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Molecular dance: How does adsorption happen quietly in nature?
Adsorption is the process by which atoms, ions or molecules of a substance in a gas, liquid or dissolved solid adhere to a surface. This process forms a thin film called adsorbate on the surface of the adsorbent. Adsorption is different from absorption, in which a liquid (absorbent) is dissolved or penetrated into a liquid or solid (absorbent). Adsorption is a surface phenomenon and is clearly distinguished from absorption.
The operation of many natural and man-made systems depends on the subtle effects of adsorption phenomena, including the adsorption of pollutants from the air and the purification of water.
The adsorption process can be divided into physical adsorption (physisorption) and chemical adsorption (chemisorption). Physical adsorption is mainly caused by weak van der Waals forces, while chemical adsorption involves strong covalent bonding. This process can also occur due to electrostatic attraction. The nature of the adsorption can affect the structure of the adsorbate; for example, physical adsorption of a polymer from solution may result in a squashed structure on the surface. This process not only exists in nature, but is also widely used in industry, such as hybrid catalysts, activated carbon, adsorption coolers, and water purification.
In the pharmaceutical industry, adsorption is also used to prolong the exposure of the nervous system to specific drugs, which is a lesser-known application.
Adsorption isotherm
The adsorption of gases and solutes is usually described by isotherms, that is, the relationship between the amount of adsorbate on the adsorbent and its pressure (for gases) or concentration (for liquid solutes) at constant temperature. Fifteen different isotherm models have been developed so far, one of the earliest of which was proposed by Freundlich in 1906.
Freundlich model
The Freundlich isotherm is based on an empirical formula involving the mass of the adsorbent, the mass of the adsorbate and its pressure, which succinctly describes the changes in the adsorption process. Although this formula cannot completely accurately describe the isotherm under certain conditions, it marks an important step forward in adsorption research.
Langmore Model
By 1918, Langmuir had developed a semi-empirical isotherm model based on statistical thermodynamics, which had a wide range of applications. The key assumption of this model is that all adsorption sites are equivalent and that one site can accommodate only one molecule. Although these assumptions are not necessarily true in reality, the Langmuir model remains the preferred choice for most adsorption models.
The Langmuir adsorption mechanism shows that gas molecules can form equilibrium with adsorption sites and adsorb and desorb at specific rate constants.
BET Model
Over time, scientists have discovered that adsorbed molecules are often not in a single layer, but can form multiple layers. In 1938, Bruner, Emmett and Taylor introduced the BET theory to explain this phenomenon. This theory modifies Langmuir's mechanism, can analyze the process of multilayer adsorption, and provide a more accurate mathematical model.
Thermodynamic properties of adsorption
The adsorption constant is an equilibrium constant and therefore follows the van't Hoff equation. This equation shows the relationship between the heat of adsorption (ΔH) and the adsorption equilibrium constant (K), thus revealing the thermodynamic characteristics of the adsorption process. In this way, scientists were able to gain a deeper understanding of the mechanisms of adsorption and its effects on the system.
As we delve deeper into the phenomenon of adsorption, we begin to realize its importance in a wide range of natural and artificial processes.
Adsorption plays an indispensable role in ecosystems, industrial processes and daily life. When we think about the technological applications of this phenomenon and its impact on the environment, we can't help but ask: How can adsorption create more possibilities for us between developing new technologies and maintaining ecological balance?
