Viscosity Of Ionic Liquids Research Articles

Interfacial viscosity and ionic reorientation probed using electrochemical surface plasmon resonance at the gold electrode interface of ionic liquids

• Interfacial dielectric relaxation of ionic liquids has been studied by ESPR. • Interfacial viscosity of ionic liquids is demonstrated to be extremely high. • Smaller ions tend to form the electric double layer with more solid-like structure. • Imidazolium ring has weak reorientation response to the potential steps. The interfacial dielectric relaxation of ionic liquids (ILs) at the gold electrode interface has been investigated using electrochemical surface plasmon resonance (SPR), by analyzing the SPR response to the potential step for four ILs with the two cations, trioctylmethylammonium and 1-butyl-3-methylimidazolium, and three amide anions, bis(fluorosulfonyl)amide, bis(trifluoromethanesulfonyl)amide, and bis(nonafluorobutanesulfonyl)amide. For all the four ILs, the SPR response to the potential step exhibits a fast relaxation process mainly ascribable to the ionic reorientation, followed by an ultraslow relaxation process ascribable to the ionic translation, with the opposite directions of SPR shift to each other. The ultraslow relaxations by the positive potential steps are always significantly slower than those by the negative steps. The average time constants of the ultraslow relaxation process and the amplitudes of the fast relaxation process are evaluated by the fitting with a multiple exponential function and are compared with the measured bulk viscosity and the dipole moments of each ion. The interfacial viscosity, estimated from the time constants of the ultraslow relaxation process, is several orders of magnitude higher than the bulk viscosity, and the viscosity increase is larger for the ILs composed of smaller ions. The amplitude of the fast relaxation, reflecting the ionic reorientation, is moderately correlated with the dipole moments of counter-ions on the electrode at the pre-step potential.

Machine learning investigation of viscosity and ionic conductivity of protic ionic liquids in water mixtures.

Ionic liquids (ILs) are well classified as designer solvents based on the ease of tailoring their properties through modifying the chemical structure of the cation and anion. However, while many structure-property relationships have been developed, these generally only identify the most dominant trends. Here, we have used machine learning on existing experimental data to construct robust models to produce meaningful predictions across a broad range of cation and anion chemical structures. Specifically, we used previously collated experimental data for the viscosity and conductivity of protic ILs [T. L. Greaves and C. J. Drummond, Chem. Rev. 115, 11379-11448 (2015)] as the inputs for multiple linear regression and neural network models. These were then used to predict the properties of all 1827 possible cation-anion combinations (excluding the input combinations). These models included the effect of water content of up to 5 wt. %. A selection of ten new protic ILs was then prepared, which validated the usefulness of the models. Overall, this work shows that relatively sparse data can be used productively to predict physicochemical properties of vast arrays of ILs.

Application of PC-SAFT EoS for calculating gas solubility and viscosity of ammonium-based ionic liquids

Application of PC-SAFT EoS for calculating gas solubility and viscosity of ammonium-based ionic liquids

The effect of protic ionic liquids incorporation on CO2 separation performance of Pebax-based membranes

• Two protic ionic liquids/Pebax blended membranes were developed for CO 2 separation. • [TMGH][Im]/Pebax blended membrane showed better CO 2 permeability and selectivity. • Gas permeability increased and CO 2 selectivity declined at higher temperatures. The separation of carbon dioxide (CO 2 ) is of great importance for environment protection and gas resource purification. The ionic liquids (ILs)-based gas separation membrane provides a new chance for efficient CO 2 separation, while high permeability and selectivity of membranes is a great challenge. In this study, the influence of two protic ILs with different anion ([TMGH][Im] and [TMGH][PhO]) on the CO 2 separation performance of the prepared ILs/Pebax blended membranes were systematically investigated at different temperature. The results showed the CO 2 permeability exhibits the rising trend for ILs/Pebax blended membranes with the increment of IL content. Especially, the [TMGH][Im] with low viscosity and high CO 2 absorption capacity leads to the blended membranes showing better CO 2 permeability and ideal CO 2 selectivity than that of membranes with [TMGH][PhO] at high IL content. Besides, with operating temperature increasing, the gas permeability of 20% (mass) [TMGH][Im]/Pebax blended membrane increases due to the decreasing viscosity of IL and the rising chain mobility of polymer. Inversely, the gas selectivity shows decreasing trend because CO 2 absorption capacity obviously decreased at higher temperature.

Surface tension, viscosity and electrical conductivity characteristics of new ether-functionalized ionic liquids

1-(2-Ethoxyethyl)-3-methylimidazolium acetic acid ([C2OC2mim][OAc]) and 1-(2-ethoxyethyl)-3-methylimidazolium lactic acid ([C2OC2mim][Lac]) ether-functionalized ionic liquids (ILs) were prepared. In terms of the standard addition method, the experimental data of density, surface tension, refractive index, viscosity and electrical conductivity of two pure ILs were obtained. Using the definition of the molar surface Gibbs energy with Lorentz-Lorenz empirical equation, the surface tensions of ether-functionalized ILs were estimated, and their predicted values were in good agreement with the experimental values. According to the Vogel-Fulcher-Tamman (VFT) equation, the variation tendency between viscosity (or electrical conductivity) and temperature of [C2OC2mim][OAc] and [C2OC2mim][Lac] was studied. The variation trend of the viscosity was opposite to electrical conductivity. Namely, the viscosity gradually decreased and electrical conductivity gradually increased with increasing temperature. The results indicated that VFT equation had an enormous application range for fitting the viscosity and electrical conductivity of ether-functionalized ILs. The activation energies of [C2OC2mim][OAc] and [C2OC2mim][Lac] were estimated by Arrhenius empirical equation. The calculated results indicated that [C2OC2mim][Lac] (Eη means the activation energy, Eη = 41.24 kJ·mol−1) was larger than [C2OC2mim][OAc] (Eη = 33.57 kJ·mol−1). This result may be related to physical size or stronger interactions.

Benchmarking machine learning methods for modeling physical properties of ionic liquids

The great importance of the ability to quantitatively predict the properties of ionic liquids (ILs) using quantitative structure–property relationships (QSPR) models necessitates the understanding of which modern machine learning (ML) methods in combination with which types of molecular representations are preferable to use for this purpose. To address this problem, a large-scale benchmarking study of QSPR models built by combining three traditional ML methods and neural networks with seven different architectures with five types of molecular representations (in the form of either numerical molecular descriptors or SMILES text strings) to predict six important physical properties of ILs (density, electrical conductance, melting point, refractive index, surface tension, and viscosity) was carried out. The datasets include from 407 to 1204 diverse ILs composed of various organic and inorganic ions. QSPR models for predicting the properties of ILs at eight different temperatures were built using multi-task learning. The best combinations of ML methods and molecular representations were identified for each of the properties. A unified ranking system was introduced to rank and prioritize different ML methods and molecular representations. It was shown in this study that on average: (i) nonlinear ML methods perform much better than linear ones, (ii) neural networks perform better than traditional ML methods, (iii) Transformers, which are actively used in natural language processing (NLP), perform better than other types of neural networks due to the advanced ability to analyze chemical structures of ILs encoded into SMILES text strings. A special “component-wise” cross-validation scheme was applied to assess how much the predictive performance deteriorates for the ILs composed of cations and anions that are not present in the dataset.

Four Phosphonium‐based Ionic Liquids. Synthesis, Characterization and Electrochemical Performance as Electrolytes for Silicon Anodes

AbstractHerein, we describe the synthesis, characterization and electrochemical performance of four phosphonium‐based ionic liquids (ILs) as electrolytes, Physicochemical properties such as viscosity, density, ionic conductivity, and thermal stability of ILs and conventional organic solvent ethylene carbonate (EC)/diethyl carbonate (DEC) were experimentally determined at different temperatures. All ILs showed thermal stability greater than 300 °C, surpassing the stability of the conventional organic solvent, whose flash points were 145 and 23 °C for EC and DEC, respectively. Nevertheless, at room temperature, all ILs are much more viscous than EC/DEC. The composite Si ‐[P2224][FSI] (triethyl‐n‐butylphosphonium bis(fluoromethylsulfonyl)imide) and Si‐EC/DEC anodes exhibit initial specific capacities at 0.15 A/g of 2409 and 2631 mAh/g, respectively. This demonstrates that despite the inferior transport properties of ILs, short alkyl‐substituted phosphonium ILs like [P2224][FSI] are potentially competitive for the new generation of electrolytes for LIBs. NMR, DSC, TGA, and galvanostatic discharged/charged were used as characterization techniques.

Machine learning for the prediction of viscosity of ionic liquid–water mixtures

In this work, a nonlinear model that integrates the group contribution (GC) method with a well-known machine learning algorithm, i.e., artificial neural network (ANN), is proposed to predict the viscosity of ionic liquid (IL)-water mixtures. After a critical assessment of all data points collected from literature, a dataset covering 8,523 viscosity data points of IL-H2O mixtures at different temperature (272.10 K-373.15 K) is selected and then applied to evaluate the proposed ANN-GC model. The results show that this ANN-GC model with 4 or 5 neurons in the hidden layer is capable to provide reliable predictions on the viscosities of IL-H2O mixtures. With 4 neurons in the hidden layer, the ANN-GC model gives a mean absolute error (MAE) of 0.0091 and squared correlation coefficient (R2) of 0.9962 for the 6,586 training data points, and for the 1,937 test data points they are 0.0095 and 0.9952, respectively. When this nonlinear model has 5 neurons in the hidden layer, it gives a MAE of 0.0098 and R2 of 0.9958 for the training dataset, and for the test dataset they are 0.0092 and 0.9990, respectively. In addition, comparisons show that the nonlinear ANN-GC model proposed in this work has much better prediction performance on the viscosity of IL-H2O mixtures than that of the linear mixed model.

Molecular Simulation and Experimental Study on Low-Viscosity Ionic Liquids for High-Efficient Capturing of CO2

In recent decades, ionic liquids (ILs) have been developed as an ideal and reversible decarbonization solvent. However, some pitfalls, such as the low CO2 capacity and high viscosity of ILs, limit their further scale up and industrial application. Therefore, in this work, three novel functional ILs [N8881][NIA], [N8881][For], and [N8881][Ac] were synthesized after using a molecular design method to capture CO2 at 303.15–333.15 K and pressures up to 1000 kPa. The experimental results illustrate that [N8881][NIA] presents the smallest viscosity and the highest CO2 solubility. The capacity order is [N8881][NIA] > [N8881][For] > [N8881][Ac] under the same experimental conditions. The CO2 recyclability experiments using [N8881][NIA] show the stability of CO2 solubility after 5 cycles. The quantum chemistry simulations at the level of DFT/B3LYP with 6-311++G(d,p) basis sets were used to study the CO2 absorption mechanism in the studied ILs from the molecular viewpoint. Simulation results illustrated that the higher interaction energy between CO2 and ILs means more CO2 absorbed in these ILs, which agreed well with the experimental results. The atoms in molecules theory and the function of the reduced density gradient were also used to calculate the interactions in these IL–CO2 systems. Results show that the majority of these interactions present an electrostatic character.

Comparison of CO2 Separation Efficiency from Flue Gases Based on Commonly Used Methods and Materials.

The comparison study of CO2 removal efficiency from flue gases at low pressures and temperatures is presented, based on commonly used methods and materials. Our own experimental results were compared and analyzed for different methods of CO2 removal from flue gases: absorption in a packed column, adsorption in a packed column and membrane separation on polymeric and ceramic membranes, as well as on the developed supported ionic liquid membranes (SILMs). The efficiency and competitiveness comparison of the investigated methods showed that SILMs obtained by coating of the polydimethylsiloxane (PDMS) membrane with 1-ethyl-3-methylimidazolium acetate ([Emim][Ac]) exhibit a high ideal CO2/N2 selectivity of 152, permeability of 2400 barrer and long term stability. Inexpensive and selective SILMs were prepared applying commercial membranes. Under similar experimental conditions, the absorption in aqueous Monoethanolamine (MEA) solutions is much faster than in ionic liquids (ILs), but gas and liquid flow rates in packed column sprayed with IL are limited due to the much higher viscosity and lower diffusion coefficient of IL. For CO2 adsorption on activated carbons impregnated with amine or IL, only a small improvement in the adsorption properties was achieved. The experimental research was compared with the literature data to find a feasible solution based on commercially available methods and materials.

Role of density and electrostatic interactions in the viscosity and non-newtonian behavior of ionic liquids - a molecular dynamics study.

Strong ionic interactions, as well as the consequent correlations between cation and anion dynamics, give ionic liquids various physical features that set them apart from ordinary organic solvents. In particular, they result in larger viscosities and larger densities than mixtures of neutral compounds with similar molecular structures. However, both the direct effect of electrostatic interactions and the increase of liquid density contribute to the high viscosity and so far no experimental or computational work enabled a clear quantification of those effects. Also, the effects over the shear thinning behavior, which may have important consequences for application as lubricants, were not considered yet. Here, these questions were tackled by performing non-equilibrium molecular dynamics (NEMD) simulations changing both the strength of ionic interactions and liquid density at several shear rates using a coarse grained model. The relative dielectric constant was adjusted to reproduce viscosity data from all-atoms simulations on both zero shear and high shear conditions. Elimination of ionic interactions results in a reduction of density and zero shear viscosity and also delays the beginning of shear thinning to higher shear rates. Restoring density to the ionic liquid's value only partially reverses the alterations. Correlations of the non-newtonian behavior and changes in the intermolecular structure and contact lifetimes were also explored.

Density, Viscosity, Crystallization Temperature, Decomposition Temperature, Ionic Conductivity, and Electrochemical Window of Ionic Liquid Based Electrolytes

Density, Viscosity, Crystallization Temperature, Decomposition Temperature, Ionic Conductivity, and Electrochemical Window of Ionic Liquid Based Electrolytes

Binary mixtures containing imidazolium ionic liquids: properties measurement

Abstract Densities and transport properties (dynamic viscosity) of pure imidazolium ionic liquids 1-butyl-3-methylimidazolium acetate and 1-butyl-3-methylimidazolium dicyanamide and their binary mixtures with water and ethanol were measured within the temperature range of 293.15—333.15 K. Obtained experimental data were used to calculate excess molar volume and viscosity deviation. For the chosen binary mixtures, variations of excess molar volume, partial molar volumes of mixture components and of the viscosity deviation with the binary mixture composition were correlated using the Redlich-Kister equation. In addition, variation of viscosity with the binary mixture composition and temperature was fitted using the Jouyban-Acree model.

Pressing matter: why are ionic liquids so viscous?

Room temperature ionic liquids are considered to have huge potential for practical applications such as batteries. However, their high viscosity presents a significant challenge to their use changing from niche to ubiquitous. The modelling and prediction of viscosity in ionic liquids is the subject of an ongoing debate involving two competing hypotheses: molecular and local mechanisms versus collective and long-range mechanisms. To distinguish between these two theories, we compared an ionic liquid with its uncharged, isoelectronic, isostructural molecular mimic. We measured the viscosity of the molecular mimic at high pressure to emulate the high densities in ionic liquids, which result from the Coulomb interactions in the latter. We were thus able to reveal that the relative contributions of coulombic compaction and the charge network interactions are of similar magnitude. We therefore suggest that the optimisation of the viscosity in room temperature ionic liquids must follow a dual approach.

On the prediction of thermo‐physicochemical properties and equilibrium absorption of CO 2 into aqueous protic monoethanolamine glycinate ionic liquid

The utilization of pure ionic liquids (ILs), as environmentally green and soft chemical solvents, is restricted in the CO2 capture industry due to their high viscosity and fair conductivity. In this paper, the novel comprehensive models were developed for accurate estimation of density, dynamic viscosity, surface tension, and electrical conductivity of a novel protic amino acid-functionalized IL, namely monoethanolamine glycinate, [MEA][GLY], in the temperature range of 303.15–323.15 K at two different intervals of water content (5–20 wt%, and 25–100 wt%). The results of the suggested models revealed that the density, viscosity, and surface tension of [MEA][GLY] decline substantially with rising temperature, while the electrical conductivity presents an adverse trend. Additionally, the impact of varying the concentration on the density and viscosity of the IL solution is more significant than the solution temperature, particularly at high concentrations of the IL. Moreover, a second-order polynomial equation was suggested to depict the equilibrium pressure dependence of CO2 loading capacity of aqueous [MEA][GLY] in IL, at various equilibrium pressures of 1–7 bar.

Influence of the alkyl chain length on the physicochemical properties and microbial biocompatibility of phosphonium based fatty acid ionic liquids

Ionic liquids (ILs) have remarkable properties and applications in many areas of science. Phosphonium ILs have become important because of their unique chemical and thermal stabilities. The present work is focused on the synthesis, characterisation, physicochemical properties, and microbial toxicity assessment of phosphonium ILs bearing seven different fatty acid anions. The structures of the synthesised ILs were confirmed by1H and13C nuclear magnetic resonance (NMR) and FTIR spectroscopy. Physicochemical properties such as density and viscosity of pure ILs were measured at temperatures ranging from 298.15 to 313.15 K. The experimental density decreased, whereas the viscosity increased with an increasing number of carbon atoms in the anion. The derived properties were also found to be anion dependent. The thermal decomposition temperature was investigated by TGA. Subsequently, the toxicity profile of the ILs was determined for selected Gram positive and Gram negative bacteria and some species of fungi in terms of minimum inhibitory concentrations (MIC). The results show that the antimicrobial activities of the ILs are strongly related to the structures of the ILs, where an increase in toxicity was observed with increasing alkyl group chain length of the fatty acid anion.

Viscosity and electrical conductivity of ether-functionalized ionic liquids-[C1OC2mim][OAc], [C1OC2mim][Pro] and [C2OC2mim][Pro

1-(2-Methoxyethyl)-3-methylimidazolium acetic acid ([C1OC2mim][OAc]), 1-(2-methoxyethyl)-3-methylimidazolium propionic acid ([C1OC2mim][Pro]) and 1-(2-ethoxyethyl)-3-methylimidazolium propionic acid ([C2OC2mim][Pro]) were successfully prepared by two-step synthesis method. Based on the standard addition method, the physicochemical parameters of the density, viscosity and electrical conductivity for ether-functionalized ionic liquids (ILs) were measured. The volume properties of three ether-functionalized ILs and their homologues were estimated by semi-empirical methods. According to the Eying viscosity equation, the relationship between viscosity and activation Gibbs energy for viscous flow (ΔG≠) was studied. The activation energy values for three ether-functionalized ILs were calculated by Arrhenius empirical equation. The activation energies of these ether-functionalized ILs are arranged in the following order: [C1OC2mim][Pro] (Eη = 39.17 kJ·mol−1) > [C2OC2mim][Pro] (Eη = 37.26 kJ·mol−1) > [C1OC2mim][OAc] (Eη = 37.23 kJ·mol−1). Futhermore, the Vogel-Fulcher-Tamman (VFT) equation is used to describe the relationship between viscosity (or electrical conductivity or molar electrical conductivity) and temperature, and the experimental results show that VFT equation has good general applicability.

A van der Waals-EoS-based model for the dynamic viscosity of ionic liquids

A density-independent thermodynamic model was presently developed to adequately represent the dynamic viscosity of pure ionic liquids (ILs). The present model adopts the simple form of the van der Waals equation of state under the basis of a previously established phenomenological similarity between the P-V-T and T-η-P surfaces for non-ionic fluids; however, we found that, in the particular case of pure ionic liquids, the surface P−f−T (where f is the fluidity, the reciprocal of viscosity) rather than the T-η-P surface conforms much better to the P-V-T surface. The resulting model was successfully validated during the representation of experimental dynamic viscosities of three families of imidazolium-based ILs ([CXmim][BF4], [CXmim][PF6] and [CXmim][Tf2N]), four pyridinium-based ILs ([bmpy][BF4], [empy][EtSO4], [Et2Nic][EtSO4] and [hemmpy][Tf2N]) and two ammonium-based ILs ([cpmam][MeSO4] and [4bam][doc]) within a temperature range varying from 0 to 80 °C and at pressures from 1 up to 3,000 bar thus covering a wide viscosity range of 10–19,610 mPa-s.

Pressure and shear rate effects on viscosity and structure of imidazolium-based ionic liquids

A lubricant film can be subject simultaneously to very high pressures and shear rates under machine operation. Due to the promising applications of ionic liquids (ILs) as lubricants and as lubricant additives, it is crucial to know how the physical properties and molecular structure of these liquids change under these extreme conditions. Non-equilibrium molecular dynamics (NEMD) simulations were performed to determine the apparent viscosity as a function of the shear rate of the ILs 1-ethyl-3-methyl-imidazolium tetrafluoroborate ([EMIM][BF4]) and 1‑butyl‑3-methyl-imidazolium tetrafluoroborate ([BMIM][BF4]) under pressures of 0.101, 507 and 1013 MPa. For comparison purpose, a similar study was also conducted for benzene. The Carreau equation was used to describe the shear rate dependence of both the viscosity and the structure. We found that the zero shear viscosity and the relaxation time exhibit a remarkable pressure dependence. Under high pressure, the shear tinning behavior starts at lower shear rates, and that correlates with the weakening of the first coordination shell of anions around cations. Both imidazolium ring and benzene assume preferentially an orientation parallel to the flow at high shear rate, and this effect is intensified at high pressure.

Comparing the Electrochemical Performance of Organic and Ionic Liquid-Based Electrolytes in Lithium-Oxygen Battery

Lithium-oxygen batteries (LOB) have been considered promising electrochemical energy storage devices to supersede current rechargeable batteries for next generation electric vehicles. Despite having tremendously high theoretical energy density (5200 Wh/kg), the practically achievable energy density (1700 Wh/kg) is far from commercial application till date. There are several challenges associated with Li anode, porous cathode and electrolyte that need to be resolved to achieve better results. The choice of suitable electrolyte for LOB is one of the biggest obstacles in its development. The various electrolytes that have been developed and studied come with limitations which includes thermal and chemical stability at higher voltages, volatility, viscosity, flammability, capability to dissolve incoming oxygen etc. Therefore, in order to improve current performance of battery, these parameters need to be optimized.This study focuses on the influence of organic and ionic liquid (IL)-based electrolytes on electrochemical performance of Li-O2 battery. [Pyr14][TFSI] and two commercially available organic solvents TEGDME and DMSO were investigated in this study. A series of tests were performed at various operating temperatures with the IL. Vulcan XC 72R coated carbon cloth with 30wt% PTFE is used as cathode. 1 mg/cm2 (+/-0.3mg/ cm2) is used as carbon loading. All experiments were conducted with pure oxygen supply at room temperature (unless stated otherwise). Li|Li Symmetric tests were also performed to measure the stability of electrolyte.The IL was tested at three different operating temperatures including room temperature (22°C), 35°C and 75°C. The results indicate that increase in temperature helps improve discharge capacity of battery. The discharge capacity obtained at three operating temperatures were 0.04 mAh/mg, 0.06 mAh/mg and 0.3 mAh/mg respectively. The improvement is in accordance with the fact that increase in temperature reduces IL viscosity and increases oxygen diffusivity. At discharge cut-off voltage of 2V, the discharge cycles did not show mass transfer limitation therefore, the subsequent tests were conducted with cut-off voltage set at 0V. This significantly improved the discharge capacity from 0.3 mAh/mg to 1.1 mAh/mg with discharge cycles limited by mass transfer effect.Two organic electrolytes TEGDME and DMSO were also tested at room temperature (22°C). The discharge capacities obtained were 4.65 mAh/mg and 5.43 mAh/mg. The higher discharge capacity of DMSO could be associated with its unique properties which includes donor number (DN), conductivity and viscosity. The DN for DMSO and TEGDME are 29.8 kcal/mol and 16.6 kcal/mol respectively. Higher DN improves discharge capacity by promoting growth of Li2O2 as bigger toroidal precipitates on cathode surface as opposed to its counterpart which produces Li2O2 insulating film. Similarly, DMSO has higher conductivity (2.11 mS/cm) and lower viscosity (1.94 cP) than TEGDME (0.3 mS/cm,4.05 cP) which improves species mobility.Finally, Li|Li symmetric tests were conducted at room temperature to measure stability of electrolytes. The results indicate, DMSO could cycle 60 cycles without significant surge in overpotential followed by TEGDME (40 cycles) and IL (20 cycles).