1. Adsorption Fundamentals
– Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface.
– It is distinct from absorption and involves the transfer of the absorbate to the surface.
– Sorption encompasses adsorption and absorption, with desorption being the reverse process.
– Adsorption is influenced by surface energy and can be classified as physisorption or chemisorption.
– The IUPAC definition states an increase in the concentration of a dissolved substance at the interface of condensed and liquid phases.
– Adsorption bonding can be due to weak van der Waals forces, covalent bonding, or electrostatic attraction.
– It is widely used in industrial applications such as heterogeneous catalysts, activated charcoal, and water purification.
2. Adsorption Isotherms
– Isotherms describe adsorption of gases and solutes based on pressure or concentration at constant temperature.
– There are 15 different isotherm models developed to describe adsorption behavior.
– Isotherms change with pressure, requiring higher pressures to saturate the surface.
– The Langmuir isotherm is a common model based on statistical thermodynamics with applications in surface kinetics and thermodynamics.
3. Adsorption Theories
– Langmuir derived the first scientifically based adsorption isotherm in 1918.
– The Langmuir isotherm assumes all adsorption sites are equivalent and accommodate only one molecule each.
– BET theory, developed in 1938, accounts for multilayer adsorption and provides a more complex model than the Langmuir isotherm.
– Kisliuk’s precursor state theory models adsorption by considering the adsorption probability around existing gas molecules on a solid surface.
– Quantum Mechanical-Thermodynamic Modelling uses the Chi hypothesis and excess surface work theories to explain adsorption.
4. Adsorbents and Applications
– Adsorbents come in various forms like pellets, rods, and monoliths and must have high abrasion resistance and thermal stability.
– Different classes include oxygen-containing, carbon-based, and polymer-based compounds.
– Activated carbon, silica gel, and zeolites are common adsorbents used in various applications.
– Adsorption is crucial in water purification, carbon capture and storage, protein adsorption, and adsorption chillers.
5. Advanced Applications of Adsorption
– Adsorption solar heating and storage using synthetic zeolites like Linde 13X for thermal energy storage.
– Carbon capture and storage using zeolites and MOFs for carbon capture with less energy-intensive regeneration.
– Protein and surfactant adsorption important in biomaterials field.
– Adsorption chillers use adsorbents and refrigerants for cooling without a compressor.
– Polymer adsorption is crucial for non-stick coatings and surface modification in material science and engineering.
Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption, in which a fluid (the absorbate) is dissolved by or permeates a liquid or solid (the absorbent). While adsorption does often precede absorption, which involves the transfer of the absorbate into the volume of the absorbent material, alternatively, adsorption is distinctly a surface phenomenon, wherein the adsorbate does not penetrate through the material surface and into the bulk of the adsorbent. The term sorption encompasses both adsorption and absorption, and desorption is the reverse of sorption.
adsorption: An increase in the concentration of a dissolved substance at the interface of a condensed and a liquid phase due to the operation of surface forces. Adsorption can also occur at the interface of a condensed and a gaseous phase.
Like surface tension, adsorption is a consequence of surface energy. In a bulk material, all the bonding requirements (be they ionic, covalent or metallic) of the constituent atoms of the material are fulfilled by other atoms in the material. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates. The exact nature of the bonding depends on the details of the species involved, but the adsorption process is generally classified as physisorption (characteristic of weak van der Waals forces) or chemisorption (characteristic of covalent bonding). It may also occur due to electrostatic attraction. The nature of the adsorption can affect the structure of the adsorbed species. For example, polymer physisorption from solution can result in squashed structures on a surface.
Adsorption is present in many natural, physical, biological and chemical systems and is widely used in industrial applications such as heterogeneous catalysts, activated charcoal, capturing and using waste heat to provide cold water for air conditioning and other process requirements (adsorption chillers), synthetic resins, increasing storage capacity of carbide-derived carbons and water purification. Adsorption, ion exchange and chromatography are sorption processes in which certain adsorbates are selectively transferred from the fluid phase to the surface of insoluble, rigid particles suspended in a vessel or packed in a column. Pharmaceutical industry applications, which use adsorption as a means to prolong neurological exposure to specific drugs or parts thereof,[citation needed] are lesser known.
The word "adsorption" was coined in 1881 by German physicist Heinrich Kayser (1853–1940).
