Thermophysical properties of ionic liquids and their mixtures from a new equation of state

In our previous work, a perturbed hard-trimer-sphere equation of state (PHTS EOS) was developed for modeling the phase equilibria of pure ionic liquids (ILs) (M.M. Alavianmehr et al., Ionics 22 (2016) 2447–2459). In this work, we have successfully extended the model to the mixtures of IL + IL and IL + solvent. Two temperature-dependent parameters appearing in the EOS are correlated with two microscopic scaling constants σ, the effective hard-sphere diameter, and ε, the non-bonded interaction energy. The overall average absolute deviation (AAD) of the estimated densities from the literature data using the proposed model with and without non-additivity parameter (λ _ij) was found to be 0.44 and 0.79%, respectively. A modified Enskog equation and rough hard-sphere (RHS) theory are combined with our proposed equation of state to calculate the viscosity coefficient of ionic liquids and their mixtures. Finally, from the results obtained, a linear relation between logarithm of surface tension and viscosity property of ionic liquid was developed.     

Modelling phase equilibria of pure ionic liquids from a new equation of state

A perturbed hard-trimer (PHT) equation of state (EOS) has been employed to model densities and some derived thermodynamic properties of 39 ionic liquids (ILs) considering a trimer expression obtained from the statistical associating fluid theory as the reference physical model. The van der Waals dispersion forces were applied as perturbation term. The proposed model was tested using ILs containing imidazolium, pyrrolidinium, pyridinium, phosphonium and piperidinium cations. Two parameters appeared in the PHT EOS which are temperature-dependent, reflecting the dispersive energy parameters among trimers, ε and the hard-core diameter, σ, were determined based on the molecular scaling parameters. The performance of the proposed PHT EOS has been evaluated by predicting the volumetric and first and second derivatives thermodynamic properties in the pressure and temperature ranges within 0.1–200 MPa and 273–472.6 K, respectively. From 6331 data points examined, the average absolute deviation (AAD) of the correlated (at 0.1 MPa) and predicted (at high pressures) densities from the experimental ones was found to be 0.18 %. Furthermore, the isothermal compressibilities and thermal expansion coefficients as well as the heat capacities were estimated through the PHT EOS with uncertainties of the order of ±11.09, ±11.76 and ±3.34 %, respectively. Further, the vapour pressures of ILs are also predicted by the proposed model. The trend of the predicted vapour pressure is in accord with those reported in literature.

     

A perturbed hard-sphere equation of state extended to imidazolium-based ionic liquids

A perturbed hard-sphere equation of state (EOS) has been previously employed to predict pressure–volume–temperature properties of some ionic liquids (ILs) with phosphonium-, pyridinium-, and pyrrolidinium cations. In this work, we have extended the considered EOS to another class of ILs in compressed states. This class consists of 14 imidazolium-based ILs. The predicted densities were compared with those obtained from the experiment, over a broad pressure range from 0.1 to 200 MPa. From 1,122 data points examined for the aforementioned ILs, the total average absolute deviation was found to be 1.05%.     

Improved equation of state for ionic liquids using surface tension

The major objective of this work was the development of a reliable model to describe volumetric properties of ionic liquids (ILs). In this regard, we have applied the Ihm–Song–Mason equation of state (EOS) to some phosphonium- and imidazolium-based ILs. Three temperature-dependent parameters in the equation of state have been scaled based on the surface tension and the liquid density at room temperature. In order to improve the predictive power of the mentioned EOS for ILs, we have proposed using a simple modification. We have taken 1,228 experimental points to show the reliability of the improved EOS. The comparison of predicted densities with literature data over a broad range of temperature, 293–472 K, and pressures up to 200 MPa led to encouraging results. The average absolute deviation of calculated densities from literature values was found to be 0.75%.     

Probing solute-solvent interaction in 1-ethyl-3-methylimidazolium-based room temperature ionic liquids: A time-resolved fluorescence anisotropy study

Rotational diffusion of two organic solutes, coumarin153 (C153) and 4-aminophthalimide (AP) has been investigated in four ionic liquids (ILs), viz. 1-ethyl-3-methylimidazolium trifluoroacetate (EMIMTFA), 1-ethyl-3-methylimidazolium ethylsulfate (EMIMESU), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMTFB) and 1-ethyl-3-methylimidazolium tetracyanoborate (EMIMTCB), as a function of temperature. Between the two probes, AP can act as hydrogen-bond-donor to the solvents having hydrogen bond acceptor ability. The results indicate that the rotational dynamics of C153 is mainly governed by the viscosity of the medium. On the other hand, the rotational motion of AP is found to be significantly hindered in the ILs depending on the nature of anions of the ILs. Rotational coupling constant values for AP in the ILs follow the order TFA?>?ESU?>?TCB?>?TFB. The slower rotational motion of AP in these ILs has been attributed to the specific hydrogen bonding interaction between AP and anions of ILs.

Physicochemical properties of ionic liquid mixtures containing choline chloride,chromium (III) chloride and water: effects of temperature and water content

The effects of water addition and temperature on some physicochemical properties of room temperature ionic liquids containing chromium chloride, choline chloride and water in the molar ratio of 1:2.5:x (where x = 6, 9, 12, 15 or 18) have been studied. The density, viscosity, surface tension and conductivity of the liquid mixtures were measured for the temperature range of 25 to 80 °C. Increasing both water content and temperature resulted in decreasing density, surface tension and viscosity and increasing electrical conductivity. The average void radii (hole sizes) for the liquid systems under study were calculated; they were in the range of 1.21 to 1.82 Å. The average hole size was stated to grow with increasing both temperature and water content in the mixture. The variation of the average void radii correlates with the change in viscosity and conductivity. The activation energies of viscous flow and conductivity diminishes with increasing water content in the liquid mixture. There is a strong linear correlation between conductivity and fluidity which indicates that the conductivity of the ionic liquid mixtures is generally controlled by the ionic mobility. A moderate viscosity and higher conductivity of the Cr(III)-containing ionic liquids with extra-water addition (at x > 9) make them suitable for the development of chromium electrodeposition processes.     

A piperidinium-based ester-functionalized ionic liquid as electrolytes in Li/LiFePO<Subscript>4</Subscript> batteries

A new functionalized ionic liquid (IL) based on cyclic quaternary ammonium cations with ester group and bis(trifluoromethanesulfonyl)imide ([TFSI]^?) anion, namely, N-methyl-N-methoxycarbonylpiperidinium bis(trifluoromethanesulfonyl)imide ([MMOCPip][TFSI]), was synthesized and characterized. Physical and electrochemical properties, including Li-ion transference number, ionic conductivity, and electrochemical stability, were investigated. The electrochemical window of [MMOCPip][TFSI] was 6 V, which was wide enough to be used as a common electrolyte material. The Li-ion transference number of this IL electrolyte containing 0.1 M LiTFSI was 0.56. The half-cell tests indicated that the [MMOCPip][TFSI] obviously improved the cyclability of a Li/LiFePO₄ cell. For the Li/LiFePO₄ half-cells, after 20 cycles at room temperature at 0.1 C, the discharge capacity was 109.7 mAh g^?1 with 98.7% capacity retention in the [MMOCPip][TFSI]/0.1 M LiTFSI electrolyte. The good electrochemical performance demonstrated that the [MMOCPip][TFSI] could be used as electrolyte for lithium-ion batteries.     

Modified equation of state extended to imidazolium-, phosphonium-, pyridinium-, pyrrolidinium- and ammonium-based ionic liquids

An analytical equation of state (EoS) has been previously employed by Hosseini and Sharafi (Ionics 17:511, 2011) for modelling of PVT data of some ionic liquids (ILs). In this work, we have extended the mentioned model to five classes of ILs by the use of alternative scaling constants for corresponding states correlation procedure. For this purpose, ILs involving imidazolium, phosphonium, pyridinium, pyrrolidinium and ammonium cations have been taken into account. From these, 1,294 experimental data points examined to show the reliability of the modified EoS. The comparison of predicted densities with the measured values over a broad range of temperature 293?C452 K and pressures up to 150 MPa led to the encouraging results. The average absolute deviation of calculated densities from literature values was found to be 0.73%. Moreover, to establish the predictive power of proposed model, the reproduced densities have been compared with those obtained by another literature work. Moreover, we have demonstrated the density behaviour of studied ILs in terms of alkyl chain length of imidazolium cation via proposed model.     

A first-principle study on the phase transition,electronic structure,and mechanical properties of three-phase ZrTi<Subscript>2</Subscript> alloy under high pressure*

We employed density-functional theory (DFT) within the generalized gradient approximation(GGA) to investigate the ZrTi₂ alloy, and obtained its structural phase transition,mechanical behavior, Gibbs free energy as a function of pressure, P-V equation of state,electronic and Mulliken population analysis results. The lattice parameters andP-V EOS for α, β and ω phases revealed by ourcalculations are consistent with other experimental and computational values. The elasticconstants obtained suggest that ω-ZrTi₂ and α-ZrTi₂ are mechanically stable, and that β-ZrTi₂ is mechanically unstableat 0 GPa, but becomes more stable with increasing pressure. Our calculated resultsindicate a phase transition sequence of α → ω → β forZrTi₂. Both thebulk modulus B and shear modulus G increase linearly withincreasing pressure for three phases. The G/B values illustrated goodductility of ZrTi₂alloy for three phases, with ω<α<β at0 GPa. The Mulliken population analysis showed that the increment of d electron occupancystabilized the β phase. A low value for B '₀ is the feature of EOS for ZrTi₂ and this softness in the EOS isrepresentative of pressure induced s-d electron transfer.     

Study on ionic conductivity and dielectric properties of PEO-based solid nanocomposite polymer electrolytes

The ionic conductivity and dielectric properties of the solid nanocomposite polymer electrolytes formed by dispersing a low particle-sized TiO₂ ceramic filler in a poly (ethylene oxide) (PEO)-AgNO₃ matrix are presented and discussed. The solid nanocomposite polymer electrolytes are prepared by hot press method. The optimum conducting solid polymer electrolyte of polymer PEO and salt AgNO₃ is used as host matrix and TiO₂ as filler. From the filler concentration-dependent conductivity study, the maximum ionic conductivity at room temperature is obtained for 10 wt% of TiO₂. The real part of impedance (Z′) and imaginary part of impedance (Z″) are analyzed using an LCR meter. The dielectric properties of the highest conducting solid polymer electrolyte are analyzed using dielectric permittivity (ε′), dielectric loss (ε″), loss tangent (tan δ), real part of the electric modulus (M′), and imaginary part of the electric modulus (M″). It is observed that the dielectric constant (ε′) increases sharply towards the lower frequencies due to the electrode polarization effect. The maxima of the loss tangent (tan δ) shift towards higher frequencies with increasing temperature. The peaks observed in the imaginary part of the electric modulus (M″) due to conductivity relaxation shows that the material is ionic conductor. The enhancement in ionic conductivity is observed when nanosized TiO₂ is added into the solid polymer electrolyte.     

A perturbed hard-sphere equation of state for phosphonium-, pyridinium-, and pyrrolidinium-based ionic liquids

In this work, a perturbed hard-sphere equation of state has been employed to predict the pressure–volume–temperature properties of some phosphonium-, pyridinium-, and pyrrolidinium-based ionic liquids in compressed states. Two temperature-dependent parameters that appeared in the equation of state have been determined using the critical properties of abovementioned ionic liquids as the scaling constant, i.e., knowing only two available scaling constants is sufficient to utilize the proposed equation of state to this class of fluids. The predicted densities were compared with those obtained from the experiment, over a broad pressure range from 1 to 65 MPa. From 550 data points examined for the aforementioned ionic liquids, the total average absolute deviation, AAD was found to be 1.12%.     

Electrolyte based on fluorinated cyclic quaternary ammonium ionic liquids

Ionic liquids, ILs, based on fluorinated pyrrolidinium and piperidinium ammonium cations and imide anion were prepared and characterized. The physicochemical and electrochemical properties of these ILs including melting point, glass transition and degradation temperatures, viscosity, ionic conductivity, and electrochemical stability were determined and compared to alkyl pyrrolidinium and piperidinium ILs. The incorporation of a CF₃ group instead of a CH₃ induces an increase of the IL viscosity, thus a conductivity decrease. However, good ionic conductivity is obtained with fluorinated pyrrolidinium IL. Cyclic amine ILs with propyl alkyl chain or fluorinated ammonium exhibit very high electrochemical stability toward oxidation. The effect of the addition of LiTFSI on the IL properties was studied with the same methodology.     

Volumetric and transport properties of <Emphasis Type="Italic">N</Emphasis>-Butyl-<Emphasis Type="Italic">N</Emphasis>-methylpyrrolidinium bis(Trifluoromethanesulfonyl)imide–methanol binary mixtures

Densities, viscosities, and ionic conductivities were measured for the binary mixtures containing the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide and methanol over the entire range of compositions at the temperature varying from 253.15 to 318.15 K. The densities and viscosities decrease monotonously with temperature and the content of ionic liquids (ILs). Furthermore, excess isobaric expansion coefficient has been calculated from the experimental densities. The dependence of temperature on the viscosity has been fitted to the Arrhenius law with high precision. The dependence of temperature on the ionic conductivity has also been gauged by both of the Arrhenius and Vogel–Tamman–Fulcher (VTF) equations. In fact, the shape of the curves shows that the temperature dependence of the conductivity does not follow a simple Arrhenius law, but a better fitting of experimental results is achieved using the VTF model. Additionally, the effects of ILs concentration on the viscosity and the conductivity have been examined using the Walden rule, which shows that the variation of conductivity is inversely proportional to viscosity. Excess molar volumes and viscosity deviations for all mixtures are evaluated and well fitted to the Redlich–Kister polynomial expansions. Physicochemical properties show two clearly distinguished behaviors corresponding to ILs-rich and methanol-rich regions, with distinct transport and volumetric properties. The obtained results are discussed in terms of dipolar interactions and hydrogen bonding establishment between ions of ILs and the methanol molecules.     

Dependence of structure factor and correlation energy on the width of electron wires

The structure factor and correlation energy of a quantum wire of thickness b ? a _B are studied in random phase approximation (RPA) and for the less investigated region r _s < 1. Using the single-loop approximation, analytical expressions of the structure factor are obtained. The exact expressions for the exchange energy are also derived for a cylindrical and harmonic wire. The correlation energy in RPA is found to be represented by ? _c (b, r _s ) = α(r _s )/b + β(r _s ) ln(b) + η(r _s ), for small b and high densities. For a pragmatic width of the wire, the correlation energy is in agreement with the quantum Monte Carlo simulation data.     

Ion dynamics in methylcellulose–LiBOB solid polymer electrolytes

A polymer electrolyte system comprising methylcellulose (MC) as the host polymer and lithium bis(oxalato) borate (LiBOB) as the lithium ion source has been prepared via the solution cast technique. The electrolyte with the highest conductivity of 2.79 μS cm^?1 has a composition of 75 wt% MC–25 wt% LiBOB. The mobile ion concentration (n) in this sample was estimated to be 5.70?×?10²⁰ cm^?3. A good correlation between ionic conductivity, dielectric constant, and free ion concentration has been observed. The ratio of mobile ion number density (n) at a particular temperature to the concentration n ₀ of free ions at T?=?∞ (n/n ₀) and the power law exponents (s) exhibit opposite trends when varied with salt concentration.     

Structural,electrical, and transport number studies of AgI-doped silver borotellurite fast ion conducting glass system

Silver ion conducting glass system composed of xAgI–(100???x)[0.444 Ag₂SO₄–0.555 (0.4TeO₂–0.6B₂O₃)] has been prepared by melt quenching method for x?=?0 to 80 in step of 10. XRD, DSC, FTIR, and SEM were carried out to understand some structural properties of prepared samples. XRD and DSC studies of the samples with x?≤?60 show predominantly glassy nature. Electrical parameters and activation energies of all the samples were evaluated by complex impedance analysis and Arrhenius plots of DC conductivity, respectively. Carrier concentration, mobility, inter-ionic distance, and ionic conductivity of samples were measured and discussed. It is observed that the conductivity varies with increasing the temperature and composition. The highest conductivity (1.8?×?10^?1 S cm^?1) and ionic current (8.33 μA) is observed for =?50 sample at room temperature; hence, it can be used as best electrolyte material for solid-state battery application.     

Design of amphiphilic poly(vinylidene fluoride-<Emphasis Type="Italic">co</Emphasis>-hexafluoropropylene)-based gel electrolytes for high-performance lithium-ion batteries

Gel polymer electrolytes (GPE) based on electrospun polymer membranes, poly(vinylidene fluoride-co-hexafluoropropylene), grafted poly(poly(ethylene glycol) methyl ether methacrylate) (PVDF-HFP-g-PPEGMA), and poly(vinylidene difluoride-co-hexafluoropropylene) (PVDF-HFP) are prepared for lithium ion batteries by incorporating with 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMITFSI). The uniform porosity and the compatibility of blend electrospun membranes avoiding the pore blocking are beneficial to enhance the electrolyte uptakes. The GPE based on the fibrous PVDF-HFP-g-PPEGMA/PVDF-HFP activated with 1 M LiTFSI (BMITFSI) show a maximum ionic conductivity of 2.3 × 10^?3 S cm^?1 at room temperature and electrochemical stability of up to 5.2 V. The Li/GPE/LiFePO₄ cells with GPE based on PVDF-HFP-g-PPEGMA/PVDF-HFP blend electrospun membrane deliver specific capacities of 163, 141, and 125 mAh g^?1 at 0.1, 0.5, and 1C rates, respectively, and remains well after 50 cycles for each rate. Therefore, the novel GPE have been demonstrated to be suitable for lithium-ion battery applications.     

High precision determination of the low-energy constants for the two-dimensional quantum Heisenberg model on the honeycomb lattice

The low-energy constants, namely the staggered magnetization density M? _s per spin, the spin stiffness ρ _s , and the spinwave velocity c of the two-dimensional (2-d) spin-1/2 Heisenberg model on the honeycomb lattice are calculated using first principles Monte Carlo method. The spinwave velocity c is determined first through the winding numbers squared. M? _s and ρ _s are then obtained by employing the relevant volume- and temperature-dependence predictions from magnon chiral perturbation theory. The periodic boundary conditions (PBCs) implemented in our simulations lead to a honeycomb lattice covering both a rectangular and a parallelogram-shaped region. Remarkably, by appropriately utilizing the predictions of magnon chiral perturbation theory, the numerical values of M? _s , ρ _s , and c we obtain for both the considered periodic honeycomb lattice of different geometries are consistent with each other quantitatively. The numerical accuracy reached here is greatly improved. Specifically, by simulating the 2-d quantum Heisenberg model on the periodic honeycomb lattice overlaying a rectangular area, we arrive at M? _s = 0.26882(3), ρ _s  = 0.1012(2)J, and c = 1.2905(8)Ja. The results we obtain provide a useful lesson for some studies such as simulating fermion actions on hyperdiamond lattice and investigating second order phase transitions with twisted boundary conditions.     

Agglomeration behavior of lipid-capped gold nanoparticles

The current investigation deciphers aggregation pattern of gold nanoparticles (AuNPs) and lipid-treated AuNPs when subjected to aqueous sodium chloride solution with increasing ionic strengths (100–400 nM). AuNPs were synthesized using 0.29 mM chloroauric acid and by varying the concentrations of trisodium citrate (AuNP1 1.55 mM, AuNP2 3.1 mM) and silver nitrate (AuNP3 5.3 μM, AuNP4 10.6 μM) with characteristic LSPR peaks in the range of 525–533 nm. TEM analysis revealed AuNPs to be predominantly faceted nanocrystals with the average size of AuNP1 to be 35?±?5 nm, AuNP2 15?±?5 nm, AuNP3 30?±?5 nm, and AuNP4 30?±?5 nm and the zeta-average for AuNPs were calculated to be 31.23, 63.80, 26.08, and 28 nm respectively. Induced aggregation was observed within 10 s in all synthesized AuNPs while lipid-treated AuNP2 (AuNP2-L) was found to withstand ionic interferences at all concentration levels. However, lipid-treated AuNPs synthesized using silver nitrate and 1.55 mM trisodium citrate (AuNP3, AuNP4) showed much lower stability. The zeta potential values of lipid-treated AuNPs (AuNP1-L-1x/200, ??17.93?±?1.02 mV; AuNP2-L-1x/200, ??21.63?±?0.70; AuNP3-L-1x/200, ??14.54?±?0.90; AuNP3-L-1x/200 ??13.77?±?0.83) justified these observations. To summarize, AuNP1 and AuNP2 treated with lipid mixture 1 equals or above 1x/200 or 1x/1000 respectively showed strong resistance against ionic interferences (up to 400 mM NaCl). Use of lipid mixture 1 for obtaining highly stable AuNPs also provided functional arms of various lengths which can be used for covalent coupling.

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Graphical abstract Agglomeration behavior of gold nanoparticles before and after lipid capping

     

Effects of ionic liquid on the hydroxylpropylmethyl cellulose (HPMC) solid polymer electrolyte

Biodegradable solid polymer electrolyte (SPE) is prepared by solution-casting technique using low-cost cellulose derivative, hydroxypropylmethyl cellulose (HPMC) as a host polymer. Owing to the hydrophobic nature of this polymer, it is predicted to exhibit low ionic conductivity upon addition of magnesium trifluoromethanesulfonate (MgTf₂) salt. Therefore, ionic liquid (IL), 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIMTf), is added in order to enhance its ionic conductivity. Based on the findings, the ionic conductivity at room temperature and the dielectric behaviors of the SPE complex improved upon incorporation of 40 wt.% IL. On top of that, addition of IL reduces the degree of crystallinity and the glass transition temperature (T _g ) of the SPE. The conductivity-temperature plot revealed that the transportation of ions in these films obey Arrhenius theory. The interaction between SPE complex, MgTf₂ salt, and BMIMTf is investigated by means of Fourier transform infrared (FTIR) spectroscopy through the change in peak intensity around 3413, 1570, and 1060 cm^?1, which are responsible for –OH stretching band, C–C and C–N bending modes of cyclic BMIM⁺, and C–O–C stretching band, respectively.