Metal-ammonia solutions volume expansion

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 volume expansions of alkali metals in liquid ammonia are discussed in the light of the current available data. Special emphasis is made of the anomalous volume minimum found with sodium-ammonia and potassium-ammonia solutions. Recent studies of potassium in ammonia at —34° C. were found to exhibit a large minimum in the volume expansion, AV, vs. concentration curve. The results of these findings were compared with the previous results of potassium in ammonia at —45° C. The volume minimum was found to be temperature dependent in that the depth of the minimum increased and shifted to higher concentrations with increasing temperature. No temperature effect was observed on either side of the minimum. These findings are discussed in light of the Arnold and Patterson and Symons models for metal-ammonia solutions. 

Solutions of alkali metals in liquid ammonia at all concentrations, with the exception of cesium, are less dense than either of the constituents. This behavior for metal ammonia solutions is unique in that the expansion in volume is much larger than that shown on forming solutions of normal electrolytes or non-electrolytes. 

A large volume expansion for solutions of sodium in ammonia was first reported by Kraus and Lucasse (17). Since this initial report, many investigations have been made of the volume expansion for a number of alkali metal-ammonia solutions. The techniques employed in these investigations have varied from density measurements for concentrated solutions using the Westphal Balance or Pycnometer to dilatometric studies for dilute solutions, which measure the volume expansion directly. 

Still there is no chance to find some experimental criterion for checking each of the described theories. It should be stressed that the polaron model of the cavity with the localized electron is definitely proved by the volume expansion of ammonia when adding the alkali metal to it (38). Besides, on the basis of any of the cavity theories it becomes comparatively easy to explain the results of investigation of e tr photoannealing—i.e., the dispersion of the trap depths, and temperature dependence of ExmaX of the electrons. In this connection it is rather desirable to find some approach to determining the sizes of the cavities for the electrons in the irradiated polar systems, as it has been done for the metal-ammonia solutions. 

Density and Volume Expansion. Kraus, Johnson, and collaborators have performed careful measurements of the densities of alkali-metal—ammonia solutions. All the measurements were carried out near the boiling point of liquid ammonia and for the concentration range from IM to saturation. They found the densities of the solutions for all three metals to be less than that of pure ammonia. It is more meaningful to consider the apparent expansion in volume per gram atom of metal dissolved which is obtained from the equation... 

Solutions of alkali metals in liquid ammonia have been studied by many techniques. These include electrical conductivity, magnetic susceptibility, nuclear magnetic resonance (NMR), volume expansion, spectroscopy (visible and infrared), and other techniques. The data obtained indicate that the metals dissolve with ionization and that the metal ion and electron are solvated. Several simultaneous equilibria have been postulated to explain the unique properties of the solutions. These are generally represented as follows ... 

A fairly comprehensive review of the information on the volume expansion exhibited by metal solutions may be found in the paper of R. Catterall (4). Our purpose here is to emphasize our most recent work on these solutions, namely, to discuss the expansion studies of potassium in liquid ammonia at —34° C. and to compare these results with the earlier investigations obtained at —45° C. 

In 1921, by dissolving an alkali metal in liquid ammonia, C Kraus and W Lucasse observed a volume expansion of the solution greater than that obtained for the dissolution of ordinary salts.They attributed this volume expansion to the formation of the solvated electron, which is regarded as a particle, since the electron itself has a negligible volume. For example, the dissolution of three moles of sodium in... 

In 1921, by dissolving an alkali metal in liquid ammonia, C. Kraus and W. Lucasse observed a volume expansion ofthe solution greater than that obtained for the dissolution of ordinary salts [3], They attributed this volume expansion to the formation of the solvated electron with a cavity, regarded as a particle since the electron itself has a negligible volume. For example, the dissolution of 3 moles of sodium in one litre of liquid ammonia induces an increase in volume of 43 cm compared to the pure liquid. Assuming that all the metal is dissociated, it may be deduced that in ammonia the electron occupies a spherical volume with a radius of 0.18 nm. In fact, the cavity radius of the solvated electron in ammonia is greater than that value and is about 0.3 nm. 

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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