A solvent is an element which dissolves something, basically the solute. Normally, all the solvents we encounter are in liquid phase. So, I was wondering if there could be any solvent in a gaseous phase. Can there be any gaseous solvent, after all?
In order for a substance to act as a solvent, there has to be a solution. According to the IUPAC Gold Book, a solution is:
A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are called solutes.
Wikipedia's article on Solutions has a section on gaseous solutions, but calls them "trivial" and admits that part of the literature does not consider solutions in the gas phase (without citation, unfortunately):
If the solvent is a gas, only gases are dissolved under a given set of conditions. An example of a gaseous solution is air (oxygen and other gases dissolved in nitrogen). Since interactions between molecules play almost no role, dilute gases form rather trivial solutions. In part of the literature, they are not even classified as solutions, but addressed as mixtures.
Of course, solutions are mixtures (specifically, homogeneous ones), as implied by the IUPAC definition talking about a phase (singular) rather than phases.
So if you go with IUPAC, gasses are not solvents because solutions are either in the liquid or the solid phase. They don't mention supercritical fluids, but neither does the OP, so those would have to be considered elsewhere.
Reference: IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "solution". doi:10.1351/goldbook.S05746
Solvents imply dissolving process as intermolecular interactions between solvent and solute, leading to disperging the solute from the condensed form.
"Permanent" gases near normal conditions do not provide such interactions. Volatile compounds evaporate with or without presence of gas. Gas is rather a bystander/spectator of such evaporation.
Fluids near critical conditions like supercritical CO2 share many properties of liquids and gases, so such interactions are present, so in some sense and with both eyes closed, one can consider it as solvent and gas.
Yes, solvents can be in gaseous form. Air itself is an example of a gas-gas solution, where other gases are "dissolved" in nitrogen. But liquids and solids can not be dissolved in gas to form a true solution, instead they form colloids.
- Physical Chemistry Textbook for Competitions by Dr. O.P. Tandon and Dr. A. S. Singh, Ninth Edition, 2008.
No, unless you use a very loose definition of solvation
The essential feature of solvation is some relatively strong molecular interaction between the solvent and the substance being solvated. Ethanol is soluble in water (and vice versa) in any proportion because their hydrogen bonds have strong intermolecular interactions. Sodium chloride is partly soluble in water because the dipolar nature of water molecules can interact sufficiently strongly with the ions of the salt to keep some of them in solution. But only up to a certain point when the water's capacity to do this is saturated and no more salt will dissolve.
These concepts are irrelevant for gases. If two compounds are a gas at a given set of conditions, they will mix. No interactions between the components are relevant to this. It is inherent to the nature of things being gases. The normal things happening in solvation are irrelevant. The mixing of gaseous substances in any ratio is a property of them both being gases and does not relate to the normal processes of solvation. So there is little benefit to describing the process as "solvation" of one gas being a solvent for another.
There is one exception to this: supercritical fluids. Above the critical point for a substance (usually at higher than normal pressures or temperatures for typical gases) there is no distinction between the gas and liquid phase. For example, Carbon dioxide is a supercritical fluid beyond about 30 °C and 7 atm pressure and is used in some industrial processes. Supercritical fluids have many of the properties of liquids (but not all eg a lack of surface tension) but, importantly, things can be solvated in them in the same way that liquids solvate solids.
Apart from the (extreme) example of supercritical fluids, it makes no sense to talk about gases being solvents.
Hot carbon dioxide gas can act very much like a solvent, and for an otherwise unlikely solute at that -- elemental carbon. See here for the not so exotic chemistry involved.
The concept of carbon "solubility" in a hot carbon-dioxide bearing gas is useful in optimizing temperature and gas composition for cleaning carbonaceous contaminants off a steel strip during some continuous annealing/hot dip coating processes.