Chemical Absorption Explained in Simple Terms

Absorption is a physical or chemical process in which one substance is incorporated into another. When people consider the term absorption, they typically think about the mass transfer of liquid particles into a solid – like a dry paper towel soaking up spilled water. But there are different types of absorption processes, including physical and chemical absorption, which can involve many substances, like gases, biological particles, chemicals, dissolved solids and more.

Here’s everything you need to know about chemical absorption, explained in simple terms.

What is absorption?

Absorption is what we call the process when one material (the absorbate) is dissolved, retained, or absorbed completely into another (the absorbent). In absorption, atoms, molecules or ions enter the bulk or volume phase of another substance, meaning a solid or liquid material. After a substance is dissolved or absorbed, molecules cannot be easily separated from the absorbent.

In pharmacology, absorption is studied as part of drug development and during clinical trials. In this context, absorption is the process by which a drug moves from the delivery site into the bloodstream. The drug’s chemical composition, formulation, and route of administration determine how that drug is ultimately absorbed. However, no matter the route of administration, all drugs must be in a solution to be absorbed. Drugs in solid forms (i.e. tablets or capsules) must be able to disintegrate and separate. Understanding the crucial role absorption plays in drug development and pharmacology could potentially help drug discovery professionals find more successful molecules earlier in the drug discovery process – saving them both time and resources. 

Absorption also plays a major role in digestion, where food is broken down into small molecules so valuable nutrients may be absorbed across the epithelial lining of the digestive tract, and used by the body for energy, growth, and repair.

What are the different types of absorption?

There are different types of absorption, physical absorption and chemical absorption. Chemical absorption is reactive, whereas physical absorption is non-reactive.

  • Chemical absorption - In chemical absorption, sometimes also called reactive absorption, there is a chemical reaction between absorbed and absorbing substances (i.e. a gas absorbed by a solid). Chemical absorption leads to the formation of chemical bonds (i.e. covalent, hydrogen, ionic, van-der-Waals).

  • Physical absorption - In physical absorption, no reaction occurs during the absorption process (i.e. ammonia dissolving into water). The process of physical absorption depends on a number of different variables and physical properties like solubility, temperature and pressure conditions.  

It’s possible for chemical absorption to sometimes combine with physical absorption. In this type of absorption, the absorbed and absorbing substances contain multiple elements which interact with one another.

Is solubility important in absorption?

Solubility describes the extent to which a substance dissolves in a solvent to form a solution. Solubility can be complete (of one fluid only - liquid or gas) or partial (i.e. oil and water dissolve only slightly). 

Absorption can sometimes depend on differences in solubility, for example the solubility of solids in liquids tends to increase with temperature, while the solubility of gases tends to decrease with temperature.

What is the difference between dissolution and absorption?

In absorption, one material (the absorbate) is retained by another (the absorbent).

In dissolution, two phases, or two homogeneous quantities of matter, are mixed to form a single, new uniform phase (the solution).

What are some examples of real world chemical absorption applications?

Chemical absorption has many different applications and, therefore, broad relevance across industries and sectors. 

Some common commercial uses of the absorption cycle include any kind of cold storage – like space cooling applications, ice production, refrigerants, or other clean-burning fuels. As another example, the process of gas absorption by a liquid can be observed in food science, where the hydrogenation of oils and carbonation of beverages are commonly used processes.

In industrial processes, one common use of reactive absorption involves the separation and/or purification of gas mixtures. For example, in ammonia production, hydrogen sulfide gas from feedstock is chemically absorbed. By this method, hydrogen sulfide comes into contact with solid zinc oxide (ZnO) in the ammonia production plant, which then forms solid zinc sulfide.

Chemical and physical absorption can be reversible or irreversible, depending on the substances in question. For example, trace amounts of oxygen in water can be reversed by heating the water. However, chemical absorption of hydrogen sulfide by zinc oxide is irreversible.

What is the difference between absorption and adsorption?

While substances enter the bulk phase of a liquid or solid during absorption, in adsorption, particles (i.e. atoms, ions or molecules) from a substance (the adsorbate) cling or adhere only to the surface of the molecule, whether liquid or solid (the adsorbent). This sticking may be permanent, or it may sometimes be reversed, either chemically or through heating the surface. Adsorption is often referred to as a surface phenomenon.

In some pharmaceutical industry applications, adsorption is used as a way to prolong drug release from specific formulations, and to achieve steady-state concentration. Adsorbent drugs (i.e. activated carbons) may also be used to trap and remove toxins.

Is absorption endothermic or exothermic?

Absorption is typically an endothermic process, which means energy is absorbed from the environment into the reaction, so the process itself either requires the absorption of heat, or is otherwise accompanied by the absorption of heat.

Adsorption is typically an exothermic process, which means energy is released from the system into the environment. 

What is the rate of absorption vs. adsorption?

Rate of absorption is the speed at which an absorbate substance moves into the bulk of an absorbent compound with the formation of chemical bonds or physical interactions. The rate of absorption usually depends on several variables – concentration and exposed surface area. Rate of absorption is not impacted by temperature. 

By contrast, surface area and temperature are the main influencing factors in the rate of adsorption. Adsorption is actually impacted by temperature, and low temperatures are favorable as particles with less thermal energy also have less kinetic energy, meaning they can better adhere to surfaces.

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