Adsorption vs Absorption

Surface chemistry is concerned with the phenomena that occur at surfaces or interfaces. The interface or surface is represented by a hyphen or a slash separating the bulk phases. The interface between a solid and a gas, for example, can be represented by solid gas or solid/gas. There is no interface between the gases due to their complete miscibility. 

In surface chemistry, the bulk phases encountered may be pure compounds or solutions. The interface is typically a few molecules thick, but its area is determined by the size of the bulk phase particles. Many important phenomena occur at interfaces, including corrosion, electrode processes, heterogeneous catalysis, dissolution, and crystallization. Both sorption processes, adsorption and absorption, are sorption processes.

What is Adsorption?

The substance whose molecules are adsorbed at the surface is referred to as the adsorbate in adsorption. The adsorbent is the substance on whose surface the process occurs. It is a surface occurrence.

The adhesion of molecules to the surface of a solid or liquid is referred to as adsorption. The molecules accumulate only at the surface and do not penetrate the adsorbing material’s bulk.

Adsorption has industrial applications such as air conditioning, adsorption chillers, synthetic resin, and water purification. Because there are no moving parts in an adsorption chiller, it is quiet. Adsorption is used in the pharmaceutical industry to extend neurological exposure to specific drugs or parts of drugs. The adsorption of molecules onto polymer surfaces is used in a variety of applications, including the development of nonstick coatings and biomedical devices.

Some basic terms commonly used in adsorption are:

• Adsorbate – Any substance that has adsorbated on the surface is referred to as an adsorbate. Charge transfer between the adsorbate and the metal occurs during the adsorption process, resulting in a dipole moment.
• Adsorbent – Adsorbents are insoluble materials with liquid coatings on their surfaces, such as capillaries and pores. When a material, such as a sponge, has the ability to contain a specific amount of liquid in small chambers, it is said to be adsorbent. Adsorbents are essential in chemical absorption, which occurs when a specific substance is trapped on the surface of a material.

Types of Adsorption

Adsorption is classified into two types based on the interaction forces between adsorbate and adsorbent.

1. Physical adsorption: Physical adsorption, also known as physisorption, is an exothermic process. It has a low adsorption enthalpy. Physisorption normally involves the accumulation of gas on a solid surface due to weak forces known as Van der Waals forces. Because the adsorbent in the given surface does not show any particular gas, physisorption lacks specificity. It is reversible in the sense that the physisorption of a gas by a solid can be reversed by the physisorption of a gas by a solid. Adsorption of gases such as hydrogen, nitrogen, and others on the surface of adsorbents such as charcoal is an example of physisorption. The surface area of the adsorbent influences physisorption. The extent of adsorption increases with increasing surface area. Finely divided metals and porous substances, for example, have a large surface area. As a result, they are regarded as good adsorbents. It also depends on the adsorbate’s composition.
2. Chemical adsorption: Chemical adsorption is another name for chemisorption. Adsorption occurs in adsorbed substances held together by chemical bonds in chemisorption. Chemisorption has a high specificity, which means that it occurs only when there is a chemical bonding between the adsorbent and the adsorbate. Chemisorption is irreversible, and it prefers high pressure. Chemisorption has a high enthalpy of adsorption due to chemical bonding. At a higher temperature, the physisorption of a gas adsorbed at a lower temperature can be converted to chemisorption. Chemisorption is proportional to the surface area. Chemisorption increases as the surface area of the adsorbent increases. Adsorption of hydrogen, nitrogen and other gases on the surface of adsorbents such as ferrous catalysts at high temperatures is an example of chemisorption.

Mechanism of Adsorption

It is an exothermic process, which means that energy is released during it. Enthalpy is the amount of heat that is released when one mole of adsorbate is adsorbed on an adsorbent. 

The enthalpy change is denoted as negative. The reason for this is that when adsorbate molecules are adsorbed on the surface, their freedom of movement is restricted, resulting in a decrease in entropy. Adsorption occurs spontaneously at constant temperature and pressure.

Examples of Adsorption

Adsorption is, at its core, a surface phenomenon. Because solids, particularly finely divided metals, have a large surface area, charcoal, silica gel, alumina gel, clay, colloids, metals in a finely divided state, and so on, act as good adsorbents. Here are a few examples:

• When a gas such as O₂, H₂, CO, Cl₂, NH₃, or SO₂ is placed in a closed vessel containing powdered charcoal, the pressure in the enclosed vessel decreases. Gas molecules congregate at the surface of the charcoal, implying that gases are adsorbed there.
• When animal charcoal is added to a solution of an organic dye, say methylene blue, and the solution is thoroughly shaken, the filtrate turns colourless. As a result, the dye molecules accumulate on the surface of the charcoal, i.e. are adsorbed.
• When an aqueous solution of raw sugar is passed over beds of animal charcoal, the colouring substances are adsorbed by the charcoal and the solution becomes colourless.
• As water molecules are adsorbed on the surface of silica gel, the air becomes dry in its presence.

What is Absorption?

In absorption, molecules are absorbed by the length rather than by the air. Adsorption is based on the surface where an adsorbate film forms, whereas absorption includes the entire volume of the absorbing agent. Absorption is the process by which a substance captures and transforms energy. The absorbent distributes the material it captures throughout the entire structure, whereas the adsorbent only distributes it on the surface. When atoms pass through or enter a bulky material, this is referred to as absorption. The molecules are completely dissolved or diffused in the absorbent during absorption, forming a solution. Once dissolved, the molecules are difficult to separate from the absorbent.

Absorption is a physical or chemical effect or mechanism that occurs when electrons, molecules, or ions join a bulk phase – a solid or liquid substance.

Absorption chillers for space cooling applications, ice production, cold storage, and turbine inlet cooling are common commercial applications of the absorption cycle. Absorption is a very good choice for consumers due to its high efficiency, environmentally friendly refrigerants, clean-burning fuels, and few moving parts that require maintenance. The absorption of a gas by a liquid is used in the hydrogenation of oils and the carbonation of beverages.

Types of Absorption:

• Physical absorption: A non-reactive process, such as when oxygen in the air dissolves in water. The process is determined by the liquid and gas, as well as physical properties such as solubility, temperature, and pressure.
• Chemical absorption: Chemical absorption is a type of absorption in which chemicals in one state are absorbed by chemicals in another state (e.g. gases absorbed by a liquid or solid).

Some basic terms commonly used in absorption are:

• Absorbate – The substance which gets absorbed is called absorbate
• Absorbent –  The substance which absorbs is called the absorbent.

When we use a paper towel to clean up split water, the paper absorbs the water, so the paper is the absorbent and water is the absorbate.

Difference between Adsorption and Absorption

Absorption	Adsorption
It is the assimilation of the molecular system throughout the bulk of the solid or liquid medium.	It is the accumulation of molecular species at the bottom instead of the liquid or solid.
It is a bulk phenomenon.	It is a surface phenomenon.
It is an endothermic process	It is an exothermic process
It is unaffected by temperature.	It is influenced by low temperature
It occurs at a uniform rate.	It increases steadily and reaches equilibrium.
It is constant throughout the medium.	The Concentration at the bottom of the adsorbent is different from that in bulk.
Because of the availability of space and the nature of the particle, substances are absorbed into an absorbent.	Substances are adsorbed onto the surface of an adsorbent because the adsorbent contains vacant spaces that encourage particle adhesion to the spaces.
The absorbed materials remain in the absorbent without interacting chemically with it.	The adsorbed materials are held to the adsorbent by Van der Wall’s forces or covalent bonds.
Based on their chemical interactions with the phases, absorbing materials can be separated into different phases.	Adsorbed materials can be separated by passing a new substance through the adsorbent’s surface and replacing the previously adsorbed material.
Absorption is used by a variety of living and non-living systems. Living systems, such as unicellular organisms, rely on the absorption phenomenon to obtain nutrients and water. Absorption is used for cold storage in non-living systems such as refrigerators.	Adsorption is used in a variety of living and non-living systems. Adsorption is a phenomenon that living systems, such as viruses, use to attach to bacteria or other organisms. Adsorption chromatography, for example, uses the principle of adsorption to separate mixtures.

Sample Questions 

Question 1: What is desorption?

Answer:

Desorption is the process by which the adsorbed substance is removed from the adsorbent’s surface.

Question 2: What is sorption processes?

Answer:

Sorption is a physical and chemical process that occurs when a substance (typically a gas or liquid) accumulates within another phase or on the phase boundary of two phases. It is the opposite of desorption.

Question 3: Differentiate between adsorption and absorption?

Answer:

Adsorption compounds cling to the molecule’s surface, whereas absorption substances enter the liquid or solid’s bulk phase.

Question 4: What are the different types of absorption?

Answer:

The two types of absorption processes are physical absorption and chemical absorption, depending on whether a chemical reaction occurs between the solute and the solvent.

Question 5: Among methane and sulfur dioxide gases, which gas adsorbs more on charcoal and why?

Answer:

As the critical temperature of sulfur dioxide is higher than that of methane, it adsorbs more.