Interfacial Phenomenon

INTERFACIAL PHENOMENA
Presented by
(Dr) Kahnu Charan Panigrahi
Asst. Professor, Research Scholar,
Roland Institute of Pharmaceutical Sciences,
(Affiliated to BPUT)
Web of Science Researcher ID: AAK-3095-2020
12/14/2021 INTERFACIAL PHENOMENON BY KC 1

INTRODUCTION
• Interface is the boundary between two phases.
• Surface is a term used to describe either a gas-
solid or a gas- liquid interface.
• Definition: Surface tension is the force per
unit length that must be applied parallel to the
surface to counterbalance the net inward pull. It
has the units of dynes/cm or N/m.
• Interfacial tension is the force per unit length
existing at the interface between two
immiscible phases (units are dynes/cm or N/m).
𝜸= F/L

• Surface tension maintains the surface area of liquid to a minimum value.
• If the surface of the liquid increases the energy of the liquid also increases.
• Because this energy is proportional to the size of the free surface, it is called
as surface free energy.
• Surface free energy is defined as the work required to increase the surface
area by 1 sq cm.
INTERFACIAL PHENOMENON BY KC
12/14/2021 3
Surface Free Energy
𝑾 = 𝜸 ∆ 𝑨
𝑾 Surfacefree energy(ergs)
𝜸 surface tension (dynes/cm)
∆ 𝑨increase in area (cm2).

• Let’s consider a ABCD rectangular wire as shown in figure.
• The side AD of length L is movable.
• A drop of soap solution is placed on frame so that it will form a film within
the frame.
• The side AD remain stable until a downward force f is applied.
• After applying force side AD move to a distance d as shown in figure.
• The work done is given by:
12/14/2021 INTERFACIAL PHENOMENON BY KC
PANIGRAHI
4
W = F X d
The soap film has two surface each having length L
F = γ X 2 L
γ = F X 2L
By putting this in equation 1
W = γ X 2L X d
W = γ X dA

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METHODS OF SURFACE TENSION MEASUREMENTS:-
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There are several methods of surface tension measurements:
1. Capillary rise method
2. Dunouy’sring method
2. Stallagmometer method

CapillaryRiseMethod
𝜸
𝒓
𝒉
𝒑
𝒈
surface tension
radius of capillary
height
density of the liquid
acceleration of gravity
This method cannot be used to obtain interfacial tensions.
When a capillarytube is placed in a liquid contained in a beaker, the liquid rises
up in the tube to a certain distance. By measuring this rise in the capillary, it is
possible to determine the surface tension of the liquid using the formula:
𝜸 = ½ 𝒓𝒉𝒑𝒈
PANIGRAHI
7
a
b
𝜽

Upward Component:-
For this purpose, a thin circular capillary is dipped into the tested liquid. If the interaction
forces of the liquid with the capillary walls (adhesion) are stronger than those between the
liquid molecules (cohesion), the liquid wets the walls and rises in the capillary to a defined
level and the meniscus is hemi spherically concave.
In the opposite situation the forces cause decrease of the liquid level in the capillary below
that in the chamber and the meniscus is hemi spherically convex.
If the cross-section area of the capillary is circular and its radius is sufficiently small, then
the meniscus is hemispherical. Along the perimeter of the meniscus there acts a force due to the
surface tension presence. 𝒄𝒐𝒔𝜽 = a / 𝜸
a= 𝟐𝝅𝒓𝜸𝒄𝒐𝒔𝜽
[r= the capillary radius, 𝜸 = the liquid surface tension, ⱺ= the wetting contact angle]
The upword force a is equilibrated by the mass of the liquid raised in the capillary to the
height h, that is the gravity force b.
12/14/2021 8

12/14/2021 9
Downward Component:-
In the case of non-wetting liquid , it is lowered to a distance h.
At equilibrium: a = b
𝟐𝝅𝒓𝜸𝒄𝒐𝒔𝜽 = 𝝅𝒓𝟐hpg
𝜸 =
𝒓𝒉𝒑𝒈
𝟐𝒄𝒐𝒔𝜽
For liquid completely wetting wall of capillary ⱺ = 0 so equation becomes 𝛄 =
𝐫𝐡𝒑𝐠
𝟐
b= 𝝅𝒓𝟐hpg
Method:-
 A clean dry capillary is fixed on a stand horizontally.
 Microscope & cross wire are adjusted to measure internal diameter(d).from which r is
estimated
 50 ml water is taken in a 100ml beaker.
 Then capillary is dipped in it such that pointer just touches the water surface.

 h1=Height of water level in capillary during attachment of beaker
h2=Height of water in capillary after removal of beaker
Rise of liquid in capillary (h)= h2-h1
 Density (p) of liquid is estimated by a specific gravity or pychnometer method.
 Using the value of r, h, p surface tension can be estimated.
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𝜸 = ½ 𝒓𝒉𝒑𝒈

The Du-Nouy Ring Method
 This method is used to measure both surface &
interfacial phenomena.
 it is a rapid process of determination & only a small
amount of liquid can be determined.
 A platinum ring can be used, which is submerged in the
liquid.
 As the ring is pulled out of the liquid, the force required
to detach it from the liquid surface is precisely measured.
 The force necessary to detach a platinum–iridium
ring immersed at the surface or interface is
proportional to the surface or interfacial tension.
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UPWARD PULL=>
When ring is pulled upward then some liquid rises from its level. And force is recorded (in
dyne)by help of torsion wire.
Upward pool =dial reading in dynes
DOWNWARD PULL=>
Weight of the liquid under ring & also interfacial tension makes a downward pool to balance the
upward pool.
Downward pool =mg = 𝜸 ∗ 𝟐𝝅𝒓 ∗ 𝟐
At equilibrium Upward pool = Downward pool
Dial reading in dynes = 𝜸 ∗ 𝟐𝝅𝒓 ∗ 𝟐
12/14/2021 12

Method
• A cleaned platinum-iridium ring is attached to the hook at the end of the torsion lever arm.
• A more dense liquid is transferred into a clean glass vessel and placed on the table.
• The table is moved beneath the ring and raised until the ring is immersed in the liquid.
• The torsion arm is released by rotating the torsion adjusting knob. The instrument is
adjusted to zero reading.
• The torsion knob is adjusted until the index and its image is exactly in line with the
reference mark on the mirror.
• The light liquid is poured on the surface of the heavier liquid.
• During the next step, two operations are done simultaneously.
(l) The sample table is lowered and therefore, the lever (ring) is pulled down.
(2) The torsion knob is adjusted so as to induce upward-pull.
• The ring at the interface between two liquids will become distended, but the index is kept
on the reference.
• These two adjustments are continued until the distended film at the interface ruptures.
• The scale reading at the breaking point of the interfacial film is the apparent interfacial
tension.
12/14/2021 13

Stallagmometric Method:-
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• For this purpose fixed volume of test liquid and standard liquid is
leaked out from glass capillary of the stalagmometer and then
were weighed.
• By using the value of w1& w2,the surface tension of liquid can be
determined.
• This method was first time described by Tate in 1864 who formed
an equation, which is now called theTate’s law.

12/14/2021 15
v1 = F/L
v1 = w1 / 2‫ח‬ r
w1 = 2‫ח‬ r v1……………………….. (1)
[w= drop weight, r = the capillary radius, v1 = surface tension of the standard liquid]
v2 = F/L
v2 = w2 / 2‫ח‬ r
w2 = 2‫ח‬ r v2……………………….. (1)
[w= drop weight, r = the capillary radius, v2 = surface tension of the test liquid]
By solving both the equation

Spreading of liquid
• When Oleic acid dropped on water, it immediately spreads on
the surface of water
• Oleic Acid – Spreading Liquid (L)
• Water – Sub-layer Liquid (S)
• Generally spreading occurs when adhesive force is more
than cohesive force
12/14/2021
PANIGRAHI
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• Work of Cohesion (W ) may be
c
defined as the surface free energy
increased by separating a column of
pure liquid into two halves
• Surface free energy increase = γ dA
• Wc = γL (dA+dA) = 2 γLdA
• Here the column is of cross sectional
area is 1cm2 (dA= 1cm2)
• Wc = 2 γL

• Work of Adhesion (Wa) may be defined
as the surface free energy increased by
separating a column of two immiscible
liquids at its boundary into two sections
• As two sections of immiscible liquids
are already separated by a boundary, the
energy requirement will be less by an
amount γLS dA
• Wa= γLdA + γS dA - γLS dA
• Here the columns are of cross sectional
area 1cm2
• Wa = γL + γS - γLS

• Spreading coefficient (S) is the difference between work of adhesion
and work of cohesion
S = Wa –Wc
= (γL + γS – γLS) - 2γL
= γS – γL – γLS
• S = γS – (γL + γLS)
• γL - Surface tension of spreading liquid
• γS - Surface tension of sublayer liquid
• γLS - Interfacialtension

• Spreading occurs when spreading coefficient S is positive i.e., γS > (γL+
γLS). When free energy of the spreading liquid and the interfacial tension
with the sub layer is less than that of sublayer the spreading becomes
spontaneous to reduce free energy of sublayer.
• If spreading coefficient S is negative ie, (γL+ γLS) > γS Spreading liquid
forms globules or floating lens means spreading will not take place

Surface ActiveAgents
• Molecules and ions that are adsorbed at interfaces are termed
surface-active agents or surfactants.
• Surfactants have two distinct functional groups in their chemical
other of
structure, one of which is water-liking (hydrophilic) and the
which is lipid-liking (lipophilic).
• These molecules are referred to as amphiphile.

Surface ActiveAgents
• When such molecule is placed in an air-water or
oil-water system, the polar groups are oriented
toward the water, and the nonpolar groups are
oriented toward the air or oil.
• When surfactants are dissolved in water they can
reduce surface tension by replacing some of the
water molecules in the surface so that the forces
of attraction between surfactant and water
molecules are less than those between water
molecules themselves, hence the contraction
force is reduced.

Classification of surface active agents
• Non-ionic surfactants
Have low toxicity and high stability and compatibility
,
e.g. Sorbitan esters (spans) and Polysorbates (tweens).
• Anionic surfactants
Have bacteriostatic action
e.g. Sodium Lauryl Sulphate
• Cationic surfactants
Have bactericidal activity
e.g. benzalkonium chloride
• Ampholytic Surfactants
Phospholipids
PANIGRAHI
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HLB System
hydrophile-lipophile balance
• Definition: The
(HLB) system is an arbitrary scale for
expressing the hydrophilic and lipophilic
characteristics of an emulsifying agent.
• Agents with HLB value of 1-8 are lipophilic and
suitable for preparation of w/o emulsion,
• Those with HLB value of 8-18 are hydrophilic
and good for o/w emulsion.

12/14/2021 25
ACTIVITY HLB
ANTIFOAMING AGENT 1-3
EMULSIFIER(W/O) 3.5-6
WETTING AGENT 7-9
EMULSIFIER(O/W) 8-18
DETERGENT 13-15
SOLUBILIZER 15-18

12/14/2021 26
TYPE NAME OF EMULSIFIER HLB
ANIONIC GLYCERYL MONOSTERATE (SELF EMULSELFING) 5.5
TRIETHNOLAMINE OLEATE 12
SODIUM OLEATE 18
SODIUM LAURYL SULPHATE 40
CATIONIC BENZALKONIUM CHLORIDE
N-CETYL,N-ETHYL MORPHOLINUM 25-30
ETHO SULPHATE
NONIONIC SORBITAN MONOOLEATE(SPAN-80) 4.3
SORBITAN MONOLAURATE(SPAN-20) 8.6
POLYOXYETHYLENE MONOSTERATE(TWEEN-60) 11.1
POLYOXYETHYLENE SORBITAN MONOOLEATE(TWEEN-
80)
15
POLYOXYETHYLENE SORBITAN MONOLAURATE(TWEEN-
20)
16.7
AMHOTERIC LECITHIN,N-ALKYL AMINOACID
STERATE 60
OLEATE 80
LAURATE 20
PALMITATE 40

• HLB = Σ (Hydrophilic group) – Σ (Lipophilic group) + 7
• Polyhydric Alcohol Fatty Acid Esters (Ex. Glyceryl monostearate)
HLB = 20 ( 1 – S / A )
S = Saponification number of the ester
A = Acid number of the fatty acid
• Surfactants with no Saponification no (Ex. Bees wax and lanolin)
HLB = (E + P) / 5
E = The percent by weight of ethylene oxide
P=The percent by weight of polyhydric alcohol group in the molecules
• Surfactants with hydrophilic portion have only oxyethylene groups
HLB =E / 5
12/14/2021 27
Method of estimation

Micellar solubilization
• Surfactants can lower surface tension & improve the
dissolution of lipophilic drugs in the aqueous
medium.
• A surfactant, when present at low concentrations in
a system, adsorbs onto surfaces or interfaces
significantly reducing the surface or interfacial free
energy.
• When the concentration of surfactants exceeds their
critical micelle concentration (CMC), micelle
formation occurs, entrapping the drugs within the
micelles.
• This process is known as micellisation and generally
results in enhanced solubility of poorly soluble drugs.
12/14/2021 28

Detergency
• Detergency is a process by which soil is removed from a surface and
undergoes solubilization or dispersion.
• The HLB requirement for detergency is about 13-16.
• It result of several physicochemical phenomenon at the interface of three
phases : surface/soil/detergent.
 Wetting of surface
 Solubilisation of dirt
 Removing soluble dirt as deflocculated particle
 Suspending particle in detergent solution
 Removing oil soluble material as emulsion
 Converting dirt into foam
 Avoiding re-deposition of soil on surface
12/14/2021 29

12/14/2021 30

Wetting phenomenon
• Wettability is defined as the tendency of one fluid to spread on or adhere to
a solid surface in the presence of other immiscible fluids.
• The tendency of a liquid to spread over the surface of a solid is an indication
of the wetting characteristics of the liquid for the solid.
• Contact angle is the angle between liquid droplet and surface over which it
spreads.

• As the contact angle decreases, the wetting characteristics of the liquid
increase.
• Complete wettability would be evidenced by a zero contact angle, and
complete nonwetting would be by a contact angle of 180° .
• Surface active agent decrease the interfacial tension and also lower the contact
angle.
• The HLB requirement to act as wetting agent is 6 to 9.
γs= γSL + γL cos ϴ
• Weknow that spreading coefficient ‘S’
S= γS – γL – γLS
• Combining both these equations, substituting value of γs in second equation
S = γL (cos ϴ-1)
• The surface tension obtained at cos ϴ =1 or ϴ = 0 is critical surface tension.
12/14/2021 32

Electrical properties
Surface charge – zeta potential :-
Let’s consider solid particle are dispersed in an aqueous solution containing
electrolyte .
Distribution of charges are shown in the fallowing fig.
Assuming the cation are absorbed at interface
PANIGRAHI
33

The interface:
• aa’ the actual plane is the solid liquid interface and it is assumed that the
cation are adsorbed in the interface and impart +ve charges.
Tightly bound layer:
• Immediately adjacent to the interface aa’ is the region of tightly bound
layer and it extend up to bb’.
• Once the adsorption is complete the cation attract few anion and repel the
approaching cation.
• Thus at equilibrium some excess anion are present at this region however
their number is less than adsorbed cation. Therefore bb’ the shear plane
still impart +ve charge.
• The degree of attraction of certain molecule and counter ion is such that
shear plane is bb’ rather than aa’.
PANIGRAHI
34

Diffused 2nd layer:
• This is the region bound by the line bb’ and cc’.
• In the layer excess –ve ion are present.
• At and beyond cc’ the charge is electrically neutral.
• As a whole the system is electrically neutral.
• Thus the electrical distribution at the interface is equivalent to the double
layer which consist of tightly bound layer and diffused 2nd layer.
• When the interface adsorbed –ve ion than aa’ is negative, bb’ is negative and
cc’ is neutral.
Nernst potential and zeta potential:
• Nernst potential is the potential at actual surface itself i.e. aa’ due to presence
of potential determining ion. It is denoted as E and also called electrodynamic
potential.
PANIGRAHI
35

• It is defined as the potential difference between the actual surface and the
electro neutral region of the solution.
• Zeta potential is the potential observed at the shear plane i.e. bb’.
• Zeta potential is also known as electrokinetic potential.
• It is defined as the potential difference between surface of tightly bound layer or
shear plane and electroneutral region.
• Zeta potential also defined as the work required to bring unit charge from
infinite to surface of particle.
PANIGRAHI
36

Adsorption on solidsurface

• Adsorption is a surface phenomenon whereas absorption is a bulk
phenomenon.
• Desorption (evaporation) is the reverse of adsorption
• Physical adsorption, in which the adsorbate is bound to the surface
through the weak van der Waals forces.
• Chemical adsorption or chemisorption, which involves the stronger
electrostatic forces.
• Adsorbate: material which get adsorb (Gas/solute) (x moles)
• Adsorbent: material on which adsorption takes place (m grams)
Introduction

• The graph between gas adsorbed per unit area or unit mass of solid vs pressure
at constant temperature is called adsorption isotherm.
• Freundlich isotherm is expressed as:
y = x/m = 𝒌𝒑𝟏/𝒏
x = weight of gas adsorbed per unit weight of adsorbent,m
p = equilibrium pressure
k and n = constants
• The equation can be converted to logarithmic form as:
𝐥𝐨𝐠
𝐱
𝐦
= 𝐥𝐨𝐠 𝐤 +
𝟏
𝒏
𝐥𝐨𝐠 𝑷
12/14/2021 39
Freundlich isotherm

Freundlich isotherm
y = x
= kp1/n
m
Log x
m n
= Log k + 1
Log p
y =
x
m
= kp1/n

Langmuiradsorptionisotherm
Fallowing are the assumption for this isotherm:
• The surface of solid has fixed no of site for adsorption
• The thickness of layer is uni molecular
• Rate of adsorption proportional to number of site unoccupied
• Rate evaporation proportional to number of site occupied
Let’s consider at a particular pressure P,
Fraction of active sites occupied = ϴ
Fraction of un-occupied sites = 1- ϴ
Rate of adsorption r1=k1(1-ϴ)p
Rate of desorption r2=k2ϴ

ϴ =
k1p
k2+k1p
ϴ = (k1/k2)p
k
k2
1+( 1)p
Replacing(k1/k2) with b and ϴ with y/ym
• y= mass of gas adsorb per gram of adsorbent at pressure p
• ym= mass of gas that 1 gram of adsorbent adsorb when
monolayer is formed
k2ϴ = k1(1-ϴ)p
At equilibrium r2=r1
k1p– k1pϴ= k2ϴ
(𝑦/𝑦𝑚) = bp/(1+bp)
y = 𝑦𝑚𝑏𝑝/(1 + 𝑏𝑝)
p/y =
1
𝑦𝑚𝑏
+
𝑝
𝑦𝑚
Inverting above equation and multiplying with p we get
1/y =
1
𝑦𝑚𝑏𝑝
+
𝑏𝑝
𝑦𝑚𝑏𝑝

• Above equation represent Langmuir isotherm.
• The plot of p/y against p gives straight line.
• ym can be obtained from slop and b can be obtained from intercept
𝒑
𝒚
=
𝟏
𝒚𝒎𝒃
+
𝒑
𝒚𝒎

BET Equation:
12/14/2021 44
Sometime gas adsorbed as multimolecular layer on solid. The expression
for this was derived by Brunaer, Ennet and Teller and termed as BET
equation which is given by
𝒑
y(po−p)
=
𝟏
ymb
+
𝒃−𝒑
ymb
𝒑
po
where,po = saturated vapour pressure
b = constant alpha heat of adsorption
y= mass of gas adsorbe per gram of adsorbant at pressure p
ym=mass of gas that 1 gram of adsorbent adsorb when monolayer is formed

12/14/2021 45
APPLICATION OF BET :
• The surface area per unit weight i.e specific surface area can be predicted.
• The type of isotherm can be determined. If b > 2.0 then type 2 and if b<2.0
then type-1 isotherm.
• The point of monolayer formation can be identified from the graph.

Adsorption Isotherm
12/14/2021 46
Adsorption isotherm are defined as the plot drawn between the amount of gas
adsorbed on a solid (y- axis) against the equilibrium pressure(x – axis) at constant
temperature. There are 5 different type of adsorption isotherm:

Type I :
• This adsorption isotherm represents a significant increase in the adsorption
with increasing pressure and fallowed by levelling off.
• This levelling off is due to saturation of available specific chemical groups
on the entire surface.
• This is same as Freundlich or Langmuir adsorption isotherm.
• e.g: Adsorption of nitrogen gas on charcoal.
Type II :
• This isotherm is ssigmoidal in shape and occur when gas undergo physical
adsorption on non-porous solid.
• The first inflection point represent formation of monolayer.
• When pressure increases multilayer formation observed.
• Described by BET equation. Constant b is greater than 2.
• Eg: Adsorption of nitrogen on platinum catalyst.

Type III:
• The heat of adsorption of gas in first layer is less than latent heat of
condensation in successive layers.
• Described by BET equation. Constant b is less than 2.
• e.g.: Adsorption of bromine on silica.
Type IV:
• This plot represents the adsorption of gas on porous solid.
• The first inflection point represent formation of monolayer.
• Condensation within the pore results multi molecular layer formation.
• E.g: Adsorption of benzene on silica.
Type V:
• Represented by capillary condensation.
• Adsorption reaches a limiting value before P0
• e.g: Adsorption of water vapour on charcoal.
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Thankyou

Interfacial Phenomenon

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