1. Lecture 1. The physico-chemical basis of surface phenomenon
Prepared by PhD Falfushynska Halina

2. Surface tension
The surface of a liquid acts as a tensional film which always tends to contract to a minimum area. It proves that surface of liquid has tension. It is called surface tension.
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects (e.g. water striders) to run on the water surface.

3. The cohesive forces between molecules down into a liquid are shared with all neighboring atoms. Those on the surface have no neighboring atoms above, and exhibit stronger attractive forces upon their nearest neighbors on the surface. This enhancement of the intermolecular attractive forces at the surface is called surface tension.
Surface tension has the dimension of force per unit length, or of energy per unit area. The two are equivalent—but when referring to energy per unit of area, people use the term surface energy —which is a more general term in the sense that it applies also to solids and not just liquids.

4. 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.
Greater surface tension reflects higher intermolecular force of attraction, thus, increase in hydrogen bonds or molecular weight cause increase in ST

5. The work W required to create a unit area of surface is known as SURFACE FREE ENERGY/UNIT AREA (ergs/cm2)
erg = dyne . cm2 G=*S
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
G=*S
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.

6. Factors affecting surface tension
The presence of impurities either on the surface or dissolved in it, affect surface tension of the liquid. Highly soluble substances increase the surface tension of water, whereas sparingly soluble substances reduce the surface tension of water.
The surface tension of a liquid decreases with increase in temperature. The surface tension of a liquid becomes zero at its boiling point and vanishes at critical temperature.

7. The surface tension of water is 72 dynes/cm at 25°C
The surface tension of water is 72 dynes/cm at 25°C . The surface tension of water decreases significantly with temperature as shown in the graph. The surface tension arises from the polar nature of the water molecule.
Hot water is a better cleaning agent because the lower surface tension makes it a better "wetting agent" to get into pores and fissures rather than bridging them with surface tension. Soaps and detergents further lower the surface tension.

8. When compared to many other liquids at room temperature, water exhibits a relatively large value for surface tension-roughly about three times as great.

9. Surface tension (mN m-1) ofsome liquids
0oC oC 40oC 60oC 80oC 100oC
H2O
Ethanol
Acetone
Toluene
Benzene
Hg (0oC) Ag (970oC) 800
NaCl (1080oC) AgCl (452oC) 125

10. Example of problem
A glass slide is in contact with a water surface along one of its sides as shown in the figure below. What is the magnitude of the force required to overcome surface tension as the slide is lifted from the water surface? The surface tension of water, for water at 20 degrees Celsius, is g/cm. The dimensions of the plate are 10 cm, by 2 cm, by 0.02 cm.

12. Methods for measuring surface and interfacial tension
Measurement of Surface and Inter­facial Tensions
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.

13. Capillary Rise Method The Principle
When a capillary tube is placed in a liquid, it rises up the tube a certain distance. By measuring this rise, it is possible to determine the surface tension of the liquid. It is not possible, to obtain interfacial tensions using the capillary rise method.

14. If a capillary tube of inside radius =r immersed in a liquid
that wet its surface, the liquid continues to rise in the tube
due to the surface tension, until the upward movement is
just balanced by the downward force of gravity due to the
weight of the liquid
The total upward force around the inside
circumference of the tube is
Where
Ө = the contact angle between the surface of the
liquid and the capillary wall
2 π r = the inside circumference of the capillary.
For water the angle Ө is insignificant, i.e. the liquid wets the capillary wall so that cos Ө = unity
σ = ρghr/(2cosθ).
Cont. angle
water and glass
a = 2 π r γ cos Ө
Cont. angle
Mercury and glass

15. Determination Methods
Capillary Rise Method
where
h = the height the liquid is lifted,
la = the liquid-air surface tension (J/m² or N/m)
θ = contact angle
ρ = density of liquid (kg/m3)
g = acceleration due to gravity (m/s²)
r = radius of tube (m)

16. 2 π r γ cos Ө = π r 2 h (p – p o) g + w
The downward force of gravity
(mass x acceleration) is given by
Where:
π r = the cross-sectional area
h = the height of the liquid column to
the lowest point of the meniscus
(p – p o) = the difference in the density of the
liquid p and its vapor po
g = the acceleration of gravity
w = the weight of the upper part of the meniscus.
At Maximum height, the opposing forces are in equilibrium
p o, Ө and w can usually be disregarded
Hence the surface tension can be calculated.
π r 2 h (p – p o) g + w
2 π r γ cos Ө = π r 2 h (p – p o) g + w
2 π r γ = π r 2 h p g
γ = 1/2 r h p g

17. γ = F / 2 π (R1 + R2) For measuring surface and interfacial tensions.
Ring (Du Nouy) Tensiometer
For measuring surface and interfacial tensions.
The principle
the principle of the instrument depends on the fact that:
the force necessary to detach a platinum-iridium ring
immersed at the surface or interface is proportional to the
surface or interfacial tension.
The force of detachment is recorded in dynes
on a calibrated dial
The surface tension is given by:
Where:
F = the detachment force
R1 and R 2= the inner and outer radii
of the ring.
γ = F / 2 π (R1 + R2)

18. Equipment for surface tension determination by ring detachment
Pt ring
rack mechanism
Balancer
Unknown sample of liquid

19. Drop Weight and drop volume method
If the volume or weight of a drop as it is detached from a tip of known radius is determined, the surface and interfacial tension can be calculated from
Where m = the mass of the drop
V = the volume of the drop
p = the density of the liquid
r = the radius of the tip
g = the acceleration due to gravity
Φ = a correction factor
The correction factor is required as not all
the drop leaves the tip on detachment
The tip must be wetted by the liquid so as
the drop doesn’t climb the outside of the tube.
γ = Φ mg = Φ V pg
2 π r π r

20. Stalagmometric method
The weight of the drops is in an equilibrium state with the surface tension.
To account the surface tension proportional to the weight of the drop, we can use a reference fluid (mostly using water as a reference) to compare with the fluid which we are interested in

21. Water has a surface tension of 0. 4 N/m
Water has a surface tension of 0.4 N/m. In a 3 mm diameter vertical tube if the liquid rises 6 mm above the liquid outside the tube, calculate the contact angle.
Data:
Surface tension (s) = 0.4 N/m
Dia of tube (d) = 3 mm = m
Capillary rise (h) = 6 mm = m

22. Formula:
Capillary rise due to surface tension is given by 
h = 4scos(q)/(rgd), where q is the contact angle.
Calculations:
cos(q) = hrgd/(4s) = x 1000 x x / (4 x 0.4) = 0.11
Therfore, contact angle q = 83.7o

23. Air is introduced through a nozzle into a tank of water to form a stream of bubbles. If the bubbles are intended to have a diameter of 2 mm, calculate how much the pressure of the air at the tip of the nozzle must exceed that of the surrounding water. Assume that the value of surface tension between air and water as 72.7 x 10-3 N/m.
Data:
Surface tension (s) = 72.7 x 10-3 N/m
Radius of bubble (r) = 1

24. Formula:
Dp = 2s/r
Calculations:
Dp = 2 x 72.7 x 10-3 / 1 = N/m2
That is, the pressure of the air at the tip of nozzle must exceed the pressure of surrounding water by 145.4 N/m2

26. Capillarity
The tube with very short diameter is called a capillary tube. When a capillary tube is put in a liquid, the liquid surface in it will change.
If liquid can wet the wall of the capillary tube, the liquid surface in the tube will go up and if the liquid cannot wet the wall, the surface will go down.
This phenomenon is called capillarity.

27. Cohesion and Adhesion
Cohesive force is the force existing between like mole­cules in the surface of a liquid
Adhesive force is the force existing between unlike molecules, such as that between a liquid and the wall of a glass capillary tube
When the force of Adhesion is greater than the cohesion, the liquid is said to wet the capillary wall, spreading over it, and rising in the tube.

28. When the attractive forces are between unlike molecules, they are said to be adhesive forces. The adhesive forces between water molecules and the walls of a glass tube are stronger than the cohesive forces lead to an upward turning meniscus at the walls of the vessel and contribute to capillary action.
concave meniscus
The attractive forces between molecules in a liquid can be viewed as residual electrostatic forces and are sometimes called van der Waals forces or van der Waals bonds.
convex meniscus

29. What is Adsorption?
Adsorption is the phenomenon of accumulation of large number
of molecular species at the surface of liquid or solid phase in comparison to the bulk.
How Adsorption occurs?
The process of adsorption arises due to presence of unbalanced or residual forces at the surface of liquid or solid phase. These unbalanced residual forces have tendency to attract and retain the molecular species with which it comes in contact with the surface.

30. Adsorption is a term which is completely different from Absorption
Adsorption is a term which is completely different from Absorption . While absorption means uniform distribution of the substance throughout the bulk, adsorption essentially happens at the surface of the substance. When both Adsorption and Absorption processes take place simultaneously, the process is called sorption.

31. Adsorption process involves two components Adsorbent and Adsorbate
Adsorption process involves two components Adsorbent and Adsorbate. Adsorbent is the substance on the surface of which adsorption takes place. Adsorbate is the substance which is being adsorbed on the surface of adsorbent. Adsorbate gets adsorbed.
Adsorbate + Adsorbent gives rise to Adsorption
Oxygen molecules (red) adsorb on a bimetallic surface of platinum (purple) and cobalt (green).
Some modern techniques have been used to study surface.
Low energy electron diffraction (LEED).
Photo electron spectroscopy (PES).
Scanning Tunneling microscopy (STM).

32. Adsorption is a spontaneous process
For reaction or process to be spontaneous, there must be decreases in free energy of the system i.e. ΔG of the system must have negative value.
Also we know, ΔG = ΔH – TΔS
And during this process of adsorption, randomness of the molecule decreases which ΔS is negative. We can rewrite above equation as

33. Types of Adsorption
Forces of attraction exist between adsorbate and adsorbent. These forces of attraction can be due to Vanderwaal forces of attraction which are weak forces or due to chemical bond which are strong forces of attraction. On the basis of type of forces of attraction existing between adsorbate and adsorbent, adsorption can be classified into two types: Physical Adsorption or Chemical Adsorption.
Physical Adsorption or Physisorption
When the force of attraction existing between adsorbate and adsorbent are weak Vanderwaal forces of attraction, the process is called Physical Adsorption or Physisorption.
It takes place at low temperature below boiling point of adsorbate. As the temperature increases in, process of Physisorption decreases.

34. Chemical Adsorption or Chemisorption
When the force of attraction existing between adsorbate and adsorbent are chemical forces of attraction or chemical bond, the process is called Chemical Adsorption or Chemisorption. Chemisorption takes place with formation of unilayer of adsorbate on adsorbent. It has high enthalpy of adsorption
Physical Adsorption vs T and Chemical Adsorption vs T

35. Comparison between Physisorption and Chemisorption
Comparison between Physisorption and Chemisorption
Physisorption
Chemisorption
Low heat of adsorption usually in the range of kJ mol-1
High heat of adsorption in the range of kJ mol-1
Force of attraction are Van der Waal's forces
Forces of attraction are chemical bond forces
It usually takes place at low temperature and decreases with increasing temperature
It takes place at high temperature
It is reversible
It is irreversible
It is related to the ease of liquefaction of the gas
The extent of adsorption is generally not related to liquefaction of the gas
It is not very specific
It is highly specific
It forms multi-molecular layers
It forms monomolecular layers
It does not require any activation energy
It requires activation energy

36. Applications of Adsorption
1. Charcoal is used as a decoloriser as it adsorbs the coloring matter from the coloured solution of sugar.
2. Silica gel adsorbs moisture from the desiccators.

37. 3. Silica and alumina gels are used as adsorbents for removing moisture and for controlling humidity of rooms.
4. Activated charcoal is used in gas masks as it adsorbs all the toxic gases and vapours and purifies the air for breathing.
5 .Adsorption processes are useful in carrying out heterogeneous catalysis.

38. Fabricate sorbents
Natural sorbents
Cellulose
Pectin
chitin

39. Factors affected Adsorption
Temperature. Adsorption increases at low temperature conditions.
Adsorption process is exothermic in nature. According to Le Chatleir principle, low temperature conditions would favour the forward direction.
Pressure
As depicted by Adsorption Isotherm, with the increases in pressure, adsorption increases up to a certain extent till saturation level is achieved. After saturation level is achieved no more adsorption takes place no matter how high the pressure is applied.
Surface Area. Adsorption is a surface phenomenon therefore it increases with increase in surface area.
Activation of Adsorbent
Activation of adsorbent surface is done so as to provide more number of vacant sites on surface of adsorbent. This can be done by breaking solid crystal in small pieces, heating charcoal at high temperature, breaking lump of solid into powder or other methods suitable for particular adsorbent.