A. K. Lyashchenko

Complex dielectric permittivity and relaxation parameters of concentrated aqueous electrolyte solutions in millimeter and centimeter wavelength ranges

Abstract The complex dielectric permittivity of aqueous electrolyte solutions in a wide range of concentrations was comprehensively studied between 7 and 120 GHz. In the first concentration region, experimental data are described in terms of the Debye or Cole-Cole relaxation models. No resonance or relaxation processes other than the Debye rotational diffusion of water molecules occur in a high-frequency part of the millimeter wavelength range. Concentrated aqueous LiCl and MgCl2 solutions exhibit two separate regions of the complex permittivity dispersion with considerably different (5–10 times) relaxation times. Decomposition of the dielectric spectra in a wide frequency range points to the occurrence of two simple relaxation processes. It is assumed that they are dominated by relaxation of water molecules incorporated in hydration shells and in ion-water clusters, respectively. The lifetime of water molecules in the clusters is 12–18 ps.

Concentration transition from water-electrolyte to electrolyte-water solvents and ionic clusters in solutions

Abstract A general model of structural and molecular-kinetic changes in transition from dilute electrolyte solutions to aqueous melts of crystal hydrates and salts is suggested. The model corresponds to the scheme: ions and complexes in the water structure→ ionic and ion-water clusters→ polymeric melt structures. The structural transition is caused by a decrease in electrostatic hydration effects and an abrupt decrease in the dielectric permittivity of electrolyte solutions beyond the concentration range of the first structural region where the water structure persists. The appearance of cluster nanostructures is considered to be a common property of highly concentrated solutions of electrolytes. Analysis of concentration structural changes is based on X-ray diffraction and spectroscopic data. Microwave dielectric properties and other physicochemical properties of solutions (electric conductivity, viscosity, density, etc.), as well as phase equilibria in water-salt systems are interpreted in terms of the concepts developed.

Computer simulation study of rotational diffusion in polar liquids of different types

Rotational diffusion in liquid acetonitrile, dimethylsulphoxide (DMSO), water, and methanol is studied with molecular dynamics simulations. The effects of hydrogen bonding and local dipole-dipole correlations (Kirkwood g-factor) on the relationship between the single molecule and collective relaxation are examined. The first rank single molecule dipole moment autocorrelation functions (ACFs) are constructed in the molecule-fixed coordinate frame and the principal components of rotation diffusion tensor are reported. Higher rank orientational ACFs are computed. These ACFs, as a rule, are strongly nonexponential (at least not single exponential) at longer times and the decomposition of these functions into a series of single exponentials results in broad distributions of relaxation times, with the broadening being particularly prominent in the case of higher rank ACFs. The rank dependence of characteristic times calculated as weighted averages over the relaxation time distributions does not follow the pattern of small angle (Debye) diffusion model for all liquids studied in this work except methanol. In contradiction, the same rank dependence computed by direct integration of ACFs leads to good agreement with the Debye diffusion model in the case of acetonitrile, DMSO, and water (but not methanol). The linear-angular momentum cross correlation functions are also computed and the effect of rototranslational coupling on reorientaional relaxation at longer times (>1.0 ps) is found to be small.

Dielectric properties, hydration and ionic association in binary and multicomponent formate water-salt systems☆

Abstract The complex dielectric permittivity of binary and multicomponent aqueous solutions of yttrium, barium and copper formates is investigated in the microwave range. The dielectric constant e s and the parameters of the process of dielectric relaxation are calculated. It is shown that the relative changes of e s in a series of considered formate solutions are connected not only with hydration processes but also with association of Y 3+ and Cu 2+ . In barium formate solutions the hydration interactions prevail. The features of the branches of the solubility isotherm for ternary water-salt systems are displayed in the concentration dependencies of the dielectric characteristics of the saturated multicomponent solutions. It is shown that the hydration and the ion-ion interaction determine a type of solubility isotherm in these water-salt systems.

Orientational relaxation in hydrogen-bonded systems: aqueous solutions of electrolytes

A model of the rotational motion of water molecules within ion hydration sheaths is proposed in the present paper. This model is based on the concept of a ‘boundary’ ion, according to which the parameters of the structural surroundings, as well as the translational, rotational and librational motions of water molecules within the first sphere, are the same as those for water molecules in water. The wide-band absorption spectra, α(ν)(0 < ν/cm–1 < 400), and the complex dielectric permittivity spectrum measured for the concentrated electrolyte solutions, are described on the basis of the model proposed in this paper. It is shown that the confined rotator/extended diffusion (CR/ED) model, which has been advanced previously for liquid water, is applicable for spectral calculations of the electrolyte solutions studied in the present paper.The typical characteristics of ions having hydrophilic and hydrophobic hydration, as well as the distinctions between the hydration sheaths of ions with negative and positive hydrations, are discussed. A new method is proposed to study the molecular nature of these phenomena at the level of elementary processes. The complex dielectric permittivity of the concentrated LiCl, NaCl, Nal, KCl and KBr solutions has been measured in the range 7–25 GHz at 25 °C. For electrolyte solutions, the wide band (0 ⩽v/cm–1⩽ 1000) dielectric spectra and the parameters of the proposed model are calculated.

Concentration dependence of dielectric spectra and molecular kinetic properties in aqueous nonelectrolyte solutions. The case of dimetyl sulfoxide

Abstract A model is suggested describing molecular-kinetic and structural changes in dimethyl sulfoxide (DMSO) aqueous solution. Dielectric response is supposed to be stipulated by libration and rotation of dipoles. The calculation scheme is proposed in which one DMSO fraction is introduced along with one or several water fractions. The changes of parameters obtained for such model of orientational motion are related to the hydrophobic hydration and to the formation of strong DMSO-H 2 O H-bonds in the solution, Dielectric spectra observed in centimeter, millimeter, submillimeter and far IR wavelength ranges are interpreted (0–200 cm −1 ). Complex permittivity/absorption spectra are calculated at various temperatures.

Microwave dielectric properties of aqueous solutions of potassium and cesium fluorides

The microwave dielectric properties of aqueous solutions of potassium and cesium fluorides are studied over a wide concentration range at frequencies of 13 to 25 GHz and temperatures of 288, 298, and 308 K. The static dielectric constant ɛs and the time τ and enthalpy of activation ΔHɛ++ of dielectric relaxation for the solutions are calculated. It is demonstrated that, for fluorides, the disruptive effect of ions on the hydrogen bond network of water is weaker than that for the other halogenides.

The Role of Concentration Dependent Static Permittivity of Electrolyte Solutions in the Debye–Hückel Theory

The Debye-Hückel theory has been extended to allow for arbitrary concentration dependence of the electrolyte solution static permittivity. The theory follows the lines advanced by Erich Hückel ( Hückel, E. Phys. Z. 1925, 26, 93) but gives rise to more general and lucid results. New theoretical expressions have been obtained for the excess free energy of solution, activity coefficient of water and mean ionic activity coefficient. The thermodynamic functions contain two terms representing interionic interactions and ion-water (solvation) interactions. The theory has been applied to calculate the activity coefficients of components in the aqueous solutions of alkali metal chlorides from LiCl to CsCl at ambient conditions making use of permittivities taken from experimental dielectric relaxation studies. Calculations without parameter adjustment have demonstrated a semiquantitative agreement with experimental data, reproducing both the nonmonotonic concentration dependence of the activity coefficients and the ordering of activity coefficients for the salts with different cations. A good agreement with experimental data is obtained for the aqueous solutions of LiCl in the concentration range up to 10 mol/kg. The nonmonotonic concentration dependence of activity coefficients is explained as a result of a balance between the effect of interionic interactions and the solvation contribution which appears quite naturally in the framework of the Debye-Hückel approach after incorporation of variable permittivity of solution.

Temperature Changes of Dielectric Permittivity and Relaxation in Aqueous Lithium Iodide Solutions

The complex permittivity of aqueous LiI solutions is studied over a wide range of concentrations at temperatures of 288–323 K in the water permittivity dispersion region at seven frequencies in the range of 7.5–25 GHz. One relaxation region describable by the Debye or Cole-Cole equation is observed in these solutions. Dielectric relaxation time τ and static permittivity ɛs are studied as dependent on temperature and concentration. The time and enthalpy of activation of dielectric relaxation decrease in going from water to solutions, which corresponds to the distortion of the initial water structure and the increasing mobility of water molecules in hydration shells of ions. In the initial concentration range, the water activity is a linear function of 1/ɛs. The negative temperature dependence of ɛs disappears in going to concentrated solutions. At high concentrations, the static dielectric constant increases in response to increasing temperature. The new trends in ɛs and τ at elevated temperatures of 313–323 K are due to the formation of ion pairs and other ion-water groups having high dipole moments.

The activity of water and permittivity of aqueous solutions of electrolytes

The relation between concentration changes in the activity of water (aw) and static permittivities (ɛ) of concentrated solutions of salts was studied over a wide concentration range. The dependence of aw on 1/ɛ was shown to be linear at 298 K for more than 15 examples. The concentration boundary of the first structural zone of solutions where this dependence was observed was established. The appearance of deviations was related to concentrations at which complex ion-water groups were present in solutions and the transition from a water-electrolyte to an electrolyte-water solvent occurred.