SURFACE TENSION, INTERFACIAL TENSION, SURFACE FREE ENERGY, Measurement of surface and interfacial tension-capillary rise method, drop number method, drop weight method, Du Nuoy tensiometer method, Spreading of liquids, spreading coefficient, surface active agents, hydrophilic-lipophilic balance, soluble monolayers, Adsorption on solid surface, Isotherms

1. Interfacial phenomena
SURFACE TENSION, INTERFACIAL TENSION, SURFACE FREE ENERGY
Measurement of surface and interfacial tension-capillary rise method, drop number method, drop weight
method, Du Nuoy tensiometer method
Spreading of liquids, spreading coefficient, surface active agents, hydrophilic-lipophilic balance, soluble
monolayers
Adsorption on solid surface, Isotherms

2. Surface Tensions &
Measurement of Surface Tensions

3. Liquid Interfaces
3
• Interface is the boundary between two phases.
• Surface is a term used to describe either a gas-solid or a
gas- liquid interface.
• Interfacial phase is a term used to describe molecules
forming the interface between two phases which have
different properties from molecules in the bulk of each
phase.

4. Surface Tension
• Molecules in the bulk liquid are surrounded in all
directions by other molecules for which they have an
equal attraction (only cohesive forces).
• Molecules at the surface can only
develop cohesive forces with
other molecules that are below
and adjacent to them; and can
develop adhesive forces with
molecules of the other phase.
4

5. Surface Tension
This imbalance in the molecular attraction
will lead to an inward force toward the
bulk that pulls the molecules of the
interface together and contracts the
surface, resulting in a surface tension.
Definition: Surface tension is the force per
unit length that must be applied parallel to
the surface to counterbalance the net
inward pull. It has the units of dynes/cm or
N/m.
5

6. Interfacial Tension
• Interfacial tension is the force per unit length existing at the
interface between two immiscible phases (units are dynes/cm or
N/m).
• The term interfacial tension is used for the force between:
6
Two liquids = γLL Solid liquids = γSL

7. Interfacial Tension
• The term surface tension is reserved for the tensions:
Liquid-vapor = γ
Solid-vapor = γ
• Interfacial tensions are weaker than surface tensions because
the adhesive forces between two liquid phases forming an
interface are greater than that between liquid and gas phase
7

8. Surface Free Energy
8
The surface layer of a liquid possesses additional energy as compared to the
bulk liquid.
If the surface of the liquid increases (e.g. when water is broken into a fine spray), the
energy of the liquid also increases.
Because this energy is proportional to the size of the free surface, it is called
a surface free energy:
𝑾= 𝜸 ∆ 𝑨
𝑾 Surfacefree energy (ergs)
𝜸 surface tension (dynes/cm)
∆ 𝑨 increase in area (cm2).
Therefore, surface tension can also be defined as the surface free energy per unit area of
liquid surface.

9. Surface Free Energy
Each molecule of the liquid has a tendency to move inside the liquid from
the surface; therefore, when the surface is increased, the liquid takes the
form with minimal surface and as a result, minimal surface energy:
9
Sphere shape

10. Measurement of Surface Tensions
𝜸 surface tension
𝒓 radius of capillary
𝒉 height
𝒑 density of the liquid
𝒈 acceleration of gravity
This method cannot be used to obtain interfacial tensions.
Capillary Rise Method
When a capillarytube is placed in a liquidcontained in a
beaker, the liquid rises up in the tube to a certain distance.
By measuring this rise in the capillary, it is possible to
using thedetermine the surface tension of the liquid
formula:
𝜸 = ½ 𝒓 𝒉 𝒑 𝒈
10

11. Measurement of Surface Tensions
The DuNoüy Ring Method
The force necessary to detach a
platinum–iridium ring immersed at the
surface or interface is proportional to the
surface or interfacial tension. The
surface tension is given by the formula:
The DuNoüy tensiometer is widely
used for measuring surface and
interfacial tensions.

12. Spreading of Liquid &
Spreading Coefficient

13. Spreading of liquid
• When Oleic acid dropped on water, it immediately spreads on
the surface of water
• Oleic Acid – Spreading Liquid (L)
• Water – Sub-layer Liquid (S)
• Generally spreading occurs when adhesive force is more
than cohesive force

14. Spreading Coefficient • Work of Cohesion (Wc) may be
defined as the surface free energy
increased by separating a column of
pure liquid into two halves
• Surface free energy increase = γ dA
• Wc = γL (dA+dA) = 2 γLdA
• Here the column is of cross sectional
area is 1cm2 (dA= 1cm2)
• Wc = 2 γL

15. • Work of Adhesion (Wa) may be defined
as the surface free energy increased by
separating a column of two immiscible
liquids at its boundary into two sections
• As two sections of immiscible liquids
are already separated by a boundary, the
energy requirement will be less by an
amount γLS dA
• Wa = γLdA + γS dA - γLS dA
• Here the columns are of cross sectional
area 1cm2
• Wa = γL + γS - γLS
Spreading Coefficient

16. Spreading coefficient
• Spreading coefficient (S) is the difference between work of adhesion
and work of cohesion
S = Wa –Wc
= (γL + γS – γLS) - 2γL
= γS – γL – γLS
• S = γS – (γL + γLS)
• γL - Surface tension of spreading liquid
• γS - Surface tension of sublayer liquid
• γLS - Interfacial tension

17. Spreading coefficient
• Spreading occurs when spreading
coefficient S is positive i.e., γS > (γL+ γLS).
When free energy of the spreading liquid
and the interfacial tension with the sub layer
is less than that of sublayer the spreading
becomes spontaneous to reduce free energy
of sublayer.
• If spreading coefficient S is negative ie,
(γL+ γLS) > γS Spreading liquid forms
globules or floating lens means spreading
will not take place

18. Spreading
• There may be saturation of the liquid with the other and there may be
change in the surface tension of the sublayer liquid
• In that case the spreading coefficient may become negative after
saturation, the spreading liquid coalesces and form a lens on the
surface of the sublayer
• In the case of a DUPLEX FILM if S become negative after saturation,
it forms a monolayer and excess liquid remains as lens on the surface

19. Spreading
• Fatty acids and alcohols have high spreading coefficient
• As non-polar chain length increases in an acid or alcohol spreading
coefficient decreases
• Propionic acid and ethyl alcohol having high spreading coefficient

20. Applications of spreading coefficients
• Absorption of medicament from creams, lotions, etc. on skin
• Stabilization of emulsions

21. Surface Active Agents

22. Surface Active Agents
• Molecules and ions that are adsorbed at interfaces are termed
surface-active agents or surfactants.
• Surfactants have two distinct functional groups in their chemical
structure, one of which is water-liking (hydrophilic) and the other of
which is lipid-liking (lipophilic).
• These molecules are referred to as amphiphile.
22

23. Surface Active Agents
When such molecule is placed in an air-water or oil-water system,
the polar groups are oriented toward the water, and the nonpolar
groups are oriented toward the air or oil.

24. Reduction of Surface Tension
Principle: When surfactants are dissolved in water they can reduce surface tension by
replacing some of the water molecules in the surface so that the forces of attraction
between surfactant and water molecules are less than those between water molecules
themselves, hence the contraction force is reduced.
24

25. Classification of surface active agents
• Non-ionic surfactants
Have low toxicity and high stability and compatibility,
e.g. Sorbitan esters (spans) and Polysorbates (tweens).
• Anionic surfactants
Have bacteriostatic action
e.g. Sodium Lauryl Sulphate
• Cationic surfactants
Have bactericidal activity
e.g. benzalkonium chloride
• Ampholytic Surfactants
• Phospholipids
25

26. Reduction of Surface Tension
Effect of Structure on Surface Activity
• The surface activity (surface tension reduction) of a particular surfactant depends
on the balance between its hydrophilic and hydrophobic properties.
• An increase in the length of the hydrocarbon chain (hydrophobic) of a surfactant
increases the surface activity.
• An increase in the length of the ethylene oxide chain (hydrophilic) of a non-ionic
surfactant results in a decrease of surface activity.
26

27. HLB
Hydrophilic-Lipophilic balance

28. HLB System
• Definition: The hydrophile-lipophile balance
(HLB) system is an arbitrary scale for
expressing the hydrophilic and lipophilic
characteristics of an emulsifying agent.
• Agents with HLB value of 1-8 are lipophilic and
suitable for preparation of w/o emulsion,
• Those with HLB value of 8-18 are hydrophilic
and good for o/w emulsion.

29. HLB System
The oil phase of an o/w emulsion requires a specific HLB, called the required hydrophile–
lipophile balance (RHLB).
% emulsifier with high HLB = RHLB- HLB Low
HLB High - RHLB

30. Monolayers

31. Monolayers
• Types of monolayers on basis
of solubility of adsorbate
(amphiphile/surfactant)
• Soluble monolayers
• Insoluble monolayers
Soluble monolayer
Insoluble monolayer

32. Soluble monolayers
Surface excess concentration ( 𝜞 ) amount
(concentration) of amphiphile per unit
area in excess of that in bulk of liquid, c
It is the difference between the amount per
unit area of a solute in the surface of a
real system and that of a hypothetical
system (without adsorption).
c, concentration of amphiphile is zero in
case of insoluble monolayers
𝜞= surface excess (g/cm2)
𝑹= gas constant (8.314 J mol–1 K–1)
T= absolute temperature (kelvins)
c= amphiphile concentration in bulk (mol m–3)
𝑑γ/dc is change in ST with chance in bulk conc.
𝜞 =
𝑐
𝑅𝑇
𝑑γ
𝑑𝑐
Surface excess is expressed by
the Gibbs adsorption equation:

33. Reduction of Surface Tension
𝑨 =
Surface Area
• The surface area (𝑨)is the area
occupied by one surfactant
molecule at the solution surface.
It can be calculated using the equation:
N = Avogadro number (6.023 × 1023 molecules /mol)
N 𝜞
𝟏
𝜞 = surface excess

34. Micellisation
• Micelles are formed when the
concentration of a surfactant reaches a
given concentration called critical
micelle concentration (CMC) in which
the surface is saturated with surfactant
molecules.
• The main reason for micelle formation is
to obtain a minimum free energy state.
• In a micelle, polar or ionic heads form an
outer shell in contact with water, while
non polar tails are sequestered in the
interior to avoid water.
Concentration
SurfaceTension

35. Micellisation
Solubilization
• Solubilization is the process where water-insoluble substances are brought
into solution by incorporation into micelle.
• Solubilization does not occur until the milcells are formed (i.e. above
CMC)
• The amount of substance solubilized increases as the number of micelles
increases.

36. Adsorption on solid surface

37. Adsorption of solute from liquid to solid surface
Types of Adsorption
Adsorption is the adhesion of atoms,
ions, or molecules from a gas, liquid,
or dissolved solid to a interface.
There are two general types of
adsorption:
1. Physical adsorption, in which the
adsorbate is bound to the surface
through the weak van der Waals
forces.
2. Chemical adsorption or
chemisorption, which involves the
stronger valence forces.

38. Adsorption of Gas
• Adsorption & Desorption (evaporation)
• Physical adsorption, in which the
adsorbate is bound to the surface
through the weak van der Waals forces.
• Chemical adsorption or chemisorption,
which involves the stronger valence
forces.
• Adsorbate: material which get adsorb
(Gas/solute) (x moles)
• Adsorbent: material on which
adsorption takes place (m grams)
m
x

39. Freundlich isotherm
y =
x
m
= kp1/n
Log
x
m
= Log k +
1
n
Log p
y =
x
m
= kp1/n

40. Langmuir adsorption isotherm
• Molecules of gas adsorb on active sites on adsorbent
• Fraction of active sites occupied (at pressure p) are ϴ
• Fraction of un-occupied sites are 1- ϴ
• Rate of adsorption is proportional to pressure and no. of un occupied
sites
• r1=k1(1-ϴ)p
• Rate of desorption (evaporation is proportional to no. of occupied sites
• r2=k2ϴ
• At equilibrium r2=r1

41. Langmuir adsorption isotherm
• ϴ =
k1
p
k2
+k1
p
• ϴ =
(k1
/k2
)p
1+(
k1
k2
)p
• y =
ym
bp
1+bp
•
p
y
=
1
bym
+
p
ym
If we replace
• (k1/k2) with b
• ϴ with y/ym
Where,
• y= mass of gas adsorb per gram of adsorbent at pressure p
• ym= mass of gas adsorb on adsorbent on 1 gram of adsorbent
when monolayer is formed
The plot of p/y against p gives straight line and ym and b can be
obtained by slop and intercept
• k2ϴ = k1(1-ϴ)p

42. Langmuir adsorption isotherm

43. Other types of adsorption
isotherm
Type I : Previous
Type II : Sigmoidal when gas undergo
physical adsorption on non-porous solid to
form monolayer. Described by BET
equation. Multilayer formation and
condensation within pores.
Type IV : Adsorption on porous solid.
Capillary condensation
Type III & Type V : Heat of adsorption of
gas in first layer is less than latent heat of
condensation in successive layers. Capillary
condensation occur in type V
Surface area can be determined where
monolayer formation is detected … Type I,
II & IV

44. Wetting

45. Wetting
• Force of attraction between solid and liquid play important role
• Angle of contact range (0° to 180 °)

46. Wetting phenomenon
• Contact angle is the angle between liquid droplet and surface over which it spreads.
• Important action of wetting is to reduce angle of contact
• Draves test for to test wetting property of wetting agent… time required to sink
weighted cotton yarn of standard solution.

47. Young’s equation
• γs= γSL - γL cos ϴ
• Where ϴ= angle of contact
• We know that spreading coefficient ‘S’
• S= γS – γL – γLS
• Combining both these equations, substituting value of γs in second
equation
• S = γL (cos ϴ -1)
• Surface tension when, cos ϴ =1 is known as critical surface tension

48. Steps of wetting
• Adhesional Wetting
• Immersional
• Spreading

49. Reference
• Martin’s Physical Pharmacy