Desorption :
Desorption incoperates both adsorption and absorption phenomenon.Concentration of molecules is not always same on the surface or in the bulk , but always wants to attain equilibrium .When this equilibrium is not attained or faces some disturbance due to some external factors (T,P) then desorption occurs.
As it is understood that molecules move from higher concentration to lower concentration,If the molecule concentration is lower in bulk ,then the molecules( adsorb or absorb) move into bulk in order to saturate.
When the surface molecules leaves they may converted into gas or liquid .It arises due to difference in bulk and surface concentration .If the binding between adsorbate and adsorbent is weak, subtances leave the surface in adsorption and bulk in absorption ..e.g when you peel out the cocumber and then smear it with salt ,water comes out of it these water molecules which were absorbed before smearing salt and now they are desorped.
The surface molecules linked to each other, when heated molecules transfer heat fom one molecule to other,it will leads to desorption .Hence temprature when raised then thermal desorption occur.The reason behind is equilibrium is disturbed as some molecules evaporated due to heating (changed their phase) .Leaving behind a system which will incoperate molecules either absorbed or adsorbed to attain stable state.
Desorption is also the ability of a substance to move freely in the bulk.More it is mobile less its chances to adsorb.Hence conduction of heat is easier.
The adsorption is indefinate as the molecules adhere to the surface as long as the temprature remains low.Rate of desorption is given by
R=rNx
x=kinetic order
N=adsorb material concentration
r =rate constant for desorption
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