Sound Velocities for Dissolving AgI + LiCl Melts - Journal of Chemical

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Sound Velocities for Dissolving AgI + LiCl Melts Victor P. Stepanov*,†,‡ and Vladimir I. Minchenko† †

Institute for High Temperature Electrochemistry, Russian Academy of Sciences, Ural Division, Akademicheskaya str. 20, Yekaterinburg 620990, Russia ‡ Ural Federal University, Mira str. 19, Yekaterinburg 620002, Russia ABSTRACT: The ultrasonic velocities of a molten stratified mixture at a molar ratio of 0.3AgI and 0.7LiCl (the composition corresponding to the top of the miscibility gap) were measured along the saturation line over a wide temperature range using the pulse method to establish the characteristics of mixing salts comprising different chemical bonds. It is shown that the coefficients of the temperature dependences for the sound velocities in the coexisting phases have opposite signs as a result of the superposition of the temperature and concentration factors. It is found that the difference, Δu, between the magnitudes of the sound velocities for the coexisting phases decreases with increasing temperature and becomes zero at 1250 K. This temperature corresponds to the critical phase transition point, Tc. The temperature dependence of the sound velocity difference, Δu, for the system studied is described by the equation Δu ≈ (Tc − T)β, where β = 0.900, which is close to the value 0.896 obtained for the previously investigated AgI+NaCl melt but is less than the value found for alkali halide melts (β = 1.02), in which long-range Coulomb forces between ions prevail. The results are discussed in terms of the peculiarity of the chemical bond in silver iodide. tally, the miscibility gaps for alkali halide melts were first observed in systems comprising lithium fluoride as one component and either chlorides, bromides, or iodides of sodium, potassium, rubidium, or cesium, as the other component.6−9 We have measured the density, electrical conductivity, and ultrasonic velocity for the coexisting phases of the alkali halide melts10−12 as well as the surface tension at the boundary between phases13 and have shown that, for a constant temperature, the differences in these properties in the coexisting phases increase when the radii of the alkali cation and halide anion increase. Recently, we measured the sound velocity in the coexisting phases for the stable diagonal of the reciprocal ternary system AgI + NaCl14 and found that the temperature dependence of the sound velocity difference, Δu, is described by the equation Δu ≈ (Tc − T)β, in which β = 0.896, which is less than that obtained for the alkali halide melts (β = 1.0211). Both silver iodide and the alkali halides completely dissociate above the melting point. Moreover, a detailed analysis of the works related to the structure and properties of these salts15−17 shows that bonding in silver iodide occupies an intermediate position between ionic compounds and fully covalently bonded compounds. It is possible that the nature of the chemical bond is responsible for the other behaviors of melts containing AgI relative to the alkali halide systems. To prove this hypothesis, we decided to measure the sound velocity along the saturation line for a stratified mixture composed of silver iodide (0.3 mol fraction) and lithium chloride (0.7 mol fraction). Both, this

1. INTRODUCTION This work investigates the properties of molten salt systems susceptible to stratification in the liquid state. Such research is essential to elucidate the potential use of molten salts as media for the accumulation and emission of heat that occurs from the phase transition of mixing and demixing in thermal devices.1 Moreover, the application of immiscible melts for concentrating any substance in one of the phases may be promising.2−4 The use of halide melts in extraction processes and subsequent utilization for actinides and fission products is especially interesting because of the unique technological properties of these media (good electrical conductivity, high thermal and radiation stability). The results of such investigations will contribute to the development of the theory underlying condensed ionic liquids at critical conditions, including the liquid−liquid phase transition. The salts studied (like alkali halides) are examples of systems that exhibit Coulomb interactions as the principle interaction between ions. In this regard, questions are raised regarding the cause and regularity of the segregation, the characteristics of the phases along the saturation line, and the criticality type of the ionic melts. Above the melting point, these salts dissociate almost entirely. Here, the solvent exhibiting a large dielectric constant is absent, the intermolecular boundaries are obliterated, and ions are able to act to a certain extent independently, being linked together by long-range Coulomb forces. For the alkali halide melts, it has been shown5 that the long-range Coulomb interaction manifests itself, specifically, as the dimensional differences of the constituent ions of the mixture, and their different shielding abilities are the main parameter of the system that determines its transition to the two-phase state at temperatures below the critical miscibility point. Experimen© 2014 American Chemical Society

Received: August 21, 2014 Accepted: October 22, 2014 Published: November 3, 2014 3888

dx.doi.org/10.1021/je500781j | J. Chem. Eng. Data 2014, 59, 3888−3893

Journal of Chemical & Engineering Data

Article

melt and the 0.5AgI + 0.5NaCl melt, are representative of systems whose upper critical temperature corresponds to the top of the miscibility gap.18 As long as the alkali cations in these melts have different diameters, it is possible to investigate the influence of the dimensions factor on the behavior of critical liquids.

2. EXPERIMENTAL 2.1. Materials. Special attention was given to the purification of the salts under study from products formed by their interaction with oxygen and water in the atmosphere and other contaminants. In the first stage, anhydrous lithium chloride (0.999 mass fraction purity) and silver iodide (> 0.99 mass fraction purity) were dried under vacuum at room temperature for 4 h and slowly heated to 500 K. Then, they were melted in an atmosphere of pure argon (0.9999 mass fraction) and, after cooling, were handled in a dry box. In the second stage, lithium chloride was subjected to zone-refinement19 three times in pure flowing argon (Table 1). For this Table 1. Provenance and Mass Fraction Purity of the Materials Studied chemical name silver iodide, AgI lithium chloride, LiCl argon, Ar

provenance Plant Rare Metals Ural Plant of Chemicals Chemical Plant

mass fraction purity

melting point, Ka

0.99 0.999

832.0 884.0

Figure 1. Scheme of the cell for ultrasound velocity measurements in stratifying salt melts: 1, transmitter of sound wave pulses and piezoelectric cell; 2, lower sound guide made of the fused quartz; 3, the fused quartz crucible with a stratified salt melt; 4, mobile upper waveguide made of aluminum oxide single crystal; and 5, piezoelectric cell with a receiver of signals; Δh is the sound wave transmission in the melt.

0.9999

a

Measurement performed with a Jupiter thermal analyzer (NETZSCH, Germany).

lowered into a tubular resistor furnace equipped with three heaters, which were supplied with currents of different strength to provide an isothermal (by 1 K) zone with a vertical extension of at least 10 cm. The temperature was measured using a Pt/PtRh thermocouple with an uncertainty of 0.5 K. Data were obtained under a pure Ar atmosphere. The sound velocity in molten AgI−LiCl was measured using a pulse method with the experimental setup that was previously described in detail.20 A major element of the experimental setup was the electronic device, which registered the decrement of the sound-pulse amplitude and the travel time through the melts between the waveguides. The autogenerator generated sinusoidal voltage pulses with a carrier frequency of 5 MHz. Our previous experiments20 show that the sound velocity in the molten salts does not depend on the frequency over a range of 1 MHz to 35 MHz. Voltage pulses were delivered to a lithium niobate piezoelectric element, where they were transformed into sound pulses. Sound pulses passed through salt melts under test, entered the piezoelectric element of the receiver and were transformed into electrical pulses. The time interval measurement unit and precision temporal shift generator measured time increments with an uncertainty of at least 10−8 s as the sound traveled a fixed distance. Earlier,20,21 this setup was tested by measuring the sound velocity for pure molten alkali halides. The values of measured sound velocities differ by less than 1 % from those reported by other authors. This is a comparatively good agreement serving as a proof of function of our experimental device. Measurements were carried out as follows. First, the travel time of a pulse over the entire acoustic track t0 was measured for the initial position of the upper sound guide h0 in one of the phases. Then, the upper sound guide was moved vertically and

purpose, the smelted salts were loaded into a nickel boat (25 cm long and 2 cm wide) and placed in a quartz tube closed at both ends. The gas space of the tube was evacuated and filled with pure argon. Local salt smelting was carried out using a portable resistance furnace. The width of the molten zone was approximately 1.5 cm, and the traverse speed of the furnace along the quartz tube was approximately 2·10−2 mm/s. After purification, the lithium chloride was almost colorless and had formed a transparent monolith. The finite part of the ingot was opaque and had a gray tint with dark patches due to the relatively high concentration of hydrolysis products and other impurities. The pH of the lithium chloride aqueous solution was 6.15 for the head of the ingot and 9.73 for the rear part. The measured melting points of the salts after purification (884.0 K ± 0.5 K for LiCl and 832.0 K ± 0.5 K for AgI) reflect the high purity of the samples and strongly agree with the reference data.15 Because lithium chloride exhibits a tendency to absorb water molecules, all of the following manipulations with salts were carried out in a dry box under Ar atmosphere. 2.2. Measuring Procedure. The scheme of the cell employed to measure the sound velocity in stratified salt melts is shown in Figure 1. The blend of salts at a molar ratio of 0.3AgI to 0.7LiCl was placed in a fused quartz crucible (3) equipped with a fused quartz waveguide (2). The lower face of the waveguide was hard-connected to a lithium niobate piezoelectric transducer (1). The lower face of the second mobile sound guide 15 mm diameter (4), composed of synthetic sapphire and connected to a piezoelectric transducer (5) at the upper end, was immersed in the studied melt. This cell was placed into a hermetic quartz container with a controlled gas space. The lower part of the container was 3889



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coaxially toward the lower guide over an arbitrary distance Δh = h − h0 by means of a cathetometer, with an uncertainty of 0.01 mm, and the overall time t was determined again. The sound velocity was calculated as u = (h − h0)/(t − t0). Within the upper phase, the sound velocity measurements were carried out at various heights. When the interface of the melt was reached, the acoustic signal was distorted and decreased. After this, the upper sound guide was transferred into the bottom phase of the melt, and the sound velocity was determined in the same way. The temperature dependences of the sound velocity were analyzed within both the increasing and decreasing regimes. Measurements were made no earlier than 40 min after a temperature change to ensure that equilibrium was reached in the system. Uncertainty in sound velocity measurements arose from an uncertainty of the sound path length in the melt (ranging from 0.1 % to 0.05 %) during the period in which the sound passes through this path (ranging from 0.06 % to 0.17 %) and from uncertainty temperature determination (0.04 % to 0.06 %). The analysis showed that the relative combined expanded uncertainty in sound velocity measurements did not exceed 2 m/s.

Figure 2. Temperature dependences for the sound velocities, u, in the AgI + LiCl melt (■, upper phase, □, lower phase) and individual LiCl(- - -) and AgI () melts.

is not linear: the temperature derivative of the velocity is greater when the temperature increases. Seemingly, a concentration factor influenced the temperature dependence of the sound velocity. Indeed, the solubility of silver iodide in the upper phase increases as the temperature increases.18 As the sound velocity in the AgI melt is lower than that in the LiCl melt (Figure 2), the phase dressing by AgI also provokes a drop in velocity in addition to the temperature effect. The sound velocity in the lower phase increases with increasing temperature. This temperature dependence of the velocity for the silver iodide-enriched melt also results from the interplay of the two processes: the weakening cation−anion bonding with increasing temperature provokes a decrease in velocity, while an increase in the lithium chloride concentration in the lower phase causes an increase in the sound velocity. As the experimental data demonstrate, concentration is a predominant factor determining the temperature dependence of the velocity in the heavy phase of the AgI + LiCl melt. For the system investigated, the sound velocities in the equilibrium phases converge when the temperature rises due to the alignment phase compositions.18 Here, the liquid−liquid equilibrium of the system studied revealed behavior typical of a partially miscible system with an upper critical mixing temperature. The temperature dependence of the difference, Δu = u1 − u2, between sound velocities, u1 and u2, of the upper and lower phases, respectively, is shown by the plots in Figure 3. It is obvious that the difference in sound velocities decreases as the temperature increases. At low temperatures, Δu depends relatively weakly on the temperature. The correlation changes considerably by only 50 K to 70 K before reaching the critical temperature. In this temperature range, the sound velocities in the phases rapidly converge. For the AgI + LiCl melt at 1250 K, the sound velocities in the coexisting phases become indistinguishable; above this temperature, absolute mixing of the components takes place. Consequently, this temperature corresponds to the critical phase transition point. Recently, the charged hard sphere model for alkali halide melts was used to interpret experimental phase diagrams with liquid−liquid equilibriums.5 The specificity of the melts was considered for mixtures in which ions have identical charges but different sizes. This model predicts that segregation occurs as a result of the different shielding abilities of the ions: the smaller the radius of the ion, the better it shields the electrostatic interactions. Therefore, the smaller-sized cations and anions

3. RESULTS AND DISCUSSION Sound velocity data were obtained in two series of experiments and demonstrated good reproducibility of the results. The primary data for saturated coexisting phases of the AgI+LiCl system at different temperatures are shown in Table 2 and Table 2. Temperature Dependence of the Sound Velocity for the Dissolving (0.3AgI + 0.7LiCl) Melta 1st series

2nd series

T/K

u1/(m s−1)

u2/(m s−1)

T/K

u1/(m s−1)

u2/(m s−1)

930 980 1035 1082 1138 1178 1198 1220 1230 1238 1247 1254

1980 1940 1875 1810 1715 1557 1477 1400 1357 1328 1292 1280

1098 1100 1105 1108 1130 1168 1200 1214 1228 1270 1272 1280

1156 1189 1209 1225 1237 1243 1248 1275

1647 1527 1445 1375 1326 1304 1290 1264

1150 1185 1208 1206 1265 1258 1276 1264

a

The u1 refers to the sound velocity in the upper phase, u2 refers to the sound velocity in the lower phase. The estimated expanded uncertainty in temperature is 0.5 K and in sound velocity is 2 m s−1.

Figure 2. The sound wave transmission in the melt was varied about 8 mm to 12 mm in 1 mm step widths. Every point of sound velocity plotted in Figure 2 and given in Table 2 is an average value of four to six measurement replicates. Figure 2 also shows the polytherms of the velocity obtained in melts of pure lithium chloride and silver iodide.21,22 It is known that the decrease in sound velocity in both the individual molten salts and completely homogeneous mixtures21 is inversely proportional to the temperature increase because of intensification of the thermal movement of the particles. The upper phase of the AgI + LiCl melt enriched by lithium chloride also demonstrates a reduction in sound velocity with increasing temperature. However, the temperature dependence 3890



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these salts the critical mixing temperature increases when the length of the side-chains grows.23 Reasons for these differences are not yet clear. A significant difference between the behavior of the alkali halides and the silver halide-containing melts consists of different manifestions of the ion sizes in the stratification phenomena for liquids. Previously, it was shown that the segregation effect in the alkali halide mixtures begins at a certain ratio of ion sizes;9 the LiF + KBr melt is a boundary between the exsolving and completely homogeneous melts. In this system, the sum of the ionic radii, ri, of the heavy component, dKBr = rK+ + rBr−, is greater than that of the light component, dLiF = rLi+ + rF− and is equal to 0.132 nm (at Pauling39). Separation between molten mixtures with a smaller difference in the sizes of the components was not observed. For the silver halide-containing melts studied, the differences in the sums of the radii for the heavier and lighter components were 0.066 and 0.101 nm for the AgI + NaCl and AgI + LiCl melts, respectively. Both values are considerably less than the difference limit in the ion sizes for the alkali halide melts. An additional factor that likely contributes to the segregation of the silver halide-containing melts, even at a relatively small difference in size between the heavy and light ions, is the lower charge density around the silver ion.17 Figure 4 shows the dependence of the difference in the sound velocity at the saturation line for the AgI + LiCl melt on

Figure 3. Temperature dependence of the difference, Δu = u1 − u2, between sound velocities, u1 and u2, of the upper and lower phases, respectively, for the AgI + LiCl (□) and AgI + NaCl (△)14 melts.

tend to segregate, causing the system to separate into a pair of phases. For the reciprocal system Ag, Li||Cl, I, of both the halides studied, lithium chloride has the smallest ionic dimensions. Obviously, the large ion potentials (ze/r, where ze is the charge and r is the radius of the ion) and the small polarizability of Li+ and Cl− provoke a removal of the metathetical reaction LiCl + AgI = AgCl + LiI toward the formation of the component with the smallest ions at temperatures lower than the critical point. As a result, the light phase forms on the base of Li+−Cl− binding with the maximal possible energetical field. In this case, the component with the largest sum of the cation and anion radii cannot prevent isolation of the medium owing to the powerful chemical bonding of Li−Cl. Instead, it is ‘‘pressed out’’ by a strong field from the volume in the surface layer and forms a second phase enriched with AgI. Temperature increases result in the destruction of the strong Li−Cl chemical bond and the redistribution of the energies in the phases, leading to complete mixing at temperatures above the critical point. A theoretical analysis of systems exhibiting Coulomb interactions between ions5 shows that an increase in the difference in ionic radii is accompanied by an increase in the critical mixing temperature according to the correlation Tc ≈ (dRX − dMY)2. Here, dRX is the sum of radii of the large ions of the mixtures, and dMY is the sum of the radii of the small ions. The predicted effect provides a very good fit to the experimental data for the AgI-containing melts as well. Figure 3 shows the temperature dependences of the difference between the sound speed of equilibrium phases both for the system studied in this work and the previously investigated mixture AgI + NaCl.14 The intersection points of these curves with the abscissa correspond to the critical mixing temperatures. It is clear from Figure 3, the critical temperature of mixing for AgI + NaCl (1064 K) is in reality essentially lower than that found for the AgI + LiCl mixture. A similar influence of the particle size on parameters of the immiscibility dome were found in molten alkali halide mixtures, in which the long-range Coulomb forces between ions are dominant,9−12 solutions of the low melting salts with large organic cations in molecular solvents,23−31 and mixtures of the molecular liquids with each other.32−37 There are some exceptions to this rule. Transition temperatures of the alcohol solutions of 1-alkyl-3-methylimidazolium tetrafluoroborates decrease with an increase in the length of the alkyl chain of the cation.23,38 However, for water and dialcohol solutions of

Figure 4. Log−log plot of the sound velocity difference, Δu, as a function of the temperature difference, ΔT = (Tc −T), for the AgI + LiCl (□) (β = 0.900) and AgI + NaCl (△) (β = 0.896)14 melts.

the temperature difference, ΔT = Tc − T, in logarithmic coordinates. The experimental points lie on a straight line described by the equation Δu ≈ (Tc − T)β, where β = 0.900. Figure 4 also shows a similar dependence for mixtures AgI + NaCl.14 The value of the critical exponent for AgI + LiCl is close to that obtained for the AgI + NaCl melt, but is less than those found for alkali halide melts (1.02), where long-range Coulomb forces between ions dominate.11 This difference in the critical exponent may result from the fact that silver halides are intermediate between the typical ionic salts and the fully covalently bonded ones.

4. CONCLUSIONS Experimentally, the sound velocity in the stratified 0.3AgI + 0.7LiCl melt was studied in the demixing region. The behavior of the mixture was, in many ways, similar to that of the earlier investigated AgI + NaCl system. For both liquid mixtures, the temperature coefficients of sound velocity in the upper and 3891



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(16) Schinke, H.; Sauervald, F. Ü ber die Volumenänderung beim Schmelzen und den Schmelzprozess bei Salzen. Z. Anorg. Allg. Chem. 1956, 287, 313−324. (17) Wilson, M.; Madden, P. A.; Costa-Cabral, B. J. Quadrupole polarization in simulations of ionic systems: application to AgCl. J. Phys. Chem. 1996, 100, 1227−1237. (18) Flor, G.; Margheritis, Ch.; Vigano, G. C.; Sinistri, C. Miscibility gaps in fused salts. XI. Systems formed with silver halides and lithium or sodium halides. Z. Naturforsch. 1982, 37a, 1068−1072. (19) Warren, R. W. Procedures and apparatus for zone purification of the alkali halides. Rev. Sci. Instrum. 1965, 36, 731−737. (20) Smirnov, M. V.; Minchenko, V. I.; Bukharov, A. N. Sound absorption in molten alkali chlorides, bromides, iodides and their mixtures. Electrochim. Acta 1988, 33, 213−220. (21) Smirnov, M. V.; Minchenko, V. I.; Stepanov, V. P. Adiabatic and isothermal compressibilities of molten alkali halides and their binary mixtures. Silic. Ind. 1976, 41, 113−121. (22) Takeda, S.; Hiraishi, I.; Kawakita, Y. Ultrasound velocity and absorption for molten silver halides mixtures. J. Non-Cryst. Solids 1999, 250−252, 496−500. (23) Wagner, M.; Stanga, O.; Schröer, W. Corresponding states analysis of the critical points in binary solutions of room temperature ionic liquids. Phys. Chem. Chem. Phys. 2003, 5, 3943−3950. (24) Heintz, A.; Lehman, J.; Wertz, Ch. Thermodynamic properties of mixtures containing ionic liquids. 3. Liquid−liquid equilibria of binary mixtures of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide with propan-1-ol, butan-1-ol, and pentan-1-ol. J. Chem. Eng. Data 2003, 48, 472−474. (25) Crosthwaite, J. M.; Aki, S. V. K.; Maginn, E. J.; Brennecke, J. F. Liquid phase behavior of imidazolium-based ionic liquids with alcohols. J. Phys. Chem. B 2004, 108, 5123−5119. (26) Domańska, U.; Marciniak, A. Liquid phase behaviour of 1hexyloxymethyl-3-methyl-imidazolium-based ionic liquids with hydrocarbons: The influence of anion. J. Chem. Thermodyn. 2005, 37, 577− 585. (27) Sahandzhieva, K.; Tuma, D.; Breyer, S.; Kamps, A.P.-S.; Maurer, G. Liquid−liquid equilibrium in mixtures of the ionic liquid 1-n-butyl3-methylimidazolium hexafluorophosphate and an alkanol. J. Chem. Eng. Data 2006, 51, 1516−1525. (28) Butka, A.; Vale, V. R.; Saracsan, D.; Rybarsch, C.; Weiss, V. C.; Schröer, W. Liquid−liquid phase transition in solutions of ionic liquids with halide anions: Criticality and corresponding states. Pure Appl. Chem. 2008, 80, 1613−1630. (29) Shiflett, M. B.; Niehaus, A. M. S.; Yokozeki, A. Liquid−liquid equilibria in binary mixtures containing chlorobenzene, bromobenzene, and iodobenzene with ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. J. Chem. Eng. Data 2009, 54, 2090− 2094. (30) Domańska, U.; Królikowski, M. Phase equilibria study of the binary systems (1-butyl-3-methylimidazolium tosylate ionic liquid + water, or organic solvent). J. Chem. Thermodyn. 2010, 42, 355−362. (31) Paduszyński, K.; Chiyen, J.; Ramjugernath, D.; Letcher, T. M.; Domańska, U. Liquid−liquid phase equilibrium of (piperidiniumbased ionic liquid + an alcohol) binary systems and modeling with NRHB and PCP-SAFT. Fluid Phase Equilib. 2011, 305, 43−52. (32) Matsuda, H.; Kitabatake, A.; Kosuge, M.; Kurihara, K.; Tochigi, K.; Ochi, K. Liquid−liquid equilibrium data for binary perfluoroalkane (C6 and C8) + n-alkane systems. Fluid Phase Equilib. 2010, 297, 187− 191. (33) Kurihara, K.; Yamanaka, Y.; Matsuda, H.; Tochigi, K.; Ochi, K.; Furuya, T. Determination and correlation of liquid−liquid equilibria for nine binary acetonitrile + n-alkane systems. Fluid Phase Equilib. 2011, 302, 109−114. (34) El-Dossoki, F. I. The influence of cation, anion, and temperature on the liquid−liquid equilibrium of some pentanols−water system. Fluid Phase Equilib. 2011, 305, 161−168. (35) Zakrzewska, M. E.; Manic, M. S.; Macedo, E. A.; Visak, V. N. Liquid−liquid equilibria of mixtures with ethyl lactate and various hydrocarbons. Fluid Phase Equilib. 2012, 320, 38−42.

lower phases have different signs. Evidently, this is as result of the superposition of the temperature and concentration factors, the effect of which on the sound velocity can be sent in one direction and in the opposite direction. In the vicinity of the demixing transition, the difference in the sound velocities for coexisting phases changes similarly for both mixtures. As such, the critical exponents of the velocity are similar for these systems (0.900 and 0.896 for the AgI + LiCl and AgI + NaCl melts, respectively). For the further development of ideas regarding the role of the structural features of the components of the reciprocal salt mixtures in the remixing phenomena, it is necessary to attract new objects and methods of research. Additionally, the theoretical models of the liquid−liquid interface should be upgraded by taking into account the non-Coulomb part of the particles’ energy.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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