Main Difference – Absorption vs Adsorption
Both Absorption and Adsorption are physio-chemical processes where a relatively small quantity of one species enters/gets attached to another species of relatively higher quantity. The way the materials get incorporated into each other clearly differentiates the two processes. When a small quantity of one material fully enters and spread through the entire volume of another material, it is called, “absorption.” In contrast, when a small quantity of one material gets attached only to the outer surface of the other material, it is called, “adsorption.” Therefore, the main difference between absorption and adsorption is, absorption is the distribution of a certain substance throughout the volume of another substrate while adsorption is the attachment of a certain substance onto the surface of another substrate.
What is Absorption
As mentioned above, when we say a certain material is absorbed by another material, it means that the first material enters and spreads fully into the second material throughout its volume occupying space depending on the concentration. Therefore, in theory, atoms, molecules, and ions can enter another gaseous, liquid or solid bulk phase. Based on the rates of absorption of different substances by absorbates (the material that absorbs the foreign material), various separation mechanisms have come into existence. For example, liquid-liquid extraction for chemical substances. In this case, a certain substance which had been absorbed by one liquid is put into contact with another liquid which has a higher affinity towards the substance and hence it gets distributed into the second liquid whereby it can be extracted.
Similarly, another example would be the absorbance of Oxygen into water where the entire amount of gas gets dissolved and equally distributed throughout the volume of water. Here, the gas initially gets dissolved after an initial contact with the surface. And the amount that gets transferred eventually depends on the characteristics of both the liquid and the gas. This type of absorption is called a ‘physical absorption.’ Absorption can be broadly categorized into physical absorption and chemical absorption where in the latter case it will be due to a chemical reaction rather than the bulk transfer depending on the characteristics of the materials involved.
a) Gas–liquid absorption b) liquid–solid adsorption
What is Adsorption
Adsorption is a “surface phenomenon” and strictly deviates from absorption which involves the entire volume of the material. Therefore, in the case of adsorption, a relatively small quantity of one substance would get attached to the surface of the bulk substrate as a result of various types of interactions. The substance that gets absorbed is called the ‘adsorbate’, and the bulk substrate that absorbs the incoming material is called the ‘absorbent.’ Adsorption can be broadly divided into two categories as physisorption and chemisorption. In the case of Physisorption, the type of interactions between the adsorbate and adsorbent are physical forces such as Van der Waals interactions. However in the case of Chemisorption, it is chemical bonds that bind the adsorbates to the adsorbent.
The reasoning behind adsorption is similar to surface tension. The molecules within the adsorbent are surrounded by other adsorbent molecules and their energy at the surfaces are neutralized. But it is not true for the outer surface layer of the adsorbent as it is not surrounded by other molecules. Therefore, the surface of an adsorbent has extra energy the inner layers. Hence, it absorbs other molecules to their surface in order to minimize the surface energy. .However, the difference between adsorption and absorption almost disappears at the micro-molecular scale when the surface area of the adsorbent is significantly large.
Difference Between Absorption and Adsorption
Definition
Absorption is the distribution of a certain substance throughout the volume of another substrate.
Adsorption is the attachment of a certain substance onto the surface of another substrate.
Separation processes
Absorbed materials can be separated between different phases due to their chemical characteristics.
The technique of adsorption can be used to separate out certain material by adsorbing them onto one surface while the flowing of another material which had previously absorbed the substance.
Energy
Materials get absorbed into other material through the availability of space and also due to the properties of each material.
The vacant surface of the adsorbent has extra energy that stimulates the attachment of foreign molecules on to it.
Bonding
Materials that get absorbed remain within the space of the absorbent without having a chemical relationship being perfectly intact as when it is free. It might only carry weak physical interactions.
In adsorption, the adsorbates bind onto the surface of the adsorbent most commonly through Van der Waals interactions or through covalent bonds.
Image Courtesy:
“Absorption vs adsorption” by Aushulz – File:Absorbimento e adsorbimento.svg. (CC BY-SA 3.0) via Commons
