Adsorption and Absorption

Absorption is the process in which a fluid is dissolved by a liquid or a solid (absorbent).

Adsorption is the process in which atoms, ions or molecules from a substance (it could be gas, liquid or dissolved solid) adhere to a surface of the adsorbent. Adsorption is a surface-based process where a film of adsorbate is created on the surface while absorption involves the entire volume of the absorbing substance.

Items	Absorption	Adsorption
Definition	Assimilation of molecular species throughout the bulk of the solid or liquid is termed as absorption.	Accumulation of the molecular species at the surface rather than in the bulk of the solid or liquid is termed as adsorption.
Phenomenon	It is a bulk phenomena	It is a surface phenomena.
Heat Exchange	Endothermic process	Exothermic process
Temperature	It is not affected by temperature	It is favored by low temperature
Rate of Reaction	It occurs at a uniform rate.	It steadily increases and reach to equilibrium
Concentration	It is same throughout the material.	Concentration on the surface of adsorbent is different from that in the bulk

Process

Gas-liquid absorption (a) and liquid-solid adsorption (b) mechanis.

Blue spheres are solute molecules

Adsorption and absorption are both sorption processes.

Absorption occurs when atoms pass through or enter a bulky material. During absorption, the molecules are entirely dissolved or diffused in the absorbent to form a solution. Once dissolved, the molecules cannot be separated easily from the absorbent.

Adsorption is generally classified into physisorption (weak van der Waal’s forces) and chemisorption. It may also occur due to electrostatic attraction. The molecules are held loosely on the surface of the adsorbent and can be easily removed.

 

 

Uses

Adsorption: Some of the industrial applications for adsorption are air-conditioning, adsorption chillers, synthetic resin and water purification. An adsorption chiller does not require moving parts and hence is quiet. In pharmaceutical industry applications, adsorption is used as a means to prolong neurological exposure to specific drugs or parts thereof. Adsorption of molecules onto polymer surfaces is used in various applications such as in the development of non-stick coatings and in various biomedical devices.

Absorption: The common commercial uses of absorption cycle are absorption chillers for space cooling applications, ice production, cold storage, turbine inlet cooling. High efficiency operation, environmentally friendly refrigerants, clean-burning fuels and few moving parts that require maintenance make absorption a very good choice for consumers. 

The process of gas absorption by a liquid is used in hydrogenation of oils and carbonation of beverages.

Video Link for comparison of adsorption and absorption http://v.youku.com/v_show/id_XNjIxNzAxOTc2.html

 

 

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Adsorption Isotherm

Adsorption isotherm is the relationship between the pressure and adsorption amount at a constant temperature. The horizontal axis is the relative pressure (P/P₀) which is the equilibrium pressure divided by the saturation pressure. The relative pressure can be 0 to 1 and P/P₀ =1.0 means that the condensation of adsorptive occurs in the sample cell. So an adsorption isotherm is the measurement of adsorptive density which becomes higher than the bulk (gas) phase density due to the interaction between the adsorptive and solid surface atoms below its condensation pressure. Adsorption amount in the vertical axis is commonly expressed as V/ml (STP)g^-1 which is expressed by the standard gas volume (at 0^oC and 1 atm).

The figure indicates the classification of adsorption isotherms defined by IUPAC. The type of adsorption isotherm is determined by the pore size and surface character of the material.

• i.    Microporous materials (e.g. Zeolite and Activated carbon)
• ii.   Non porous materials (e.g. Nonporous Alumina and Silica)
• iii.  Non porous materials and materials which have the weak interaction between the adsorbate and adsorbent (e.g. Graphite/water)
• iv.  Mesoporous materials (e.g. Mesoporous Alumina and Silica)
• v.   Porous materials and materials that have the weak interaction between the adsorbate and adsorbent (e.g. Activated carbon/water)
• vi.  Homogeneous surface materials (e.g. Graphite/Kr and NaCl/Kr)

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