surface and interfacial intension

SURFACE & INTERFACIAL TENSION

Md. Asif Hasan niloy

Pharmacy 3rd semester (24 Batch)

1SURFACE

When two phases are in contact with each other and one of the two phases is in gas or,

vapor or, air than the boundary between them is called surface.

TYPES

There are two types of surface is possible.

Name of Surface Phases Example

i. Solid Surface Solid and gas Table Top

ii. Liquid Surface Liquid and gas Body of water exposed to atmosphere

INTERFACE

When the phase exists together, the boundary between two of them is termed as interface.

The phases must be immiscible.

TYPES

Interface may be divided into two types.

Name of interface Phases Example

i. Solid interface Solid - Solid Powder particles in with each other

Solid - Liquid Suspension

ii. Liquid interface Liquid - Liquid Emulsion




2SURFACE TENSION

Surface free energy per unit of surface of the liquid is called surface tension. Surface tension is

the phase per unit length that must be applied parallel to the surface so as to counterbalance

to net in ward pull.

Unit: Unit of the surface tension is dyne/cm.

Surface tension = where, f = force, L = length

Example: Surface tension of H2O at 20℃ is 72.8 dyne/cm.

INTERFACIAL TENSION

Interfacial tension is the force per unit length existing in the interface between to immiscible

liquid phase.

Unit: Unit of the interfacial tension is dyne/cm.

In the general science all tension may be referred to as interfacial tension.

Example: The interfacial tension of Benzene against water at 20℃ is 35.0

ADHESIVE FORCE

The forces which act between molecules of different phases are called adhesive force.

Adhesional force tends to increase the affinity of two phases.

For Example: The force acting between water and oil is called adhesive force.

COHESIVE FORCE

The force acting between the molecules of the same phase is termed as cohesive force.

For Example: The force acting between water molecules. Cohetional force tends to

keep the phase separated.




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CONTACT ANGLE

The contact angle is an angle that a liquid

creates with a solid surface or capillary walls of

a porous material when both materials come in

contact together. This angle is determined by

both properties of the solid and the liquid and

the interaction and repulsion forces between

liquid and solid and by the three phase interface

properties (gas, liquid and solid).

METHODS OF MEASURING SURFACE TENSION

There are methods available for the measurement of surface tension. Such as-

i. Withelmy Plate Method.

ii. Ring Method (Du Nouy Tensiometer).

iii. Drop Weight and Drop Volume Methods.

iv. Capillary Rise Method.

v. Dynamic Methods

SURFACTANTS / SURFACE ACTIVE AGENTS / AMPHIPHILES

Surfactants are compounds that lower the surface tension of a liquid, allowing easier

spreading, and lowering of the interfacial tension between two liquids, or between a liquid

and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents,

and dispersants.

Characteristics: i. Hydrophilic and Lipophilic.

ii. Reside at interfaces and lower the interfacial tension.

iii. Can be Synthetic or natural.




4CLASSIFICATION OF SURFACTANT

Depending on their ionization in aqueous solutions surfactant can be classified as:-

i. Anionic Surfactant.

ii. Cationic Surfactant.

iii. Non-ionic Surfactant.

iv. Amphoteric Surfactant.

Anionic Surfactant:

They ionize in aqueous solution into a large anion which is responsible for their emulsifying

ability.

Characteristics: i. Has a negative charge.

ii. Widely used in the pharmaceutical and cosmetic company.

iii. Have an unpleasant test and have skin irritation potential.

iv. Not compatible with cationic surface active agents.

v. Compatible with non-ionizes and amphoteric surface active agents.

Example: Potassium stearate, Calcium and Aluminum stearate, Oleic acid.

Amphoteric Surfactant:

Amphoteric surfactants are the substances whose ionic characteristics depend on the pH of

the system. Below a certain pH, these are cationic while above a defined pH, these are

anionic. At intermediate pH these behave as zwitterions.

Characteristics: i. can be cationic, anionic or zwitterionic.

ii. Compatible with all types of surface active agents.

Example: Lecithin and N-dodecyl alanine.

Cationic Surfactant:

They ionize in aqueous solution into a large cation which is responsible for their

emulsifying ability.

Characteristics: i. Has a positive charge.

ii. Can be used as bactericidal agents and secondary emulsifying agents.

iii. Absorb onto negatively charged surfaces.

iv. Are used as hair conditioners and fabric softeners.

v. Are electrolytes.

vi. Compatible with nonionic.




5Example: Cetrimide, Germicide, ammonium bromide,.

Non-ionic Surfactant:

Non-ionic surfactants comprise the largest group of surface active agents that is used

pharmaceutically.

Characteristics: i. Has no charge.

ii. Not electrolytes.

iii. Are not affected much by the presence of salts or charges in pH.

iv. Hydrophilic group may contain hydroxyl groups, saccharides.

Example: glycerol monostearate and propylene glycol monostearate.

Advantage: The advantage of these agents includes their compatibility with both

anionic and cationic surfactants, their resistance to pH change and effects

of electrolytes and lower irritancy as compared to other surfactants.

Disadvantage: These agents have tendency to inactive preservatives having phenolic or

carboxylic acid groups when present in excess quantities.

SURFACTANT CLASS SURFACE ACTIVE AGENT

1. Anionic

a. Alkali Soaps Potassium Stearate

b. Organic Sulphates Sodium lauryl sulphate

c. Organic Sulphonates Sodium cetylsulphonate

2. Cationic a. Quaternary ammonium compounds Cetrimide

b. Pyridinium compounds Dodecyl pyridinium chloride

3. Ampholytic Amino acids N-dodecyl-alanine

4. Non-ionic

a) Alcohol-poly-ethylene glycol ethers Polyethylene glycol

1000 monocetyl ether

b) Fatty acid poly-ethylene glycol esters Polyethelene glycol

40 monostearate

c) Fatty acid poly hyhydric alcoholesters Sorbitan mono-oleate




6MEASUREMENT OF SURFACE AND INTERFACIAL TENSION

CAPILLARY RISE METHOD

This method is suitable for measuring the surface tension and not for interfacial tension.

When a capillary tube is placed in a liquid contained in a beaker, the liquid immediately rises

up the tube to a certain height.

A capillary tube of radius r is vertically inserted into a liquid. The liquid rises to a height h

and form a concave meniscus. The surface tension (γ) acting along the inner circumference of

the tube exactly supports the weight of the liquid column.

By definition, surface tension is force per 1 cm acting at a tangent to the meniscus surface. If

the angle between the tangent and the tube wall is θ, the vertical component of surface

tension is γcosθ. The total surface tension along the circular contact line of meniscus is 2πr

times. Therefore,

Upward Force = 2πrcosθ

Where r is the radius of the capillary. For most liquids, θ is essentially zero, and cosθ = 1.

Then the upward force reduces to 2πrγ.

The downward force on the liquid column is due to its weight which is mass × g.

thus,

Downward Force = hπr2dg

Where d is the density of the liquid.

But Upward Force = Downward Force

2πrnγ = hπr2dg

γ = dynes/cm

In order to know the value of γ, the value of h is found with the help of a travelling

microscope and the density (d) with a pyknometer.




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Precautions:

i. The capillary tube used for the experiment should be of circular cross-section and

should have a uniform diameter throughout its length.

ii. The vessel used during the experiment should have a large diameter.

iii. The capillary tube and the other apparatus should be absolutely clean.

iv. The height of the column should be measured accurately.

v. The temperature should be maintained constant throughout the experiment.

Related math

Q. Determine the surface tension of a 0.5% solution of a surfactant that hasa density of 1.012 g/cm3 which rises to a height of 3.4 cm in a capillary tubehaving an inside diameter of 0.05cm.

Solution

Density of the surfactant solution (ρ) = 1.012 g/cm3

Inside diameter of the capillary tube = 0.05 cm

Inside radius of the capillary tube (r) = . = 0.025 cm

Height to which liquid rises in the capillary tube (h) = 3.4 cm

Acceleration due to gravity (g) = 981 cm/sec2

Surface tension of the surfactant solution:

γ = ℎ= × 0.025 × 3.4 × 1.012 × 981

= 42.19 dynes/cm




8DROP WEIGHT & DROP COUNT METHODS

If a liquid is allowed to fall slowly through a capillary tube, the liquid first forms a drop at

the tip of the tube which gradually increases in size and finally detaches from the tip when

the weight of the drop equals the total surface tension at the circumference of the tube.

Expressing mathematically,

W = 2 r

Where, w is the one weight of one drop of the liquid

r is the radius of the capillaryγ is the surface tension of the liquid.

Surface tension of a liquid can be determined using the above principle by either of the

following two methods using a drop pipette (Stalagmometer).

1. Drop weight Method

2. Drop count Method

Drop weight Method

The drop pipette or stalagmometer consists of a glass tube

with a bulb blown approximately in the middle of the

tube. There are two markings A and B on the tubes, one

above the bulb and the other below it. There is a capillary

bore at the tip of the stalagmometer.

The stalagmometer is clamped vertically and the given

liquid, whose surface tension is to be determined, is sucked

into it upto the mark A. The liquid is then allowed to drop slowly from the tipoff the

pipette. Twenty to thirty drops are collected from the pipette into a clean tarred vessel and

the weight of one drop of the liquid is determined (w). The surface tension of the liquid is

then given by

W = 2 rOr, =




9It is a general practice to determine and report the surface tension of a liquid with respect to

water. For this, the liquid is first taken in the pipette. The weight of one drop of liquid (w1) is

determined as described above. Similarly water is taken up in the pipette and the weight of

one drop of water (w2) is obtained.

The relative surface tension of the liquid

=

= =

Drop Count Method

The method is similar to the drop weight method except that the number of drops of the

liquid formed is counted instead of weighing.

In this method, the given liquid is sucked into the stalagmometer upto the mark A.

Keeping the pipette vertically, the number of drops formed when the liquid level falls from

the mark A to B is counted.

We know that,

γ =

=

Where m is the mass of 1 drop, g is the gravitational force and n is the number of drops.

Or, γ =

Where, vd = volume × density = mass (m)

For determination of relative surface tension of a liquid, the number of drops of the liquid

(n1) as well as water (n2) formed for the same volume are determined.




10The relative surface tension of the liquid is given by:

=

=

= = ×

Precautions:

i. The tip of the pipette should have no imperfections in the outer circumference and

should be completely weighted by the liquid.

ii. The drops should be allowed to form slowly.

iii. About 20 to 30 drops should be collected and from this, the average weight of

one drop should be determined.

iv. The temperature should be maintained constant throughout the experiment.

HLB System

Surfactants comprise of both polar and non-polar groups in their molecule. Surfactants with

more polar groups are predominantly hydrophilic while those with more non-polar groups

are predominantly lipophilic. The balance between the hydrophilic and lipophilic nature of

the surfactant may be given by means of HLB system. It was first introduced by Griffin in

1949 for classification of non-ionic surfactants




11HLB Value

HLB value probably estimated by employing the following formula:

HLB = 20 (1 – S/A)

Where,

S is saponification number of the ester

A is the acid number of the fatty acid.

Example: oleic acid’s HLB value is 1.0

##Not Need: For materials such as beeswax lanolin derivatives with which it is not possible

to obtain good saponification number, the following formula may be used:

HLB =

Where,

E is % w/w of oxyethelene chain and

P is % w/w of polyhydric alcohol group in the molecule.

SURFACE ACTIVE AGENT HLB value

Oleic acid 1.0

Glyceryl monostearate (pure) 3.8

Sorbitan monooleate (Span 80) 4.3

Sorbitan monostearate (Span 60) 4.7

Glyceryl monostearate (Self emulsifying) 5.5

Gum acacia 8.0

Sorbitan monolaurate (Span 20) 8.6

Triethanolamine oleate 12.0

Poltoxyethylene sorbitan monooleate (Tween 80) 15.0

Polyxyethylene sorbitan monolaurate (Tween 20) 16.7

Potassium oleate 20.0

Sodium lauryl sulphate 40.0




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