here we address this topic on the microscopic level by employing quasielastic neutron scattering (QENS) and provide information about the stochastic ionic motions in the N-alkylpyridinium based ILs in a relatively expanded time range, from short time (subpicosecond) particle rattling to long time diffusive regime (hundreds of picoseconds). We have systematically investigated the effect of the alkyl chain length on the picosecond dynamics by employing partial deuteration of the samples and varying the number of carbon atoms in the alkyl substituent. The localized dynamics of the side groups have appeared to be enhanced for bulkier cations, which is opposite to the trend observed for the translational motion. This result highlights the role of the conformational flexibility of the alkyl group on the dynamical properties of ILs.

Poly(ethylene oxide) Mushrooms Adsorbed at Silica-Ionic Liquid Interfaces Reduce Friction

The adsorbed layer conformation and lubricity of 35, 100, and 300 kDa PEO adsorbed to ionic liquid (IL)-silica interfaces from 0.01 wt % solutions have been investigated using colloid probe atomic force microscopy. The ILs used were propylammonium nitrate (PAN) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), which are protic and aprotic ILs, respectively. Normal force curves reveal steric interactions consistent with adsorbed polymer layers which are best fit using the mushroom model. Friction measurements show that the adsorbed polymer layer markedly reduces friction compared to surfaces sliding in the pure ILs and that lubricity increases with polymer length. When polymer is adsorbed to the sliding surfaces, friction is controlled by the creation and disruption of intermolecular interactions between entangled chains and the dragging of polymer chains through the interpenetration region. These experiments show that added polymer can reduce friction while maintaining the useful properties of ILs as lubricants.

Synthesis and characterization of protic ionic liquids as thermoelectrochemical materials

PILs have been designed and synthesized for energy harvesting applications. The module exhibited a maximum S_e value of 420 μV K⁻¹ which is the highest reported for PILs with the I⁻/I₃⁻ redox couple.

Tribotronic control of friction in oil-based lubricants with ionic liquid additives

Atomic force microscopy (AFM) reveals that tribotronic control of friction using an external potential applied to a gold surface is possible for ionic liquid (IL) concentrations as low as 5 mol% in hexadecane.

Near surface properties of mixtures of propylammonium nitrate with n-alkanols 2. Nanotribology and fluid dynamics

Colloid probe friction force microscopy (FFM) has been used to study the lubricity of propylammonium nitrate (PAN) mixed with n-alkanols confined between sliding silica and mica surfaces.

Near surface properties of mixtures of propylammonium nitrate with n-alkanols 1. Nanostructure

In situ amplitude modulated-atomic force microscopy (AM-AFM) has been used to probe the nanostructure of mixtures of propylammonium nitrate (PAN) with n-alkanols near a mica surface.