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The secret of adsorption: Why do gases and liquids form thin films on surfaces?
In our daily lives, the ubiquitous membrane makes us curious about its existence. These films are primarily made of atoms, ions or molecules from gases, liquids or dissolved solids, and the process that creates this is called adsorption. By exploring the mechanisms of adsorption, we not only understand this physical and chemical process but also reveal its importance in various industrial and natural systems.
Adsorption is a surface phenomenon. Simply put, it is the process that occurs when a substance attaches to the surface of another substance.
In contrast to absorption (which is the absorption of a liquid or solid substance into its entire volume), adsorption is simply the formation of a thin film of a substance on a surface. The basic concept of this process is that surface energy makes it easier for atoms or molecules to attach to the surface of a material. When atoms on a surface bond with those inside, their bonding requirements are not fully met, allowing them to attract atoms or molecules from a gas or solution.
The nature of adsorption depends on the specific interactions between the substances involved. Generally speaking, adsorption processes are classified as physical adsorption (with weak van der Waals forces) and chemisorption (with covalent bonding properties). In some cases, electrostatic attraction also affects the type and intensity of adsorption.
During the adsorption process, the structure of the attached material will be affected. For example, physical adsorption of polymers in solution causes them to form flat structures on surfaces.
The adsorption phenomenon not only exists in nature, but is also widely used in various industrial processes, from heterogeneous catalysts and activated carbon to water purification. These applications demonstrate the relative importance of adsorption in daily life, for example, in air conditioning systems where the adsorption phenomenon is exploited to capture and utilize waste heat to provide cooling water.
Exploration of adsorption isotherms
In order to describe the adsorption process of gases and solutes, scientists have developed a series of models that represent the amount of adsorbed material on the adsorbent through isotherms. These models help understand how adsorption occurs at different pressures or concentrations, and 15 different isotherm models have been proposed to date.
Frendlich and Langmuir model
The early mathematical models were mainly proposed by Freundlich and Langmuir. Langmuir's isotherm model is based on statistical thermodynamics and takes into account the homogeneity of the adsorption sites and the degree of adsorption. Although this model is widely used in practical applications, many assumptions do not strictly hold in actual situations, so further exploration and adjustment are needed.
The basic assumptions of the Langmuir model include: all adsorption sites are equivalent, each site can only accommodate one molecule, and only one layer of molecules is formed at maximum adsorption.
However, in some cases, multiple layers are formed in succession, and the Langmuir model is no longer applicable. As a result, the BET theory was born, taking multi-layer adsorption into consideration to better describe the adsorption process.
Exploration of adsorption thermodynamics
The thermodynamics of the adsorption process are relatively complex and usually follow the van't Hoff equation. By analyzing the thermodynamic constants of the adsorption process, more important information about the adsorption mechanism can be obtained and used to predict behavior under different conditions.
Future directions of discussion
Although there have been many advances in the understanding of adsorption phenomena, specific decisions in practical applications require further research and exploration. In the future, we should focus on the development of various adsorption materials to improve efficiency and effectiveness in applications such as water treatment and gas filtration.
In-depth study of adsorption phenomena will not only deepen our understanding of basic physical and chemical processes, but may also bring new opportunities for future scientific and technological development. How do you think the role of these films in our lives will change over time?
