What kind of solution is formed when alkali metals are dissolved at high concentration in liquid ammonia?

Question

When an alkali metal is dissolved in liquid ammonia, it results in the formation of a deep blue coloured solution. $$\ce{M + (x + y) NH3 -> M+(NH3)_x + e^-(NH3)_y}$$ The ammoniated electrons absorb energy corresponding to red region of visible light. Therefore, the transmitted light is blue in colour. At a higher concentration (3 M), clusters of metal ions are formed. This causes the solution to attain a copper–bronze colour and a characteristic metallic lustre.

Source: byjus

So when the concentration of a solution of liquid ammonia is above $\pu{3 M}$, the alkali metal ions form clusters which give the solution a copper-bronze color.

Can I call such a solution an associated sol (solid in liquid colloid) similar to micelle formation in water?

Textbook I am using "Sudarshan Guha's J. D. Lee Concise Inorganic Chemistry"

Answer 1

Before asking what the nature of the solution is, let's fact-check the unsupported statements made:

[OP] when the concentration of a solution of liquid ammonia is above 3 M, the alkali metal ions form clusters which give the solution a copper-bronze color.

The phrase "solution of liquid ammonia is above 3 M" is unclear. From the context, it should probably read "solution of alkali metals in liquid ammonia with a concentration above 3 mol/L". The statement "the alkali metal ions form clusters" is misleading; according to Zahn (DOI: 10.1039/C7RA11462A), single metal ions are surrounded by ammonia and amide ($\ce{NH2-}$), forming clusters. This is different from metal clusters, which have metal-metal interactions.

Finally, if there are no metal-metal interactions, it is unlikely that the color is related to them. A source referencing the different colors in this Libretext page, which mentions a threshold of 3 mol/L for a color and conductivity change. The conductivity is due to the solvated electrons, so maybe the colors are also related to them rather than the metal ions.

This paper concludes about the changes at high concentrations:

The present study shows that the electrolyte-to-metal transition in increasingly concentrated alkali metal–liquid ammonia solutions is a gradual process rather than an abrupt first-order transition, which is in line with previous suggestions (1). From the molecular point of view, this transition may be understood in a simplified way as gradual coalescence of individual solvated electrons and dielectrons upon increasing alkali metal doping, with the metallic behavior appearing around the percolation threshold.

For readers outside of the specialty (like me), the paper is difficult to read without further study of the topic and the cited literature, but the quoted conclusion shows that it is complicated, and seems to imply that it is the combination of electrons and metal cations that results in the metallic behavior.

[OP] Can I call such a solution an associated sol (solid in liquid colloid) similar to micelle formation in water?

No, the metal ions are more like hydrated ions, with solvent molecules forming a first and maybe second shell of solvation. Micelles have multiple solutes "clumped" together, so that is not the best analogy. The metal ions are not solids, and the particles (clusters?) are not large enough to be called colloids. Liquids containing solids are not translucent (like milk or blood), with particles whose size is larger than the wavelength of light.

Answer 2

From content by Rachel Bain, at the University of Wisconsin, to quote:

When sodium metal is added to ammonia, some of the sodium dissolves. Each of the dissolving sodium atoms loses an electron and becomes a cation. Both the cation and the free electron are solvated by ammonia molecules.

$\ce{Na(s) + (x + y) NH3(l) → Na(NH3)^x+(aq) + e^-(NH3)y(aq) (blue)}$

The blue color is characteristic of a solution containing solvated electrons; as the solution becomes more concentrated, it takes on a bronze color. The bubbles appearing as the reaction proceeds are hydrogen gas that is formed in a second reaction between the sodium and ammonia:

$\ce{2 Na(s) + 2 NH3(l) → 2 NaNH2(aq) + H2(g)}$

And, further in a narration section:

A small piece of sodium is cut to expose a fresh surface. The sodium is dropped into liquid ammonia at a temperature of approximately -33 degrees Celsius. Some of the sodium dissolves, forming sodium cations surrounded by ammonia molecules and electrons surrounded by ammonia molecules. The solvated electrons give the blue color to the solution. Because of the mobility of the electrons, the solution is a good electrical conductor. Bubbles of hydrogen gas are formed by a second reaction that also produces sodium amide. More concentrated solutions appear bronze-colored and have a conductivity similar to metals.

which is all in accord with provided comments above.