Metal-ammonia solutions, concentrated properties

The metallic nature of concentrated metal-ammonia solutions is usually called "well known." However, few detailed studies of this system have been aimed at correlating the properties of the solution with theories of the liquid metallic state. The role of the solvated electron in the metallic conduction processes is not yet established. Recent measurements of optical reflectivity and Hall coefficient provide direct determinations of electron density and mobility. Electronic properties of the solution, including electrical and thermal conductivities, Hall effect, thermoelectric power, and magnetic susceptibility, can be compared with recent models of the metallic state. 

The overall changes in magnetic and transport properties of metal-ammonia solutions from the dilute to concentrated regimes are shown in Fig. 4. 

Fig. 4. The concentration dependence of various electronic properties of metal-ammonia solutions, (a) The ratio of electrical conductivity to the concentration of metal-equivalent conductance, as a function of metal concentration (240 K). [Data from Kraus (111).] (b) The molar spin (O) and static ( ) susceptibilities of sodium-ammonia solutions at 240 K. Data of Hutchison and Pastor (spin, Ref. 98) and Huster (static, Ref. 97), as given in Cohen and Thompson (37). The spin susceptibility is calculated at 240 K for an assembly of noninteracting electrons, including degeneracy when required (37).

Na-NH3, 1.02 x 108 ohm-1 cm2 mol-1 for Hg, 0°C, 0.16 x 10 ohm-1 cm2 mol-1). In the intermediate composition range 1 to 7 MPM, a NM-M transition occurs, and changes in the electronic, thermodynamic, and mechanical properties of the system are equally impressive (35, 37, 124, 154). A detailed discussion of the concentration dependence of various properties of metal-ammonia solutions is given in the book by Thompson (164). In addition, a recent review (60) at Colloque Weyl V also summarizes the available data for lithium-methylamine solutions (10, 11, 63, 127, 128, 166). 

Since the first preparation of potassium-ammonia solution (Sir Humphrey Davy, in 1808) alkali metal-ammonia solutions have been at the centre of much theoretical and experimental interest. Novel properties include low density, high electrical conductivity, liquid-liquid phase separation, and a concentration driven metal-nonmetal transition [35]. 

The proceedings of a conference on metal-ammonia solutions have been published, featuring reviews of the physical properties of dilute and concentrated solutions, electrical, n.m.r., i.r., and Raman spectroscopic studies of diffusion, the solvated electron, kinetics, and solution structure."" Electron spin resonance in metallic Li-NHa systems has been investigated from 12 to 296 K. In the liquid solutions and in the cubic phase of Li(NH3)4 the conduction e.s.r. lineshapes are in agreement with theory. To a good approximation the solvated ions are the only spin scatterers in the liquid state. The paramagnetic susceptibility of liquid Li(NH3)4 indicates that the concentration of localized moments is low and they order antiferromagnetically below 20 K." ... 

Of the two latest reviews dealing with properties of metal-ammonia solutions, one deals mainly with optical and magnetic properties and the other with electric conductance properties. In addition to these, a listing of the various properties of these solutions can be found in a number of recent papers which attempt to explain the properties using the proposed models for the structure of the solutions. The present section will be divided into nine parts. Parts A to H will deal with the properties of solutions of weak and medium concentration. In Part I, we shall list the properties of concentrated solutions, which have been less extensively studied than dilute solutions. 

Color. The most remarkable property of metal-ammonia solutions is their color. In dilute solutions the color is blue, as is also found for solutions in methylamine and other amines. In concentrated solutions, the solution has a metallic copper-like appearance and reflects light at normal incidence much more than the non-metallic solutions and liquids. 

It was from a consideration of the above successes and failures of the cluster and cavity models in explaining the observed optical properties that Symons was led to propose the unified model which assumes the presence of monomer clusters, one-electron cavities, and dimer clusters in the metal-ammonia. solutions in addition to the metal ions. The one-electron cavities predominate in dilute solutions and explain the observed infrared band. In the more concentrated solutions the dimer clusters produce the optical band. The monomers produce a band in the neighborhood of 12,500 cm-, but they are too scarce in dilute solutions for this band to be visible, while in the more concentrated solutions this band is probably swamped by the broad optical band. 

The M-NM transition has been a topic of interest from the days of Sir Humphry Davy when sodium and potassium were discovered till then only high-density elements such as Au, Ag and Cu with lustre and other related properties were known to be metallic. A variety of materials exhibit a transition from the nonmetallic to the metallic state because of a change in crystal structure, composition, temperature or pressure. While the majority of elements in nature are metallic, some of the elements which are ordinarily nonmetals become metallic on application of pressure or on melting accordingly, silicon is metallic in the liquid state and nonmetallic in the solid state. Metals such as Cs and Hg become nonmetallic when expanded to low densities at high temperatures. Solutions of alkali metals in liquid ammonia become metallic when the concentration of the alkali metal is sufficiently high. Alkali metal tungsten bronzes... 

Liquid ammonia s ability to dissolve alkali and alkaline-earth metals has been well known for a long time. In concentrated solutions, the metals largely remain in the metallic state. The magnetic properties and the electrical conductivity, which is comparable to that of mercury, confirm this. In the more dilute blue solutions, the metals are completely dissociated to positive metal ions and solvated electrons [1415]. The ammoniacal solutions allow preparation of many compounds otherwise unobtainable... 

There is evidence for formation of metal ion clusters as the concentration of metal increases. At concentrations 371/ or above, the solutions are copper-colored and have a metallic luster, and in various physical properties such as their exceedingly high electrical conductivities they resemble liquid metals. When 20% solutions of Li in liquid ammonia are cooled, a golden-yellow conducting solid, Li(NH3)4, is obtained.9 Similar behavior is also shown by the Group II elements, which also give solids, usually non-stoichiometric but approximating to M(NH3)6. 

CHEMICAL PROPERTIES pure mercury does not tarnish on exposure to air at ordinary temperatures slowly oxidizes to mercuric oxide when heated to boiling point forms alloys with most metals except iron combines with sulfur at ordinary temperatures reacts with nitric acid and hot, concentrated sulfuric acids does not react with dilute hydrochloric acids, cold sulfuric acid, or alkalies reacts with ammonia solution in air to form Hg2NOH. 

From the meager data available, the observed analogy in the variation of the conductance in the methyl amine solutions with concentration and temperature with that in the ammonia solutions suggests a close similarity in the constitution of both solutions, as also do the optical data discussed in Part III-B. The observed conductance in methylamine solutions with concentration greater than 0.3M is smaller by about a factor of 100 than the conductance in the corresponding ammonia solutions. This can probably be explained by the smaller solubility of the metals in amine solutions. This would lead to a larger value for the number of methylamine molecules per atom of metal dissolved relative to ammonia solutions (about 6 in saturated sodium-ammonia solutions as compared to 23 in sodium solutions in methylamine). However, in very dilute solutions, the smaller value of the equivalent conductance in the methylamine solutions would have to be explained by differences in physical properties such as viscosity and dielectric constant of ammonia and methylamine. 

The interpretation of these remarkable properties has excited considerable interest whilst there is still some uncertainty as to detail, it is now generally agreed that in dilute solution the alkali metals ionize to give a cation M+ and a quasi-free electron which is distributed over a cavity in the solvent of radius 300-340 pm formed by displacement of 2-3 NH3 molecules. This species has a broad absorption band extending into the infrared with a maximum at 1500nm and it is the short wavelength tail of this band which gives rise to the deep-blue colour of the solutions. The cavity model also interprets the fact that dissolution occurs with considerable expansion of volume so that the solutions have densities that are appreciably lower than that of liquid ammonia itself. The variation of properties with concentration can best be explained in terms of three equilibria between five solute species M, M2, M+, M and e ... 

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