When phases exist together, the boundary between two of them is known as interface. When the phase is in contact with atmosphere it is termed as surface.

1. Prepared and Presented by:- Ms. Kajal A. Pradhan
Assistant Professor
Smt. R. D. Gardi College of Pharmacy
Surface and Interfacial Tension
Surface and Interfacial tension

2. Topic:
 Liquid Interface
 Surface and Interfacial tensions
 Surface free energy
 Measurement of surface and interfacial tension
 Spreading co-efficient
 Adsorption at liquid interface
 Surface active agents
 HLB scale
 Solubilisation
 Detergency
 Adsorption at solid interface
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3. What is interface and surface?
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 When phases exist together, the boundary between two of them is known as interface.
 The properties of the molecules forming the interface are often sufficiently from those in the bulk of
each phase that they are referred to as forming an interfacial phase
 When the phase is in contact with atmosphere it is termed as surface.
OR
 The term surface is normally used to denote interface, when one of the phase is Gas.
 All surface are interface but not all interface are surface

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L-G
L-L
L-S
S-S
Ex:- powder granule for
tablet
Ex:- Emulsion containing
drug particles

5. Different types of interface
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6. Common terms:
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 Cohesive forces:
“Intermolecular attractions between like molecules”
 Adhesive forces:
“Intermolecular attractions between unlike molecules”
 Liquid interface:
The boundary between 2 immiscible liquids is termed as
liquid interface.

7. Surface tension:
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Force applied not
equal in all direction
Force applied equal in all
direction thus in
equilibrium
Ex:-

8. Surface tension:
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 a force pulling the molecules of the interface together resulting in a contracted
surface.
 Force per unit area applied parallel to the surface.
 Unit in dynes/cm or N/m
OR
 The force in dynes acting on the surface of the liquid at right angle to any line
in 1 cm is known as surface tension.

9. Interfacial tension:
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 It is the force per unit length that exist at the interface between two
immiscible liquid phases.
 Interfacial tensions are less than surface tensions because an adhesive
forces, between the two liquid phases forming the interface are greater
than when a liquid and a gas phase exist together.
 If two liquids are completely miscible, no interfacial tension exists between
them.

10. Importance of Interfacial phenomena in pharmacy:
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 Adsorption of drugs into solid adjuncts in dosage forms
 Penetration of molecules through biological membranes
 Emulsion formation and stability
 The dispersion of insoluble particles in liquid media for suspensions.

11. Surface Free Energy:
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 “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=surface free energy (ergs)
𝜸=surface tension (dynes/cm)
∆𝑨 increase in area (cm2 ).

12. Surface Free Energy:
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 Therefore, surface tension can also be defined as the surface free energy per unit area of
liquid surface.
 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:

13. Problem sum:
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 What in the work required to increase area of a liquid droplet by 10
cm sq if the surface tension is 49 dynes/cm?
W = ϒ * Δ A
= 49 dynes/cm x 10 cm sq
= 490 ergs

14. Method for measurement of Surface & interfacial
tension:
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15. The choice of the method for measuring surface and
interfacial tension depend on:
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 Whether surface or interfacial tension is to be determined.
 The accuracy desired
 The size of sample.

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 If a capillary tube of inside radius =r immersed in a liquid that wet its surface, the liquid continues
to rise in the tube due to the surface tension, until the upward movement is just balanced by the
downward force of gravity due to the weight of the liquid
 Surface tension at any point of circumference of the capillary tube = γ cos Ө
 Total upward force f1 = 2 π r γ cos Ө
 Where Ө = the contact angle between the surface of the liquid and the capillary wall
 2 π r = the inside circumference of the capillary.
Conti....
(1)

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 This method was first time described by Tate in 1864 who formed an equation, which
is now called the Tate’s law.
 Up to 40% of the drop volume may be left on the stalagnometer tip. Therefore a
correction f has to be introduced to the original Tate's equation.
 𝑾′ = 𝟐𝝅𝒓𝜸𝒇
 the weight of the falling drop W' is lower than W.
 f expresses the ratio of W’/ W.
 Harkins and Brown found that the factor f is a function of the stalagmometer tip radius,
volume of the drop v, and a constant, which is characteristic of a given stalagmometer,
f = f (r, a, v).

23. By using this principle surface tension can be found
by 2 methods:
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1. Drop weight method:-
Relative surface tension
Where w1= wt. of drop of sample
w2= wt. of drop of distilled water

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2. Drop count method:-
Relative surface tension
Where n1= no of drop of sample
n2= no of drop of distilled water

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