Surfactants
Mr. K. K. Mali
Assistant Professor,
YSPM’s Yashoda Technical Campus, Satara
CONTENTS

Introduction of surfactant

Classification of Surfactant

Properties of surfactant

Phase behaviour of Surfactant
INTRODUCTION




Surface active agents
Lowers surface tension of water
Any material that makes a surface contribution to the free energy of
the surface phase of two component system.
Surfactants is a Amphiphilic compound that






Is soluble in at least one phase of system
Forms oriented monolayers at phase interface
Exhibits equilibrium concentrations at phase interfaces higher than those in
the bulk solution & forms micelles at specific concentration.
Exhibits characters- detergency, foaming, wetting, emulsifying, solubilizing
& dispersing
Tail or hydrophobic group this group is usually hydrocarbon (alkyl)
chain
Head or hydrophilic group can be neutral or charged
INTRODUCTION

Their surface activity arises from
adsorption at the solution air interface –
the means by which the hydrophobic
region of the molecule ‘escapes’ from
the hostile aqueous environment by
protruding into the vapour phase above.

Adsorption at the interface between
aqueous and nonaqueous solutions
occurs in such a way that the
hydrophobic group is in the solution in
the nonaqueous phase, leaving the
hydrophilic group in contact with the
aqueous solution.
INTRODUCTION
Surfactants - behavior
SURFACE AND INTERFACIAL TENSION;
SURFACE AND INTERFACIAL FREE ENERGY








‘‘surface’’ is usually reserved for the region between a condensed phase
(liquid or solid) and a gas phase or vacuum.
‘‘interface’’ is normally applied to the region between two condensed
phases.
liquid–gas interface, molecules of the liquid in the boundary can only
develop attractive cohesive forces with molecules situated below and
adjacent to them.
They can develop attractive adhesive forces with molecules of the
gaseous phase.
At the gas–liquid interface, these adhesive forces are quite small.
The net effect is that molecules at the surface of the liquid have potential
energies greater than those of similar molecules in the interior of the
liquid and experience an inward force toward the bulk of liquid.
This force pulls the molecules of the interface together and the surface
contracts.
Thus, the surface of a liquid behaves as if it were in a state of tension—
the surface tension ()—due to the contracting force acting in all
directions in the plane of the surface.
SURFACE AND INTERFACIAL TENSION;
SURFACE AND INTERFACIAL FREE ENERGY






In order to extend the surface of a liquid it is necessary to
bring molecules from the interior to the surface against the
inward pull.
The work required to increase the surface area by unit area is
termed the surface free energy.
At the interface between two condensed phases, the
dissimilar molecules in the adjacent layers facing each other
have potential energies greater than those of similar
molecules in the respective bulk phases.
This is due to the fact that cohesive forces between like
molecules tend to be stronger than adhesive forces between
dissimilar molecules.
The interfacial tension is the force per unit length existing at
the interface between two immiscible or partially miscible
condensed phases.
The interfacial free energy is the work required to increase
the interface by unit area.
SURFACTANT CLASSIFICATION

Depending on their charge characteristics the
surface-active molecules may be
Anionic: SLS
 Cationic: QAC
 Zwitterionic (ampholytic): N-dodecyl-N,Ndimethylbetaine
 Nonionic: Sorbitan esters, Polysorbates, Poloxamer

SURFACTANT CLASSIFICATION

Examples
GRIFFIN'S
SCALE OF HLB
PROPERTIES OF SURFACTANT

Wetting

Emulsification

Detergency

Solubalization

Micellization
PROPERTIES OF SURFACTANT

Wetting
PROPERTIES OF SURFACTANT

Emulsifying Agent
PROPERTIES OF SURFACTANT

Detergency
• Detergents are surfactants used for removal of dirt.
• Detergency involves
Wetting of the dirt particles
Removing the insoluble dirt
as a deflocculated particle or
as a emulsion (oil soluble material)
•
Washing
PROPERTIES OF SURFACTANT

Solublisation

Process of preparing clear solution
Microemulsion
 Swollen micelle


Phenolic compound such as cresol, thymol, chlorocresol chloroxylenol
stabilized to form clear solution used as disinfection.

Low solubility of steroids in water is major problem in ophthalmic formulation.
So use of non ionic surfactant produce a clear solution which are stable to
sterilization.

Polysorbate used in preparation of aqueous injection of water insoluble
vitamins A,D,E & K.
PROPERTIES OF SURFACTANT

Solublisation

Micelle used in targeted drug delivery system .

Micelle is used to encapsulate antibiotic and anti-cancer drug.


Amphotericin is encapsulated in a deaggregated form in micelle of
monomethoxy poly Phospholipids formed by solvent evaporation

Complex cisplatin block copolymer is used in cancer therapy.
Drug activity & absorption


Low concentration of surfactant increases absorption due to the
enhanced the contact of drug with absorbing membrane.
Concentration above the CMC either produce additional effect or
cause decrease absorption because drug held within the micelle so
that the concentration available for absorption is reduced.
MICELLIZATION

Micelle Formation






In dilute aqueous solution, amphiphiles tend to concentrate on
surface.
As concentration increased- surface become more crowded-till no
more space in surface layer.
Then forced to remain in aqueous solution and causes disruption
of the hydrogen bonding between water molecules.
To minimize this disruption amphiphile molecules tend to
aggregate into multiple molecular structures.
It disrupts water-water attractions.
So driving force for formation of micelles-entropy gain from
disruption of the water structure.
MICELLIZATION
STRUCTURE OF MICELLES
core
Cross section of micelle
Shear
surface
MICELLIZATION

Micellization
•





As concentration of surfactant increased there is alteration in
physical properties of solution.
Self-association of the amphiphile into small aggregates
called micelles.
Concentration of surfactant at which micelles first appear in
solution is called as CMC
Reason for micelle formation is the attainment of a minimum
free energy state.
Driving force for the formation of micelles is the increase of
entropy that occurs when the hydrophobic regions of the
surfactant are removed from water and the ordered structure
of the water molecules around this region of the molecule is
lost.
Most micelles are spherical and contain between 60 and 100
surfactant molecules.
MICELLIZATION

Critical Micelle Concentration

Minimum concentration at which surfactants molecules begin to
form micelles
MICELLIZATION

Critical Micelle Concentration
Extremely dil.
Dil. solution
Solution at CMC
Above CMC
MICELLIZATION
MICELLIZATION

Micelles Structure

Critical packing parameter

The shape of the micelle formed by a particular surfactant is
influenced to a large extent by the geometry of the surfactant
molecule, as can be seen if we consider the packing of space-filling
models of the surfactants. The dimensionless parameter of use in
these considerations is called the critical packing parameter (CPP)
and is defined as

where v is the volume of one chain,
a is the cross-sectional area of the surfactant head group
lc is the extended length of the surfactant alkyl chain


MICELLIZATION

Micelles Structure

Critical packing parameter

Structure of the aggregate that will be formed in solution.
Consideration of the packing of molecules into spheres shows that
when CPP ≤ 1/3, which is the case for surfactants having a single
hydrophobic chain and a simple ionic or large nonionic head group, a
spherical micelle will be formed.
Ionic Micelle
MICELLIZATION

Micelles Structure

Critical packing parameter

It is easily seen that if we double volume (v) by adding a second alkyl
chain then the value of CPP will exceed 1/3 and nonspherical
structures such as bilayers (CPP = 1) will form in solution, from which
vesicles are formed .
MICELLIZATION

Micelles Structure

Critical packing parameter

The ‘effective’ cross-sectional area of the surfactant molecule is
strongly influenced by the interaction forces between adjacent head
groups in the micelle surface. These forces are decreased by
addition of electrolyte, leading to a decrease of a, an increase of the
CPP, and a change of shape of the aggregate,
MICELLIZATION


Micelles Structure
ionic surfactants consists of:
Hydrophobic core composed of the hydrocarbon chains of the
surfactant molecule
 a Stern layer surrounding the core, which is a concentric shell
of hydrophilic head groups with (1 – )N counterions, where is
the degree of ionisation and N is the aggregation number
(number of molecules in the micelle). For most ionic micelles
the degree of ionisation is between 0.2 and 0.3; that is, 70–
80% of the counterions may be considered to be bound to the
micelles
 a Gouy–Chapman electrical double layer surrounding the
Stern layer, which is a diffuse layer containing the N
counterions required to neutralise the charge on the kinetic
micelle. The thickness of the double layer is dependent on the
ionic strength of the solution and is greatly compressed in the
presence of electrolyte.

MICELLIZATION

Micelles Structure

ionic surfactants

When concentration increased
converted spherical to
cylindrical
MICELLIZATION

Micelles Structure

Nonionic surfactants
are larger than their ionic counterparts and may sometimes be
elongated into an ellipsoid or rod-like structure.
 attributable to the removal of electrical work which must be done
when a monomer of an ionic surfactant is added to an existing
charged micelle.
 nonionic micelles are frequently asymmetric due to its size.
 have a hydrophobic core formed from the hydrocarbon chains of
the surfactant molecules surrounded by a shell (the palisade
layer) composed of the oxyethylene chains of the surfactant and
entrapping a considerable number of water molecules, which is
highly hydrated.

MICELLIZATION
MICELLIZATION

Importance



Micelles make insoluble material soluble in water.
The structure of the micelles can affect the viscosity of the
solution.
Micelles are reservoirs of surfactants.
MICELLIZATION
1.
Nature of hydrophilic group
2.
Nature of hydrophobic group
Factors
3.
Nature of counter ion
4.
Effects of electrolyte
Affecting
CMC and
Micellar size
5.
Effect of temperature
6.
Effect of pressure
MICELLIZATION

Factors Affecting CMC and Micellar size

Nature of hydrophobic group
•Increase in length of the HC results
in:
• decrease in CMC, which for
compounds with identical polar
head groups is expressed by the
linear equation:
log [CMC] = A – Bm
where m is the number of
carbon atoms in the chain and A
and B
are constants for a homologous
series.
• corresponding increase in
micellar size.
•Branching of HC increases CMC
•Unsaturation of HC increases CMC
MICELLIZATION

Factors Affecting CMC and Micellar size

Nature of hydrophobic group
MICELLIZATION

Factors Affecting CMC and Micellar size

Nature of hydrophilic group
Non-ionic surfactants generally have very much lower CMC
values and higher aggregation numbers than their ionic
counterparts with similar hydrocarbon chains.
 An increase in the ethylene oxide chain length of a non-ionic
surfactant makes the molecule more hydrophilic and the CMC
increases.

MICELLIZATION

Factors Affecting CMC and Micellar size

Type of Counterion
Micellar size increases for a particular cationic surfactant as the
counterion is changed according to the series Cl < Br < I , and for
a particular anionic surfactant according to Na < K < Cs .
 Ionic surfactants with organic counterions (e.g. maleates) have
lower CMCs and higher aggregation numbers than those with
inorganic counterions.

−
+
−
+
−
+
MICELLIZATION

Factors Affecting CMC and Micellar size

Addition of electrolyte
Electrolyte addition to solutions of ionic surfactants decreases the
CMC and increases the micellar size. This is because the
electrolyte reduces the forces of repulsion between the charged
head groups at the micelle surface, so allowing the micelle to
grow.
 At high electrolyte concentration the micelles of ionic surfactants
may become non-spherical.

MICELLIZATION

Factors Affecting CMC and Micellar size

Effect of Temperature
•Aqueous solutions of many nonionic surfactants become turbid at a
characteristic temperature called
the cloud point.
•At temperatures up to the cloud
point there is an increase in
micellar size and a corresponding
decrease in CMC.
•Temperature has a comparatively
small effect on the micellar
properties of ionic surfactants.
MICELLIZATION

Factors Affecting CMC and Micellar size

Effect of Pressure
•Increase in CMC with pressure
upto 150 Mpa followed by a CMC
decreases at high pressure.
MICELLIZATION

Thermodynamics of Micelle Formation

Mass action model
Micelles are in equilibrium with unassociated surfactant.
 For nonionic surfactant, n [the aggregation number] molecules
of the monomeric un ionised surfactant (S) react in a single step
to form a micelle, M
nS  M
 Equilibrium constant for micelle formation Km is given as

Km 

[M ]
[ S ]n
Term in bracket refers molar concentration of species.
MICELLIZATION

Thermodynamics of Micelle Formation

Mass action model

When n is large – the free energy of micellization at the CMC is

Gm0  RT ln[ S ]CMC
G0m represents the standard free energy change for transferring 1
mol of S from the aqueous solution into its micellar form.
MICELLIZATION

Thermodynamics of Micelle Formation

Mass action model
For ionised surfactant, ionic micelle derived from an anionic
surfactant is formed by the association of n surfactant ions S- and
of [n-p] firmly bound counterions (X+) as follows:
nS- + (n-p)X+ M-p
 Equilibrium constant for micelle formation is given by

[M  p ]
K   n  n p
[S ] [ X ]
2
m

When aggregation number n is large, the free energy of
micellization at or near the CMC reduces to
p

Gm0   2   RT ln[ S  ]CMC
n

MICELLIZATION

Thermodynamics of Micelle Formation

Phase –Separation model
considers micelles as a separate phase at CMC.
 Hence s and m0 are defined as the chemical potentials [per mol]
of the free surfactant in the aqueous phase and of the associated
surfactant in the micellar phase respectively
 At equilibrium s = m0
 If activity coefficients are ignored, s is related to the surfactants
standard state s0 by

 s   s0  RT ln S
MICELLIZATION

Thermodynamics of Micelle Formation

Phase –Separation model
On the other hand, micellar material is in standard state, and m =
m0
 The standard free energy of micellization is

Gm0   m0   s0

The analogous approach for an ionized surfactant yields
Gm0  [1  (n  p )] RT ln[ S  ]CMC
PHASE BEHAVIOUR

Equilibrium phase structures






As the concentration of a surfactant solution is increased,
different structures encount ered.
At concentrations well above the CMC, a more ordered
structuring of the solution occurs.
Two main types of liquid crystalline phases
Middle phase, M, exhibiting a hexagonal array of indefinitely long,
mutually parallel rods; and the neat phase, G, with a lamellar
structure.
The liquid crystalline hexagonal phase, like the micellar phase,
can exist either in a normal or reverse orientation.
The order of phase structures formed upon increasing surfactant
concentration generally follows a well defined sequence with a
‘‘mirror plane’’ through the lamellar phase in such a way that
normal phase structures can be considered to be ‘‘oil-in-water’’
and the reverse structures to be ‘‘water-in-oil.
PHASE BEHAVIOUR

Modified phase structures

Vesicular forms of surfactants are generally formed by
dispersing lamellar phases in an excess of water (or nonaqueous polar solvents such as ethylene glycol or
dimethylformamide) or, in the case of reversed vesicles, in an
excess of oil.
With most surfactants, vesicles are non-equilibrium structures
that will eventually re-equilibrate back into the lamellar phases
from which they originated.
Vesicles are structural analogs of liposomes; they are
approximately spherical structures and have the ability to
‘‘solubilize’’ both lipid soluble and water soluble agents.


PHASE BEHAVIOUR

Formation of liquid crystals and vesicles
Lyotropic liquid crystals
 The liquid crystalline phases that occur on increasing
the concentration of surfactant solutions are referred to
as lyotropic liquid crystals.

PHASE BEHAVIOUR

Formation of liquid crystals and vesicles

Lyotropic liquid crystals
Increase of concentration of a surfactant solution frequently
causes a transition from the typical spherical micellar structure
to a more elongated or rod-like micelle.
 Further increase in concentration may cause the orientation
and close packing of the elongated micelles into hexagonal
arrays; this is a liquid crystalline state termed the middle phase
or hexagonal phase.
 With some surfactants, further increase of concentration
results in the separation of a second liquid crystalline state –
the neat phase or lamellar phase.
 In some surfactant systems another liquid crystalline state, the
cubic phase, occurs between the middle and neat phases

PHASE BEHAVIOUR

Formation of liquid crystals and vesicles

Lyotropic liquid crystals
The lyotropic liquid crystals are
anisotropic, that is, their physical
properties vary with direction of
measurement.
The middle phase, for example,
will only flow in a direction
parallel to the long axis of the
arrays. It is rigid in the other two
directions.
The neat phase is more fl uid and
behaves as a solid only in the
direction perpendicular to that of
the layers.
Plane-polarised light is rotated
when travelling along any axis
except the long axis in the middle
phase and a direction
perpendicular to the layers in the
neat phase.
PHASE BEHAVIOUR

Formation of liquid crystals and vesicles

Thermotropic liquid crystals
Thermotropic liquid crystals are produced when certain
substances, for example the esters of cholesterol, are heated.
 The arrangement of the elongated molecules in thermotropic
liquid crystals is generally recognisable as one of three
principal types
 Nematic liquid crystals:
 Groups of molecules orientate spontaneously with their
long axes parallel, but they are not ordered into layers.
 Because the molecules have freedom of rotation about
their long axis, the nematic liquid crystals are quite mobile and
are readily orientated by electric or magnetic fields.

PHASE BEHAVIOUR

Formation of liquid crystals and vesicles

Thermotropic liquid crystals
Smectic liquid crystals:
 Groups of molecules are arranged with their long axes
parallel, and are also arranged into distinct layers.
 As a result of their two-dimensional order the smectic
liquid crystals are viscous and are not orientated by magnetic
fields.

PHASE BEHAVIOUR

Formation of liquid crystals and vesicles

Thermotropic liquid crystals







Cholesteric (or chiral nematic) liquid crystals:
Are formed by several cholesteryl esters.
Can be visualised as a stack of very thin two-dimensional nematic-like
layers in which the elongated molecules lie parallel to each other in the
plane of the layer.
The orientation of the long axes in each layer is displaced from that in
the adjacent layer and this displacement is cumulative through successive
layers so that the overall displacement traces out a helical path through the
layers.
The helical path causes very pronounced rotation of polarised light,
which can be as much as 50 rotations per millimeter.
The pitch of the helix (the distance required for one complete rotation)
is very sensitive to small changes in temperature and pressure and
dramatic colour changes can result from variations in these properties.
The cholesteric phase has a characteristic iridescent appearance when
illuminated by white light due to circular dichroism.
PHASE BEHAVIOUR

Formation of liquid crystals and vesicles
Vesicles
 Vesicles are formed by phospholipids and other surfactants
having two hydrophobic chains. There are several types:
 Liposomes




Liposomes are formed by naturally occurring phospholipids such as
lecithin (phosphatidyl choline).
They can be multilamellar (composed of several bimolecular lipid
lamellae separated by aqueous layers) or unilamellar (formed by
sonication of solutions of multilamellar liposomes).
They may be used as drug carriers; water-soluble drugs can
be entrapped in liposomes by intercalation in the aqueous
layers, whereas lipid-soluble drugs can be solubilised within
the hydrocarbon interiors of the lipid bilayers.
PHASE BEHAVIOUR

Formation of liquid crystals and vesicles





Surfactant vesicles and niosomes
Formed by surfactants having two alkyl chains.
Sonication can produce single-compartment vesicles.
Vesicles formed by ionic surfactants are useful as
membrane models.
Vesicles formed from non-ionic surfactants are called
niosomes and have potential use in drug delivery.
PHASE BEHAVIOUR

Formation of liquid crystals and vesicles






Monoolein vesicles
Polar amphiphilic lipids such as glyceryl monooleate (monoolein)
form bilayers, the nature of which depends on the temperature
and concentration.
phase formed at monoolein concentrations of 60–80% w/w is the
bicontinuous cubic phase.
The structure of this phase is unique and consists of a curved
bicontinuous lipid bilayer extending in three dimensions,
separating two networks of water channels with pores of about 5
nm diameter.
On dilution, these structures coexist with excess water and there
is the formation of dispersed cubic phase vesicles or cubosomes.
Cubic phases have been shown to incorporate and deliver small
molecule drugs and large proteins by oral and parenteral routes,
in addition to local delivery in vaginal and periodontal cavities.
PHASE BEHAVIOUR
Binary system
Ternary system
Cloud point
Emulsifieroil-water
Phases
Best
Example
PHASE BEHAVIOUR
Liquid crystalline
phase
PHASE BEHAVIOUR
Example of ternary
system
PHASE BEHAVIOUR
Factors
Ionic surfactant
Non-Ionic surfactant
Nature of
solublizate
Nature of oil
phase
Chain length
Effect of
temperature
Effect of
molecular
structure
Effect of
HLB
PHASE BEHAVIOUR
Best example
Tetrachlorom
ethane
Methyloctanoate
Caprylic acid
APPLICATIONS

Surfactant as emulsifying agent.

Surfactant for contact lenses cleaning.

Surfactant in drug absorption from rectal suppositories.

Surfactant as flocculating agent