2. INTRODUCTION
○ Surface chemistry deals with the phenomenon that occur at the surface/interface.
○ surface is basically the separating medium between two phases
○ Separation between two surfaces is denoted by putting a hyphen or a slash
between both the interfaces.
○ Eg: solid-liquid
There is usually no interface between two gases cause they are very miscible and
therefore get mixed with each other very quickly.
3. Adsorption
● A surface phenomenon
● It is the accumulation of molecular species at the surface rather than in the bulk of a
solid or liquid.
○ Eg: dirt particles on the hand.
■ Bulk here means inside the volume of the substance.
● Molecular species which GET ACCUMULATED on the surface → ADSORBATE
● Whichever substance adsorbs the molecule → ADSORBENT
○ Leaf: adsorbent ; water: adsorbate
Adsorption of hydrogen over Pt is called occlusion.
4. Examples of adsorption
1.charcoal in gas
● take a gas [h2,o2,co etc] in a container, this gas exerts pressure on the sides of the
container.
● Add charcoal powder to it, the pressure decreases. Why?
○ The powdered charcoal adsorbs the gas and now the gas out free is decreased.
2.organic dye and charcoal
● Methylene blue + animal charcoal → the blue colour of the solution disappears
○ The molecules of the dye that gives the color to methylene blue gets adsorbed to the surface of charcoal.
3.silica gel
● Silica gel adsorbs the water molecules around it and takes the moisture away from it
surroundings.
Therefore, solids hold liquid/gas molecules well.
6. Adsorption vs absorption
Adsorption
● The molecules only stay on the surface of the
adsorbent.
● Here the dyed water is just on the surface of
the plastic paper, it just got adsorbed and
didnt get coloured/dyed.
Absorption
● They get into the bulk
● Here the tissue absorbed the coloured water as the
tissue got dyed, meaning the dye got into the bulk of
the tissue
7. SORPTION
When both adsorption and absorption takes place simultaneously.
Eg: dipping a tissue in water → the water gets on the surface of the tissue as well as
into the bulk.
8. Adsorption mechanism
-the molecules in the bulk of the substance have 4 other molecules around each
molecule so,
○ There is a pull and a push from all the sides as there are neighbouring particles on all four sides.
○ So the forces applied get balanced out and the net force is 0.
● But the molecules on the surface have only 3 particles on their 3 sides as the top
part is vacant.
○ So when all the three other particles exert force on THE particle, there is an unbalanced force called
RESIDUAL ATTRACTIVE FORCES.
○ Now this residual force starts attracting adsobents to the surface.
9. ● Adsorption increases with increase of surface area at a given temperature and
pressure.
● Adsorption is an exothermic process[release of heat] but how?
○ The surface energy decreases as new particles keep filling the surface due to the residual forces,and
this decrease of surface energy is released in the form of heat.
○ Delta H is negative for exothermic process.[decrease in enthalpy]
○ When a gas is adsorbed the freedom of the movement of a molecule is restricted as they are now
bound to a surface therefore there is decrease in entropy .
○ Looking at gibbs free energy: for adsorption to be a spontaneous
process g has to be negative so here to make g negative we
should make sure the H is a very high negative value as s is
also -ve
10. Types of adsorption
● Adsorption takes place due to physical causes/contact →PHYSISORPTION
○ Accumulation of gas on the surface of a solid occurs on account of weak vander waal’s forces
○ Require low temperature
● Adsorption takes place due to formation of chemical bond → CHEMISORPTION
○ Accumulation of gas on the surface of a solid occurs on account of chemical bonds
○ For bonds to form, a high energy is needed therefore this takes place in the presence of high
temperature and is hence called ACTIVATED ADSORPTION.
● Sometimes physisorption at increased temperature may pass into chemisorption.
11. Features of physisorption
● Lack of specificity
○ The surface of the adsorbent does not show any preference while attracting cause van der waals
forces is what is attracting it and van der waals force by itself is UNIVERSAL.
● Nature of adsorbate
○ Rate of adsorption depends on the nature of the adsorbate. How?
■ Easily liquefiable gases are readily adsorbed as van der waals’ forces are stronger near critical
temperatures.
● Critical temperature → the temperature above which a gas cant be liquified.
● High critical temperature more easily liquefiable a gas is.
● Decreases with increase in temperature following the le-chatelier’s principle.
○ It also involves weak forces which decrease with increase in temperature
● Nature of adsorbent:
○ Same gas may be adsorbed to different extents on different adsorbent.
12. ● Surface area of adsorbent
○ More the surface area → more is the adsorption.
● Enthalpy of adsorption
○ Heat release is low. Why?
■ Physisorption takes place due to a weak force and this weak force only decreases the surface
energy very little.
● No activation energy is required.
● Happens in low temperature
13. Chemisorption- features
● High specificity
○ It is only possible if a chemical bond is possible between adsorbent and adsorbate.
● Irreversible in nature
○ A whole bond is being formed so it can't be reversed.
● Surface area
○ ↑ area of the adsorbent ↑ is the adsorption
● Enthalpy of adsorption
○ The enthalpy is high as there is a large decrease in the surface energy as it involves formation of
bond and a lot of energy is released.
● High activation energy is needed
● High temperature is needed [if the temperature is low the process is very slow]
14. isotherms
● Adsorption at a constant temperature
● Adsorption Isotherms are those curves that are used to express the variation in the
amount of gas absorbed by the adsorbent with pressure at constant temperature.
●
K and n are constants and p isi pressure
Check tb
15. Both the conditions are
supported by
experimental results. The
experimental isotherms
always seem to approach
saturation at high
pressure. This cannot be
explained by Freundlich
isotherm. Thus, it fails at
high pressure.
16. Adsorption in solution phase
● ↑in temperature → ↓ in adsorption
● ↑ in surface ↑ in adsorption
● Extent of adsorption depends on the conc of solute in solution
● Extent of adsorption depends on the nature of adsorbent and adsorbate
Check tb
17. colloids
Homogenous mixture: have a uniform composition throughout
Heterogenous mixture:have non uniform composition, you can differentiate its components
Colloids: contain particle size is between 1nm-1000nm
18. ● Colloid is a heterogeneous system in which one substance is DISPERSED PHASE [the
particle being dispersed] and the other being the medium it is getting dispersed into
called dispersion medium [getting dispersed into it]
● Eg: milk,whipped cream,smoke,dust,blood
19. Classification of colloids
1.based on physical state of dispersed medium and dispersed phase
Two gases never form
colloids as two gases form
a homogenous mixture
20.
21. 2.based on nature of interaction between dispersed phase and medium
Types:
1.lyophilic sols /reversible sols [solvent attracting] like the dispersed medium
2.lyophobic [solvent repelling]dislike the dispersed medium
If the liquid is water then its called hydrophilic and hydrophobic
–lyophilic means to love liquid. It is basically a solid in liquid mixture
some substances which can from colloids directly on mixing them with a suitable liquid(dispersion medium). These colloids are called lyophilic
colloids.
Eg: gum,gelatin,starch
–if separated forcefully but given a chance they come vack to each other very quickly.
They are quiet stable
Cannot be easily coagulated [turning liquid to solid or semi solid] cause the dispersion phase really likes the medium so
they just stay with them
Even when the dp and dm are separated, they come back so they are called reversible
22. Lyophobic sols
Eg:metals , thier sulphides mixed with a dispersion medium.
They readily coagulate on addition of small amount of heat or eleectrolytw
Not stable
Also called Irreversible sols cause once ppted they cant be brought back to normal
Stabilising agents are used to preserve them
23. 3.based on type of particles of the dp
Types:
1.Multimolecular eg:gopld sol , sulphur sol[made up of 1000s of s8 molecues]
2.macromolecular eg:cellulose,starch
Man made: polysterene,synthetic rubber
Quite stable and resemble true solutions
25. Formation of micells
○ Soap is the sodium or potassium salt of fatty acid and may be represented as RCOO- Na
(e.g. sodium stearate, (CH3(CH2)16COO-Na+]) .
○ When dissolved into water , it dissociates into RCOO- and Na+ ions
26. At lower conc:
As conc increases, the whole ion is pulled into the water and form an aggregate with head facing away
and tails joined together this is called ionic miscelle.
28. Electric disintegration or bredig’s arc method
Involves condensation nd dispersion
Condensation : small particles come together to form colloidal solutions
1:20:29 to 1:22:59
29. Soap cleansing action
● Miscelle forms around dirt or oil
○ The tail is around the oil droplet and the head is facing outwards
30. peptization
Converting a freshly prepared ppt into a colloidal sol by mixing it with a dispersion medium in
the presence of some electrolyte.
The ppt adsorbs the ions and is now surrounded by ions on all 4 sides and a charge around it is
seen. The charge on teh colloid depends on the common ion [next slide]
Now these start breaking up into colloidal particle sized particles and a colloidal sol is formed.
This is a chemical despersion as in dispersion a big particle breaks into colloids and it is chemical
cause chemical reactions take place to split it.
32. Purification of colloids
Why purify them?
● While preparing them sometimes a bit of impurity might get mixed
● A bit of electrolytes more than needed might be left
○ Colloids do need electrolytes in them but more than a certain amount can make them unstable as
coagulation might occur [colloids start sticking to each other]
The process ued for reducing the amount of impurities to the minimum → purification
of colloidal solution
1.dialysis
2.electrodyalsis
3.ultrafiltration
33. dialysis
● Dissolved substance in the colloidal solution can be removed by passing through a
suitable membrane →diffusion [only solution p[articles will pass through and not
the colloidal solution’s]
● The apparatus used is called DIALYSER
In a true solution, the particles are <1nm [size of colloidal particles]
–suitable membrane here can be LIKE A CELLOTAPE
34. Take a bag made up of suitable membrane → dialysis memebrane. It contains teh
solution and is kept inside a tub containing water
The dialysing membrane makes sure it only lets the smaller particles than the colloidal
particles.
So at the end of the process only the impurities are filtered out of the solution.
This is a very slow process
35. Electro dialysis
Speeding up the process by applying electric field → electrodialysis
● Used only to remove EXCESS ELECTROLYTES cause only electrolytes [contain
ions] move towards the electric field.
Construction:
● Bag made up of dialysing membrane with the solution in it
● A vessel with water into which the bag willbe put
● Anode and cathode set up on either sides through which electric field will be
applied
36. ● When the electric field is applied the ions go to their respective electrodes.
● This is just to make it the movement and the whole process faster
37. ultrafilterations
● Using a specially designed filter paper
○ It stips the flow of colloidal particles throught hem,only solvent and impurities go through them
■ It is made by soaking a filter paper in a collodion solution →hardening by formaldehyde
→drying it
■ Collodion solution: 4% solution of nitrocellulose in a mixture of alcohol adn ether.
● This is a slow process too but can be fastened by applying pressure.
38. Properties of colloids
1.colligative properties
● Colligative →latin word meaning BOUND TOGETHER
● Depends on the no.of solute particles and not the nature.
● No.of dolute particles in a true solution is more than that of a colloidal solution
cause the size of colloidal particles is larger
● Therefore compared to a true solution the colligative properties of a colloidal sol
is lesser.
39. 2.Tyndall effect
Scattering of light by particles ina colloid or in a fine suspension.
Since the particle size is bigger, they scatter light in all directions
● Criterias to be satisfied for tyndall effect to take place:
○ Diameter of dp should not be much smaller than the wavelength of light used.
○ Dp and dm should have a high difference in their magnitude of refractive index.
41. 4.Brownian movement
Random movement of colloidal particles due to collision b/w particles and molecules
of the liq/gas[dispersion medium].
It is observed when seen under a compound microscope.
BM is:
● independent of the nature of the colloid
● but is dependant on the size of particle and viscosity
↓size , ↓viscosity =↑motion
42. 5.charge on colloidal particles
All the particles always carry the same charge
Due to this there is a replusion b/w them and they wouldnt want to be close to each
other, therefore due to the charge only they are stable and dont coagulate.
1.negatively charged colloids
2.postively cahrged
44. 6.electrophoresis
The movement of colloidal particles under the influence of an electric field is called electrophoresis.
Negatively charged particles move towards the cathode and Positively charged particles moves
towards anode.
Electrophoresis is due to electrical charge on the colloidal particles.
Positively charged things are going
towards anode
45. ● But now if the movement of the particles are forcefully stopped due to an external
force, what will happen?
○ The DISPERSION MEDIUM STARTS MOVING. But towards which side, should move towards
anode itself but what if it is moving towards the cathode
○ It starts moving otwards rthe cathode because as the +vely charged colloid particles are mized in the
dm, it kinda makes the dm -vely charged [neg attracts pos] since now it is -vely charged as soon as
it gets a chance to move, iot starts moving towards the cathode and the dp is stationary
○ Why is it moving?
■ Due to the electric field pulling it.
○ THIS MOVEMENT OF DM IS CALLED ELECTROOSMOSIS
46. 7.COAGULATION
The phenomenon of the precipitation of the colloidal solution by addition of
electrolytre.
When the particles are charged they dont get closer to each other due to repulsion but
once the charge is removed → they start coming closer → form aggregate →settle
down →form precipiates.
This process is called COAGULATION [process of settling down of colloidal particl]
47. Coagulation of lyophobic sols [the charge on the dp must be
removed]
1.electrophoresis
Particles start moving towards respective electrode→get discharged →start
aggregating →ppt forms.
2.mixing two oppositely charged sols
Mixing them in equal propotion → neutralise each other →charges are kinda
removed →partial /cpmplete ppt →called mutual coagulation
eg:
Mixing them together forms their ppt
48. 3.boiling
● Boiling→reandom motion and collision →collides with dm’s molecules→charges
get removed→ coagulation
4.persistent dialysis
● dialysis →Removes excess electrolyte BUT Continuous dialysis →removes all the
electrolytes →gets discharged →becomes unstable →coagulation
A bit of electrolyte is always
needed in the sol to keep it
stable as electrolytes contain
ions and these ions provide
stability.
49. 5. Addition of electrolyte
● Excess of electrolyte being added → neutralising of the charge on the particle
→make it unstable →coagulation
● Green colour →excess electrolyte
Mixing with coloide and gets neutralised
50. Hardy schulze rule
● Talks about the power of coagulation of the coagulating ion [the ion that causes
coagulation in the colloidal particles by neutralising it [previous slide]]
○ Greater is the valency of coagulating ion, faster is coagulation
● eg:
51. Coagulation of lyophilic sols
Stability of these sols is due to :
● Charge of particles
● Solvation of the particles
So both should be destroyed.
Methods:
1.adding electrolyte [discharges]
2.adding suitable solvent
● Eg: alcohol →dehydrates the dp→ breaks the bond between dp and dm → add
electrolyte now →coagulation
52. Protective colloids
● Lyophilic are more stable than lyophobic
○ As there are 2 reasons to make lyophilic stable : charge and solvation
● Lyophilic have a property of PROTECTING LYOPHOBIC COLLOIDS from the
electrolytes when they are together.
When a lyophilic sol is added to lyophobic sol, the lyophilic particles (colloids) form a
layer around the particles of lyophobic sol
