7 surface interfacial phenomena

Outline of the Learning Topics:
• Interface
• Surface Active Agents
• Factors affecting Spreading Coefficient
• Application of Spreading Coefficient in Pharmacy
• Classification of Surface Active Agents
• Structural Classifications of Surface Active Agents
• Oriented Adsorption of Surfactant Interfaces
• Incompatibilities Involving Surfactants

 Interface is the boundary between two or more phases
exist together
 The properties of the molecules forming the interface are
different from those in the bulk that these molecules are
forming an interfacial phase.
 Several types of interface can exist depending on whether
the two adjacent phases are in solid, liquid or gaseous state.
 Importance of Interfacial phenomena in pharmacy are the following:
 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
Surface and Interfacial Tensions
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.
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
 The net effect is that the molecules at the surface of the
liquid experience an inward force towards the bulk of the
liquid and pull the molecules and contract the surface with
a force (F) .

To keep the equilibrium, an equal force must be applied to oppose the inward
tension in the surface.
Thus SURFACE TENSION [γ ] is the force per unit length that must be applied
parallel to the surface so as to counterbalance the net inward pull and has the
units of dyne/cm
INTERFACIAL TENSION is the force per unit length existing at the interface
between two immiscible liquid phases and has also units of dyne/cm.
Invariably, interfacial tensions are less than surface tensions because 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.
Greater surface tension reflects higher intermolecular force of attraction, thus,
increase in hydrogen bonds or molecular weight cause increase in surface tension

The work (W) required to create a unit area of surface is known as
SURFACE FREE ENERGY/UNIT AREA (ergs/cm2)
erg = dyne . cm
Its equivalent to the surface tension γ
Thus the greater the area (A) of interfacial contact between the phases,
the greater the free energy.
W = γ ∆ A
For equilibrium, the surface free energy
of a system must be at a minimum.
Thus Liquid droplets tend to assume a
spherical shape since a sphere has the
smallest surface area per unit volume.

Methods for measuring surface and interfacial tension
1. Capillary rise method
2. Ring (Du Nouy) tensiometer
3. Drop weight method (Stalagmometer)
The choice of the method for measuring surface and interfacial tension
depend on:
 Whether surface or interfacial tension is to be determined.
 The accuracy desired
 The size of sample.

Capillary Rise Method
When a capillary tube is placed in a liquid, it rises up
the tube a certain distance. By measuring this rise, it is
possible to determine the surface tension of the liquid. It is
not possible, to obtain interfacial tensions using the
capillary rise method.
 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 wet the capillary wall,
spreading over it, and rising in the tube.
The Principle

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
a = γ cos Ө
a = 2 π r γ cos Ө
The upward component of the force resulting from
the surface tension of the liquid at any point on the
circumference is given by:
Thus the total upward force around the inside
circumference of the tube is
Where
Ө = the contact angle between the surface of the
liquid and the capillary wall
2 π r = the inside circumference of the capillary.
For water the angle Ө is insignificant, i.e. the liquid wets the
capillary wall so that cos Ө = unity
Cont. angle
water and glass
Contact angle
Mercury and glass

The downward force of gravity
(mass x acceleration) is given by
Where:
π r 2 = the cross-sectional area
h = the height of the liquid column to
the lowest point of the meniscus
(p – p o) = the difference in the density of the
liquid p and its vapor po
g = the acceleration of gravity
w = the weight of the upper part of the meniscus.
At Maximum height, the opposing forces are in equilibrium
p o, Ө and w can usually be disregarded
Hence the surface tension can be calculated.
π r 2 h (p – p o) g + w
2 π r γ cos Ө = π r 2 h (p – p o) g + w
2 π r γ = π r 2 h p g γ = 1/2 r h p g

Ring (Du Nouy) Tensiometer
 the principle of the instrument depends on the fact that:
the force necessary to detach a platinum-iridium 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
 The surface tension is given by:
Where:
F = the detachment force
R1 and R 2= the inner and outer radii of the ring.
γ = F / 2 π (R1 + R2)
 For measuring surface and interfacial tensions.
The principle

If the volume or weight of a drop as it is detached from a tip of
known radius is determined, the surface and interfacial tension can be
calculated from
Where 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
 The correction factor is required as not all
the drop leaves the tip on detachment
 The tip must be wetted by the liquid so as
the drop doesn’t climb the outside of the tube.
γ = Φ mg = Φ V pg
2 π r 2 π r
Drop Weight and drop volume method

A surfactant molecule is depicted schematically as a cylinder
representing the hydrocarbon (hydrophobic) portion with a sphere
representing the polar (hydrophilic) group attached at one end.
The hydrocarbon chains are straight because rotation around carbon-carbon bonds
bends, coils and twists them.
Sodium Lauryl
Sulfate molecule

Molecules and ions that are adsorbed at interfaces are
termed surface active agents, surfactants or amphiphiles
The molecule or ion has a certain affinity for both polar and
nonpolar solvents.
Depending on the number and nature of the polar and
nonpolar groups present, the amphiphilic may be hydrophilic,
lipophilic or be reasonably well-balanced between these two
extremes.
It is the amphiphilic nature of surface active agents which
causes them to be adsorbed at interfaces, whether these be
liquid/gas or liquid/liquid.

A scale showing classification of
surfactant function on the basis of HLB
values of surfactants.
The higher the HLB of a surfactant
the more hydrophilic it is.
Example: Spans with low HLB are
lipophilic. Tweens with high HLB are
hydrophilic.

Determination of HLB
 Polyhydric Alcohol Fatty Acid Esters (Ex. Glyceryl monostearate)
HLB = 20 ( 1 – S / A )
 Surfactants with no Saponification no (Ex. Bees wax and lanolin)
S = Saponification number of the ester
A = Acid number of the fatty acid
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

When a liquid is placed on the surface of other liquid, it will spread
as a film if the adhesive force is greater than the cohesive forces.

As surface or interfacial work is equal to surface
tension multiplied by the area increment.
The work of cohesion, which is the energy required to separate the
molecules of the spreading liquid so as it can flow over the sub-layer
=
Where 2 surfaces each with a surface tension = γ L
The work of adhesion, which is the energy required to break the
attraction between the unlike molecules =
Where: γ L =the surface tension of the spreading liquid
γ S =the surface tension of the sub layer liquid
γ LS =the interfacial tension between the two liquids.
Spreading occurs if the work of adhesion is greater than the work of
cohesion, i.e. Wa > Wc or Wa - Wc > 0
Wc = 2 γ L
Wa = γ L + γ S - γ LS

Spreading Coefficient is The difference between the work of
adhesion and the work of cohesion
S = Wa - Wc = (γ L + γ S - γ LS ) - 2 γ L
S = γ S - γ L - γ LS
S = γ S – (γ L + γ LS )
Spreading occurs (S is positive) when the surface tension of the sub-
layer liquid is greater than the sum of the surface tension of the spreading liquid
and the interfacial tension between the sub-layer and the spreading liquid.
If (γ L + γ LS ) is larger than YS , (S is negative) the substance forms globules or a
floating lens and fails to spread over the surface.
Liquid Substrate

Factor affecting Spreading Coefficient
Molecular Structural:
o The greater the polarity of the molecule the more positive [SC]
as ethyl alcohol and propionic acid
o Non polar substances as Liquid petrolatum have negative [SC] fail
to spread on water
o For organic acids, as Oleic acid,
the longer the carbon chain decrease in polar character decrease [S]
o Some oils can spread over water because they contain polar groups
as COOH and OH
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
The requirement of film coats to be spreaded over the
tablet surfaces
The requirement of lotions with mineral oils to spread on
the skin by the addition of surfactants

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

Wetting agents
 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.
 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.

According to the nature of the liquid and the solid, a drop of liquid placed
on a solid surface will adhere to it or not, which is the wettability between liquids
and solids.
When the forces of adhesion are greater than the forces of cohesion, the
liquid tends to wet the surface and vice versa.
Place a drop of a liquid 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.

.
no
wetting
incomplete wetting
complete
wetting
= 0° < 90° = 90° > 90° = 180°ӨӨӨӨӨ
γ s– γsL< 0γ s – γsL ≈ 0γ s – γsL> 0

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.

Micellar Solubilization
Surfactant molecules accumulate in the interfaces between
water and water insoluble compound. Their hydrocarbon
chains penetrate the outermost layer of insoluble compound
which combine with the water-insoluble molecules. Micelles
form around the molecules of the water-insoluble compound
inside the micelles’ cores and bring them into solution in an
aqueous medium. This phenomenon is called micellar
solubilization.
The inverted micelles formed by oil-soluble surfactant which
dissolves in a hydrocarbon solvent can solubilize water-soluble
compound which is located in the center of the micelle, out of
contact with the solvent.

Micelles of nonionic surfactants consist of an outer shell
containing their polyethylene glycol moieties mixed with water and
an inner core formed by their hydrocarbon moieties. Some compounds
like phenols and benzoic acid form complexes with polyethylene
glycols by hydrogen bonding and/or are more soluble in liquids of
intermediate polarity like ethanol or ethyl ether than in liquids of low
polarity like aliphatic hydrocarbons. These compounds locate in the
aqueous polyethylene glycol outer shell of nonionic micelles on
solubilization.
Drugs which are soluble in oils and lipids can be solubilized by
micellar solubilization.

As Micellar solubilization depends on the existence of micelles; it
does not take place below the CMC. So dissolution begins at the
CMC. Above the CMC, the amount solubilized is directly
proportional to the surfactant concentration because all surfactant
added to the solution in excess of the CMC exists in micellar form,
and as the number of micelles increases the extent of solubilization
increases .
Compounds that are extensively solubilized increase the
size of micelles in two ways:
o The micelles swell because their core volume is
augmented by the volume of the solubilizate.
o The number of surfactant molecules per micelle

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 and Anti Foaming agents
 Foaming agents
Many Surfactants solutions promote the formation of foams and
stabilize them, in pharmacy they are useful in toothpastes compositions.
Anti Foaming agents
They break foams and reduce frothing that may cause problems as in
foaming of solubilized liquid preparations. in pharmacy they are useful
in aerobic fermentations, steam boilers.

Detergents
 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
solubilisation of the dirt particles
•Finally washing away the dirt.

Structural Classification
 A single surfactant molecule contains one or more
hydrophobic portions and one or more hydrophilic
groups.
 According to the presence of ions in the surfactant
molecule they may be classified into:
 Ionic surfactants
o Anionic surfactants: the surface active part is anion
(negative ion ) e.g. soaps, sodium lauryl sulfate
o Cationic surfactants: the surface active part is cation
(positive ion) e.g. quaternary ammonium salts
o Ampholytic surfactants: contain both positive and
negative ions e.g. dodecyl-B-alanine.

Ionic surfactants
They are the metal salts of long - chain fatty acids as lauric acid.
Sodium dodecyl sulfate or Sodium Lauryl Sulfate is used in
toothpaste and ointments
Triethanolamine dodecyl sulfate is used in shampoos and other
cosmetic preparations.
Sodium dodecyl benzene sulfonate is a detergent and has
germicidal properties.
Sodium dialkvlsulfosuccinates are good wetting agents.
Anionic surfactants

Cationic surfactants
These are chiefly quaternary ammonium compounds. They have
bacteriostatic activity probably because they combine with the
carboxyl groups in the cell walls and of microorganisms by cation
exchange, causing lysis.
Among the most popular antiseptics in this category are
benzalkonium chloride, cetylpyridinium chloride and
cetyltrimethylammonium bromide,
Ampholytic Surfactants
These are the least common, e.g. dodecyl-β-alanine

Non-ionic surfactants
Widely used in pharmaceutical formulations e.g. Tweens,
Spans, Brij and Myrj.
They are polyethylene oxide products.
Surfactants based on sorbitan are of pharmaceutical importance.
Esterification of the primary hydroxyl group with lauric,
palmitic, stearic or oleic acid forms sorbitan monolaurate,
monopalmitate, monostearate or monooleate
These are water-insoluble surfactants called Span 20, 40, 60 or
80, respectively.
Addition of about 20 ethylene oxide molecules produces the
water-soluble surfactants called polysorbate or Tween 20, 40. 60
or 80.

At water-air interface
Surface-active molecules will
be adsorbed at water-air
interfaces and oriented so
that the hydrocarbon chains of are pushed out of the water and rest on the surface,
while the polar groups are inside the water. Perhaps the polar groups pull the
hydrocarbon chains partly into the water.
At oil-water interface
Surface-active molecules will be oriented so that the hydrophobic
portion is inside the oil phase and the hydrophilic portion inside
the water phase.
As a Surface active substance contains a hydrophilic and a
hydrophobic portions, it is adsorbed as a monolayer at the interfaces.

At low surfactant concentrations:
The hydrocarbon chains of surfactant molecules adsorbed in the
interface lie nearly flat oh the water surface.
At higher concentrations:
They stand upright because this permits more surfactant molecules
to pack into the interfacial monolayer.
As the number of surfactant molecules adsorbed at the water-air
interface increased, they tend to cover the water with a layer of
hydrocarbon chains. Thus, the water-air interface is gradually
transformed into an non polar-air interface. This results in a decrease in
the surface tension of water.

At a given concentration, temperature, and salt content, all micelles
of a given surfactant usually contain the same number of molecules,
i.e. they are usually monodisperse.
For different surfactants in dilute aqueous solutions, this number
ranges approximately from 25 to 100 molecules.
The diameters of micelles are approximately between 30 and 80
Ao. Because of their ability to form aggregates of colloidal size,
surfactants are also called association colloids.
Micelles are not permanent aggregates. They form and disperse
continually.

a- Cone-shaped surfactant resulting in b-normal micelles
c- Hampagne cork shaped surfactant resulting in d-reverse
micelles with control of their size by the water content
e- Interconnected cylinders .
f- Planar lamellar phase .
g- Onion-like lamellar phase.
Surfactant shapes in colloidal solution

 Normal spherical micelles
In dilute aqueous solutions micelles are approximately spherical.
The polar groups of the surfactants are in the periphery and the
hydrocarbon chains are oriented toward the center, forming the
core of the micelles
 Inverted spherical micelles
In solvents of low polarity or oils micelles are inverted.
The polar groups face inward to form the core of the micelle
while the hydrocarbon chains are oriented outward
 Cylindrical and lamellar micelles
In more concentrated solutions
of surfactants, micelles change
from spherical either to cylindrical or
lamellar phase.

 Nonionic surfactants have few incompatibilities with drugs and
are preferred over ionic surfactants. even in formulations for external
use, except when the germicidal properties of cationic and anionic
surfactants are important.
 Nonionic surfactants form weak complexes with some
preservatives as phenols, including esters of p-hydroxybenzoic acid
(Parabenzes) and with acids like benzoic and salicylic via hydrogen
bonds. This reduces the antibacterial activity of these compounds.
Nonionic surfactants

Ionic surfactants
 Ionic surfactants capable of reacting with compounds possessing ions
of the opposite charge. These reactions may bind the surface active
ions, sometimes with precipitation. The compounds which react with
the surface active ions are also changed, and this may be harmful from
the physiological or pharmacological point of view.
Incompatibility of surface active quaternary ammonium compounds
with bentonite, kaolin, talc, and other solids having cation exchange
capacity.

 Define the following terms:
[Interface, absorption, penetration, dispersion, suspension, solution, emulsion, cohesive,
adhesive, float, radius, diameter, hydrophobic, hydrophilic, lipophilic, amphiphilic, wetting,
solubilization, emulsion, dispersion, suspension, defloculation, foamation, detergent, concave,
convex, micelle, surfactant, anion, cation, ampholyte, colloid, etc]
Respond to the following questions:
Give a descriptive account of different forms interface types, tensions, forces that affects material
substances
Describe the different Methods and respective choices made for measuring surface and
interfacial tension
Give a detailed account of different types of surface active agents commonly used in
pharmaceutical processes
 Elaborate on factors affecting spreading coefficient during the use of surface active agents
Elaborate on classification of surface active Agents that have pharmaceutical uses
Group work discussional questions:
Give a descriptive account of different forms of interface types, tensions, forces that affects
material substances interactions
Give a detailed account of different types of surface active agents commonly used in
pharmaceutical processes
 Elaborate on factors affecting spreading coefficient during the use of surface active agents
Elaborate on classification of surface active Agents based on chemical structures, chemical
shapes, chemical compatibilities and pharmaceutical uses

7 surface interfacial phenomena

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