Absorption (chemistry)

Absorption is a fundamental process in chemistry and physics. It occurs when atoms, molecules, or ions enter the bulk phase of a material. This bulk phase can be a gas, a liquid, or a solid. It is important to distinguish absorption from a similar process called adsorption. In adsorption, particles only stick to the surface of a material. In absorption, the particles are taken up into the entire volume of the substance.

This distinction is vital for understanding how materials interact with their environment. Absorption can even involve the capture and transformation of energy.

There are two primary ways that absorption occurs: physical and chemical. Physical absorption is a non-reactive process. In this type, a substance moves from one phase to another without a chemical reaction. For example, scientists can perform liquid-liquid extraction to move a solute between liquids. Noble gases and osmium tetroxide are examples of substances that can be extracted this way. Chemical absorption is also known as reactive absorption. This occurs when a chemical reaction happens between the absorbing substance and the substance being absorbed.

Reactive absorption depends on the concentration of the reactants and the stoichiometry of the reaction.

When absorption is a purely physical process, it often follows the Nernst distribution law. This law describes the ratio of concentrations of a solute in two different bulk phases. When these phases are in contact and at equilibrium, this ratio remains constant. This constant value is called the partition coefficient, noted as KN. The value of this coefficient depends on the temperature of the system. This law is valid if the concentrations are not too high. It also requires that the species does not change its form in either phase.

If a molecule undergoes association or dissociation, the law still describes the equilibrium for that specific form.

In industrial settings, chemical absorption is often used to replace physical processes. A common example is the absorption of carbon dioxide by sodium hydroxide. Because this is an acid-base process, it does not follow the Nernst partition law. To manage these reactions, engineers use specialized equipment. These units are often called plate columns or packed columns.

These columns allow for different types of phase flows and interactions. In the case of gas absorption, scientists can calculate concentration using the Ideal gas law. They might also use partial pressures to find these values.

Many solids exhibit a strong ability to absorb water. These are known as hydrophilic solids. Many biological materials are hydrophilic. This happens because of polar interactions between the water and the molecules of the solid. These interactions favor the partition of water into the solid. Because of this, these materials can absorb significant amounts of water vapor. This can happen even when the humidity in the air is relatively low.

We can observe the effects of absorption in the textile industry. Most fibers are hydrophilic materials. Even when a fiber feels dry to the touch, it usually contains some water. This phenomenon is known as moisture regain. Moisture regain is the proportion of a material's weight that consists of water. Scientists can measure this by using an oven to heat the fiber. The heat drives the water off, which causes a measurable decrease in weight.

If the fiber is returned to a normal atmosphere, it will gradually regain that weight.

Understanding absorption is essential for many scientific fields. It connects to concepts in physical chemistry, gas technologies, and the study of gases. The term "sorption" is a broader term used to cover absorption, adsorption, and ion exchange. By studying how substances enter the bulk phase of others, we can better understand how energy and matter move through our world.