Gas Sorption

First : What is a gas ?

Unfortunately, the term gas is used in many different ways, both in science and in general use.

Some definitions refer to the gas state, and include condensible vapours, either pure or in mixtures, as distinct from the liquid state and the solid state.
Other definitions differentiate a gaseous phase from a vapour state : here, a vapour can be condensed to become a liquid at the same temperature, by increasing only the pressure, while a gas remains a gas because the temperature is above the Critical Temperature.
In common use, we may also refer to the normal state of the substance under ambient temperature and pressure, or to the form in which they are supplied.

Here we are including both gases and vapours, but we will make it clear when the possibility of condensation is important.

Definitions : What is Gas Sorption ?

Gas Sorption can be defined as the relative accumulation of mobile gas molecules due to the presence of a more or less static condensed phase, which may be a solid or a liquid. The molecules may accumulate on the surface, at an interface, or in the bulk of the condensed phase

In the field of Gas sorption, we tend to categorise it in many different ways, according to the strength, the type, and the location of the interaction.

Absorption: involves transfer of the gas molecules into the bulk of the solid or liquid. In general, this involves an initial stage of Adsorption.
Adsorption: most often refers to the accumulation of molecules on the surface of a solid. Adsorption also occurs at internal interfaces, and at the surface of liquids.
Physisorption: involves relatively weak interactions, of comparable strength to the interactions between the molecules in a liquid, without formation of strong chemical bonds.
Chemisorption: Both Adsorption and Absorption may involve chemisorption, where strong chemical bonds are formed between the free molecules and the condensed phase. In the bulk of a material, it can yield complete or partial transformation of the bulk to a new structure, as when hydrogen is absorbed in some alloys to form a solid solution, followed by separation into one or more hydride phases.; Surface Catalysis is a special case where chemisorption allows an adsorbed molecule to be transformed, or to react with another adsorbed molecule, to yield a new molecule, which is then released.; Irreversible chemisorption occurs when the chemical bond is so strong that the molecule is not released back to the gas phase, and may continue until the surface is fully covered.
Adsorption in Pores: In porous materials, the surface available for sorption can be much greater than the apparent surface of the visible particles. This increases the amount of matter which can be taken up by the material, and also gives an increase of the strength of interaction relative to an equivalent flat surface. When a vapour is adsorbed inside pores, the surface tension of curved surface of the adsorbed phase can lower its saturation vapour pressure so that the pores fill with liquid.

What is Gas Sorption, again ?

Molecules of gas arrive at the surface of a solid or a liquid.

The molecules may :

bounce directly off the surface
react chemically with the surface, and then:
- be trapped in position
- rearrange to make a new molecule, and be released
be held at the surface by a weak attractive force, characteristic of “physical adsorption”, and then :
- move around on the surface
- be released again
molecules may meet, and react with each other.
any molecules on the surface may absorb into the bulk, with a weak physical or a strong chemical interaction.
If there are pores or roughness, then the molecules may end up inside the pores, where they are held more strongly.

The number of molecules builds up, until it reaches equilibrium, when the rate of arrival of molecules equals the rate of loss.

The time taken to reach equilibrium may depend on :

the arrival rate of gas molecules at the surface
the rates of any reactions with the surface, with the bulk, with other molecules
the rates of movement, or diffusion, of the molecules :
- across the surface
- through the bulk
- inside the pores.

All of these effects depend quite strongly on the temperature and the pressure.

Why measure Gas Sorption ?

What is important in Gas Sorption ?

Depending on the field of study, every aspect of sorption might be of interest :

The rates of transport of matter, diffusion, permeation
The rates of absorption, reaction, equilibration
The nature and strength of the interaction
The final composition at equilibrium

In some cases, the sorption or transport process is being studied, in other, the sorption interaction is used to characterise the material.

A few examples may give an idea of the range of applications…

Gas solubility

The amount of gas which can dissolve into the bulk of a solid or liquid is often of interest in itself.

From the use of Ionic Liquids as a medium for chemical reactions, to the curing of concrete structures by incorporation of Carbon Dioxide, dissolved gas has many effects.

Measurement of powder and porous properties by Gas physisorption

This is probably the most common type of sorption measurement performed, except for the simple gravimetric measurement of absorbed moisture by weighing a sample before and after thermal drying.

Adsorption of Nitrogen at a temperature of 77 Kelvin is extremely widely used for measurement of surface area by BET analysis
Porous properties can also be measured, from micropores up to mesopores measuring 500 nm or more, by physisorption measurements using a variety of adsorbates. With some knowledge of the sample properties, it is possible to obtain the total pore volume, and distributions of the pore volume or area as a function of pore size.

Adsorptive Separation, purification, and gas scrubbing

Both the amount of sorption on a substrate, and the rate or speed of sorption, are dependent on the gas species. This gives a wide range of very important applications for sorption :

to purify raw materials, to increase efficiency, to reduce energy consumption, or to avoid contamination or production of undesirable by-products.
to separate components from mixtures, for example, as a lower energy or higher throughput replacement for distillation.
to filter or scrub an undesirable impurity or contaminant from a gas stream, to avoid release to the environment

Gas sorption can separate different gases or vapours by a number of effects :

trapping on or in a strongly sorbing material. This may be irreversible, requiring periodic replacement of the filter, or may need a chemical or heat treatment to regenerate the material.
Selective adsorption, where the material absorbs one component more strongly than another.
Kinetic separation - where one component is absorbed very rapidly, while another is absorbed slowly.
Transport limited adsorption, as is used in isotopic separation.

A continuous output flow is often obtained by using multiple beds of sorbent material, where one is used for production, while the other is being regenerated, either by purging with clean gas, by reducing pressure, or by increasing temperature.

A typical case is the use of Carbon molecular sieve materials which adsorb oxygen in preference to Nitrogen. A Pressure Swing Adsorption (PSA) process allows generation of Nitrogen and Oxygen gases from air, for industrial, medical, and laboratory use.

Sorption isotherms, kinetics, and breakthrough curves, both of individual components, and of mixtures, are used for development of processes, for selection of materials, and to determine process parameters.

to absorb gases or moisture inside enclosures or packaging, to protect and preserve foods, works of art, historical artefacts, and other items

Membrane separation and barrier properties

Membranes are widely used to control the permeation of gases and vapours.

allow gases to pass through packaging but exclude moisture and retain flavour compounds.
allow escape of humidity from buildings
use of moisture permeable membranes for drying of compressed air or other mixtures

Permeability of a substance in a material depends on the rate of diffusion and the solubility. The sorption isotherm gives the solubility, and the kinetic equilibration curve gives the diffusion coefficient.

Gas storage and transport

A sorbent material can allow equivalent gas storage with a reduced containment pressure. The desorption kinetics can regulate, or hinder, the release of the gas.

Some metals and alloys are used to store hydrogen as hydride compounds - a chemisorption process.
Porous materials, especially activated carbons, are used for low pressure storage of other hydrocarbons.

Catalysis and chemisorption

Many catalysts take the form of porous materials with small clusters of metal atoms. Reduction of the metal atoms by hydrogen is often used to activate the metal atoms, before adsorption of reactive molecules to measure the number of sites available for reaction.

Metal hydrides are used in many modern battery technologies.