Surface chemistry is that branch of chemistry which deals with study of the phenomena occuring at the surface
or interface, i.e. at the boundary separating two bulk phases. In this chapter our main emphasis will be on three
important topics related to surface chemistry, viz., adsorption, colloids and emulsions.
When the particles of the adsorbate are held to the surface of the adsorbent by the physical forces such as van
der Waal’s forces, the adsorption is called physical adsorption or vanderwaals adsorption
The extent of adsorption of a gas on a solid surface is affected by the following factors
The variation of extent of adsorption (x/m) with pressure (P) was given mathematically by Freundlich
It is based on the assumption (a) that every adsorption site is equivalent and (b) that the ability of a particle to
bind there is independent of whether or not nearby sites are occupied
The process of adsorption can take place from solutions also. It is observed that solid adsorbents
adsorb certain solutes from solution in preference to other solutes and solvents
In gas masks : Activated charcoal is generally used in gas masks to adsorb poisonous and toxic gases from air.
These masks are commonly used by the miners because there are poisonous gases like CO, CH4 etc. in the
atmosphere in coal mines
Positive Catalysis : A substance which increase the rate of chemical reaction is called positive catalyst
and this process called positive catalysis
Substance which themselves are not catalyst but its presence can increase
the catalytic activity of catalyst
A Catalyst remains unchanged in mass and chemical compositions at the end of reactions. However its
physical state can be change
Adsorption Theory of Heterogeneous Catalyst
Some Industrial Catalytic reactions
Colloid State : A substance is said to be in colloidal state when the size of the particle of
disperse phase is greater than particle of true solution and less than that of suspension solution
particle
A colloid is a heterogeneous system in which one substance is dispersed (dispersed
phase) as very fine particles in another substance called dispersion medium. The solution
and colloid essentially differ from one another by particle size
On the bases of physical state of D.P. and D.M. colloidal solution may be divided into eight system.
DISTINCTION BETWEEN LYOPHILIC AND LYOPHOBIC
COLLOIDS
Multimolecular, macromolecular and associated colloids
There are some substances which at low concentrations behave as normal strong electrolytes but at higher
concentrations exhibit colloidal behaviour due to the formation of aggregated particles. The aggregated particles
thus formed are called micelles
It has been mentioned earlier that a micelle consists of a hydrophobic hydrocarbon like central core. The cleansing
action of soap is due to these micelles, because oil and grease can be solubilised in their hydrocarbon, like
centres which are not otherwise soluble in water. This is shown diagrammatically in Fig. The dirt goes out along
with the soap micelles
Preparation of lyophobic colloidal sols :
Condensation methods
In these methods large particles of the substance are broken into particles of colloidal dimensions in the presence
of dispersion medium. These are stabilized by adding some suitable stabilizer. Some of the
methods employed are given below
The colloidal sols obtained by various methods are impure and contain impurities of electrolytes and other
soluble substances. These impurities may destabilise the sol. Hence, they have to be removed. A very important
method of removal of soluble impurities from sols by a semipermeable membrane is known as dialysis
Colligative Properties :
Colloidal sols show the colligative properties viz. relative lowering of vapour pressure, elevation in boiling point,
depression in freezing point and osmotic pressure. However, due to high average molecular masses of colloidal
particles, mole fraction of the dispersed phase is very low. Hence, the values of the colligative properties observed
experimentally are very small. Only osmotic pressure measurements are used in determining the molecular
mass of polymers
Brownian movement: Robert Brown, a botanist, discovered in 1827 that pollen grains placed in water do not
remain at rest but move about continuously and randomly. Later on, this phenomenon was observed in case of
colloidal particles when they were seen under an ultramicroscope
The particles of the colloids are electrically charged and carry positive or negative charge. The dispersion medium
has an equal and opposite charge making the system neutral as a whole. Due to similar nature of the charge
carried by the particles, they repel each other and do not combine to form bigger particles. That is why, a sol is
stable and particles do not settle down.
Electrosmosis
Sedimentation potential or Dorn potential
Isoelectric point
Streaming potential
Colloidal particles are either positively charged or negatively charged. This charge is due to preferential adsorption
of either positive or negative ions on their surface
Electric Double Layer Theory or Helm-holtz Electric double layer
The presence of small amounts of appropriate electrolytes is necessary for the stability of the colloids. However,
when an electrolyte is added in larger concentration; the particles of the sol take up the ions which are oppositely
charged and thus get neutralised
Nature of sols : The lyophobic colloid can easily coagulate because it is a less stable colloid, but lyophilic
colloids coagulate hardly by the addition of electrolyte due to protective layer of D.M. surrounding the colloidal
particle
Lyophilic colloidal sols are much more stable than lyophobic colloidal sols. This is due to the
extensive solvation of lyophilic colloidal sols, which forms a protective layer outside it and thus
prevents it from forming associated colloids
Zpsigmondy (1901) introduce a term called gold number it is defined as ‘’the minimum amount of the protective
colloid in milligrams which when added to 10 ml of a standard gold sol is just sufficient to prevent a colour change
from red to blue on the addition of 1 ml of 10% sodium chloride solution
Colloids including emulsions find a number of uses in our daily life and industry. Some of the uses are given
below.
The size and shape of the colloidal particles is determined with the help of an electron microscope which has much more
