Properties of metal-ammonia solutions

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. 

Dye, J. L., Electrochemical Properties of Metal-Ammonia Solutions E.M.F. and Transference Numbers in Metal-Ammonia Solutions, Physicochemical Properties, Colloque Weyl, G. Lepoutre, M. J. Sienko, eds., p. 137, Benjamin Press, New York, 1964. 

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). 

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. 

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. 

In this section, the physical properties of metal-ammonia solutions other than electric, magnetic and spectroscopic will be listed. While the properties listed in this section do not give definitive evidence for or against various models that have been proposed for the structure of these solutions, they do supplement the evidence provided by the spectroscopic, magnetic and electrical properties. A listing of experimental results concerning these properties observed prior to 1944 can be found in a book by Yost and Russell. References 4 and 5 list the results of some measurements later than 1944. 

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. 

After having listed the observed properties of metal-ammonia solutions in Section I and describing the various models for them in Section II, the important job now remains of discussing the explanation of the observed properties by the proposed models. 

In Part I- F the magnetic properties of metal-ammonia solutions were listed. As we have seen, the obseiwed magnetic properties consisted of results of total susceptibility measurements, spin susceptibility measurements using electron spin resonance techniques, dynamic features of electron spin resonance involving measurements on the relaxation times, and nuclear resonance studies. We shall first take up the explanation of the susceptibility data using the cavity, cluster, and unified models and subsequently consider the interpretation of the results of resonance studies. 

List four significant properties of metal-ammonia solutions of the alkali metals and, in an answer of one or two sentences at the most, sketch out how each property is accounted for by the physical model or theory of these solutions. 

The proceedings of a conference on Electrons in Fluids have been published. They include papers on the reaction rates of solvated electrons, on the conduction properties of metal-ammonia solutions, and on electron mobility in non-polar fluids, relevant to the question of possible long-range electron transfer between oxidizing and reducing centres in solution. Measurements of hole mobility and of hole quenching by various reagents in cyclohexane as solvent have been interpreted in terms of electron-transfer reactions such as equation (19) (DMA=A A -dimethyl-... 

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