Important for Physical Pharmaceutics. 3rd Sem. B.pharm

1. SURFACE AND
INTERFACIAL
PHENOMENON

2. INTRODUCTION
❑The boundary between two phases which exist together, called “interface”.
❑It may be also depending on two adjacent
Phases also solid, liquid & gaseous states.
❑“Every surface is an interface”.
❑ It may be divided in two interface:
 Solid interface.
 Liquid interface.

3. LIQUID INTERFACE
Liquid interface deals with association of liquid-gas or liquid-liquid phase.
➢ It has applications infiltration process, biopharmaceutical study & preparation of suspensions
& emulsions.

4.  SURAFACE TENSION
The tension that exist between solid-gas & liquid-gas phase is known as “surface tension”.
 In other words;
“ A tensile force acting at the surface of liquid which is in contact with gas or it may
be between
two immiscible liquid.”
❑Its unit is N/m.
❑CGS unit is dyne/cm.
N/m = 1  103 dyne/cm.

5.  INTERFACIAL TENSION
It is the force per unit length that exist at the interface between two immiscible liquid phases
knowns as “interfacial tension”.
Interfacial tension is less than surface tension because the two liquid phases form an interface
are greater than when a liquid & gas phase exist together.
It is useful in spreading, emulsification & analyzing fluid reforming.
❑Its SI unit is N/m.
❑CGS unit is dyne/cm.

6. SURFACE FREE ENERGY
“The molecule near the surface of liquid have more potential energy that mean surface of liquid is
increases. “
This energy is proportional to the size of the free surface, it is called ‘surface free energy’.
For Example:
 A liquid droplets assume a spherical shape. That increase the surface of liquid , work
done against the surface tension.
When the bar is at a position & mass Is added to extend the surface by a distance ds, the work dW is,

7. dW = f x ds
We know that γ = fb /2L
So, dW = γ x 2L x ds
As 2L x ds is equal to the increase in surface area, dA, produced by soap film.
Therefore dW = γ. dA
For finite change,
W = γ. dA
Where
W= work done, or surface free energy increase.
γ = surface tension in dynes/cm.
∆A = increase in area in cm3 .
Surface tension can be defined as the surface free energy change per unit area.

8. ❑MEASURMENT OF SURFACE FREE
ENERGY
Following methods used to measure free surface energy of solid material –
[A]. Dyne pen method.
[B]. Contact angle method.
[C]. Interfacial tensiometer method.

9. MEASURMENT OF SURFACE TENSION
There are various methods of determination:
01. Capillary Rise Method.
02. Drop Weight & Drop count Method.
03. Wilhelmy Plate Method.
04. Ring Detachment Method ( Du Novy tensiometer).

10. Capillary Rise Method
It measure surface tension only.
Principle: Capillary placed in liquid container, liquid rise up to certain height. Rising liquid due to
adhesive force b/w liquid & capillary wall.
Rise liquid continue upward movement it balanced
by downward force of gravity due to weight of liquid.
This method is very accurate & also used for many liquids.

11. Drop Weight & Drop count Method
‘Stalagmometer’ used in this method.
❑ It consists of glass tube with one marking A above bulb &
other B below bulb. At the tip the capillary is attached.
The liquid whose surface tension to be measure put into
Beaker.
❑Capillary sucks the liquid up to mark A &
allow the liquid fall through capillary tube.
The drops collected in vessel. Weight of one drop
of liquid can be measured.
It calculate following equation;
γ = w/2𝝅r

12. Wilhelmy Plate Method
The liquid whose surface tension is measured put into container. A rectangular plate which
made of glass, platinum is suspended vertically.
➢ The plate is attached to torsion balance. The container is gradually lowered so that plate
detaches from surface of liquid.
➢ The reading on the balance is recorded.
The surface tension multiplied by perimeter
of surface detached.
WL – W = 2(L+T) γ

13. Du Novy tensiometer
 The liquid whose surface tension is measured put into container. A platinum ring s
suspended in liquid. The ring is attached to scale through torsion wire.
 The ring just touch the surface of liquid.
 The force is required to detached the ring is noted from
the scale.
 The detachment force is given by;
γ = P
2𝝅 ( r1 + r2)

14. MEASURMENT OF INTERFACIAL TENSION
There are following methods to measure the interfacial tension =
❑ Drop volume Method.
❑ Spinning Drop Method.

15. Drop volume Method
oDrop volume tensiometer is used in this method. Liquid drop produce capillary surrounding second
liquid.
oLiquid is introduced into bulk
Phase through capillary.
Interfacial tension is calculate by –
 = V∆pg
𝝅𝒅
V= drop volume.
d= inside diameter of capillary.

16. Spinning Drop Method
▪Spinning drop tensiometer used in this method. Horizontal capillary is filled bulk phase &
lighter drop phase is set in rotation.
▪Diameter of drop elongated by centrifugal
force correlates with interfacial tension.
It also calculate by;
 = r3 ꙍ2 ∆p
4

17. Spreading Coefficient:
The ability of one liquid to spread over another is calculate as “spreading coefficient”.
Oleic acid drop place on the water surface it will spread as a film.
❖The value should be ‘positive’ or ‘zero’.
❖Its applications for absorption of drug & also stabilization of emulsion.
❖Energy involved to separate two immiscible liquid at the interface into two sections called
“work of adhesion”.
Wa = γL + γS – γLS
γS = Interfacial tension of sublayer.
γLS = Interfacial tension of solid/liquid interface.

18. Table: Spreading coefficient
liquid Spreading coefficient (s) in dyne/cm.
Benzene 8.8
Ethyl alcohol 50.4
Oleic acid 24.6
Chloroform 13
Acetic acid 45.2
Liquid paraffin -13.4

19. Adsorption At Liquid Interfaces
Positive absorption:
Some molecules & ions, dispersed in the liquid, are partitioned in favor of
the interface. This phenomenon is called “ positive absorption”.
The ‘surface free energy’ & ‘surface tension’ in negative absorption.
Negative absorption:
Inorganic electrolyte dispersed in the liquid, are partitioned in favor of the
bulk. This phenomenon is called “negative absorption”.
The ‘surface free energy’ & ‘surface tension’ in negative absorption.

20. surface active agents [surfactant]
Agents which used to lower surface tension of liquid & reduces interfacial tension between two
liquids.
They contain ‘hydrophilic’ & ‘hydrophobic’ groups.
Surfactants which have both polar & non polar groups called “Amphiphiles”.
Surfactant
Anionic Ampholytic Non-ionicCationic

21. Table: Surface Active Agents
TYPES OF
SURFACTANTS
EXAMPLES
Anionic surfactants Ammonium.
Cationic surfactants Cetrimide, benzalkonium &
benzethonium chloride.
Ampholytic surfactants Glycerol, glycol ester, spans &
tweens.
Non-ionic surfactants Lecithin, N-dodecyl alanine.
❑ The surfactants are used in cosmetic
products, detergents, floor cleaner,
toothpaste & shampoos etc.
❑ Surfactants are the substance, which
exhibit superficial or interfacial activity.

22.  HLB Scale 
HLB system consist of arbiratory scale in which value are
assigned to different surfactants according to their nature.
The value of 1 to 20 on HLB scale represent lipophilic &
hydrophilic part.
Higher number indicates that agent is hydrophilic, low HLB
Value indicates that agent is lipophilic.
❑ HLB value calculate by;
HLB = E + P
5

23. HLB values of some Amphiphilic agents
SUBSTANCE HLB
oleic acid 1
Sorbitan tristearate 2.1
Glyceryl monostearate 3.8
Sorbitan monooleate (Span 80) 4.3
Diethylene glycol monolaurate 6.1
Gelatin (Pharmagel B) 9.8
Methyl cellulose (Methocel 15 cps) 10.5

24. ❑ SOLUBILISATION
#. The process of increasing the solubility of organic compounds in aqueous system due to
presence of surfactants is called “solubilization”.
#. Non-polar molecules are dissolved in non-polar core of micelle.
#. Polar molecules absorbed at micellar surface.
#. Amphiphilic group arranged in such a way so that polar groups is towards aqueous phase
while lipophilic group is inside the micelles.

25. detergency
❑ It is process involving the removal of foreign matter from surfaces.
❑Surfactants used removal of dirt through the detergency effect.
❑ They have good ‘wetting’ properties.
❑ After this surfactant get absorbed to the particle surface & develop charge there which prevent the
deposition of dirt on the solid surface.

26. Adsorption at Solid Interface 
The accumulation of gas or liquid to the surface of solid.
It is an surface phenomenon.
The principle of solid-liquid adsorption used in decolorizing solutions, detergency & wetting.
The liquid-gas Interface :
Dependent on chemical nature of the absorbent & the adsorbate.
It also recognized as following;
- Physical adsorption.
- Chemisorption.

27. ❑ Adsorption Isotherm
The amount of gas adsorbed on solid & equilibrium concentration at constant temperature
yields an “adsorption isotherm”.
[A]. Freundlich adsorption isotherm =
Y = x/m = kp1/n
[B]. Langmuir adsorption isotherm.