In chemistry, absorption is a physical or chemical phenomenon or a process in
which atoms, molecules or ions enter some bulk phase – gas, liquid or solid
material. This is a different process from adsorption, since molecules
undergoing absorption are taken up by the volume, not by the surface. A more
general term is sorption, which covers absorption, adsorption, and ion
exchange. Absorption is a condition in which something takes in another
substance. In many processes important in
technology, the chemical absorption is used in place of the physical process,
e.g., absorption of carbon dioxide by sodium hydroxide – such acid-base
processes do not follow the Nernst partition law.
For some examples of this effect, see liquid-liquid extraction. It is possible
to extract from one liquid phase to another a solute without a chemical
reaction. Examples of such solutes are noble gases and osmium tetroxide.
The process of absorption means that a substance captures and transforms
energy. The absorbent distributes the material it captures throughout whole
and adsorbent only distributes it through the surface. The reddish color
of copper is an example of this process because it is caused due to its
absorption of blue light. However, it should be remarked that the
distinction between adsorption and absorption vanishes as we go from
perfectly crystalline macroscopic materials to structured materials,
aggregates and composites made out of increasingly smaller grains, viz.,
micron-sized particles to nanoparticles, sub-nano particles and finally
molecules. In such nano-composites, the internal surface area of particulate
matter is very very large. Then the adsorption on internal surfaces simply
becomes absorption. Then the distinction between adsorption and absorption
vanishes. On the other hand, the distinction is clearest between bulk
solids without internal structure, but having only surfaces where only
adsorption can occur on the outer surfaces, and nanocomposites or
aggregates with internal structure where absorption by the host material is
simply adsorption on internal surfaces of the host material. As an example, we
may consider a crystalline piece of silicon dioxide which can adsorb water
molecules on its surface. However, if the quartz is ground into very fine
sand, the pile of sand has a very large internal surface area. A very large
amount of water can be adsorbed by the "internal" surfaces of the grains in the
pile of sand, and this absorption is simply "internal adsorption. If water is
made to flow thorugh such a pile of sand, ions and toxins may be
preferentially adsorbed by the surfaces of the grains of sand, providing a
simple, well-known water purification application.
Equation If absorption is a physical process not
accompanied by any other physical or chemical process, it usually follows the
Nernst distribution law: "the ratio of concentrations of some
solute species in two bulk phases in contact is constant for a given solute
and bulk phases": The value of constant KN depends on
temperature and is called partition coefficient. This equation is valid if
concentrations are not too large and if the species "x" does not change its form
in any of the two phases "1" or "2". If such molecule undergoes association or
dissociation then this equation still describes the equilibrium between "x" in
both phases, but only for the same form – concentrations of all remaining forms
must be calculated by taking into account all the other equilibria.
In the case of gas absorption, one may calculate its concentration by using,
e.g., the Ideal gas law, c = p/RT. In alternative fashion, one may use partial
pressures instead of concentrations. Types of absorption
Absorption is a process that may be chemical or physical.
= Physical absorption = Physical absorption or non-reactive
absorption is made between two phases of matter: a liquid absorbs a gas, or a
solid absorbs a liquid. When a liquid solvent absorbs a gas
mixture or part of it, a mass of gas moves into the liquid. For example,
water may absorb oxygen from the air. This mass transfer takes place at the
interface between the liquid and the gas, at a rate depending on both the gas
and the liquid. This type of absorption depends on the solubility of gases, the
pressure and the temperature. The rate and amount of absorption also depend on
the surface area of the interface and its duration in time. For example, when
the water is finely divided and mixed with air, as may happen in a waterfall
or a strong ocean surf, the water absorbs more oxygen.
When a solid absorbs a liquid mixture or part of it, a mass of liquid moves into
the solid. For example, a clay pot used to store water may absorb some of the
water. This mass transfer takes place at the interface between the solid and the
liquid, at a rate depending on both the solid and the liquid. For example, pots
made from certain clays are more absorbent than others. Absorption is
essentially a molecule attaching themselves to a substance and will not
be attracted from other molecules. = Chemical absorption =
Chemical absorption or reactive absorption is a chemical reaction
between the absorbed and the absorbing substances. Sometimes it combines with
physical absorption. This type of absorption depends upon the
stoichiometry of the reaction and the concentration of its reactants.
References