Lecture 2. Adsorption on the
interphase of liquid-gas
Prepared by PhD Falfushynska Halina
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
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.
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
Comparison between
Physisorption and Chemisorption
Physisorption
Chemisorption
1.Low heat of adsorption usually in theHigh heat of adsorption in the range of
range of 20-40 kJ mol-1
40-400 kJ mol-1
2.Force of attraction are Van der Waal'sForces of attraction are chemical bond
forces
forces
3.It usually takes place at low
temperature and decreases withIt takes place at high temperature
increasing temperature
4.It is reversible
It is irreversible
5.It is related to the ease of liquefactionThe extent of adsorption is generally not
of the gas
related to liquefaction of the gas
6.It is not very specific
It is highly specific
7.It forms multi-molecular layers
It forms monomolecular layers
8.It does not require any activation
It requires activation energy
energy
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.
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.
Natural sorbents
Cellulose
Pectin
chitin
Fabricate sorbents
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.
The surface-active agent is called surfactant or a
wetting agent.
Surfactants are compounds that lower the surface
tension of a liquid, the interfacial tension between two
liquids, or that between a liquid and a solid.
Surfactants may act as detergents, wetting agents,
emulsifiers, foaming agents, and dispersants. For
example, organic acid and soap are active agent of
water; salt is non-active agent of water.
Molecules and ions that are adsorbed at interfaces are
termed surface active agents, surfactants or amphiphile
The molecule or ion has a certain affinity for both polar and
nonpolar solvents.
Depending on the number and nature of the polar and
nonpolar groups present, the amphiphile may be hydrophilic,
lipophilic or be reasonably well-balanced between these two
extremes.
It is the amphiphilic nature of surface active agents which
causes them to be adsorbed at interfaces, whether these be
liquid/gas or liquid/liquid.
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A surfactant molecule is depicted schematically as a cylinder
representing the hydrocarbon (hydrophobic) portion with a
sphere representing the polar (hydrophilic) group attached
at one end.
The hydrocarbon chains are straight because rotation around
carbon-carbon bonds bends, coils and twists them.
Sodium Lauryl
Sulfate molecule
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How does surfactant work?
How Surfactants Work
Іsoterms of surface tension
1 – for solution of surface active substance,
2 – surface inactive substance,
3 – surface neutral substance
The Influence of Surfactants on
Surface Tension
Critical Micelle Concentration
(CMC)
Minimum concentration at which
surfactants molecules begin to
form micelles
Effect of Surfactant’s structure on CMC
Branched chain systems and double bonds raise CMC
Since the chains must come together inside the micelles
Length of hydrocarbon chain and polarity of Surfactants
Increase in chain length of hydrocarbon facilitate the transfer from
aqueous phase to micellar form cause decrease in CMC
Greater interaction with water will retard micelle formation thus
ionized surfactants have higher CMC in polar solvents than nonionic
Surfactants.
In polar solvents:
Polarity of Surfactant molecules
CMC
Length of hydrocarbon chain
CMC
In non-polar solvents:
Polarity of Surfactant molecules
CMC
Length of hydrocarbon chain
CMC
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Surfactant Behavior
Structures of Micelles
Types of Surfactants
Ionic Surfactants
Anionic Surfactants
Carboxylates
Soaps
LABS
FAS
Alkyl benzene Sulphonates
Ionic Surfactants
Anionic Surfactants
Sulfosuccinate
Diester
Sulfosuccinate
Monoester
Ionic Surfactants: Uses
Anionic Surfactants
• Cleansing Formulation
– Shampoo
– Hand wash
– Bath gels
– Tooth Paste
– Soaps & Detergents
Cationic Surfactants(1)
Cationic Surfactants(2)
Important Property
Substantivity
Cationic Surfactants
Ionic Surfactants
Amphoteric Surfactants
Cocobetaine (CB)
CH3
N+
CH2COO-
CH3
Cocoamidopropylbetaine ( CAPB)
CH3
CONH(CH2)3 N+
CH3
CH2COO-
Nonionic Surfactants
Nonionic Surfactants
Water Solubility of Nonionics
Reason
Interface Tension and
Spreading
When a liquid is placed on the surface of other liquid, it will
spread as a film if the adhesion force is greater than the
cohesive forces.
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As surface or interfacial work is equal to surface
tension multiplied by the area increment.
The work of cohesion, which is the energy required to
separate the molecules of the spreading liquid so as it
can flow over the sub-layer=
Wc = 2 γ
L
Where 2 surfaces each with a surface tension = γ L
The work of adhesion, which is the energy required to
break the attraction between the unlike molecules=
Wa = γ L + γ S - γ LS
Spreading occurs if the work of adhesion is greater
than the work of cohesion, i.e. Wa > Wc
Wc > 0
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or
Wa -
Problem
Calculate the work of adhesion of water on
four solids, where the equilibrium contact
angles are 30o, 60o, 120o, and (a
hypothetical) 180o. The surface tension of
the air/water interface is 72 mN m–1.
Solution
We can use the Young–Dupré equation to calculate the
work of adhesion per unit area of contact between water
and the solid. For the first solid (θ = 30o):
Using the same procedure for the other solids.
We note that the work of adhesion falls to zero when the
contact angle is 180o, but emphasize that such angles are
never observed as it would imply that there is no interaction
between the liquid and solid surface.
Spreading Coefficient is The difference between
the work of adhesion and the work of cohesion
S = Wa - Wc = (γ L + γ S - γ LS ) - 2 γ L
S = γ S - γ L - γ LS
S = γ S – (γ L + γ LS )
Spreading occurs (S is positive) when the surface tension
of the sub-layer liquid is greater than the sum of the surface
tension of the spreading liquid and the interfacial tension
between the sub-layer and the spreading liquid.
If (γ
L
+ γ
LS
) is larger than YS , (S is negative) the substance
forms globules or a floating lens and fails to spread over the
surface.
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Liquid Substrate
Spreading Coefficient, S
in Water at 20oC
S =
S=
 WA   OA   OW 
liquid B
n-hexadecane 72.8-(30.0+52.1) = - 9.3 Drop on water surface
n-octane
72.8-(21.8+50.8) = + 0.2
Spreading
n-octanol
72.8-(27.5+8.5) = +36.8 Spreading against
inpurity
Factor affecting Spreading Coefficient
Molecular Structural:
o The greater the polarity of the molecule
the more positive [S]
as ethyl alcohol and propionic acid
o Non polar substances as Liquid petrolatum have negative [S] fail
to spread on water
o For organic acids, as Oleic acid,
the longer the carbon chain
decrease in polar character
decrease [S]
o Some oils can spread over water because they contain polar groups
as COOH and OH
Cohesive forces:
Benzene spreads on water not because it is polar but
because the cohesive forces between its molecules are much
weaker than the adhesion for water.
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Application of Spreading coefficient in pharmacy
 The requirement of film coats to be spreaded over the
tablet surfaces
 The requirement of lotions with mineral oils to spread on
the skin by the addition of surfactants
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Contact angles and wetting
Wetting is the displacement from a surface of one fluid by another. It
involves, therefore, three phases, at least two of which must be
fluids.
The following account will be restricted to wetting in which a gas
(usually air) is displaced by a liquid at the surface of a solid. A
wetting agent is a (surface-active) substance which promotes this
effect. Three types of wetting can be distinguished:
1. Spreading wetting.
2. Adhesional wetting.
3. Immersional wetting.
Spreading wetting
In spreading wetting, a liquid already in contact with the solid spreads
so as to increase the solid-liquid and liquid-gas interfacial areas and
decrease the solid-gas interfacial area.
Wetting (A) and unwetting (B) solid
by liquid
Gas
Gas
Liquid
θ Liquid
Cos θ = 0÷1
А)
θ
Cos θ = -1÷0
B)
complete
wetting
incomplete wetting
no
wetting
.
Ө
= 0°
γ s – γsL> 0
Ө
< 90° Ө
= 90° Ө
γ s – γsL ≈ 0
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> 90° Ө
= 180°
γ s– γsL< 0
Surface active agent concentration
dependence of the adsorption describes by
Gibb’s equation:
C d
Г 

RT dC
Г – adsorption, mole/m2;
С – concentration SAA, mole/L;
d - surface tension of SAA, J·m/mole;

dC
Т – temperature, К; R = 8.314 J/(mole·К)
Concentration dependence
of surface tension
1
1
2
2
3
1, 3 – Henry’s direct dependence:
σ0- σ = КС
2- Shyshkovski equation
σ = σ0 – a ln (1+ bc)
3