DR. JAGADEESH. G. HIREMATH DEPARTMENT OF ......DR. JAGADEESH. G. HIREMATH DEPARTMENT OF...

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INTERFACIAL PHENOMENON

DR. JAGADEESH. G. HIREMATH

DEPARTMENT OF PHARMACEUTICS

CHAPTER 02

23 June 2012

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Phase Type of interface Example

Gas-Gas No interface possible

γLV Gas–Liquid Liquid surface Body of H2O exposed to

atmosphere

γSV Gas-Solid Solid surface Solid Body is exposed to

atmosphere

γLL Liquid-Liquid Liquid-Liquid interface Emulsion

γLS Liquid-Solid Liquid-solid interface Suspension

γSS Solid-Solid Solid-solid interface Powder particles contact

INTERFACIAL PHENOMENA

Interface: The boundary that is forms between two phases, say solid &

liquid is called as interface.

Several types of interface exist depending on whether the two adjacent

phase are in the solid, liquid or gaseous state.

Classification of interfaces

CHAPTER 02 23 June 2012

Surface tension:

This term is used to denote an interface when one of the phases

is gas Gas-solid or Gas-liquid interface.

Units: dynes/cm in CGS system and SI system N/m

In a liquid, molecules experience greater attraction from the

neighboring molecules. Intermolecular attractions between like

molecules are called cohesive forces while attractions between

the unlike molecules called adhesive forces.

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On the other hand, molecules at the surface develop only attractive

cohesive forces with other liquid molecules that are situated below

and adjacent to them.

The net effect is that the molecules at the surface of the liquid

experience an inward force towards the bulk of liquid. Such a force

pull the molecules of the interface together and as a result the surface

contract resulting in surface tension.

Figure.1 represents the unequal attractive force acting on molecules at the surface of a liquid as compared with molecular forces in the bulk of the liquid.


Liquid droplets tend to assume spherical shape, since the molecules

on the surface try to contract the surface, there by assuming the

small possible surface area.

Ex: When benzene and water are mixed together they remain

immiscible, form an interface its shows interfacial tension.

Interfacial tension:

Interfacial tension is the force per unit length existing at the interface

between two immiscible liquid phases and like surface tension has

the units of dynes/cm. In SI system N/m


Surface tension and interfacial tension of various liquids (against

water) at 20o C

Surface

tension

Dynes/cm

Interfacial

tension

against water

Dynes/cm

substance substance

Water 72.8 Mercury 375

benzene 28.9 Benzene 35

Chloroform 27.1 Chloroform 32.8

Olive oil 35.8 Olive oil 22.9


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Surface tension versus interfacial tension

*Surface tension is the tension between liquid-vapor

(gas) γ LV and solid-vapor γ SV; they are written γ L and

γ S.

*Interfacial tension is the tension between 2 immiscible

liquids γ LL, 2 solids γ SS or liquid-solid γ LS.


EXAMPLE FOR SUREFACE TENSION MEASUREMENT


A soap film is formed over the area of ABCD and can stretched by

applying the force f (such as hanging mass) to the movable bar,

length L, which acts against the surface tension of the soap film.

When the mass is removed, the film will contact owing to its

surface tension.

The surface tension γ of the solution forming the film is then a

function of the force that must be applied to break the film over the

length of the movable bar in contact with the film. Because the

soap film has two liquid–gas interfaces (one above and one below

the plane of the paper), the total length of contact is in fact equal to

twice the length of the bar.


Thus,

γ = f/2L……………………………………..Equation -1

Example: Calculating the surface tension of water

If the length of the bar L is 5cm and the mass required to break a

soap film is 0.50g. What is the surface tension of the soap solution

Recall: that the downward force is equal to the mass multiplied by

the acceleration due to gravity, Then,

γ = 0.5g x 981cm/sec2 / 10 cm = 49dynes/cm


SURFACE FREE ENERGY To a molecule from the inner layers to the surface work needs to be done against the force of the surface tension.

Each molecule on the near surface of the liquid creates a potential energy (PE) as compared to molecules in the bulk of the liquid.

BECAUSE: SURFACE AREA

Higher the surface area PE EL (energy of liquid)

Lower the surface area PE EL (energy of liquid)


To evaluate the amount of work done in increasing the

surface area, we can write Equation-1: f= γ x 2L

Recall from the Figure 1

when bar is at position AD and a mass is added to extend the surface

by a distance ds, the work dW( force multiplied by distance) is

dW= f x ds= γ x 2L x ds ……………………..Equation:2

and, because 2L x ds is equal to the increase in surface area, dA,

produced by extending the soap film,

dW= γ x dA…………………………………..Equation:3

For a finite change:

W= γ∆A:………………………………………Equation:4

W= work done or surface free energy increases expressed in ergs, γ

is the surface tension in dynes/cm and ∆A is the increase in surface

area in cm2


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CAUSE OF SURFACE TENSION

Surface tension is caused by the attraction between the liquid's molecules by various intermolecular forces.

Vander walls forces

In the bulk of the liquid: each molecule is pulled equally in all directions by neighboring liquid molecules net force of zero .


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At the surface of the liquid, molecules are pulled

inwards by other molecules deeper inside the liquid and

are not attracted as intensely by the molecules in the

neighboring medium. i.e.:

They develop attractive cohesive forces with other liquid

molecules situated below & adjacent to them.

They create adhesive forces of attraction with the other

phase involved at the interface.


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FACTORS AFFECTING SURFACE TENSION

Temperature

Surface active agents


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Surface tension of liquids decreases with increase in temperature.

E.g. γ of H2O = 75.6 dynes/cm at 0°C

= 72.8 dynes/cm at 20°C

= 63.5 dynes/cm at 75°C Why?

When temperature increase, The intermolecular cohesion forces decrease and the surface tension decrease.

TEMPERATURE


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SURFACE ACTIVE AGENTS

By adding surface active agents in water, there is a

decrease in surface tension of water.


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MEASUREMENT OF SURFACE AND INTERFACIAL

TENSION

Capillary rise method

Drop weight or drop number methods (Stalgmometer)

Ring method (du Nuoy tensiometer)


23 June 2012 CHAPTER 02 19

23 June 2012 CHAPTER 02 20

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Capillary rise method

When a capillary tube is placed in a liquid, the liquid rises

up the tube a certain distance.

Why?

The force of adhesion between the liquid molecules and

the capillary wall is greater than the cohesion between the

liquid molecules. This will cause an upward force on the

liquid at the edges and result in a meniscus which turns

upward.


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The surface tension acts to hold the surface intact, so

instead of just the edges moving upward, the whole liquid

surface is dragged upward.

*Surface tension, but not interfacial tension, can be

determined by this method


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1. Capillary rise method (cont.)


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1. Capillary rise method (cont.)

Due to surface tension (upward force), the liquid continues to rise in the tube.

Because of the weight of the liquid, the upward movement is balanced by the downward force of gravity.

Upward force = downward force

S.T. x circumference = mass x acceleration due to gravity

2 π r γ = π r2 h p g

γ = ½r h p g


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2. Drop weight method or drop number method

A known volume of a liquid is allowed to fall as drops from a dropper or a graduated pipette under the effect of gravity and the No. of drops is counted.

The drop will fall when 2 forces are balanced:

- Force due to surface tension= γ x L

= γ x 2π r

- Gravitational force = m g

γ x 2π r = m g CHAPTER 02 23 June 2012

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Taking the surface tension of distilled water as standard reference(72.8 dyne/cm at 20°C), the surface tension of other liquids can be calculated:

γ 1 / γ 2 = m1 / m2

γ 1 / γ 2 = V1 p1 / V2 p2

Where, p1/p2 can be disregarded

γ 1 / γ 2 = n2 / n1

Where n2 / n1 are the mean number of drops of two liquids.


CHAPTER 02 27 23 June 2012

DR. JAGADEESH. G. HIREMATH DEPARTMENT OF ......DR. JAGADEESH. G. HIREMATH DEPARTMENT OF...

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