Reversible and irreversible adsorption
The adsorption is reversible, if the adsorbent can be easily removed from the surface of the adsorbent by physical methods. It is called irreversible adsorption, if the adsorbate can not be removed from the surface of the adsorbent.
A gas adsorbed on a solid surface can be completely removed in vacuum. It is, therefore, reversible adsorption. Examples of irreversible adsorption are adsorption of oxygen on tungsten adsorbate and adsorption of CO on tungsten surface
Adsorbent, Adsorbate and Interface
Physisorption and Chemosorption:
Only van der Waals force are present between adsorbate and surface of adsorbent
Chemical bonds are formed between adsorbate and surface of adsorbent
Low enthalpy of adsorption ie, in the order of 20 kjmol-1.
High enthalpy of adsorption i.e, order of 200 kjmol-1.
It is usually takes place at low temperature and does not require any activation energy.
It takes place at high temperature and require activation energy..
Multi molecular layer of adsorbate are formed on the surface
Only monomolecular layers are formed.
If A, B & AB represents the adsorbed, absorbent and the absorbed – adsorbent complex then,
A + B ↔AB
ka = Equilibrium constant for adsorption = [AB]/[A][B]
kd = Equilibrium constant for desorption = [A][B]/[AB]
K = Distribution coefficient = ka/kb
Θ = Fraction of the surface of adsorbent available for adsorption.
P = pressure
Θ= KP/(1+KP) (Langmular Equation)
x= Mass of the gas adsorbed
m = Mass of absorbent
p = Pressure
K and n = constants
x/m =k.p(1/n) [n >1]
log x/m = log k + 1/n log P
Factors Affecting Adsorption:
The phase which is dispersed or scattered through the dispersion medium is called Dispersed phase or discontinuous phase.
The phase in which the scattering is done is called the dispersion medium or continuous medium.
Gold sol, Mud, Fe(OH)3 sol,
Gems, Ruby glass, Minerals
Smoke (Carbon in air) Volcanic dust
Curd, Cheese, Jellies
Milk, water in benzene, cream
Clouds, fog (water in air) mist
Froth on beer , whipped cream
Lyophobic and Lyophilic Colloids:
Those substance whose colloidal solution cannot be prepared by bringing them in contact with a solvent are called Lyophobic (disliking, fearing or hating a liquid). On the other hand those substances whose colloidal solutions can be prepared by bringing them in contact with a liquid solvent are called lyophilic colloids (loving a solvent).
Difference between True Solutions, Suspension & Colloids
Particle size less than 1nm
Particle size more than 1000nm
Particle size between 1-1000nm
Don’t settle down
Settle down under the influence of gravity
Don’t settle down
Complements cannot be separated out by filtration
Can be filtered
Can be filtered using special filter papers
Don’t show tyndrall effect
Show tyndrall effect
Show tyndrall effect
Methods of preparation of colloids
An electric arc is struck between two metallic electrodes immersed in dispersion medium. The arc produced vapourises the metal which on further condensation produces particles of colloidal size.
Hardy Schulze Rule:
substances which gets preferentially adsorbed at the air – water and solid – water interfaces forming an oriented monolayer where the hydrophilic groups point towards the aqueous phase and the hydrocarbon chain point towards the air or towards the oil phase.
Aggregates formed when the surfactant molecules in the water air interface become so packed in the monolayer that no more molecules can be accumulated with ease they accumulate in the bulk of the solution.
At a given temperature and concentration, a micelle of a surfactant of monodispersed i.e., they contain same number of molecules usually between 25 to 100.
Critical concentration for micelle formation decreases as the molecular weight of hydrocarbon chain of surfactant grows because in this case true solubility diminishes and the tendency of surfactant molecules to associate increases.