surface chemistry

1. 1
Introduction to
Surface Chemistry
Subject: Surface Chemistry
Lecture 01
Dr. Z. A Raza
NTU, Faisalabad

2. Background
•Surfaces reactions everywhere
• Washing clothes
• Brushing teeth
• Preventing food to stick to pans
•Surface chemistry important to a wide
range of technologies
• Catalysis
• Semiconductor Fab
• Paints & finishes
• Consumer products
2

3. Why are surfaces important
Catalysis
Pollution
Corrosion
Sensors
Energy
Conversion
Electronic
Devices
Surface
Processes

4. Surface Chemistry
the study of physical and chemical phenomena that occur at
the interface of two phases, including solid-liquid interfaces, solid-
gas interfaces, solid-vacuum interfaces, and liquid-gas interfaces

5. Phenomena in Surface
Chemistry
1. Wetting, Spreading and Penetration
2. Foam Breaking in Aqueous Systems
3. Solubilization
4. Rheological Effects in Surfactant Phases

6. Surface Chemistry in
Important Technologies
▪ Surface Chemistry in Pharmacy
▪ Surface Chemistry in Food and Feed
▪ Surface Chemistry in Detergency
▪ Surface Chemistry in Agriculture
▪ Surface and Colloid Chemistry in Photographic Technology
▪ Surface Chemistry in Paints
▪ Surface Chemistry of Paper
▪ Surface Chemistry in the Polymerization of Emulsion
▪ Colloidal Processing of Ceramics
▪ Surface Chemistry in Dispersion, Flocculation and Flotation
▪ Surface Chemistry in the Petroleum Industry

7. Analysis and Characterization in
Surface Chemistry
▪ Measuring Equilibrium Surface Tensions
▪ Measuring Dynamic Surface Tensions
▪ Measuring Contact Angle
▪ Determining Critical Micelle Concentration
▪ Measuring Micelle Size and Shape
▪ Identification of Lyotropic Liquid Crystalline Mesophases
▪ Characterization of Microemulsion Structure
▪ Measuring Particle Size by Light Scattering
▪ Measurement of Electrokinetic Phenomena in Surface
Chemistry
▪ Measuring Interactions between Surfaces
▪ Measuring the Forces and Stability of Thin-Liquid Films
▪ Measuring Adsorption

8. 1. Gas-Liquid and Liquid-liquid
Interfaces

9. Surface Tension, 
an effect within the surface layer of a liquid that causes that layer to
behave as an elastic sheet

10. Surface Tension as a Force
➢ caused by the attraction between the molecules
of the liquid by various intermolecular forces

11. 11
❑ Interface is the boundary between two or more phases
exist together
❑ The properties of the molecules forming the interface are
different from those in the bulk that these molecules are
forming an interfacial phase.
❑ Several types of interface can exist depending on whether
the two adjacent phases are in solid, liquid or gaseous state.
❑ Important of Interfacial phenomena in pharmacy:
❖ Adsorption of drugs onto solid adjuncts in dosage forms
❖ Penetration of molecules through biological membranes
❖ Emulsion formation and stability
❖ The dispersion of insoluble particles in liquid media to form
suspensions.

12. 12
LIQUID INTERFACES
Surface and Interfacial Tensions
In the liquid state, the cohesive forces between
adjacent molecules are well developed.
For the molecules in the bulk of a liquid
They are surrounded in all directions by other molecules for
which they have an equal attraction.
For the molecules at the surface (at the liquid/air interface)
▪ Only attractive cohesive forces with other liquid molecules
which are situated below and adjacent to them.
▪ They can develop adhesive forces of attraction with the
molecules of the other phase in the interface
▪ The net effect is that the molecules at the surface of the
liquid experience an inward force towards the bulk of the
liquid and pull the molecules and contract the surface with
a force F .

13. 13
To keep the equilibrium, an equal force must be applied to
oppose the inward tension in the surface.
Thus SURFACE TENSION [γ ] is the force per unit length that must
be applied parallel to the surface so as to counterbalance the net
inward pull and has the units of dyne/cm
INTERFACIAL TENSION is the force per unit length existing at the
interface between two immiscible liquid phases and has the units
of dyne/cm.
Invariably, interfacial tensions are less than surface tensions
because an adhesive forces, between the two liquid phases
forming the interface are greater than when a liquid and a gas
phase exist together.
If two liquids are completely miscible, no interfacial tension exists
between them.
Greater surface tension reflects higher intermolecular force of
attraction, thus, increase in hydrogen bonds or molecular weight
cause increase in ST

14. 14
The work W required to create a unit area of surface is
known as SURFACE FREE ENERGY/UNIT AREA (ergs/cm2)
erg = dyne . cm
Its equivalent to the surface tension γ
Thus the greater the area A of interfacial
contact between the phases, the greater the free energy.
W = γ ∆ A
For equilibrium, the surface free energy
of a system must be at a minimum.
Thus Liquid droplets tend to assume a
spherical shape since a sphere has the
smallest surface area per unit volume.

15. 15
Methods for measuring surface and interfacial tension
1- Capillary rise method
2- Ring (Du Nouy) tensiometer
3- Drop weight method (Stalagmometer)
The choice of the method for measuring surface
and interfacial tension depend on:
➢ Whether surface or interfacial tension is to be determined.
➢ The accuracy desired
➢ The size of sample.

16. Adsorption
The phenomenon of higher concentration of any
molecular species at the surface than in the bulk
Adsorbent
The substance on the surface of which adsorption takes
place is called adsorbent
Adsorbate
The substance which is being adsorbed on the
surface of another substance.
Desorption
The process of removal of an adsorbed substance from
the surface on which it is absorbed

17. • Activated Carbon
• Activated Alumina
• Silica Gel
• Molecular Sieves (Zeolites)
• Polar and Non-polar adsorbents
Adsorbent Materials

18. • Made from nutshells, wood, and petroleum, bituminous
coal by heating in the absence of oxygen to dehydrate
and carbonize (remove volatile components),
• "Activation" is the process that produces the porous
structure essential for effective adsorption by oxidation
of carbon with water vapor or CO2.
• Activated carbon attracts non-polar molecules such as
hydrocarbons.
• Typical surface areas are 300 to 1500 m2/g.
Activated carbon

19. Adsorption vs absorption

20. Types of Adsorption
Positive adsorption occurs when the
concentration of adsorbate is higher on the
surface of adsorbent than in the bulk.
Negative adsorption occurs when the
concentration of adsorbate is less on the
surface of adsorbent than in the bulk.

21. Types of adsorption
S. No. Physical adsorption Chemical adsorption
1 Caused by intermolecular van der
Waals' forces
Caused by chemical bond formation
2 It is not specific It is highly specific
3 It is reversible It is irreversible
4 Heat of adsorption is low (20-40 kJ/mol) High heat of adsorption (80-240 kJ/mol)
5 Low temperature is favourable Increases with high temperature
6 Results multilayer adsorption Results unimolecular layer
Comparison between physisorption and chemisorption
1. Physical adsorption
2. Chemical adsorption

22. Factors affecting adsorption
Effect of adsorbate: The easily liquifiable gases like NH3,
HCl, CO2 etc. are adsorbed to a greater extent than the
permanent gases such as H2 ,O2, N2, etc.
Effect of specific area of the absorbent: The greater
the specific area of the solid, the greater would be its
adsorbing capacity.
Effect of temperature:adsorption decreases with increase
in temperature.
Effect of pressure: An increase in pressure causes an
increase in the magnitude of adsorption of an adsorbent.

23. 1/n
x
K p
m
= 
Over a narrow range of p
Freundlich Isotherm
A graph between the amount (x/m) adsorbed by an
adsorbent and the equilibrium pressure of the adsorbate
at constant temperature is called adsorption isotherm
At low pressure the graph is nearly straight line
At high pressure x/m becomes
independent of p
0
x
p
m

1
x
p
m


24. Freundlich Isotherm
X 1
log logK logp
M n
= +

25. Rate of adsorption a a
r =k p(1-f)
Rate of desorption d d
r =k f
At equilibrium, ra = rd; a
a d
k p
f = .........(i)
k p+k
Mono-layer coverage '
a
x
=k f .........(2)
m
m: mass of adsorbate adsorbed per
unit mass of adsorbent
f: fraction of surface area covered
f
1-f
p: partial pressure of the adsorbate
Langmuir isotherm

26. Langmuir adsorption isotherm:
ap
x /m
1 bp
=
+
The values of constants ‘a’ and ‘b’ depend
upon the nature of adsorbate, nature of solid
adsorbent and temperature.
a = ka x ka’/kd
b = ka/kd
Combining equations (1) and (2):

27. Summary of adsorption isotherms
Easy to fit
adsorption data
Chemisorptions and
physisorption
Freundlich
Useful in analysis of
reaction mechanism
Chemisorption and
physisorption
Langmuir
Note
Application
Isotherm
equation
Name
1/n
x
= Kp
m
x ap
=
m 1+bp

28. Adsorption isobar
Graph between the amount adsorbed(x/m) and temperature
at a constant equilibrium pressure of adsorbate gas is known
as adsorption isobar
Chemisorption isobar shows an initial increase with temperature and
then expected decrease .The initial increase is because of the fact that
the heat supplied acts as activation energy required in chemisorption.

29. Application of Adsorption
✓ In clarification of sugar
✓ In gas masks
✓ In catalysis
✓ In adsorption indicators
✓ In chromatographic analysis
✓ In softening of hard water
✓ In preserving vacuum
✓ In paint industry
✓ In removing moisture from air in the storage of
delicate instruments