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. Liquid interface
• Different types of interface can exist depending on whether the two adjacent phases are
in the solid, liquid, or gaseous state.
• For convenience, these various combinations are divided into two groups such as liquid
interfaces and solid interfaces.
• Solid interfaces will deal with systems containing solid–gas and solid–liquid interfaces.
• In the liquid state, the cohesive forces between adjacent molecules are well developed.
Molecules in the bulk liquid are surrounded in all directions by other molecules for
which they have an equal attraction.
• But molecules at the surface (i.e., at the liquid–air interface) can only develop attractive
cohesive forces with other liquid molecules that are situated below and adjacent to
them. There are various interfaces exist. The classification is shown in Table 1:
5. Table 1: Classification of Interfaces
Phase Types and Examples of Interfaces
Gas-Gas No interface possible
……..
Gas-Liquid Liquid interface Body of water exposed to atmosphere
Gas-Solid Solid surface Tablet top
Liquid-Liquid Liquid-Liquid interface Emulsion
Liquid-Solid Liquid-Solid interface Suspension
Solid-Solid Solid-Solid interface Powder particles
6. SURAFACE TENSION
The tension that exist between solid-gas & liquid-gas phase is known as “surface tension”.
In other words;
The tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the
bulk of the liquid, which tends to minimize surface area is known as surface tension.
Molecules in the bulk liquid have an equal attraction because in all directions by other molecules are
surrounded. Molecules at the surface (i.e., at the liquid–air interface) creates attractive cohesive forces.
Hence the cohesive forces between liquid molecules are responsible for the phenomenon of surface tension.
Its unit is N/m.
CGS unit is dyne/cm.
N/m = 1 103 dyne/cm.
7. 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.
8. Table 3: The surface tensions of some common liquids and their interfacial tensions against
water at 200
C
Liquids Surface tension (dynes/cm) Interfacial tensions against water
(dynes/cm)
Water 72 ------
n-Octanol 27 8.5
CCl4 27 45
Olive oil 36 33
n-Hexane 18 51
9. 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’.
Each molecule of the liquid has a tendency to move inside the liquid from the surface; therefore, the liquid
takes form with minimal free surface and with minimal surface energy. Surface free energy describes the
excess energy, that the surface has compared to the bulk of the material. This energy is proportional to the size
of the free surface; it is called a 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,
10. W = f x ∆x
We know that γ = fb /2L
So, W = γ x 2L x ∆x
As 2L x ∆x is equal to the increase in surface area, ∆A= 2l ∆x
where ΔA is the change in area due to the expansion of the surface, it may be
concluded that
, produced by soap film.
Therefore W = γ. ∆A
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.
11. 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.
12. MEASURMENT OF SURFACE TENSION
There are various methods of determination:
1. Capillary Rise Method.
2. Drop Weight & Drop count Method.
3. Wilhelmy Plate Method.
4. Ring Detachment Method ( Du Novy tensiometer).
13. CaPil l ar Y 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.
If the contact angle of liquid is not 0 (Fig. 2), except the vertical component of F, which opposes the weight of the
column, is F cos θ and, therefore surface tension is given by
γ = rdgh
2cosθ
If the contact angle of liquid is 0 then surface tension is given by
γ = ½ rdgh
Where
r = Radius of capillary tube d = Density of the liquid
h = Height of the liquid rise g = Acceleration due to gravity γ = Surface tension of liquid.
14. Dr oP WEighT & Dr oP 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;
15. 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
16. 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)
17. MEASURMENT OF INTERFACIAL TENSION
There are following methods to measure the interfacial tension =
Drop volume Method.
Spinning Drop Method.
18. Dr oP vol umE 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.
19. SPinning Dr oP 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
20. SPr Eading 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.
22. Adsor PTion AT Liquid InTEr FacEs
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.
23. SURFACE ACTIVE AGENTS
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-ionic
Cationic
24. Tńble: SurfaceActive 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.
25. HLB Scal E
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;
27. 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.
28. 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.
29. Adsor PTion aT Sol id InTEr FacE
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.
30. 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.
