12 Surfactants

8/3/2019 12 Surfactants

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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

12 Surfactants

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