Ch3. interfacial phenomena

Bule Hora University
College of Health and Medical Sciences
Department Of Pharmacy
CHAPTER 3
INTEGRATED PHYSICAL PHARMACY AND PHARMACEUTICS I
INTERFACIAL PHENOMENA
By: Aliyi Gerina [BSc, B.pharm]
4/5/2022
Interfacial Phenomena by Aliyi Gerina Bule Hora University 1

 Surface/interfacial tension
 Measuring surface/interfacial tension
 Surface free energy
 Spreading coefficient
Liquid interface
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Interfacial Phenomena by Aliyi Gerina Bule Hora
University

Interfacial Phenomena
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 Substance can exist in any of 3 phases: gas, liquid or solid.
 Surface is a term used to describe either a gas-solid or a gas-
liquid interface.
 Interface is the boundary between two phases exist together.
 Interfacial phase is a term used to describe molecules forming
the interface between two phases which have different properties
from molecules in the bulk of each phase.

Interfacial Phenomena,…
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 Several types of interface can exist, depending on whether
the two adjacent phases are
 in the solid, liquid or gaseous state.
 For convenience, we shall divide these various
combinations into two groups, namely
 liquid interfaces and
 solid interfaces.

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Classification of Interfaces

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 Important of Interfacial phenomena in pharmacy:
 Adsorption of drugs onto solid adjuncts in dosage forms
 Penetration of molecules through biological membranes
 Emulsion formation and stability
 The dispersion of insoluble particles in liquid media to form
suspensions.

Liquid Interfaces
SurfaceTensions
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 In the liquid state, the cohesive forces between adjacent molecules
are well developed.
 For the molecules in the bulk of a liquid
 They are surrounded in all directions by other molecules for
which they have an equal attraction (only cohesive forces)..
 For the molecules at the surface (at the liquid/air interface)
 Only attractive cohesive forces with other liquid molecules
which are situated below and adjacent to them.
 They can develop adhesive forces of attraction with the
molecules of the other phase in the interface.

Liquid Interfaces,…
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Fig. Unequal attractive forces acting on molecules at
the surface of a liquid as compared with molecular
forces in the bulk of the liquid.

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 The net effect is that the molecules at the surface of the liquid
experience an inward force towards the bulk of the liquid and
contract the surface with a force F, resulting in a surface tension..
 To keep the equilibrium, an equal force must be applied
 to oppose the inward tension in the surface.

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 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.

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 Visualization of surface tension as a person
lifting a rock up the side of a cliff by pulling the
rope in a horizontal direction.

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Interfacial Tension
 In the case of interface the molecules at the interface will be
pulled by both faces into the bulk
 Since COHESIVE FORCE (between like molecules) are
stronger than ADHESIVE FORCE (between unlike
molecules) the net pull will be into the bulk of same phase.

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 Interfacial tension is the force per unit length existing
at the interface between two immiscible phases.
 It has the units of dyne/cm.

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 The term interfacial tension is used for the force
between:
Two liquids =γLL Solid liquids =γSL
 The term surface tension is reserved for the tensions:
 Liquid-vapor = γ
 Solid-vapor = γ

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 Interfacial tensions are weaker than surface tensions
 because the adhesive forces between two liquid phases
forming an interface are lower than that between liquid and
gas phase.
 If two liquids are completely miscible,
 no interfacial tension exists between them.

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Surface Tension and Interfacial Tension (Against Water) at 20°C*
Substance
Surface Tension
(dynes/cm)
Substance
Interfacial Tension
(dynes/cm)
Water 72.8 Mercury 375
Glycerin 63.4 n-Hexane 51.1
Oleic acid 32.5 Benzene 35.0
Benzene 28.9 Chloroform 32.8
Chloroform 27.1 Oleic acid 15.6
Carbon tetrachloride 26.7 n-Octyl alcohol 8.52
Caster oil 39.0 Caprylic acid 8.22
Olive oil 35.8 Olive oil 22.9
Cottonseed oil 35.4 Ethyl ether 10.7
 Greater surface tension reflects higher intermolecular force of
attraction,
 thus, increase in hydrogen bonds or molecular weight cause
increase in ST.

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Surface Free Energy
 To move a molecule from the inner layers to the surface,
additional energy(work) needs to be done against the force of
surface tension.
 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.

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𝑾= 𝜸 ∆ 𝑨
𝑾 = Surface free energy (ergs)
𝜸 = surface tension (dynes/cm)
𝑨 = increase in area (cm2).
 Therefore, surface tension can also be defined as the
surface free energy per unit area of liquid surface.

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 Thermodynamically a system is stable when the free energy
is minimum.
 So systems will try to reduce the surface free energy
 by contracting the surface area and attain stability.
 Thus the greater the area of interfacial contact between the phases,
the greater the free energy.
Eg: When liquids suspended in air or immiscible liquids
 it assumes a minimum surface area to volume ( spherical shape).

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Measurement of ST and IT
 Methods for ST and IT:-
 Capillary rise method
 Ring (Du Nouy) tensiometer
 Drop weight method (Stalagmometer)
 The choice of the method for measuring surface and
interfacial tension depends on:
 Whether surface or interfacial tension is to be determine
 The accuracy desired
 The size of sample

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Capillary rise method(ST)
 When a capillary tube of inside radius ,r 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.
 It is not possible, to obtain interfacial tensions.

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 Cohesive force is the force existing between like
molecules in the surface of a liquid.
 Adhesive force is the force existing between unlike
molecules, such as that between a liquid and the wall
of a glass capillary tube.
 When the force of Adhesion is greater than the
cohesion,
 the liquid is said to be wet the capillary wall,
 spreading over it, and rising in the tube.

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 The liquid continues to rise in the tube due to the
surface tension until the upper movement is just
balanced by the downward force of gravity due to the
weight of the liquid.
At equilibrium
 Upward force (surface tension) = Downward force
(mass of liquid)

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𝜸 = ½ 𝒓 𝒉𝒑𝒈
 𝜸=surface tension
 r=radius of capillary
 h=height
 p= density of the liquid
 g=acceleration of gravity

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Ring Method (The DuNouy Tensiometer)(ST/IT)
 For measuring surface and interfacial tensions.
 The force necessary to detach a platinum ring immersed
at the surface or interface is proportional to the surface or
interfacial tension.
 The force of detachment is recorded in dynes on a
calibrated dial.

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• The ST or IT is the maximum
force needed to detach the ring
from the aqueous liquid surface.
• In effect, the instrument measures
the weight of liquid pulled out of
the plane of the interface
immediately before the ring
becomes detached from surface .

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 The surface tension is given by the formula:

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Drop weight method (Stalagmometer) (ST)
 If a liquid fall slowly from tube, forms a drop at the
end of the tube which increases in size and finally
detaches from the end when the weight of the drop
equals the surface tension.

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 If the volume or weight of a drop as it is detached from
a tip of known radius is determined, the surface tension
can be calculated from:
γ = Φ mg = Φ V pg
2 π r 2 π r
m = the mass of the drop V = the volume of the drop
p = the density of the liquid r = the radius of the tip
g = the acceleration due to gravity Φ = a correction factor.

Factors affecting surface tension
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 Surface tension will be reduced when the temperature
of the liquid increased.
 This is due to the thermal expansion of liquids
 Water has ST 76, 72,63 dyne/cm at 0˚,20 ˚, and 75 ˚ C
 This continues till the temperature of the liquid
reaches the CRITCAL TEMPERATURE of the liquid
 At this point Surface tension becomes zero
 The surface energy may altered by the addition of
solutes that migrate to the surface and modify the
molecular forces , surface active agents.

Do U know that
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Walking on water
 Small insects such as the water strider can walk on water
because their weight is not enough to penetrate the surface.
Floating a needle
 If carefully placed on the surface, a small needle can be made
to float on the surface of water even though it is several times
as dense as water. If the surface is agitated to break up the
surface tension, then needle will quickly sink.
Soaps and detergents
 help the cleaning of clothes by lowering the surface tension of
the water so that it more readily soaks into pores and soiled
areas.

Cont’d,…
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Washing with cold water
 The major reason for using hot water for washing is that its
surface tension is lower and it is a better wetting agent. But if
the detergent lowers the surface tension, the heating may be
unneccessary.
Surface tension disinfectants
 Disinfectants are usually solutions of low surface tension.
This allow them to spread out on the cell walls of bacteria and
disrupt them. One such disinfectant, S.T.37, has a name which
points to its low surface tension compared to the 72 dynes/cm
for water.

Spreading of Liquid &
Spreading Coefficient
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Spreading of Liquid
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 When a liquid is placed on the surface of other liquid,
it will spread as a film.
 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.

Spreading Coefficient “S”
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Work of Cohesion (Wc)
 The energy required to separate the molecules of the
spreading liquid so it can flow over the sub-layer.
 The surface free energy increased by separating a column of
pure liquid into two halves.
 Surface free energy = γ dA
Wc = γL (dA+dA) = 2 γLdA
 Here the column
cross sectional area is 1cm2 (dA=1cm2).
Wc = 2 γL

Spreading Coefficient “S”,…
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Work of Adhesion (Wa)
 The energy required to break the attraction between the unlike
molecules.
 The surface free energy increased by separating a column of two
immiscible liquids at its boundary into two sections.
 γLS is destroyed after separation and γL and γS are created.
 Wa = γLdA + γS dA - γLS dA
 Here the columns
cross sectional area 1cm2
Wa = γL + γS - γLS

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 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 sub-layer liquid
• γLS - Interfacial tension

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 Spreading occurs when spreading coefficient S is +ve
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 sub-layer
 the spreading becomes spontaneous to reduce free
energy of sub-layer.

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 If spreading coefficient S is -ve
 i.e,(γL+ γLS) > γS
 Spreading liquid forms globules or floating lens means
spreading will not take place.

Factor affecting Spreading Coefficient
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Molecular structural
 The greater the polarity of the molecule
=>the more positive [S] spread on water
 ethyl alcohol and propionic acid
 Non polar substances
=>have negative [S] fail to spread on water
 Liquid petrolatum
 For organic acids, as Oleic acid,
 the longer the carbon chain
=>decrease in polar character decrease [S]
 Some oils can spread over water
 because they contain polar groups as COOH and OH.

Factor affecting Spreading Coefficient,…
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Cohesive forces
 Benzene spreads on water not because it is polar but
 because the cohesive forces between its molecules are
much weaker than the adhesion for water.

Application of Spreading coefficient
in pharmacy
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 To prefer suitable blend of oil/water and emulsifying
agent to get STABLE emulsion.
 The requirement of film coats to be spreaded over the
tablet surfaces.
 Spreading of Creams/lotions on skin is a must to be
absorbed.

• Surfactants/SAA
Adsorption at Liquid Interfaces
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Adsorption at Liquid Interfaces
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 Certain molecules and ions, when dispersed in the liquid, move
of their own accord to the interface.
 Their concentration at the interface then exceeds their
concentration in the bulk of the liquid.
 Obviously, the surface free energy and surface tension of the
system are automatically reduced.
 Such a phenomenon, where the added molecules are partitioned
in favor of the interface, is termed adsorption, or, more correctly,
positive adsorption.
 Other materials (e.g., inorganic electrolytes) are partitioned in
favor of the bulk, leading to negative adsorption and
 a corresponding increase in surface free energy and surface
tension.

Surfactants/
Surface Active Agents
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 are substances that adsorbed to surfaces or interfaces,
 causing a marked decrease in the surface tension.
 They have a certain affinity
 for both polar and nonpolar solvents.
 Depending on the number and nature of the polar and nonpolar
groups present, the amphiphile may be predominantly
 hydrophilic (water-loving): If high number of polar groups
 Lipophilic (oil-loving): If high number of nonpolar groups
 Reasonably Well balanced molecule between these two extremes.

Surfactants,…
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 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.

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How does surfactant
work?
 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.
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MICELLE
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o In dilute solution, Amphiphiles tend to reduce Surface
tension.
o As concentration molecules of amphiphiles goes on
increasing they disturb hydrogen structure,
o to minimize the disturbance molecules tend to form aggregate
into a structure.
o Structure is called micelle and
o Amphiphilic molecule is known as Surface Active Agent.
o A micelle is an aggregate of monomer surfactant molecules
dispersed in a liquid colloid.

Types of micelle
Oil in Water Type
 Hydrophilic head region in contact
with surrounding solvent in the
micelle center.
 Capable to hold lipidic nature drug at
center .
Water in oil type
 Reverse micelles have the head
group at the center with the tails
extending out.
 Hold large amount of water in their
interior.
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Structure of micelle
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MICELLE FORMATION
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1. The process of forming micelle is AKA micellization.
2. Typical micelle is Spherical in structure which contain
50-100 monomers.
3. Number of monomers to form micelle is called as
aggregation number.

Process of micellization
SAA bulk Concentration
Surface excess
Surface saturated with SAA
Excess in the bulk
Micelles( colloidal aggregates 50 -100nm)
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Critical micelle concentration (CMC)
 The lowest concentration at which
micelles first appear is called the
 critical concentration for micelle
formation.
 The CMC is the point at which
surfactant molecules aggregate
together in the liquid to form
groups known as micelles.
 The CMC of a surfactant indicates
the point at which surface active
 properties are at an optimum and
 performance is maximized. 4/5/2022
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Critical micelle concentration (CMC),…
 The CMC is the concentration
above surfactant when micelles
will form spontaneously.
 Increase in concentration of
surfactant beyond CMC
 change number of size or shape
but not provide increase in conc.
of monomeric species.
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Factors Affecting CMC
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• Structure of hydrophobic group
– length of hydrocarbon chain
Micelle size CMC
• Addition of Electrolyte
Micelle Size CMC
• Effect of Temperature up to cloud point
Micelle Size CMC

Classification of surfactants
HLB System Classification
Definition:
 The hydrophile-lipophile balance (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 (more oil
soluble),
 suitable for preparation of w/o emulsion,
 Those with HLB value of 8-18 are hydrophilic (good water
solubility) and
 good for ppn o/w emulsion.
Example:
 Spans with low HLB are lipophilic.
 Tweens with high HLB are hydrophilic
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HLB,…
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HLB,…
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 The oil phase of an o/w emulsion requires a specific HLB,
called the required hydrophile–lipophile balance (RHLB).
% Emulsifier with high HLB = RHLB - HLB Low
HLB High – HLB Low

Structural classification
Surfactants can be classified based on
charge groups present in their head
 A nonionic surfactant do not have
any charge groups over its head
 The head of an ionic surfactant
carries a net charge.
 If the charge is negative, the surfactant is
more specifically called anionic and
 if the charge is positive, it is called cationic
 If a surfactant contains a head with two
oppositely charged groups, it is termed
zwitter ion
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Non-ionic surfactants
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 Those surfactants do not have any electrical charge,
which makes them resistant to water hardness
deactivation
 They are less irritant than other anionic or cationic
surfactants
 The hydrophilic part contains the polyoxyethylene
,polyoxypropylene or polyol derivatives
 The hydrophobic part contains saturated or
unsaturated fatty acids or fatty alcohols They are
excellent grease/oil removers and emulsifiers.

Non-ionic surfactants,…
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 The non ionic surfactant can be classified as
 Polyol esters ,
 polyoxyethylene esters ,
 Poloxamers
 The Polyol esters includes glycol and glycerol esters
and sorbitan derivatives
 Polyoxyethylene esters includes polyethylene glycol
(PEG 40,PEG -50 ,PEG- 55).
 The most commonly used non-ionic surfactants are
esters of fatty Alcohols

Anionic surfactants
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 In solution, the head is negatively charged.
 These surfactants are the most widely used type
of surfactant for
 preparing shampoos because of its excellent
cleaning properties and high hair conditioning
effects.
 Anionic surfactants are particularly effective at
oil cleaning and oil/clay suspension.

Anionic surfactants,…
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 The most commonly used anionic surfactants are alkyl
sulphates, alkyl ethoxylate sulphates and soaps.
 Most of the anionic surfactants are carboxylates ,
sulfates and sulfonate ions
 The straight chain is a saturated /unsaturated C12-C18
aliphatic group.
 The water solubility potential of the surfactant is
determined by the presence of double bonds

Cationic surfactants
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 In solution, the head of the cationic surfactant is positively
charged.
 Cationic surfactants are quaternary ammonium compounds and
they are mostly used for their disinfectant and preservative
properties as they have good bactericidal properties.
 They are used on skin for cleansing wounds or burns. Mostly
used cationic surfactants are cetrimide which has tetradecyl
trimethyl ammonium bromide with minimum amount of
dodecyl and hexadecyl compounds

Amphoteric Surfactants
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 These surfactants are very mild, making them particularly
suited for use in personal care preparations over sensitive
skins.
 They can be anionic (negatively charged), cationic (positively
charged) or non-ionic (no charge) in solution, depending on the
acidity or pH of the water.
 These surfactants may contain two charged groups of different
sign, Where the positive charge is almost always ammonium
but the source of the negative charge may vary (carboxylate,
sulphate, sulphonate).

Amphoteric Surfactants,…
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 These surfactants have excellent dermatological properties.
 They are frequently used in shampoos and other cosmetic
products, and also in hand, dishwashing liquids because of
their high foaming properties .
 Alkylbetaines

Functional Classification
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According to their pharmaceutical use, surfactants
can be divided into the following groups:
 Wetting agents
 Solubilizing agents
 Foaming and antifoaming agents
 Detergents

Wetting agent
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 Wetting agent is a surfactant that when dissolved in
water, lower the contact angle and
 aids in displacing the air phase at the surface and
 replacing it with a liquid phase. OR
 Surfactants are used to aid wetting of powders, because
they are capable of:
1. Lowering the interfacial tension.
2. Lowering of the contact angle between the solids and
liquids.
3. Displacing the air and permit the intimate contact.

Wetting agent,…
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 Solids will not be wetted if their critical surface tension is
exceeded than the surface tension of the liquid.
 Thus water with a value of 72 dynes/cm will not wet
polyethylene with a critical surface tension of 31 dynes/cm.
 Based on this concept we should expect a good wetting agent
to be one which reduces the surface tension of a liquid to a
value below the solid critical surface tension.

Wetting agent,…
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This process is important in many ways in
pharmacy:
 Intimate contact of solids or liquids
 an initial step towards the preparation of suspension and emulsion.
 In case of granulation prior to tableting,
 the powder are mixed with the liquid binding agents.
 The success of this process in part depends on
 the wetting and spreading of the liquid over the solid.
 Film coating requires
 the wetting and spreading of liquids (containing the coating material )
over the tablet surface.
 Dissolution of the tablet or a capsule
 necessitates the penetration of the liquid into the pores of the dosage
form.

Wetting agent,…
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 Contact angle
 is the angle between liquid droplet and surface over which it
spreads.
 is used as an indicator to evaluate the efficiency of wetting agent.
 The contact angle can be taken up any value between 0o and 180o .

Wetting agent,…
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 When a drop of a liquid placed on a smooth surface of a solid.
 According to the wettability, the drop will make a certain
angle of contact with the solid.
 A contact angle is lower than 90°, the solid is called
wettable
 A contact angle is wider than 90°, the solid is named non-
wettable.
 A contact angle equal to zero indicates complete
wettability.

Wetting agent,…
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Complete wetting incomplete wetting wetting
Ө = 0° Ө < 90° Ө = 90° Ө > 90° Ө = 180°
– γsL> 0 γ s – γsL ≈ 0 γ s– γsL< 0

Wetting agent,…
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Young’s Equation
 The contact angle may have any value between the
limits: 0-180o.
 At equilibrium, the surface and interfacial tensions
can be resolved:
γSV = γSL + γLV cos θ ,Where ϴ= angle of contact

Wetting agent,…
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 When the forces of adhesion are greater than the
forces of cohesion,
 the liquid tends to wet the surface and vice versa.
 The surface of liquid water (meniscus) has a
concave shape
 because water wets the surface and creeps up the side
 The surface of Mercury has a convex shape it does
not wet glass
 because the cohesive forces within the drops are
stronger than the adhesive forces between the drops
and glass.

Solubilizing agents (micellar solubilization)
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 When a surfactant is placed in water it forms micelles at
concentrations above its critical micelle
concentration(CMC)
 In a micelle,
 the hydrophobic tails flock to the interior in order to
minimize their contact with water, and
 the hydrophilic heads remain on the outer surface in order to
maximize their contact with water .

Solubilizing agents,…
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 Critical micellar concentration is the concentration at which
the monomeric surfactant molecules associates into small
aggregates called micelles
 Diluting the surfactant solution to below the CMC causes the
micelles to disperse or break up into single or non associated
surfactant molecules
 Micelles are not static aggregates but dissociate, regroup and
reassosciate rapidly
 There is a dynamic equilibrium between single surfactant
molecules and micelles
 The shape of micelles in dilute surfactant solutions is
approximately spherical.

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 At surfactant concentrations above the CMC the
solubility increases linearly with the concentration of
surfactant,
=> indicating that solubilization is related to micellization
 The lower the CMC value and higher the aggregation
number , the more stable are the micelles

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Pharmaceutical Examples of Solubilization
 The solubilization of phenolic compounds such as cresol,
chlorocresol, chloroxylenol and thymol with soap to form
clear solutions for use in disinfection.
 Solubilized solutions of iodine in non-ionic surfactant
micelles (iodophors) for use in instrument sterilization.
 Solubilization of drugs (for example, steroids and water
insoluble vitamins), and essential oils by non-ionic
surfactants (usually polysorbates or polyoxyethylene
sorbitan esters of fatty acids).

Foams and Antifoaming Agents
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Foams
 are dispersion of a gas
 in a liquid (liquid foams as that formed by soaps and
detergents ) or
 in a solid (solid foams as sponges ).
Foaming agents
 Many Surfactants solutions promote the formation of
foams and stabilize them,
 in pharmacy they are useful in tooth pastes compositions.

Foams and Antifoaming Agents,…
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Anti Foaming agents
 They break foams and reduce frothing that may cause
problems in foaming of solubilized liquid ppn.
 Agents such as alcohol, ether , castor oil and some
other surfactants used be break the foam.

Detergents
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 Detergents are surfactants used for removal of dirt.
 Detergency involves:
 Initial wetting of the dirt and the surface to be cleaned.
 Deflocculation and suspension, emulsification or solubilization
of the dirt particles.
 Finally washing away the dirt.

• Solid/gas interface
• Adsorption isotherms
• Solid/liquid interface
Adsorption at solid interfaces
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Adsorption
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 Adsorption should not be confused with absorption.
 The former is solely a surface effect, whereas in
absorption, the liquid or gas being absorbed means
penetrates into the capillary spaces of the absorbing
medium.
The taking up of water by a sponge is absorption;
The concentrating of alkaloid molecules on the
surface of clay is adsorption.
 Adsorption(surface) vs absorption (bulk)
Adsorption
Absorption
(“partitioning”)
University

Adsorption,…
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 Adsorption is the adhesion of atoms, ions, or molecules from a
gas, liquid, or dissolved solid to a interface.
 Desorption is the reverse process of adsorption, i.e. the process
in which adsorbed molecules escape from solid surfaces.
 Adsorbent (also called substrate) - The solid that provides
surface for adsorption
high surface area with proper pore structure and size
distribution
good mechanical strength and thermal stability are
necessary
 Adsorbate - The gas or liquid substances which to be adsorbed
on solid

Adsorption,…
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Types of adsorption
1. Physical adsorption(Physisorption)
 which the adsorbate is bound to the surface through
the weak van der Waals forces.
2. Chemical adsorption (chemisorption)
 which involves the stronger valence forces.

Adsorption,…
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Adsorption at Solid Interface
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 Adsorption of material at solid interfaces
 can take place from either an adjacent liquid or gas phase.
 The most notable adsorbents are,
 highly porous solids, such as charcoal and silica gel
 which have large internal surface areas and
 finely divided powders.

Adsorption at Solid/Gas interface
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 If a gas or vapor is brought into contact with a solid,
 some of it will become attached to the surface.
 The study of adsorption of gases on Solid arises in such diverse
applications as
 the removal of objectionable odors from rooms and food,
 the operation of gas masks, and
 the measurement of the dimensions of particles in a powder.

Adsorption at Solid/Gas interface,…
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 The degree of adsorption of a gas by a solid
depends on
 chemical nature of the adsorbent (the
material used to adsorb the gas) and
 adsorbate (the sub. being adsorbed),
 surface area of the adsorbent,
 temperature, and
 partial pressure of the adsorbed gas.

Adsorption isotherm
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 The relationship between the amount of gas
physically adsorbed on a solid and the equilibrium
pressure or concentration at constant temperature
yields an adsorption isotherm.
 The term isotherm refers to a plot at constant
temperature.
 Adsorption isotherms mathematical models:
 Freundlich isotherm
 Langmuir isotherm and
 Brunauer-Emmett-Teller (BET)

Adsorption isotherm,…
Fruendlich isotherm
 Some cases of isothermal adsorption of a
gas on a solid can be explained by the
empirical Freundlich equation
Where
 Y is the mass of gas x adsorbed per
unit mass m of adsorbent
 p partial pressure of gas
 K and n are adsorption and capacity
constants , respectively, for a
particular system at constant
temperature
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x/m = K P 1/n
Log x/m = LogK + 1/n LogP

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 The Freundlich equation
 is simple, however, it does not exactly describe the
isotherm in a wide pressure range.
 It was rather used to describe adsorption qualitatively.
 Limitation of the Freundlich adsorption isotherm:
 ¨ It is applicable within certain limits of pressure.
 At higher pressure it shows deviations.
 ¨ The values of constants k and n change with the
temperature.
 ¨ The Freundlich isotherm is an empirical one and it does
not have any theoretical basis.

Langmuir Isotherm
 Langmuir developed an equation based on
 the theory that the molecules or atoms of gas are
adsorbed on active sites of the solid to form a layer- one
molecule thick .
 Often indicative of chemisorption.
 The simplified equation of Langmuir isotherm is
 where
P is the partial pressure of gas
Y is the mass of gas adsorbed per unit mass of
adsorbent
Ym is the maximum mass of gas that a unit mass of
adsorbent can absorb when monolayer is complete
b is the affinity or binding constant
A plot of p/y against p yields a
straight line with 1/ym as the slope
and 1/bym as the intercept
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Three assumptions :
1. Adsorption cannot proceed beyond monolayer coverage
2. All surface site are equivalent and the surface is uniform
3. The ability of a molecule to adsorb at the given site is
independent of the occupation of neighboring sites
 It is assumed that gas molecules striking the surface have a
given probability of adsorption.
 Molecules already adsorbed similarly have a given probability of desorption.
 At equilibrium, equal numbers of molecules desorb and
adsorb at any time.
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Brunauer-Emmett-Teller (BET) Isotherm
 The isotherm derived by Brunauer, Emmett and Teller is known
as the BET isotherm.
 It is widely used as a standard method of determining surface
area for solids.
 BET isotherms occur when gases undergo physical adsorption
onto nonporous solids to form a monolayer followed multi layer
formation.

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The assumptions underlying the simplest BET isotherm are:
1. Gas adsorbs on a flat, uniform surface of the solid with a uniform
heat of adsorption due to van der Waals forces between the gas and
the solid.
2. There is no lateral interaction between the adsorbed molecules.
3. After the surface has become partially covered by adsorbed gas
molecules, additional gas can adsorb either on the remaining free
surface or on top of the already adsorbed layer.
4. The adsorption of the second and subsequent layers occurs with
a heat of adsorption equal to the heat of liquefaction of the gas.
multi-layers adsorption

X = bcP
M (po – P)[1+(c-1)P/Po]
 b is the amount of gas that has to be
adsorbed to cover all
 M mass of the solid by one monolayer
 p is the pressure,
 Po is the saturation vapor pressure, and
 c is a constant
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Adsorption at the Solid/Liquid Interface
 The principles of solid on liquid
adsorption are used in
 decolorizing solutions,
 adsorption chromatography,
 detergency, and
 wetting.
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Adsorption at the Solid/Liquid Interface,…
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 A known mass of the adsorbent material is shaken with a solution
of known concentration at a fixed temperature.
 When a substance moves away from a solution and accumulates
at the surface of a solid,
 the concentration of the solute remaining in solution is in dynamic
equilibrium with the accumulated concentration at the surface.
 The concentration of the supernatant solution is
 determined by either physical or chemical means until equilibrium
conditions have been established.

 Isothermal adsorption can be expressed by Langmuir
equation in the following form
21
where
c is the equilibrium conc. of the solute in solution and replaces.
 A plot of c/y against c yields a straight line, and ym and b can
be obtained from the slope and intercept of this plot.
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 Many pharmaceutical systems deal with the adsorption
of solutes onto solid surfaces:
 Drugs or preservatives in solution may adsorb onto containers
 The concentration of the drug in solution reduced.
 The preparation left unpreserved and susceptible to microbial
attack.
 Bactericidal compounds (cationic surfactants):
 adsorbed onto surface of the bacteria.

Factors Affecting Adsorption
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Solubility of the adsorbate
The extent of adsorption of a solute is inversely proportional
to its solubility in the solvent from which adsorption occurs.
Nature of the adsorbent
 The extent of adsorption is proportional to the specific SA.
Temperature
 ↑ To normally leads to a decrease in the amount adsorbed.
pH
 For simple molecules adsorption increases as the ionisation
of the drug is suppressed.

Activated Carbon
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 The most common adsorbent which is used to remove
various toxic substances.
 Activated carbons have
 unique porous structures,
 large specific surface area and porosity, and
 various surface functional groups.
 These physical and chemical properties make activated
carbons the most commonly employed adsorbents.
 for removal of substances from gaseous and liquid phases.
 It is used as an antidote for poisonings due to many
substances,
 including drugs (sulfonylureas, acetaminophen, phenobarbital, etc.).

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