Metal-ammonia solutions properties

Finally, a striking property of metal-ammonia solutions is the large expansion of the liquid due to the solvated electrons. The apparent volume of the solvated electron remains roughly constant up to the metallic range, then shows a slight increase. It is about 100 cm3 mol. It is this effect that has led to the hypothesis that the electron forms a cavity for itself a cavity of radius 3.2 A accounts quantitatively for the excess volume. A model in which the electron moves in a cavity, and the surrounding liquid is polarized or solvated as it is round a cation, was first put forward by Jortner (1959), who showed that it was able to account for the absorption spectrum. Jortner s model, as modified by Mott (1967), Cohen and Thompson (1968) and Catterall and Mott (1969), will now be described. 

The simple model described is successful in accounting for the properties of liquid metals (45) and has been applied with some success to the study of the conductivity of saturated metal-ammonia solutions (27). 

Lepoutre, G., Sienko, M. J., Metal-Ammonia Solutions Physicochemical Properties, W. A. Benjamin, New York, 1964. 

Arnold, E., Patterson, A. J., Calculation of Conductivity in Sodium-Liquid Ammonia Solutions in Metal-Ammonia Solutions, Physicochemical Properties, Colloque Weyl, G. Lepoutre, M. J. Sienko, eds., p. 160, Benjamin Press, New York, 1964. 

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 unique properties of dilute metal-ammonia solutions depend not upon the nature of the metal species, but upon the solvated electron common to all these solutions. Thus, the electron-in-a-cavity model (17, 19, 21) seems best suited to describe the species present in these solutions since the model is independent of the type of cation present. Jortner and his associates (15, 16) have extended this model by assuming that the cavity arises from polarization of the medium by the electron. The energy levels of the bound electrons are obtained by using a potential function containing the static and optical dielectric constants of the bulk medium as parameters. Using one-parameter hydrogen-like wave functions for the first two bound states of the electron, the total energy of the ith state is expressed as... 

The physical properties of these intriguing systems have been studied in serious vein since the 1920s (111,164). At the present time, this interest continues unabated. In its more recent development, the multidisciplinary nature of metal-solution investigations has also spilled over into the wider fields of excess electrons in disordered media (38, 39, 99, 103). The recent Colloque Weyl V conference on metal-ammonia solutions and excess electrons in liquids (39) brought together almost 200 scientists from all disciplines. The present review... 

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).

In the weak-interaction model (85) developed in the previous section to explain ion-pairing in metal-ammonia solutions, aggregation interactions involving Ms+ and es are relatively weak, and leave the isolated solvated electron properties virtually intact. However, a major difficulty (29,54,134) arises with the type of model when one considers the precise nature of the corresponding electron spin-pairing interaction in ammonia solutions. It is worth expanding on this issue because it probably remains one of the fundamental dilemmas of metal-ammonia solutions in the dilute range (54). 

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). 

The precise nature of the electronic interactions between centers must obviously change dramatically at the NM-M transition, e.g., from van der Waals type interaction to metallic cohesion (112). These gross changes in electronic properties at the transition are sufficient to noticeably influence the thermodynamic features of the system (86,87). The conditions therefore appear highly conducive for a thermodynamic phase transition to accompany the electronic transition at the critical density. In fact, the transition to the metallic state in metal-ammonia solutions is accompanied by a decrease in both enthalpy and entropy (146, 149), and it has been argued convincingly (124, 125) that the phase separation in supercritical alkali metals and metal solutions is... 

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." ... 

In 1954, Platzmann and Frank indicated the possibility of using the so-called radiationless theory of transitions developed by Lax for polyatomic molecules, and by Pekar for polar crystals, to the process involving charge transfer in liquids. The most general method in the theory of the radiationless transitions was suggested by Kubo and Toyozawa in 1955. Subsequently it was used in many other works. The first calculations for processes in polar liquids in the framework of the polaron theory were performed by Davydov and Deygen, " who investigated the properties of metal-ammonia solutions. 

Numerous reactions have been carried out with metal-ammonia solutions, which have strongly reducing properties. They have been extensively reviewed ... 

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. 

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