2. The boundary that forms between two phases like
solid and liquid is called interface.
3. Surface Tension: it is defined as the force, in
dynes, acting on the surface of the liquid at right
angles to any line of length of surface, 1
centimeter.
Interfacial tension: It is defined as the force per
unit length parallel to the interface to counter
balance the net inward pull.
Cohesive forces: Intermolecular attraction between
like molecules called cohesive forces.
Adhesive forces: Attraction between unlike
molecules are called adhesive forces.
4. SURFACE TENSION:
The unit of surface tension is dy/cm (CGS System) and
Newton/ meter in MKS system.
the phenomenon of surface tension is responsible for the
following processes
Formulation of spherical globules in emulsion.
Formation of nearly spherical shape of falling water
droplets.
Formation of spherical shape of mercury particles on a
flat surface .
Rise of liquid in a capillary tube.
It is also important physicochemical property and a
characteristics of a liquid.
5. A number of methods are available to determine
surface and interfacial tension:
Capillary rise.
Tensiometer.
Drop weight method.
Drop count method.
6. Capillary Rise Methods
When a capillary tube is placed in the liquid contained
in a beaker, the liquid rises up in the tube to a certain
distance.
This is because the adhesive forces between the liquid
molecules.
The glass are stronger than the cohesive forces between
liquid molecules.
The rise in the tube continue until the upward
movement is just balanced by the down-ward force of
gravity due to the weight of the liquid.
7. Example:
Underground water rising on to the surface.
Oil rising through wick in a lamp.
To certain extent, water rising from soil to the branches
above the ground in plants.
8.
9. Derivation:
Upward component:
this force represents the surface tension of liquid (γ) at any
point on the circumference.
This is given by the equation…………….
Upward component,
a = γ . Cos θ --------- (1)
Where, γ = surface tension of the liquid.
θ = contact angle between the surface of the liquid and
capillary wall
The total upward force around the inside circumference (2 π r)
of the tube
10. Up ward component :
a = γ . 2 π r. Cos θ ------------- (2)
Where, r = inside diameter of the capillary tube.
For water θ = 0, because water completely wets the capillary
wall.
The contact angle is very small Cos θ = 1
so the, Up ward component,
a = γ . 2 π r -------------- (3)
11. Downward Component:
the counteracting force is gravity and depends on the
weight of liquid in the capillary rice.
Downward component,
b = mass of the liquid in capillary X acceleration
= volume X density X acceleration
= cross-section area X height X density X acceleration
= π r2. h .ρ .g ----------------- (4)
At the equilibrium,
upward component = downward component
12. γ . 2 π r = π r2. h .ρ .g ------------------ (5)
γ = ½ r h .ρ .g ------------------- (6)
This equation is used to estimate the surface tension of
liquids by the capillary rise methods.
13. Surface Active Agent
Molecules and ions that are adsorbed at the
interface are called surface active agent or
surfactant.
Surface active agent reduce the interfacial tension.
Surface active agents are polymer like substances
which have both polar and non polar group.
Surface active agents are amphiphiles.
14.
15. Amphiphiles are molecules or ions which have certain
affinity for both polar and nonpolar solvent.
In water, at low concentration surface active agents exist
individually as monomers.
As concentration increases, the monomers aggregate
themselves over a narrow range of concentration.
These aggregates are called as micelles.
Each micelle may contain around monomers and size may
be about 50 A.
16. Critical micelle concentration is defined as the
concentration range of a surfactant at which micelles
start formation.
Amphiphiles are molecules or ions which have a certain
affinity for both polar and nonpolar solvent.
The hydrocarbon chain is designated as the tail and polar
portion is indicated by head.
Below the CMC surface active agents get adsorbed at air
water interface.
As the concentration of surfactant increases, molecules
get accumulated progressively at the interface.
17. the formation of micelles is so spontaneous that it
is difficult to differentiate the concentration related
to saturated phase and micelle phase.
Example of surface active agents are: sodium
lauryl sulphate, cetyl trimethyl ammonium
bromide, polyoxethylene lauryl ether. etc
18. Application of Surface active agent:
Pharmaceutical adjuvants: They are added to
the drugs in order to improve the product
characteristics in the design of dosage forms in a
variety of ways
Solubilizing agents, wetting agents, detergents,
suspending agents, emulsifying agents, Foaming
agents.
Influence on drug action: Surfactants at low
concentrations enhance the penetration of
Hexylresorcinol into the pinworm, Ascaris.
They reduce the interfacial tension between the
liquid phase and the cell wall of the organism.
19. Antibacterial activity: ionic surfactants adsorbed on the
cell surface by electrostatic interaction.
As a result, cell surface loos its integrity and the essential
materials are lost through the leaks.
Thus the antibacterial action is observed
20. Surface Free Energy
Surface free energy is defined as the work required to
increase the area of a liquid by 1 sq cm.
As per the definition, surface free energy is equal to
the surface tension.
21. Estimation of Surface free Energy
ABCD is a rectangular wire. The side of AD = L.
L is movable.
A drop of soap solution is placed on the frame, so that
it forms a film within the frame.
The side AD remains stable on account of surface
tension exerted by the soap film.
When the force is applied downward, the film gets
stretched as the movable bar AD goes down until the
film breaks.
22. If force ‘f’ is applied on AD ( Downward
component), it shifts the movable wire to a distance ‘d’
to A’ D’.
The work done ‘W’ is given by
Work done (W) = force X distance moved
W = f X d
The above force acts against the surface tension
(upward component) ‘γ’ of the liquid
Again f = γ X 2L
Where L = length
Substituting the equation it gives
W = γ X 2L X d
23. 2L x d is equal to increase in surface area ‘ΔA’
Produced by extending the soap film, so the equation is
W = γ X ΔA
ΔG = γ X ΔA
W = Work done.
ΔG = Surface free energy
24. Hydrophilic lipophilic Balance (HLB)
HLB is an arbitrary scale that indicates the extent of
hydrophilic lipophilic balance.
Surfactant such as Spans ( Sorbitan ester) are lipophilic
and have low HLB value ( 1.8 to 8.6)
Tweens ( poly-oxy-ethylene derivatives of Span) are
hydrophilic and have high HLB values ( 9.6 to 16.7).
The higher the HLB of an agent, the more the
hydrophilicity.
A HLB value of 1 implies that it is soluble in oil.
HLB value 20 implies that it is soluble in water.
25. Method Of Estimation
Method I :
the structure of a surfactant molecules is split into
different component groups.
Each group is assigned a number.
The addition of these numbers for their respective
groups permits the calculation of its HLB value.
HLB = ∑ ( hydrophilic group number) - ∑ ( lipophilic
group number) + 7
26. Method 2 : In this method, each atom or group has
been assigned a constant.
Example: when a surfactant contains
polyoxyethelenr chains,
The HLB value can be estimated by the formula
HLB = E + P / 5
Where, E = % weight of ethylene oxide chains.
P = % weight of polyhydric alcohol groups.
27. Method 3 : If surfactant contains ester functional
groups, such as glyceryl monostearate, the HLB
value can be estimated using the formula
HLB = 20 ( 1- S / A)
S = Saponification number of the ester.
A = Acid number of the fatty acid.
28. Disadvantages of the HLB Systems:
1. this system provides information regarding the
nature of the surfactant used, but the concentration of
such surfactant is equally important.
The HLB system neglects the concentration of
surfactant required for the optimum stability of
emulsion.
through HLB of a surfactant indicates the solubility
properties, it is an over-simplification.
It cannot be always realistic. Because solubility
depends on the nature of the solvent, temperature and
presence or absence of additives.
29. Adsorption At Liquid Interfaces
Certain molecules or ions when added to a liquid,
they may modify the interface in different ways.
When the added molecules move on their own
accord to the interface, this process is called
positive adsorption or adsorption.
30. Adsorption at Solid Interface
Adsorption of a gas or a liquid onto a solid surface is
important in pharmacy. The material used to adsorbed
gases or liquids is termed as adsorbent.
the substance that is attached to the surface of the solid
is called adsorbate.
The degree of adsorption of gas by a solid depends on
the
1. nature of adsorbent and its surface area.
2. nature of adsorbate and the partial pressure of gas.
3. temperature.
31. Depending on the nature of interaction, Adsorption is
classified into physical adsorption and chemical
adsorption.
32. Detergency
Surfactants in aqueous solutions are used to remove
the dirt from substrates such as glass, fabric, skin etc.
Effective detergents are required for the cleaning of
production equipment, containers for packing and
maintain hygiene in the industry.
33. Steps of DETERGENCY are:
Initial wetting of the dirt from the surface.
Solubilizing of the dirt.
Removing the insoluble dirt as deflocculating particles.
Suspending the particles in the detergent solution.
Removing the oil soluble materials and convert into
emulsion.
Converting the dirt into foam so as wash easily.
The HLB requirement for the detergency is about 13 to
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