SKKU Physical Pharmacy Laboratory
성균관대학교 물리약학연구실
INTRODUCTION
• Surfactant molecules
• Surface and interfacial tension
SUBJECTS
• Behavior of surfactants & Gibbs equation
• Insoluble monolayers
• Adsorption
• Micellisation
• Liquid crystals & Vesicles
• Solubilisation
CONCLUSION
• Summary
2
SKKU Physical Pharmacy Laboratory
성균관대학교 물리약학연구실
Surfactant molecules
Surface & interfacial tension
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
Characterized by two distinct regions in structure
Hydrophobic region
Hydrophilic region
Amphipathic molecule
Properties of surfactants
* Physiochemical Principles of Pharmacy 4th edition
Alexander T Florence and David Attwood
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
Generally classified by Hydrophilic group
 Anionic
 Cationic
 Zwitterionic
 Nonionic
* Physiochemical Principles of Pharmacy 4th edition
Alexander T Florence and David Attwood
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
Imbalance between attractive forces
[Air or immiscible phase]
[Water]
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Interfacial tension


Generally between the values of surface tension of involved
liquids.

n-Octanol against water
▪ Much lower than pure liquid
▪ Hydrogen bonding between liquids
→ Cohesive force ↑
* Physiochemical Principles of Pharmacy 4th edition
Alexander T Florence and David Attwood
SKKU Physical Pharmacy Laboratory
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Behavior of surfactants
Gibbs equation
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
Tendency to accumulate the boundary between two
phases
 Escape from a hostile environment
 Minimize free energy state
 Reduce interfacial tension
* Physiochemical Principles of Pharmacy 4th edition
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
Increasing the concentration of surfactants
 Surface becomes saturated with surfactant
molecules.
 Form small spherical aggregates or micelles
 Critical Micelle Concentration (CMC)
 Calculate the area occupied by a surfactant molecule
using the Gibbs equation.
Air
Surface
Water
* Physiochemical Principles of Pharmacy 4th edition
Alexander T Florence and David Attwood
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
Surfactant molecule equilibrium between surface and bulk
solution
Γ=−
1 𝑑γ
𝑐 𝑑γ
=−
𝑅𝑇 𝑑 ln 𝑐
𝑅𝑇 𝑑𝑐
γ : surface tension
Γ : the amount of component in the surface (# of moles / m2)
c : concentration of surfactant
* Physiochemical Principles of Pharmacy 4th edition
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
Application of the Gibbs equation
 Just below the cmc, surfactant molecules are closely packed in
the surface.
 The area A that each molecule occupies at the surface
1
𝐴=
𝑁𝐴 Γ
1m
1m
Γ : the amount of component in the surface (# of moles / m2)
NA: the Avogadro constant
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
Amphipathic nature of the drugs
* Physiochemical Principles of Pharmacy 4th edition
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Insoluble monolayer
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
Insoluble amphiphilic compounds can
form films on water surfaces.

Dissolve the surfactant in a volatile
solvent and carefully injecting the
solution onto the water.
* J. Phys. Chem. B, 2004, 108, 6351-6358
Yoshikito Moroi, Muhammad Rusdi, and Izumi KUbo
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
Langmuir trough
 Apparatus for study of monolayers on a laboratory scale
Measuring
surface pressure
Monolayer
Surface pressure π=γ0-γm
γ0: Surface tension of the clean surface
γm : Surface tension of the film-covered surface
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
Three types of monolayer states
1. Solid or condensed monolayers
2. Gaseous monolayers
3. Liquid or expanded monolayers
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1. Solid or condensed state
 Rises abruptly when the
molecules become tightly packed.
 At high pressures, the molecules
are in contact and orientated
vertically in the surface.
 The extrapolated surface area is
very close to the cross-sectional
area of molecule.
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2. Gaseous monolayers
 The molecules move around in the film, remaining a sufficiently
large distance apart.
 Upon compression, there is a gradual change in the surface
pressure.
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3. Expanded monolayers
 Intermediate states between
condensed and gaseous films
 Close packing is prohibited by
bulky side-chains, or cisconfiguration.
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
Transition between monolayer states
 As the film is compressed, transition between phases can occur.
Condensed phase
Begin to stand upright
Lying along the surface
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Alexander T Florence and David Attwood
Gaseous phase
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
Study of polymers used as packaging materials and
film coatings
 To assess the suitability of polymer as
potential enteric and film coatings
 At pH 3.1, more tightly packed film
 Restrict dissolution in the
stomach
 At pH 6.5, more expanded film
 Allow penetration and
disintegration
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
Cell membrane models
 Useful models for studying drug-lipid interactions
TFP  remains in the monolayer
CPZ  excluded from
the interface
* Physiochemical Principles of Pharmacy 4th edition
Alexander T Florence and David Attwood
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Adsorption at the solid/liquid interface
Adsorption isotherms
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Adsorption at the solid/liquid interface
25

Adsorption vs. Absorption
Adsorption
• The process of accumulation at
an interface
Absorption
• The penetration of one
component throughout the body
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Adsorption at the solid/liquid interface
26

Two general types of adsorption
 Physical adsorption
▪ Adsorbate is bound to the surface through the weak van der Waals
forces
 Chemical adsorption (Chemisorption)
▪ Involves the stronger valence forces
▪ Usually involves an ion-exchange process
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Adsorption at the solid/liquid interface
27

Adsorption isotherms
 Langmuir equation
▪ Monolayer adsorption
Freundlich
Langmuir
 Freundlich equation
▪ Multilayer adsorption
The concentration of solute
adsorbed onto the solid phase
* Vadose Zone Journal, 2007, 6(3), 407-435
Sabine Goldberg, Louise J. Criscenti and Kirk J. Cantrell
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
Langmuir equation
𝑥
𝑎𝑏𝑐
=
𝑚 1 + 𝑏𝑐
a: related to surface area
b: related to the enthalpy of adsorption
c: concentration of solution
𝑐
1
𝑐
=
+
𝑥/𝑚 𝑎𝑏 𝑎
Linear form
* Physiochemical Principles of Pharmacy 4th edition
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
Increase of concentration
 Deviations from the typical Langmuir plot can occur.
 Formation of multi adsorption layers
Freundlich equation
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
Freundlich equation
𝑥
= 𝑎𝑐1/𝑛
𝑚
a, n: constants
1/n: related to the intensity of drug adsorption
c: concentration of solution
log 𝑥 𝑚 = log 𝑎 + (1 𝑛) log 𝑐
* Physiochemical Principles of Pharmacy 4th edition
Alexander T Florence and David Attwood
SKKU Physical Pharmacy Laboratory
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1. Solubility of adsorbate
2. pH
3. Nature of adsorbent
4. Temperature
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Factors affecting adsorption
32

Lundelius’s Rule
 Adsorption of a solute is inversely proportional to its solubility.
 Ex) Adsorption of iodine onto carbon
▪ CCl4:CHCl3:CS2 = 1:2:4.5  Inverse ratios for the solubility
 The greater the solubility, the stronger are solute-solvent bonds
▪ Solute-solvent bonds must first be broken for adsorption.
 The smaller the extent of adsorption!
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Factors affecting adsorption
33

Adsorption increases as the
ionisation of the drug is
suppressed.

The extent of adsorption
reaches a maximum when the
drug is completely uninonised.
* Physiochemical Principles of Pharmacy 4th edition
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Factors affecting adsorption
34

Surface area of the adsorbent
 The most important property affecting
adsorption
 Extent of the adsorption is
proportional to the specific surface
area.

Adsorbent-adsorbate interactions
 Particular adsorbents have affinities
for particular adsorbates
▪ Ex) Digoxin - Antacids
* Physiochemical Principles of Pharmacy 4th edition
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Factors affecting adsorption
35

Adsorption is generally an exothermic process
 Increase in temp  Decrease in the amount adsorbed
 Small variations in temp tend not to alter the adsorption process
to a significant extent
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
Adsorption of poisons/toxins
 Activated charcoal, magnesium oxide, tannic acid

Taste masking
 Diazepam

Haemoperfusion
 Carbon haemoperfusion

Adsorption in drug formulation
 Improved dissolution rate, the stabilisation of suspensions
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Micellisation & Micellar structures
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
Micelles
 Small aggregates formed after the cmc
 Critical Micelle Concentration (CMC)
▪ Concentration over which micelles are formed
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
In dynamic equilibrium with free molecules in
solution
 Association colloids

Driving force for micelle formation
 To attain a state of minimum free energy
 Remove the hydrophobic group from
the aqueous environment
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
Critical packing parameter (CPP)
 Consider the geometry of the surfactant molecule
𝑣
𝐶𝑃𝑃 =
𝑙𝑐 𝑎
v: volume of one chain
a: cross-sectional area of surfactant head group
lc: extended length of the surfactant alkyl chain
 CPP determines the preferred association structures assumed
for each molecular shape.
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
CPP ≤ 1/3
 Single hydrophobic chain
 Simple ionic or large nonionic head group
 Spherical micelle
 Most surfactants of pharmaceutical interest fall into this category.
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
1/3 < CPP ≈ 1
 Additional second alkyl chain
 Bilayer (non-spherical structures)
 Form vesicles
* Physiochemical Principles of Pharmacy 4th edition
Alexander T Florence and David Attwood
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
1 < CPP
 In nonaqueous media, reverse (or inverted) micelles may form.
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
Stern layer
 For most ionic micelles, the degree of ionisation (α) is between 0.2 ~
0.3; 70~80 % of the counterions may be bound to the micelles

Gouy-Chapman electrical double layer
 Outer surface of the Stern layer
 Contain 20~30 % counterions to neutralise the charge on the micelle
Gouy-Chapman layer
Stern layer
* Fast track – Physical Pharmacy
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
In highly concentrated solution
 The micelles elongating to form cylindrical structures with many
ionic systems.
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
Larger than ionic micelles
 Absence of an electrical work for additional monomer into ionic
micelle
 Frequently asymmetric

Hydrophobic core surrounded by a shell of oxyethylene
chains
 The palisade layer (highly hydrated)
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•
•
•
•
•
Structure of the hydrophobic group
Nature of the hydrophilic group
Type of conterion
Addition of electrolytes
Temperature
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Factors affecting CMC and size
48

Compounds with rigid aromatic or heteroaromatic
ring structures
 Purines, pyrimidines, etc.
 Face-to-face stacking of molecules one on top of the other
 Do not exhibit cmc

Length of Hydrocarbon chain
 Increase length  Increased hydrophobicity  Decreased cmc
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Factors affecting CMC and size
49

Effect of substituents on hydrophobicity can be
roughly estimated.
Hydrophobicity
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Factors affecting CMC and size
50

Nonionic surfactants
 Not involve any electrical work
 Much lower CMC and higher aggregation number
 Increase in the ethylene oxide chain length
 Make more hydrophilic and the CMC increases
* Physiochemical Principles of Pharmacy 4th edition
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Factors affecting CMC and size
51

Cationic surfactant
 Cl- < Br- < I-

Anionic surfactant
Increase in micellar size
 Na+ < K+ < Cs+

The weakly hydrated ions can be adsorbed more
readily in the micellar surface
 Decrease the charge repulsion between the polar groups
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Factors affecting CMC and size
52

Reduction of repulsion forces by electrolytes
 Lower CMC and higher micellar size
Decrease
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Increase
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Factors affecting CMC and size
53

Cloud point
 Temperature over which aqueous solutions of nonionic surfactants
become turbid
 Reversible process of phase separation
Increase in the micellar aggregation number
Mechanism
Change in micellar interactions
The dehydration process
* J. Chromato. A, 2000, 902, 251-265 SKKU Physical Pharmacy Laboratory
R. Carabias-Martinez, E. Rodriguez-gonzalo,,
and B. 물리약학연구실
Moreno-Cordero
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Factors affecting CMC and size
54

Comparatively small effect on ionic surfactants
Ionic surfactant
Nonionic surfactant
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Liquid crystals
Vesicles
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
State of matter that have properties between those of a
conventional liquid and those of a solid crystal
Thermotropic liquid crystal
• Phase transition into liquid crystal phase as
temperature is changed.
Lyotropic liquid crystal
• Phase transition as a function of both
temperature and concentration
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
Produced when certain substances are heated

Three types of thermotropic liquid crystals
1. Nematic (soap-like) liquid crystals
▪ Orientate with long axes parallel, but not ordered into layers
▪ Mobile and orientated by electric or magnetic fields
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Parallel
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2. Smectic (thread-like) liquid crystals
▪ Arrange with long axes parallel, also arranged into layers
▪ Viscous and not oriented by magnetic fields
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3. Cholesteric (chiral nematic) liquid crystals
▪ Formed by several cholesteryl esters
▪ Stack of very thin two-dimensional nematic-like layers
* Physiochemical Principles
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4th edition
SKKU Physical
Pharmacy
Laboratory
Alexander T Florence성균관대학교
and David Attwood
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60

The liquid crystalline phases that occur on increasing
the concentration of surfactant solutions

As increase of concentration of surfactant
 Spherical micelle  elongated or rod like micelle
 hexagonal phase (middle phase)
 cubic phase (with some surfactants)
 neat phase (lamellar phase)
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
Formed by phospholipids and other surfactants
having two hydrophobic chains (CPP≈1)

Liposomes
 Multilamellar or unilamellar
 Used as drug carriers
 Disadvantages
▪ Phospholipids  Oxidative degradation
 Nitrogen atmosphere
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
Surfactant vesicles
 Formed by surfactants having two alkyl chains
 Sonication  Single-compartment vesicles
 Use of vesicles formed by ionic surfactants
 membrane models (due to toxicity)
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
Surfactant vesicles
 Niosome
▪ Nonionic surfactants-based vesicles  Less toxic and potential use
in DDS
▪ Behave in vivo like liposomes
* Physiochemical Principles of Pharmacy 4th edition
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Solubilisation
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
Process whereby water-insoluble substances are brought into
solution by incorporation into or onto micelles.

Solubilisate
 Incorporated substance

Maximum additive concentration (MAC)
 Maximum amount of solubilisate that can be incorporated into system
 Determining of MAC
▪ By visual inspection
▪ From extinction of turbidity measurement on the solutions
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
Related to the chemical nature of the solubilisate
 Nonpolar solubilisates
 Hydrocarbon core
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
Related to the chemical nature of the solubilisate
 Water insoluble compounds containing polar groups
 Polar group at the core-surface
interface
 Hydrophobic group buried inside
the hydrocarbon core
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
Related to the chemical nature of the solubilisate
 Water soluble molecules
 In the polyoxyethylene shell
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1. Nature of the surfactant
2. Nature of the solubilisate
3. Temperature
SKKU Physical Pharmacy Laboratory
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Factors affecting CMC and size
70

Chain length of hydrophobe
(when solubilisate is located within the core)
 Increase in alkyl chain length
 Solubilisation capacity increases
C12
Length
C18
* Physiochemical Principles of Pharmacy 4th edition
Alexander T Florence and David Attwood
SKKU Physical Pharmacy Laboratory
성균관대학교 물리약학연구실
Factors affecting CMC and size
71

Ethylene oxide chain length
 Increase in the hydrophilic chain length
 Solubilisation capacity increases
* Physiochemical Principles of Pharmacy 4th edition
Alexander T Florence and David Attwood
SKKU Physical Pharmacy Laboratory
성균관대학교 물리약학연구실
Factors affecting CMC and size
72

Increase of alkyl chain length  Decrease in
solubility

Effect of steroid structure on solubilisation
 More hydrophilicity of the substituent in C17 of the ring
 Lower quantity of surfactant required
Increase in solubility
C
-COCH3 < -OH < -COCH2OH
* Physiochemical Principles of Pharmacy 4th edition
Alexander T Florence and David Attwood
17
SKKU Physical Pharmacy Laboratory
성균관대학교 물리약학연구실
Factors affecting CMC and size
73

Increase of temperature  Increase of solubilisation

Complicating factor when considering the effect of
temperature on the amount solubilised is the
aqueous solubility of solubilisate.
* Physiochemical Principles of Pharmacy 4th edition
Alexander T Florence and David Attwood
SKKU Physical Pharmacy Laboratory
성균관대학교 물리약학연구실
Factors affecting CMC and size
74

Study of the solubilisation of benzoic acid
Increase in aqueous solubility
* Physiochemical Principles of Pharmacy 4th edition
Alexander T Florence and David Attwood
Increase in solubilisation
SKKU Physical Pharmacy Laboratory
성균관대학교 물리약학연구실
75

Solubilisation of phenolic compounds
 Form clear solutions  For use in disinfection

Solubilisation of iodine in nonionic surfactant
micelles (iodophors)
 Reduction of corrosion problems  For use in instrument
sterilisation

Solubilisation of drugs
 Steroids, water-insoluble vitamins, etc.
SKKU Physical Pharmacy Laboratory
성균관대학교 물리약학연구실
Summary
SKKU Physical Pharmacy Laboratory
성균관대학교 물리약학연구실
77

Surface and interfacial tensions arise because of an
imbalance of attractive forces on the molecules.

Surfactant molecules have hydrophilic and
hydrophobic regions and adsorb at interfaces for
attaining minimum free energy state.

The extent of adsorption at the interface can be
calculated using the Gibbs equation
SKKU Physical Pharmacy Laboratory
성균관대학교 물리약학연구실
78

Insoluble amphiphilic compounds will form films
on water surfaces and these may be tightly packed.
 Condensed, Expanded, Gaseous film

Adsorption of solutes onto solid surfaces from
solution can occur by physical adsorption
 Langmuir equation (monolayer), Freundlich equation (multilayer)

Micelles form at the critical micelle concentration.
 The main driving force for the formation is the increase entropy
when the hydrophobic regions of the surfactant are removed
from water.
SKKU Physical Pharmacy Laboratory
성균관대학교 물리약학연구실
79

Micellar structure
 Ionic micelle  Stern layer, Gouy-Chapman layer
 Nonionic micelle  Palisade layer (highly hydrated), Cloud point
 Critical Packing Parameter (CPP)  Preferred association
structure

An important property of surfactant micelles is their
ability to solubilise water-insoluble compounds
 The location of solubilisates is related to the chemical nature of
molecule.
SKKU Physical Pharmacy Laboratory
성균관대학교 물리약학연구실