The confusion between adsorption and absorption is obvious – both sound very similar. However, the two are vastly different processes.
Adsorption is the adherence of ions, atoms, molecules to the surface of the materials rather than accumulation inside the bulk. Whereas absorption involves the entering of a substance into the bulk of another substance.
Adsorption vs Absorption
|Accumulation of the molecular species (ions, atoms, or molecules) at the surface of the materials rather than in the bulk especially on solids or liquids is known as adsorption||Accumulation of substances (gas, liquids, or dissolved solids) throughout the bulk of the material, especially liquids and gases is called absorption|
|The adsorbent has vacant spaces that stimulate the adhesion of particles onto the surface||Absorption occurs due to the availability of molecular spaces and the nature of the particles|
|It is a surface phenomenon||It is a bulk phenomenon|
|The adsorbate (adsorbed material) remains attached to the adsorbent with either Van der Wall’s forces or covalent bonds||The absorbate (absorbed material) remains in the absorbent without having any chemical interactions with the absorbent|
|Adsorption is an exothermic process||Absorption is an endothermic process|
|It is a temperature-dependent process. Low temperature favors adsorption||It is not affected by temperature|
|The adsorbate is more concentrated on the surface than the other parts of the adsorbent after adsorption||The concentration of the absorbate in the absorbent is uniform after absorption|
|The adsorption steadily increases and reaches equilibrium eventually||The absorption process occurs uniformly|
|Adsorbed material (adsorbate) can be separated by passing new substance through the surface of the adsorbent (adsorbing material), which replaces the previously adsorbed material||Absorbing material (absorbate) can be separated into different phases based on its chemical interaction with the phases|
|Examples include water purification, gas masks, chromatographic analysis, etc||Examples include cold storage, ice production, etc|
What is adsorption?
In simple words, adsorption is the deposition of molecular species (ions, atoms, or molecules) on the surface/interface. The species that gets adsorbed on the surface is called adsorbate and the surface on which adsorption occurs is known as adsorbent.
Hydrogen, oxygen, and nitrogen gases are adsorbed by activated charcoal. Here the gases H2, O2, and N2 are adsorbates and the surface where the adsorption occurs (charcoal surface) is the adsorbent.
Unlike absorption, adsorption is a surface phenomenon, and it often precedes absorption.
There are two types of adsorptions:
Physical adsorption, also known as physisorption, is due to weak Van der Waals forces between adsorbate and adsorbent. For example, adsorption of gases like hydrogen or nitrogen on the surface of charcoal, etc.
Chemical adsorption, also known as chemisorption, is due to strong chemical forces (bonding) between adsorbate and adsorbent. For example, adsorption of hydrogen or nitrogen on the surface of adsorbent like ferrous catalyst at high temperatures, etc.
Generally, a common term known as sorption is used for both adsorption and absorption. Desorption, opposite of sorption, is a process by which a substance is released from or through the surface.
The most commonly used adsorbents are silica, activated charcoal, activated alumina, zeolites, polymers and resins, and clay.
Applications and uses of adsorption
1. Water adsorption
Adsorption of water is of great importance in many fields including but not limited to chemical engineering, materials science, and catalysis. It is also called surface hydration. Surface hydration is important in understanding interface properties, chemical reaction pathways, and catalytic performance in a wide range of chemical systems.
2. Polymer adsorption
Adsorption on polymers is useful in many ways such as in the development of non-stick coating and biomedical devices, etc.
3. Gas masks
Gas masks are used in coal mines to adsorb poisonous gases. They are based on the adsorption principle and work by purifying the air for breathing.
4. Silica gels
Silica gels are used to adsorb moisture and thus, reduce humidity. They are commonly put with newly made products, especially shoes, handbags, pharmaceutical drugs, etc.
5. Separation of noble gases
Noble gases can be separated using charcoal as an adsorbent.
6. Adsorption chillers
Adsorption chillers combine adsorbents with refrigerants and use heat to provide a cooling effect.
7. Adsorption in viruses
Getting adsorbed on a bacteria (as a bacteriophage) is the first step in the viral life cycle.
8. Chromatographic analysis
Chromatography is based on the adsorption process. The running solvent (mobile phase) when run through a stationary phase, it desorbs the adsorbed mixture substances and the separation (purpose of chromatography) occurs. For example, paper chromatography, thin layer chromatography (TLC), etc.
9. Purification of water
When alum stone is added to water, impurities get adsorbed on the alum surface and the water gets purified.
10. Ion exchange method
The ion exchange method is used to remove the hardness of water. During ion exchange, the calcium and magnesium ions are adsorbed on the surface of ion exchange resins, thus purifying water.
What is absorption?
Absorption is a phenomenon in which atoms, ions, or molecules enter the bulk phase of liquid or solid material. The species that get absorbed on the surfaces is called absorbate and the surface on which this absorption occurs is known as absorbent. Unlike adsorption, absorbate is taken up by the whole volume, not just the surface.
A familiar example is the absorption of water by a sponge. Similarly, soaking up water by a towel, tissue paper, filter paper, and clothes are all examples of absorption.
Anything having the capacity to absorb something, liquid or energy is absorbent. Kitchen towels, cotton wool, and first aid bandages are all absorbents.
Importance and uses of absorption
1. Absorption in the digestive system
The small intestine absorbs digested food particles and passes them to other parts of the body for storage and further biochemical reactions. Some specialized cells help absorbed materials to cross the intestinal lining into the bloodstream. Moreover, the lymphatic system absorbs fatty acids and vitamins, etc.
2. Amine scrubbing
At present, the most widely used organic absorption technology is amine washing technology.
3. Membrane gas absorption
Membrane gas absorption is mostly used in industrial wastewater treatment and CO2 absorption from greenhouse gases.
4. Intake of nutrients and water
Unicellular organisms use the absorption phenomenon for the intake of water and nutrients.
Refrigerators utilize the absorption process for cold storage, cooling, ice production, etc.
- Adsorption is an exothermic process while absorption is an endothermic process.
- Absorption is a slow process while adsorption is relatively a fast process.
- The thermodynamic difference between adsorption and absorption is that the entropy decreases for adsorption. This makes ΔG negative too, and thus, adsorption is a spontaneous process, while absorption process does not have any influence on spontaneity.
What is the entropy change (ΔS) for adsorption?
The entropy change for the adsorption phenomenon is negative. This is one of the places where the concept of negative entropy (negentropy) exists.
Does adsorption violate thermodynamics?
One may think that the entropy being decreased here, is violating the second law of thermodynamics. However, if we look at the bigger picture, adsorption is an exothermic process, which means that the entropy ( heat ↔ chaos ) of the surroundings increases. This increase in entropy balances the decrease in entropy and we are able to say that, in a bigger overview, adsorption does not violate the laws of thermodynamics.
What is the difference between adsorption and desorption?
Adsorption is the accumulation of a substance at the surface of another whereas, the release of the accumulated substance is termed desorption.
Why adsorption is a fast process while absorption is slow?
Adsorption is a surface phenomenon. The atoms and molecules are exposed to the surface and usually have higher energy due to surface tension. The area exposed to the outside is also wider. Hence, adsorption is relatively faster. On the other hand, absorption is a bulk phenomenon, and the molecules absorbed inside face opposition from the interacting forces, existing in the absorbing material, thus slowing down the absorption process.
What happens when some chalk is dipped in ink. Is its absorption or adsorption?
When chalk is dipped in ink, the surface of the chalk retains the color of ink due to adsorption of colored molecules while the ink also goes deeper into the chalk due to absorption. So, both absorption and adsorption are there.
What is the difference between ion exchange and adsorption?
Ion exchange is an example of the adsorption phenomenon.
What is the difference between adsorbate and adsorbent?
Adsorbate refers to the material that gets adsorbed on the surface whereas, the material on which adsorption occurs is known as adsorbent.
What is meant by re-absorption?
Re-absorption means being absorbed again. For example, our kidneys selectively reabsorb substances that have already been secreted out into the renal tubules, such as glucose, protein, sodium, etc. These reabsorbed substances are returned to the bloodstream.
Can activated charcoal absorb CO?
Yes, activated charcoal absorbs CO.
What is the adsorption of hydrogen by palladium called?
The adsorption of hydrogen by palladium is called occlusion.
Principles of Adsorption and Adsorption Processes by Douglas M. Ruthven (University of New Brunswick, Fredericton, Canada)