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Surfactants, Micelles,Emulsions
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Surfactants
Surface active agents
Amphiphiles
Detergents
TensidesIn most cases, solvent is water
hydrophilic (polar) group (head group) hydrophobic alkyl chains (tail group)
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Surfactants
anionic
cationic
nonionic
amphoteric (zwitterionic)
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head group negatively charged
e.g. carboxylate, sulfonate, sulfate
most commonly used surfactants
example SDS (C12H25OSO3Na)
Anionic Surfactants
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Headgroup positively charged
not so common (low biodegradability)
example: DTAB C12H25N(CH3)3Br
Cationic Surfactants
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Uncharged, but polar headgroup
second most common used surfactants
example: Alkylethylene oxides as e.g.
C10H21(OCH2CH2)8OH, also writen asC10E8
Nonionic Surfactants
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Headgroup contains both positive and
negative charge
seldom used (more expensive)
examples: mainly lipids
Amphoteric Surfactants
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Surfactants
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Surfactants
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Micelles
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Structure of Surfactants in Solution
Micelles
Cylinders
Bilayers
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Critical Micellation ConcentrationCMC
1 10 100 100030
40
50
60
70
Surfacetensio
n(mJ/m-2)
Concentration (mM)
SDS
Surface tension
Solubilty
Turbidity
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What happen near cmc?
see Excel sheet
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Micelles Hydrocarbon chains inside,polar head
groups outside Spherical object of typically 30100
surfactant molecules,oily phase inside
(polydisperse) Typical diameters 36 nm.
Interiors show liquid phase properties
Micelles are dynamic structures.Exchange at s timescale.
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Temperature DependenceIonic surfactants
weak dependence
at low T, precipitation as crystals
Krafft temperature: solubility = CMC consequence: low efficiency below Krafft
point
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Temperature DependenceNonionic surfactants
at high T, formation of separate phase cloud point
Pluronic
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Thermodynamics of Micelles Entropy: bringing hydrocarbon tails out of
the water (hydrophobic effect) -> decreaseof CMC with increasing tail length
Lateral repulsion of headgroups: hydrationforce, steric effects
Electrostatic repulsion for charged
surfactants -> influence of saltconcentration
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Influence of chain length and salt
concentration on the CMC
Example:alcylsulfatein NaCl at21C
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Structure of Surfactants in Solution
Micelles
Cylinders
Bilayers
Determined by the
surfactant parameter
(packing ratio)
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Surfactant Parameter
00
C
C
C
S
VN
L A A=
VC
= Volume of the hydrocarbon tail
LC = Length of hydrocarbon tailA
0= Area per head group
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Surfactant Parameter
small values: high curvature
values ~ 1: small curvature high values: inverse micelles
00
C
C
C
SVN
L AAA
=
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Structures of Surfactants Micelle
InvertedMicelles
Cylinder
or rod-like
aggregate
Bilayer
Vesicle
orliposome
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Calculation NS
( ) 3029.0027.0 nmnV CC +
( )nmnL CC 15.0127.0 +
0.2
0.205
0.21
0.215
0 5 10 15 20
n
AC
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Example
( )
( )37.0
62.015.0127.011
056.0027.011
2
3
0
=+
+==
nmnm
nm
AL
VN
C
CS
SDS: A0
= 0.62 nm2
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Biological Membranes
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Functions of biological membrane
Effective barrier for ionic transfer and charges
Eq. 12.8:compare dissociation energy in both media
Receptor proteins may be triggered to openbarrier
2 2
0 0
338 8
diss dissoil water
water oil
e e E E kT
= =
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Bicontinuous structures
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Emulsions
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Emulsions
Dispersion of two immiscible liquid phases
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Emulsions
Oil-in-water Water-in-oil
in this context, oil may denote any liquid not miscible with water!
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Volume fraction d
Determines many properties, e.g.
Viscosity
Conductivity
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Polydispersity
=2
2
2
)ln(lnexp
2
1
R
RR
R
P
Lognormal distribution
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Types of emulsionsMacroemulsions
Only kinetically stable -> demulsification
0.5-10 m size of droplets
external driving force
Microemulsions
Thermodynamically stable
very small droplet size (nm)
equilibrium as driving force
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Formation of Macroemulsions
R
VG
em
3
=
Energy required depends on surface
tension between liquids -> surfactants In practice higher energies are necessary
6.04.0 WR
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oil-in-water or water-in-oil?
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oil-in-water or water-in-oil?Volume fraction has little influence!
Dependence mainly on NS for NS < 1, mainly oil in water
for NS > 1, mainly water in oil
On stirring, W/O and O/W both are formed
Criterion: which has lowest stability, disappears
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Aging of emulsions Flocculation
Creaming
Coalescence
Phase separation
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Stabilization of EmulsionsEmulsifiers usually surfactants
Hydration force for oil-in-water Steric force for water-in-oil
Electrostatic forces for charged surfactants
Polymers steric force
Powders hydrophobic force
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Microemulsions Thermodynamic equilibrium
Spontaneous formation
react on external changes
droplet size 5-100 nm form for high surfactant concentrations
(complete coverage of interphase)
driving force is the spontaneous curvatureof the surfactants
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Estimation of drop size
3
34 RNV d =
24 NRLVSS
=
S
dSLR
3=
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Phase behavior of Microemulsions
Ionic surfactants: salt concentration(changes headgroup area)
Nonionic surfactants: temperature(changes hydration and fluctuations)
-> Phase Inversion Temperature
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Phase diagram: Example
Water / octanemicroemulsion
with alkylethyleneoxide
(C12E5)
PIT = 32C
curvature toolarge to contain all
oil in droplets
more surfactant:
easier to containall oil in droplets
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Phase transition When T , then
size of head group (less hydration)
tail widens (thermal fluctuations)
Result: phase inversion through lamellar phase
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Conclusion No foams
Surfactants form all kind of aggregates controlled by relative size of components
Macro-emulsions very important for practice Micro-emulsions:
specialty
modern research area
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