Solvated Electron

1 Nature of Electronide and Spinide Solutions of Alkali Metals in Liquid Ammonia and Similar Solvents CHARLES A. KRAUS and E. CHARLES EVERS

Downloaded by 80.82.77.83 on January 21, 2018 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0050.ch001

Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pa. 19104

The conductivity of salts in solvents of low dielectric constant, and of metals in liquid ammonia, exhibit minima which may be explained in terms of an equilibrium between ions and a coulombic compound of two ions, or "ion pairs." This equilibrium conforms to the law of mass action. A t limiting conductance in solutions of sodium in liquid ammonia, part of the current is carried by metal ions, but seven-eighths is carried by electrions. Following the BLA model, it is assumed that when two ion pairs, consisting of a sodium ion and an electron, come together, the spins of the two electrons couple to form disodium spinide. Increase in conductivity past the minimum is assumed to be caused by dissociation of d i sodium spinide into sodium ions and spinions.

Jor this discussion, coulombic compounds between single electrons and metal ions shall be termed "electronides" and their negative ions "electrions"; similar compounds between metal ions and spin-coupled electrons shall be termed "spinides" and the negative ions "spinions."

Conductance

and

Structure

In the solid state a metallic element such as sodium consists of an ordered network of positively charged sodium ions and spin-coupled electrons which move freely through the field owing to the positive ions. In this state the conductance is high. 1

Hart; Solvated Electron Advances in Chemistry; American Chemical Society: Washington, DC, 1965.

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In the crystalline state sodium chloride consists of an ordered net work of sodium and choride ions. T h e ions in this network are almost completely fixed in position, and they have a very low conductance which, however, increases slightly with increased temperature. T h i s conduct ance increase contrasts with the behavior of metallic sodium i n which the conductance decreases with increasing temperature. When sodium chloride is carried past its melting point, the molten compound becomes an excellent conductor, and conductance proceeds by an ionic process. A s the current is passed between electrodes in the melt, reactions occur at the two electrodes, sodium metal being produced at the cathode and elemental chlorine at the anode. B y contrast, when current is passed through metallic sodium, whether solid or liquid, no electrochemical reactions occur. It is characteristic of metals that the current passes from one metal to another without any material effects. Coulombic

Interactions

When sodium chloride is dissolved in water at ordinary temperatures, it is practically completely dissociated into sodium and chloride ions which, under the action of an external field, move in opposite directions and independently of each other subject to coulombic interactions. If, however, sodium chloride is dissolved in a solvent of lower dielectric constant, and if the solution is sufficiently dilute, there is an equilibrium between ions and a coulombic compound of the two ions which are com monly termed " i o n pairs." T h i s equilibrium conforms to the law of mass action when the interaction of the ions with the surrounding ion atmos phere is taken into account. In solvents of very low dielectric constant, such as the hydrocarbons, sodium chloride is not soluble; however, many quaternary ammonium salts are quite soluble, and their conduct ance has been measured. Here at very low concentrations, there also is an equilibrium between ions and " i o n pairs" which conforms to the law of mass action; but at higher concentration, i n the neighborhood of 1 X 10 ~ W , or below, a minimum occurs in the conductance. Thereafter, it may be shown that the conductance increases continuously up to the molten electrolyte, provided that a suitable electrolyte and solvent are employed which are miscible above the melting point of the electrolyte. Sodium

in Liquid

Ammonia

Sodium is quite soluble in liquid ammonia, forming a saturated solu tion which contains 5.7 molecules of ammonia per atom of sodium. T h e density of the saturated solution is 0.578 g./cc. at — 3 3 . 8 ° C ; the specific conductance is 5047, and the atomic conductance is 0.800 Χ 10 . C o n siderable expansion occurs when a metal is dissolved in ammonia, and it may be pointed out that a saturated solution of lithium in ammonia is the lightest liquid known at room temperature, having a density of only 0.477 g./cc. 4



1.

KRAUS AND EVERS

Electronide and Spinide Solutions

3

W i t h decreasing concentration the atomic conductance of the sodium solution decreases to a minimum of 540 at 0.04ΑΓ. Beyond the minimum, the conductance increases with increasing dilution and approaches a limiting value of 1022. It is obvious that between the minimum and infinite dilution, phenomena are occurring which are very similar to what one finds in all electrolyte solutions. In short, the conductance varies much like that of an ordinary electrolyte in liquid ammonia and similar solvents. As stated above, the limiting conductance at infinite dilution has been determined to be 1022. T h e limiting conductance for the sodium ion is approximately 130. There can be no doubt therefore, that i n a dilute solution of sodium or other alkali metal in liquid ammonia, part of the current is carried by metal ion. In the case of metallic sodium at the limiting conductance, one-eighth of the current is carried by the sodium ion; the other seven-eighths of the current is carried by the elect rions. Thus, in a dilute solution, the conductance process is in part purely elec trolytic. IS at ure of the

Current

Although seven-eighths of the current is carried by electrions, this process is not the same as that by which a current is carried i n a metal. T h e low value of the conductance (about 900) of the electrion in dilute solution shows clearly that in its motion, the electron interacts with solvent molecules in such a way as to reduce its mobility. T h e existence of such interaction between the electron and ammonia is confirmed by the magnetic resonance and optical properties of alkali metal solutions. In one respect, the current passing through a solution of sodium in liquid ammonia is metallic in nature. T h e current passes from solution to metal and vice versa without any evidence of material effects at the electrodes. T h e process on electrolysis of a dilute sodium solution may be followed readily if the solutions used are sufficiently dilute so that the electrodes may be observed visually and the potential is sufficiently high to bring about concentration changes in a reasonable period of time. O n electrolyzing a solution of sodium in liquid ammonia i n this way, the density of color owing to the presence of the metal increases at the cathode and, in time, the density of color i n the anode compartment decreases. If the electrolysis is continued for a sufficiently long time, the metal is carried almost completely from anode compartment to the cathode compartment. In electrolyzing the solution the sodium ions are carried to the cathode where, when they reach the electrode, they permit spincoupled electrons in the metal to come into the solution as single elec trions. BLA

Model

As pointed out above, for a solution of say, sodium chloride in a solvent of lower dielectric constant than that of water, such as alcohol,


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an equilibrium exists between ions and " i o n pairs" which is formed as a result of coulombic interactions between the ions. There is reason to believe that an equilibrium of this type exists be tween the sodium ions and the electrion to form an " i o n p a i r " as a result of coulombic interactions. If the conductance data for sodium are used to determine the equilibrium constant of sodium i n liquid ammonia for computing the constant of the " i o n p a i r " equilibrium, the experimental data do not conform to values required for such an equilibrium. T h i s is because electrons in dilute solutions exhibit magnetic properties, from which we may conclude that, at very low concentrations, the electron has a spin of / Bohr unit. It is, therefore, necessary to take into account the effect of the decreasing proportion of electrons that may be spincoupled and interacting with the positive ions of the solvent. One of us (Evers) made the simplest possible assumption, following a model pro posed by Becker, Lindquist, and Alder ( B L A ) , namely that when two " i o n pairs," consisting of a sodium ion and an electron, come together the spins of the two electrons couple to form disodium spinide, and that this cou lombic compound is not dissociated into ions at low concentrations.


1

2

Calculations Assuming that in equilibrium reactions the undissociated disodium spinide exists i n equilibrium with the two sodium electronides which, i n turn, are in equilibrium with the free ions, the dissociation constants and the limiting conductance A of the metal in solution in liquid ammonia have been determined. F o r this purpose, the data employed were those which were originally obtained by one of us (Kraus) 50 years ago. T h e data in question were obtained in five independent series of measurements involving a total of 37 different observations which covered a range of concentration from 2.5 X 10 ~ °N to near the minimum of approximately 0.04ΛΓ. T h e values obtained for the equilibria constants, ions to electronide, and electronide to spinide, and the limiting conductance value A were, respectively, 7.23 X 10 " , 27.0, 1022 based on the B L A model. O n computing the conductance of these solutions of sodium i n ammonia on the basis of the above constants, the calculated values differed from the observed values by a little less than 1 % on the average. Considering the wide concentration range covered and their distributions, the data obtained certainly lend strong support to the underlying assumptions made i n the calculations. Results obtained by D y e based on transport measurements and activity calculations, as well as resonance data, by Pitzer, Hutchison, Hughes, and others, also support this view. 0

r

3

0

Spinide

Dissociation

A t a concentration of approximately 0.04iV, solutions of sodium in liquid ammonia, like those of other alkali metals, pass through a mini m u m ; thereafter the conductance increases very rapidly up to the saturated solution. T h i s conducts nee increase beyond the minimum can


1.

KRAUS AND

EVERS

Electronide and Spinide Solutions

5


only be caused by the presence of a new " i o n i c " species whose mobility is not reduced by its interactions with the solvent molecules. T h e conductance increase probably is caused by the dissociation of the disodium spinide into sodium ions and spinions, and the latter ions do not interact appreciably with ammonia molecules at the minimum nor at concentrations beyond the minimum up to the saturated solution. Evidence exists that points toward the dissociation of the spinide and the formation of spinions moving freely through the solution much as they do in the metal. T h e amount of metal existing in the form of electronide in the neighborhood of the minimum cannot be very high since a very large amount is undoubtedly present as disodium spinide. T h e increased conductance can be caused only by the dissociation of the spinide. T o account for the dissociation of the spinide at higher concentration, we must consider the action of the ion field on the spinide. In electrolyte solutions where the minimum appears in solvents of low dielectric constant, the authors account for the increasing conductance of the electrolyte beyond the minimum by considering the action of the fields of individual ions on " i o n pairs" which lie within the field. As Wien has shown, the dissociation of a partially dissociated electrolyte increases with the field if an external field is applied. Since the coulombic force acting between two ions is decreasing owing to the action of an ionic field, it may be dissociated into its components by the impact of a solvent molecule whose energy would not be high enough to bring about dissociation in the absence of a field. T h e authors have designated this action of the ionic fields on the dissociation of electrolytes as a micro Wien effect. It is is obvious that, owing to the micro Wien effect, the dissociation of an electrolyte will increase very rapidly as ions are formed as a result of the ionic fields. T h e role played by the micro Wien effect in dissociating the disodium spinide with increasing concentration beyond the conductance minimum is strongly indicated by the temperature coefficient of the conductance of solutions of sodium and potassium in liquid ammonia. Here we have data for the temperature coefficient of sodium and potassium from relatively dilute solutions up to saturation. T h e fluidity of ammonia increases about 1.5% per degree, and in the dilute range the temperature coefficient of metal solutions is of this order of magnitude; but from a concentration of approximately 0.9iV onward the temperature coefficient of sodium and potassium begins to increase, reaching a maximum of about 3.6% for sodium and 4.6% for potassium. T h e conductance increase owing to temperature increase can only be caused by an increased dissociation of sodium spinide. It follows that the conductance increase with increasing concentration of sodium solutions is to be expected and conforms with the assumptions of a micro Wien effect. Conclusion In this report we have emphasized the conductivity of these systems, only touching on other important physical properties. We have, how-


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ever, attempted to introduce what we consider to be a logical method of naming certain species which have a high probability of existing in dilute solution; and we have proposed a mechanism to account for the minimum i n the conductance curve, based on rather simplified concepts relating to the application of mass action theory. 1965.


R E C E I V E D May 13,


Solvated Electron

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