Temperature Dependent Dielectric Relaxation in Solvent Mixtures at Microwave Frequencies

By the use of time domain reflectometry method, dielectric measurements were carried out on dimethylformamide‐2‐nitrotoluene solvent mixtures in the frequency range 10 MHz‐20 GHz, at various temperatures from 15 °C to 45 °C. These solvent mixtures as well as pure solvents display a Debye type dispersion. Their frequency dependent dielectric properties can be summarized by the three parameters in the Debye equation: a static permittivity, permittivity at high frequency and a dielectric relaxation time constant. The free energy of activation for dipolar relaxation process and the Kirkwood correlation factor were determined using these fitting parameters for these solvent mixtures at various concentrations and temperatures. By using these dielectric parameters, the excess permittivity and excess inverse relaxation time is obtained. The excess permittivity is found to be positive for all concentrations and temperatures whereas the excess inverse relaxation time is negative.     

Relative permittivity data of binary mixtures containing 2-butanol, 2-butanone,and cyclohexane

Relative permittivity measurements were made on binary mixtures of (2-butanol + 2-butanone) and (2-butanol or 2-butanone + cyclohexane) for various concentrations at T = (298.2, 308.2, and 318.2) K. Some experimental results are compared with those obtained from theoretical calculations and interpreted in terms of homo- and heterogeneous interactions and structural effects. The molecular dipole moments were determined using Guggenheim–Debye method within the temperature range of (298.2 to 318.2) K. The variations of effective dipole moment and correlation factor, g, with the mole fraction in these materials were investigated using Kirkwood–Frohlich equation. The pure compounds showed a negative and small temperature coefficient of effective dipole moment. In order to obtain valuable information about heterogeneous interaction (interactions between the unlike molecules), the Kirkwood correlation factor, the Bruggeman dielectric factor and the excess permittivity were calculated. In order to predict the permittivity data of polar–apolar binary mixtures, five mixing rules were applied.     

Dielectric data of binary mixtures of 1,2-butanediol with 2-ethyl-1-hexanol and 1,4-dioxane at T = (298.2, 308.2, and 318.2) K

Relative permittivity measurements were made on binary mixtures of (1,2-butanediol + 2-ethyl-1-hexanol) and (1,2-butanediol + 1,4-dioxane) for various concentrations at T = (298.2, 308.2, and 318.2) K. The molecular dipole moments were determined using Guggenheim–Debye method in the temperature range of (298.2 to 318.2) K. The variations of effective dipole moment and correlation factor, g, with the mole fraction in these materials were investigated using Kirkwood–Frohlich equation. The pure compounds showed a negative and small temperature coefficient of effective dipole moment. In order to obtain valuable information about heterogeneous interaction (interactions between the unlike molecules), the Kirkwood correlation factor, the Bruggeman dielectric factor and the excess permittivity were calculated. In addition, in order to predict the permittivity data of polar-apolar binary mixtures, five mixing rules were applied.     

Dielectric relaxation and hydrogen bonding studies of chlorobutane-dioxane mixtures using a time domain technique

Complex permittivity spectra have been computed for the binary mixtures of Chlorobutane (CLB) with 1, 4-Dioxane (DX) using Time Domain Reflectometry (TDR) for different concentrations and temperatures in the frequency range from 10 MHz to 30 GHz. The static dielectric permittivity and relaxation time have been obtained by fitting complex permittivity spectra to the Debye relaxation using least squares fit method. The Kirkwood correlation factor have been determined at various concentrations of 1, 4-dioxane. The Bruggeman model for the non-linear case has been fitted to the dielectric data for the mixtures.     

Dielectric Study of n-Butyl Acetate–Alcohol Mixtures by Time-Domain Reflectometry

Using picosecond time-domain reflectometry (TDR), dielectric relaxation studies have been carried out on binary mixtures of n-butyl acetate with methanol, ethanol, and 1-propanol, over the frequency range from 10 MHz to 20 GHz, at various concentrations and temperatures. The excess permittivity, excess inverse relaxation time, Kirkwood correlation factor, and thermodynamic parameters have been obtained. The excess permittivity for all the systems is negative. The values of static permittivity and relaxation time decrease with an increase in the percentage of n-butyl acetate in the mixtures.     

Dielectric properties of binary mixtures of three butanediols with 1,4-dioxane and 2-ethyl-1-hexanol at T = 298.2 K

Experimental results of dielectric investigations for solutions of the three butanediols {2,3-butanediol (2,3BD), 1,3-butanediol (1,3BD), and 1,4-butanediol (1,4BD)}, in 1,4-dioxane (1,4DX) are reported for various mole fractions at T = 298.2 K. Values of relative permittivity were measured at 100 kHz. The molecular dipole moments were determined using Guggenheim method. The variations of effective dipole moment and correlation factor, g, with mole fraction in these materials were investigated using Kirkwood–Frohlich equation. Dielectric measurements were also carried out on binary polar mixtures of the butanediols with 2-ethyl-1-hexanol (2EH) for various concentrations at T = 298.2 K. The Kirkwood correlation factor, the Bruggeman factor, and the excess permittivity were determined.     

Study of intermolecular interactions through dielectric properties of the mixtures consisting of 1,4-butanediol,primary amyl alcohols and 1,4-dioxane at various temperatures

This paper presents relative permittivities, excess permittivities, effective dipole moments, and excess Kirkwood correlation factors of binary mixtures of 1,4-butanediol with two primary pentanol isomers [1-pentanol (amyl alcohol) + 3-methyl-1-butanol (isoamyl alcohol)] from T = (298.15 to 318.15) K at p = 101.3 kPa over the entire composition range. Experimental permittivity values for polar–non-polar binary systems of (1,4-dioxane + amyl alcohol or isoamyl alcohol) were also obtained as a function of composition at the same range of temperatures. The experimental permittivity data were fitted using Redlich–Kister equation to evaluate the adjustable parameters and the standard errors. From the experimental data, the excess parameters were calculated. In this work, variations of effective dipole moment and correlation factor were investigated using Kirkwood−Frohlich equation. The experimental data of measurements were used in the analysis of the homo- and hetero interactions occurring in these binary solutions.     

Density,speed of sound,refractive index and dielectric permittivity of (diethyl carbonate + n -decane) at several temperatures

Density, speed of sound and refractive index values of (diethyl carbonate  + n -decane), were measured at the temperatures (288.15, 293.15, 298.15, and 308.15) K and atmospheric pressure. In addition, dielectric permittivities have been measured for the same mixture and at the same temperatures except at T =  293.15 K. Excess molar volumes, changes of isentropic compressibility on mixing, changes of refractive index on mixing and changes of dielectric permittivity on mixing were computed from the experimental data. The excess molar volumes were compared with predictions from the Nitta–Chao model.     

Dielectric Relaxation and Thermodynamic Studies of Binary Mixtures of 2-Nitrotoluene with Primary and Secondary Alcohols at Different Temperatures

The dielectric complex spectra of 2-nitrotoluene with primary or secondary alcohol binary mixtures were studied over the frequency range of 10 MHz to 20 GHz for the whole solute mole fraction range at four different temperatures. An unusual suppression phenomenon was observed in the real and imaginary parts of the mixture complex spectrum, which are smaller than those for the pure alcohols, at low solute concentrations. The dielectric constant and dielectric relaxation time values were obtained by fitting the complex dielectric spectrum data to the single Debye model using a non-linear least squares method. The dielectric constant of mixtures decrease with the increasing mole fraction of 2NT in both the primary alcohols and secondary alcohols; the dielectric relaxation time decreases for all the five binary systems. Using the dielectric data, derived dielectric parameters, namely: the excess dielectric constant, excess inverse relaxation time, effective Kirkwood correlation factor, molar activation enthalpy and molar activation entropy, were calculated. The non-linear variation of permittivity (?₀) reveals the change in size and shape of hetero-molecular complex due to intermolecular H-bond interaction. The negative variation of the excess permittivity constant confirms that the dipoles form multimer structures with anti-parallel ordering of unlike dipoles. The molar activation enthalpy was found to be higher at 0.2 mol fraction of 2NT for primary alcohol binary system. To confirm the molecular function group interaction, a FT-IR spectroscopy study was carried out at 298 K. The FT-IR analysis confirmed the formation of hydrogen bonds between the hydrogen atom of hydroxyl groups of the alcohols and the oxygen atom of nitro groups of 2NT in the binary mixtures.     

Micropolarity and aggregation behavior in ionic liquid + organic solvent solutions

UV–vis spectroscopy and conductivity measurement techniques were used to study the physicochemical and structural properties of the binary or ternary mixtures of 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) + organic solvent and 1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]) + organic solvent systems. The solvents involved were acetonitrile, water, ethanol, ethyl acetate, and tetrahydrofuran. It was indicated that the micropolarity and the aggregation behavior of the mixtures depend strongly on the dielectric constants of the solvents and the composition of the mixtures.     

Dielectric Relaxation Studies of Binary Mixture of α‐Picoline and Methanol Using Time Domain Reflectometry at Different Temperatures

Complex permittivity spectra of binary mixtures of varying concentrations of α‐picoline and methanol (MeOH) were obtained using time domain reflectometry (TDR) technique over frequency range 10 MHz to 25 GHz at 283.15 K, 288.15 K, 293.15 K and 298.15 K temperatures. The dielectric relaxation parame‐ ters namely static permittivity (σ₀), high frequency limit permittivity (σ_oo1) and the relaxation time (ρ) were determined by fitting complex permittivity data to the single Debye/Cole‐Davidson model. Complex non linear least square (CNLS) fitting procedure was carried out using LEVMW software. The excess static permittivity (σ₀^E) and the excess inverse relaxation time (1/ρ)^E which contains information regarding mo‐ lecular structure and interaction between polar — polar liquids, were also determined. From the experimental data, effective Kirkwood correlation factor (g^eff) and Bruggeman factor (f_B) were calculated. Excess parameters were fitted to the Redlich‐Kister polynomial equation. The values of static permittivity and relaxation time increase non‐linearly with increase in the mol fraction of MeOH at all temperatures. The values of excess static permittivity (σ₀^E) and the excess inverse relaxation time (1/ρ)^E are negative for the studied α‐picoline — MeOH system at all temperatures.     

Dielectric relaxation spectroscopy of macroporous IER beads suspensions dispersed in primary alcohols and water–ethanol mixtures

Dielectric measurements were first carried out on suspensions of ion-exchange resin beads dispersed in primary alcohols and water–ethanol mixtures in the frequency range 40 Hz–110 MHz. Due to the large bead radius, only Maxwell–Wagner (M–W) dielectric relaxations were observed. Regular dielectric behaviors were observed and phase parameters concerning constituent phases’ properties were determined through dielectric analysis, which revealed that the properties and the dispersing state of the bead are strongly dependent on the properties of dispersion medium. It is also found that dry beads cannot be completely soaked by alcohols with long aliphatic chain, and that suspension in water-rich mixture has similar dielectric behavior as in pure water due to the mixture's molecular construction. Then the dielectric behaviors of the following suspensions were measured as a function of time: suspensions of beads that have been equilibrated with water/ethanol redispersed in ethanol/water. The dielectric behaviors showed remarkable time dependency, characterized by distinct transitions on the curves of time dependent relaxation parameters. Based on the above understandings, the time dependent dielectric behaviors were analyzed in detail. It is showed that the time dependency directly reflected such processes as ion diffusion, solvent diffusion and solvent uptake that the systems undergo.     

Complex Permittivity Spectra of Binary Pyridine–Amide Mixtures Using Time–Domain Reflectometry

Frequency spectra of the complex permittivity for pyridine–amide binary mixtures have been determined over the frequency range 10 MHz to 10 GHz, at 5, 15, 25, and 40°C, using the time–domain reflectometry method, for 11 compositions of each pyridine–amide system, e.g., formamide, N-methylformamide, and N,N-dimethylformamide. The relaxation in these systems can be described by a single relaxation time using the Debye model. The static dielectric constant, relaxation time, the corresponding excess dielectric properties, Kirkwood correlation factor, and molar activation energy of the mixtures have been determined. The excess permittivity is found to be positive in the amide-rich region and negative in the pyridine-rich region. The excess inverse relaxation time is negative, except in the pyridine-rich region. The static dielectric constants for the mixtures have been fitted with the modified Bruggeman model. The temperature-dependent relaxation times show the expected Arrhenius behavior.     

Dielectric relaxation study of hexamethylphosphoramide–1,4-dioxane mixtures using time domain reflectometry (TDR) technique

Dielectric relaxation study on hexamethylphosphoramide–1,4-dioxane binary mixtures has been carried out at 16 concentrations over the frequency range of 10?MHz to 30?GHz at different temperatures using time domain reflectometry technique. The mixtures exhibit a principal dispersion of the Debye relaxation type at microwave frequencies. The concentration-dependent static dielectric constant, excess dielectric properties and thermodynamic parameters have been determined. The hydrogen-bonded theory is applied to compute the Kirkwood correlation factors for the mixture. The average numbers of the hydrogen bonds between hexamethylphosphoramide–hexamethylphosphoramide and hexamethylphosphoramide–1,4-dioxane pairs are estimated from the dielectric constant.     

Isothermal (vapour + liquid) equilibrium and thermophysical properties for (1-butyl-3-methylimidazolium iodide + 1-butanol) binary system

Experimental isothermal (vapour + liquid) equilibrium (VLE) data are reported for the binary mixture containing 1-butyl-3-methylimidazolium iodide ([bmim]I) + 1-butanol at three temperatures: (353.15, 363.15, and 373.15) K, in the range of 0 to 0.22 liquid mole fraction of [bmim]I. Additionally, refractive index measurements have been performed at three temperatures: (293.15, 298.15 and 308.15) K in the whole composition range. Densities, excess molar volumes, surface tensions and surface tension deviations of the binary mixture were predicted by Lorenz–Lorentz (n_D-ρ) mixing rule. Dielectric permittivities and their deviations were evaluated by known equations. (Vapour + liquid) equilibrium data were correlated with Wilson thermodynamic model while refractive index data with the 3-parameters Redlich–Kister equation by means of maximum likelihood method. For the VLE data, the real vapour phase behaviour by virial equation of state was considered. The studied mixture presents S-shaped abatement from the ideality. Refractive index deviations, surface tension deviations and dielectric permittivity deviations are positive, while excess molar volumes are negative at all temperatures and on whole composition range. The VLE data may be used in separation processes design, and the thermophysical properties as key parameters in specific applications.     

Thermodynamics of isomeric hexynes + MTBE binary mixtures

The vapor pressures of liquid hex-1-yne or hex-2-yne + methyl 1,1-dimethylethyl ether (MTBE) binary mixtures and of the three pure components were measured by a static method at several temperatures between 263 and 343 K. These data were correlated with the Antoine equation. Excess molar Gibbs energies G^E were calculated for several constant temperatures, taking into account the vapor-phase imperfection in terms of the second molar virial coefficients, and were fitted to the Redlich–Kister equation. Calorimetric excess enthalpy H^E measurements, for these binary mixtures, are also reported at 298.15 K. The experimental VLE and H^E data were used, examining the binary mixtures hex-1-yne or hex-2-yne + MTBE in the framework of the DISQUAC and modified UNIFAC (Do) models. The DISQUAC calculations, reporting a new set of interaction parameters for the contact carbon–carbon triple bond/oxygen ether, is regarded as a preliminary approach.     

The mixing properties of 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl) cyclotrisiloxane with various organosilicon compounds at different temperatures

The density and refractive index were determined for four binary mixtures of 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl) cyclotrisiloxane with octamethyl-cyclotetrasiloxane, hexamethyldisiloxane, 2,4,6,8-tetramethyl-cyclotetrasiloxane and 2,4,6,8-tetramethyl-2,4,6,8-tetraethenylcyclotetrasiloxane at different temperatures T = (308.15, 313.15, 318.15, 323.15 and 328.15) K and atmospheric pressure using a DMA4500/RXA170 combined system. The excess molar volume, partial excess volume at infinite dilution, isobaric coefficient of thermal expansion, excess refraction indices, Lorentz–Lorenz molar refraction and the deviation in molar refraction have been calculated using this data. The results have been incorporated into the Redlich–Kister equation and used to estimate the binary interaction parameters and standard deviation. The values of partial excess volume at infinite dilution and excess refraction indices for the four binary systems at different temperatures were calculated using the adjustable parameters of the Redlich–Kister smoothing equation. The factors that affect these excess quantities are discussed.     

Experimental determination of the isothermal (vapour + liquid) equilibria of binary aqueous solutions of sec-butylamine and cyclohexylamine at several temperatures

The vapour pressures of (sec-butylamine + water), (cyclohexylamine + water) binary mixtures, and of pure sec-butylamine and cyclohexylamine components were measured by means of two static devices at temperatures between 293 (or 273) K and 363 K. The data were correlated with the Antoine equation. From these data, excess Gibbs functions (G^E) were calculated for several constant temperatures and fitted to a fourth-order Redlich–Kister equation using the Barker’s method. The (cyclohexylamine + water) system shows positive azeotropic behaviour for all investigated temperatures. The two binary mixtures exhibit positive deviations in G^E for all investigated temperatures over the whole composition range.     

Gelation of the organic liquid electrolytes and the conductivities as gel electrolytes

The gelation of multi-component solvent mixtures with lithium salts was investigated in the presence of gelator 4, 4′bis(stearoylamino)diphenyl ether (BSDE). Ordered structures can be formed by non-covalent interactions in both miscible and non-miscible solvent mixtures. In a non-miscible solvent mixture, there was a soluble competition for BSDE, which can be explained by the different polarities of the solvents. The presence of lithium salt in the solvent mixtures seems to not affect the microscopic morphologies of the aggregates. TEM and SEM images of the gels indicated that BSDE aggregated and self-assembled into flexual, wrinkled and interconnected sheets in the solvent mixtures. These sheet-like aggregates are different from the fibril-like aggregates formed in single component solvents as reported previously. The ion conductivity of the organogels formed by these solvent mixtures confirms that the three-dimensional network does not affect the ions diffusion in the large interconnected liquid domains. The organogels of solvent mixtures containing lithium salts exhibit high conductivities which are in the range of 10⁻¹–10⁻² S cm⁻¹ at room temperature, and in the range of 10⁻³–10⁻⁴ S cm⁻¹ at −40 °C.     

Is there any sense to investigate volumetric and acoustic properties of more binary mixtures containing Ionic Liquids?

The excess speed of sound, excess molar volume and excess molar isentropic compressibility of 52 binary mixtures containing Ionic Liquids at T = 298.15 K were calculated using selected literature speed of sound and density data. The second components were alcohols: methanol, or ethanol, or 1-propanol, or 2-propanol, or 1-butanol or other solvents: acetone, acetonitrile, tetrahydrofuran, dichloromethane and dimethylsulfoxide. The Balankina’s relative excesses, X_bal, i.e. the ratios between excess and ideal quantities X^E/X^id were also determined to reduce the structural impact of pure components to absolute excesses. Analysis of quantities determined shows some patterns for concentration dependences of large groups of mixtures; thus, the scheme for influence of anion or cation of Ionic Liquids and solvent on Balankina’s relative excesses was proposed. It seems that presented analysis provide the knowledge about absolute and relative excess quantities for other mixtures without doing the experimental work. It is also visible that analysis of excess molar quantities and X_bal parameters can support the interpretation of interactions which occur between Ionic Liquids and solvent.