Previously unknown information about the phase transition of wetting in two-phase liquid media is obtained by analyzing the work of adhesion of salt melts to liquid metals. The objects of study are systems composed of molten alkali metal halides and liquid bismuth, whose work of adhesion is calculated using experimental data on the surface tensions of metals and salts at an interface with a gas phase, and the interfacial tension between the metal and salt. A regular change in the work of adhesion on temperature, electric potential, and the nature of the contacting phases is shown. The transition from incomplete to complete wetting is established. It is shown that the transition from partial to complete wetting of the surface is facilitated by a rise in the temperature, electric potential, and polarizability of salt phase ions.

—The results of studying the properties of the interfacial layer in immiscible mixtures LiF + KBr, LiF + CsCl, LiF + RbBr, LiF + KI, LiF + CsBr, LiF + RbI, LiF + CsI, LiCl + AgBr, LiCl + AgI, and NaCl + AgI are summarized. The interfacial layer thickness is found to increase with temperature and to tend toward infinity near the critical mixing temperature. At the same temperature and ion size ratio, the interfacial layer thickness in the melts containing silver halides turns out to be lower than that in the mixtures of alkali metal halides. The surface formation energy for the mixtures of alkali metal halides decreases when temperature increases, obeying a power equation with a critical exponent of 1.5 in the vicinity of the critical mixing temperature. The critical exponent of the Galvani potential for the mixtures containing silver halide turns out to be 18% lower than the critical exponent of the interfacial tension, which is related to the peculiarities of the chemical bond of silver halides. At the same temperature, an increase in the difference between the mixed-ion sizes is shown to cause an increase in the energy and a decrease in the diffusivity of the transition layer because of a decrease in the mutual solubility of the components in this direction.

The results of hydrostatic weighing investigation of the phase density along the two-phase saturation line for molten immiscible mixtures LiF + KBr, LiF + CsCl, LiF + RbBr, LiF + KI, LiF + CsBr, LiF + RbI, LiF + CsI, LiCl + AgBr, and NaCl + AgI are summarized. The ratio of the equilibrium phase densities significantly depends on the sizes of the mixed ions. The dependences of changes in the density far away from and near the critical mixing point on the nature of the chemical bond between the ions are discussed.

Structural transformations in the adsorption layer caused by an electric potential are investigated using the experimental data on the capacitance of a double electric layer for a rhenium electrode in molten sodium, potassium and cesium chlorides at 1093 K. Likening the double electric layer to a flat capacitor, as well as the effective length of the shielding of the electrode charge and changes in the charge sign depending on the applied potential are estimated. It is found that near the minimum potential of the capacitance curve, the shielding length decreases proportionally to the square of the potential due to the deformation of the double layer. The deformation reaches critical values at the potentials of −0.65, −0.38 and −0.40 V for the Re|NaCl, Re|KCl and Re|CsCl systems respectively, and decreases sharply at more positive potentials. The analysis of the dependence of the charge density on the electrode revealed the effect of shielding of potential-induced rhenium cations by salt phase anions. The strong Raman-active Re–Cl stretching mode was observed at 292 cm−1. This can be explained by the transfer of anions across the interface resulting in the formation of ordered layers of ion associations (possibly, ReXn(n − 1)−) on a positively charged surface.

The density, ρ, of a limited mixing LiF + AgI and LiBr + AgI melts was measured in the temperature range from the melting point to the critical mixing temperature by means of the hydrostatic weight procedure. It is shown that the density of LiBr + AgI melt along the saturation line is due to both by the intensity of the thermal movement of ions and the composition of the phases. The difference between the densities for the equilibrium phases ∆ρ is found to decrease when the temperature elevated tending to zero at Tc = 984 K. The critical index for the density of the melt studied (0.45) is shown to be 10% less than that found for the mixtures of alkali halides. The melt containing LiF exists in two phases up to temperature of 1223 K. The covalent bonding between the silver and halide ions can be understood as causing the difference between the critical exponents of the alkali halide melts and those of silver iodide-containing mixtures.

The adiabatic compressibility along the two-phase saturation line was calculated for nine molten immiscible mixtures, namely, LiF + KBr, LiF + CsCl, LiF + RbBr, LiF + KI, LiF + CsBr, LiF + RbI, LiF + CsI, LiCl + AgBr, and NaCl + AgI, using experimental data on the sound velocity and density. It is shown that the ratio of compressibility of the equilibrium phases depends significantly on the sizes of the mixed ions. The dependence of the changes in compressibility in the distance and in the vicinity of the critical mixing point on the characteristics of the chemical bond between the ions is discussed.

The potential dependence of the wetting angle on a gold electrode in sodium, potassium, and cesium bromide and iodide melts was studied to establish the laws of wetting solid surfaces with ionic melts. It is found that the forms of the contact angle vs. the potential curve for the gold electrode change from a convex to a camel-like shape with two maxima in the transition from sodium halide to potassium and cesium halide melts. This phenomenon is explained by the mutual polarization of gold and halogen ions, X−, at the place of their direct contact, which leads to the formation of ordered layers of ion associations (possibly, AuXn(n-1)–) on a positively charged surface. The hypothesis proposed agreed with the spectral observations of the electrode surface.

The propagation of sound vibrations in two-phase molten mixtures of alkali halides with each other and with silver halides is studied. The experimental setup and methods for the measurement of speed of sound at high temperatures are described. The experimental data are presented in the form of tables and graphs. The critical indicators of system mixing are given. The results are discussed in terms of a model of charged solid spheres of arbitrary diameter.

The speed of sound in binary molten KCl–KBr, KCl–KI, KBr–KI, KF–KCl, KF–KBr, and KF–KI mixtures has been measured by the pulse-temporal method in wide temperature and composition ranges. The adiabatic compressibility and molecular speed of sound have been calculated based on the lite-rature data on density. The results are analyzed from the point of view of structural transformation in ionic melts, which accompanies the mixing of salts with different ion sizes.

The adiabatic compressibility, beta, of the immiscible liquid mixture (LiF + CsCl) along the saturation line was experimentally investigated in the temperature range from the melting point to the critical mixing temperature using sound velocity values, u, measured by the pulse method, and density quantities, rho, determined using the hydrostatic weight procedure based on the relationship beta = u(-2)rho(-1). The coefficients of the temperature dependencies for the compressibility, sound velocity and density of the upper and lower equilibrium phases have opposite signs because of the superposition of the thermal motion intensity of the ions and the change in the phase compositions. The differences Delta beta, Delta u and Delta rho for the equilibrium phases decrease with temperature elevation. The temperature dependencies of the compressibility, sound velocity and density difference are described using the empirical equations Delta beta approximate to (T-c - T)(1.004), Delta u approximate to (T-c - T)(1.003) and Delta rho approximate to (T-c - T)(0.504). (C) 2019 Elsevier Ltd.

Using the phase-time approach, the dependences of the speed of sound on the temperature and composition are measured in CsCl–CsBr, CsCl–CsI, and CsBr–CsI molten binary mixtures. The adiabatic compressibility of these mixtures is calculated from the experimental values of speed and density. Negative deviations from the additive values of the speed of sound and positive deviations from the additive values of adiabatic compressibility are observed. The magnitude of deviation depends on the ratio of the sizes of anions in the mixtures.

To establish the patterns of the wettability of solid surfaces by ionic melts, the wetting energy of a gold electrode by lithium, sodium, potassium, and cesium chloride melts was studied as a function of the electrical potential applied to the electrode. It was established that when the potential was shifted in the positive direction relative to the zero-charge potential of gold in the corresponding melt, the shape of the curve that defined the dependence of the wetting energy on the electrical potential varied according to the salt composition: for lithium and sodium chlorides, the wetting energy increased monotonically, while in potassium and cesium chloride melts there was a maxima. This phenomenon was explained from the standpoint of the mutual polarization of gold and chlorine ions in the place of their direct contact. At a certain electric field strength in the double electric layer and at a certain binding energy of the melt particles, the resultant mutual ion polarization led to the formation of ordered layers of ionic associations (presumably AuCln(n − 1)-) on the anode surface, which shielded the electrode charge.

In this paper, we experimentally studied the adiabatic compressibility β of an exfoliating liquid mixture, LiF +KBr, on the saturation line in a temperature range from the melting point to the critical mixing temperature using the sound velocity u measured by the pulse method and the density ρ determined by hydrostatic weighing based on the ratio β = u–2ρ–1.The coefficients of the temperature dependences of the compressibility and density of the upper and lower equilibrium phases are shown to have opposite signs due to the superposition of the thermal motion of ions and changes in the phase composition. The reduced differences β* and ρ* ​​for the contacting phases decrease with decreasing reduced temperature T* in accordance with the empirical exponential equations β* ≈ T*1.017 and ρ* ≈ T* 0.494.

The potential dependence of the contact angle between a gold electrode and lithium, sodium, potassium, rubidium, and caesium chloride melts was studied using the meniscus weight method to establish the patterns of wettability of solid surfaces by ionic melts when changing the composition of the salt phase and the jump of the electric potential. It is found that the forms of the contact angle versus the potential curve of Au change from a convex to a camel-like shape with two maxima upon replacing the lithium chloride with the caesium chloride melt. This phenomenon is explained by the assumption that the adsorption of the halide anions at the positively charged electrode surface has a chemical rather than electrostatic character. The adsorption process is accompanied by a charge transfer through the interface and the formation of covalent bonds between the adsorbent and adsorbate.

Values of the contact angle of wetting of a surface of glassy carbon by molten chlorides of lithium, sodium, potassium, and cesium are measured by the meniscus weight method to determine the common factors of wettability of solid surfaces by ionic melts upon a change in the salt phase composition and a jump in electric potential. It is found that with a potential shift in the positive direction the shape of the curve of the contact angle’s dependence on the potential varies upon substitution of one salt by another: the angle of wetting shrinks monotonously in lithium chloride but remains constant in molten cesium chloride. This phenomenon is explained by the hypothesis that the nature of the halide anion adsorption on the positively charged surface of an electrode is chemical and not electrostatic. It is shown that the adsorption process is accompanied by charge transfer through the interface, with covalent bonding between the adsorbent and adsorbate.

An electrochemical method was used to study the Galvani potential between two coexisting equilibrium phases of molten AgI + LiCl and AgBr + LiCl systems. The method measures the electromotive force (emf) of a cell with two silver electrodes: Ag | x ′ AgI (or AgBr) + (1 − x ′ ) LiCl | x ″ AgI (or AgBr) + (1 − x ″ ) LiCl | Ag. The measurement was performed beginning with the melting point of the mixtures until reaching the temperature of full mixing, T c , to establish the Galvani potential magnitude and its temperature dependence near the critical point of mixing. It is shown that in the temperature range of 200 K below the critical mixing point, the temperature dependence of the Galvani potential for both systems is described by a universal equation, ∆E ≈ τ n , where τ = (T c  − T)/T c , with the critical exponent n = 1.228.

The liquid–liquid phase equilibrium for the melted AgI + NaCl mixture was studied by the impedance method over a wide range of temperatures along the saturation line to correlate the charge transfer in two-phase ion systems with the nature of the chemical bond. The temperature dependences of the specific electric conductivity κ of the coexisting phases have opposite signs because of the superposition of the temperature and concentration factors that act in the same or opposite directions. The difference between the κ values for the equilibrium phases decreases with increasing temperature, becoming zero at 1064 K. Near the critical mixing point, the temperature dependence of the difference between the conductivities of the coexisting phases is described by the exponential equation with a critical exponent of 0.91, which is 8% smaller than that found for alkali metal halides with primarily Coulomb forces acting between the ions.

We used the impulse-temporal method to measure the speed of sound in NaCl–NaBr, NaCl–NaI, and NaBr–NaI binary molten mixtures with respect to temperature and composition. Using the literature data on density, we calculated the adiabatic compressibility of these systems. We obtained the negative deviations of the speed of sound and the positive deviations for the compressibility from the additive values; the deviation values depend on the size ratio of the mixed anions.

The adiabatic compressibility, beta, of the immiscible liquid mixture 0.52 LiCl + 0.48 AgBr (the top of the miscibility gap) was experimentally investigated in the temperature range from the melting point to the critical mixing temperature using the sound velocity values, u, measured by the pulse method, and the density quantities, rho, which were determined using the hydrostatic weight procedure based on the relationship beta = u (2)rho(-1). It is shown that the coefficients of the temperature dependencies for the compressibility and density of the upper and lower equilibrium phases have opposite signs because of the superposition of the intensity of the thermal motion of the ions and the change in the composition of the phases. The differences, Delta beta and Delta rho, in the magnitudes of the compressibility and density for the equilibrium phases decrease with temperature elevation. The temperature dependencies of the compressibility and density -difference are described using the empirical equations Delta beta approximate to (T-c-T)(0.438) and Delta rho approximate to (T-c-T)(0.439).