Adsorption is the adhesion of a chemical species onto the surface of particles

Adsorption is the adhesion of a chemical species onto the surface of particles. The gas or liquid particles that bind to the solid or liquid surface, termed as adsorbent. It is different from absorption, in which a substance diffuses into a liquid or slid to form a solution. In most natural physical, biological and chemical systems the adsorption is operative and is used widely in industrial applications such as activated charcoal, synthetic resin and water purification.
The type of interaction between the absorbed molecule and the solid surface varies over a wide range from weak non-polar van der Waals forces to strong covalent bond. The adsorption can be classified in two types, physisorption and chemisorption.
Physisoption is a type of adsorption in which the adsorbate adheres to the surface only through van de Waals interaction. The adsorption takes place in any system at a low or moderated temperature and is associated with low heats of adsorption.
Chemisorption is the type of adsorption where by a molecule adheres to a surface through the formation of a chemical bond. It is generally defined by high heat of adsorption. The chemisorption depends on the chemical properties of both the surface molecule and the adsorbed molecules and is highly specific in nature.
The Adsorption isotherm is the amount absorbed per gram of solid depends on the specific are of the solid, the equilibrium solute concentration in the solution, the temperature, and the nature of the molecules involved. From measurements at constant temperature, one can obtain a plot of N, the number of moles absorbed per gram of solid, versus c, the equilibrium concentration.
Often it possible to represent experimental results over a limited range by an empirical isotherm suggested by freundlich: N = KCa, where k and a are constants which have no physical significant.
The theory derived by Langmuir can be applied, which is restricted to cases where only one layer of molecules can be absorbed at the surface. The form appropriate to adsorption from solution: 2= kc/((1+kc)) , where 2 is the friction of the solid surface covered by adsorbed molecules k is a constant at constant temperature.
2= N/N_m , where N is the number of moles adsorbed per gram of solid at an equilibrium solute concentration c and Nm is the number of moles per gram required to form monolayer.
c/N = c/Nm +1/kNm
The plot of c/N versus c form a straight line with slope 1/Nm, if the Langmuir isotherm is an adequate description of the adsorption process.
The known area occupied by an absorbed molecule on the surface A = NmN0 ?× 10-20
Where A specific area in m2g
N0 Avogadros number
? is given in ?2


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