Revision Notes on Surface Chemistry:

Adsorption

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

• The substances upon whose surface the change of concentration occurs, is called absorbent.

• The substance taken up on the surface is call adsorbate.

• The common surface between the two phases where the adsorbed molecules concentrate is called the interface.

Physisorption and Chemosorption:

Langmular Isotherm:

If A, B & AB represents the adsorbed, absorbent and the absorbed – adsorbent complex then,

A + B ↔AB

k_a = Equilibrium constant for adsorption = [AB]/[A][B]

k_d = Equilibrium constant for desorption = [A][B]/[AB]

K = Distribution coefficient = k_a/k_b

Θ = Fraction of the surface of adsorbent available for adsorption.

P = pressure

So,

Θ= KP/(1+KP)     (Langmular Equation)

Freundlich Isotherm:

x= Mass of the gas adsorbed

m = Mass of absorbent

p = Pressure

K and n = constants 

x/m =k.p^(1/n)                 [n >1]

or

log x/m = log k + 1/n log P

Factors Affecting Adsorption:

• Temperature: An increase of temperature leads to a decrease in amount adsorbed and vice – versa.

• Pressure or concentration: It has been found that in most cases,  the adsorption is reversible and an increased pressure of a gases vapour or an increase in concentration of a solute causes increased adsorption.

• Nature of Adsorbate and Adsorbent: The amount of the gas adsorbed depends upon the nature of adsorbent and the gas (adsorbate), which is to be adsorbed. It has been found that easily liquifiable gases such as NH₃, HCl, Cl₂ , SO₂ CO₂  etc. are more readily adsorbed than so the called permanent gases such as O₂,N₂, H₂ etc. This is because that molecules of the former type of gases have greater Vander waal’s or molecular force of attraction.

Colloids

Dispersed Phase:

The phase which is dispersed or scattered through the dispersion  medium is called Dispersed phase or discontinuous phase.

Dispersion Medium:

The phase in which the scattering is done is called the dispersion medium or continuous medium.

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).

Emulsions:

• Emulsion of oil in water: Those emulsions in which the dispersed phase is oil and water is the dispersion medium. These emulsion are generally represented as O in W emulsions. Examples are milk, vanishing cream etc.

• Emulsions of water in oil: Those emulsion in which the dispersed phase is water while oil is the dispersion medium. These emulsion are generally represented as W in O emulsions. Examples are butter, ice cream etc.

Difference between True Solutions, Suspension & Colloids

Methods of preparation of colloids

Chemical Methods: 

Bredig's method:

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.

• Peptization: 

Process of converting a precipitate into colloidal sol by shaking it with electrolyte

in dispersion medium.

Hardy Schulze Rule:

• Ion carrying charge opposite to the colloidal particle has capacity to coagulate the colloid.

• Greater the valency of ion, greater will be  the coagulating power.

• Gold Number:  The minimum amount of lyophilic colloid in milligrams which can prevent the coagulation of 10 ml gold sol against 1 ml of 10% NaCl solution. 

Surfactants

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.

• Anionic surfactants : NSodium salts of higher fatty acids such as sodium palmitate (C₁₅H₃₁COONa), sodium stereate (C₁₇H₃₅COONa) and sodium Oleate (C₁₇H₃₃COONa).

• Catiuonic Surfactants: Those which dissociates in water to yield positively charged ions  examples: C₁₈H₃₇ , C₁₆H₃₃(CH₃)₃  etc.

• Non ionogenic: Those whose molecules cannot undergo dissociation when an alcohol having a higher molecular weight reacts with several molecules of ethylene oxide, a non – ionogenic surfactant is produced.

Micelle: 

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.