Download 4.4.2 Phase diagrams

4.4.2 Phase diagrams
ordered structures at high concentrations: „lyotropic phases“
interfacial curvature may be changed by varying concentration
because effective cross-sectional area of head group changes
normal structures:
for surfactants
having a head group
area larger than the
cross-section area
of the tail
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micelles: large mean and Gaussian curvature
• at low concentrations: L1
(micelles with no long-range translational order
• at high concentrations:
micelles packed in cubic structure: I1, e.g. bcc
or rod-like micelles in hexagonal structure: HI
bilayers in lamellar structure: Lα
saddle-splay surfaces in bicontinuous phases
e.g. gyroid phase: V1, three-fold connection nodes
two continuous channels of water,
separated by bilayer of surfactant molecules
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usual sequence of phases:
L1
a
HI
b
Lα
c
HII
d
L2
surfactant concentration
a-d: intermediate phases
a: often cubic micellar structure
b: often bicontinuous cubic structure
inverse structures when solvent in minority phase:
L2: inverse micellar solution
HII: inverse hexagonal phase
c: often inverse bicontinuous phase V2
d: often inverse micellar cubic phase I2
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Phase diagram of SDS/water system
solubility
curve
SDS: anionic surfactant
phase boundaries vertical as in prediction
Krafft point quite high
→ large regions of hydrated crystal phases
CMC
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Phase diagram of nonionic surfactants
phase diagrams of CmEn
constant hydrophobic chain length m
increasing EO chain length n
(biphasic regions not indicated)
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temperature plays important role
e.g. solubility of poly(oxyethlene)
decreases with increasing temperature
→ phase boundaries not vertical
for short E chains (C12E4):
preferred mean interfacial curvature = 0
→ lamellar (Lα) and inverse micellar phases (L2) with Ns ≥ 1
longer E chains (C12E6, C12E8):
increasing tendency for normal (L1, H1) phases
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4.4.3 Membranes
bilayers of surfactants formed for Ns ≅ 1
e.g. for double-tailed surfactants:
membranes formed right above CMC
mean and Gaussian curvatures = 0
Lα phase:
strong thermal fluctuations at RT
→ may lead to sponge phase
stiffness may be controlled by charges
spacing d
undulation mode:
F ~ d-3
peristaltic mode:
F ~ d-5
fluctuations → entropic force, i.e. effective repulsion between bilayers
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Applications of membranes for DNS delivery
transfer and expression of extracellular DNA
to cell nucleus to replace defective gene
use viruses or synthetic nonviral vectors
cationic liposomes
• attach to anionic animal cells
• low toxicity
• nonimmunogenicity
• easy production
synthetically based carriers
of DNA vectors for gene therapy
made from cationic liposomes
→ liposomes change to
birefringent liquid-crystalline
condensed globules
→ multilamellar structure with
alternating lipid bilayer
and DNA monolayers
J.O. Rädler et al., Science 275, 810 (2001)
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Vesicles
vesicle:
hollow aggregate
shell: one or several bilayers
unilamellar
vesicle
liposome:
vesicle formed by lipids
→ simple model for cell
→ cosmetics, drug delivery
MLV: multilamellar vesicle
SUV: small unilamellar vesicle
LUV: large unilamellar vesicle
optical micrograph
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Preparation of vesicles
vesicles not in thermodynamic equilibrium
but often kinetically stable
sonication of dilute lamellar phases/mechanical shear
→ lamellae break up
→ reassemble as vesicles
→ small vesicles with broad size distribution
dissolution of dry phospholipids in water
→ multilamellar vesicles
dispersion of lamellar phase formed
at high concentration by excess of water
dispersion of surfactant in organic solvent
then addition of excess of water
…
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Drug delivery using vesicles
• liposome formation in presence of drug
• injection into bloodstream - drug is protected by vesicle
• liposome binds to cell wall → delivery of drug directly to cell
• incorporation of membrane proteins → specific targeting
cancer therapy:
Ø of liposome < 200 nm
cannot penetrate
endothelial wall of
healthy blood vessels
but can penetrate
the leaky vessels in tumors
liposomes are also of use
for oral delivery of
dietary/nutritional
supplements
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• coarse dispersion of gas in liquid
• liquid is minority phase
• usually not thermodynamically stable
surfactant: foaming agent
→ retard drainage of liquid from foam
gas content
4.4.4. Liquid foams
polyhedral
cells
spherical
bubbles
→ prevent rupture
→ metastable foams
in vertices („plateau borders“):
liquid pressure lower than in channels
→ liguid flow
→ rupture
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Gibbs and Marangoni effects
surfactants form lamellae
parallel to liquid film surface
→ excess of surfactant at liquid film surface
→ destabilization
Gibbs effect: draining
→ strong thinning of film
→ increase of surface area
→ decrease of surface excess
concentration of surfactant
→ increase of surface tension („Gibbs effect“)
→ opposes thinning
Marangoni effect:
surfactant flows to regions
of reduced surface excess
to restore original (lower) surface tension
(convection of surfactant along interface)
Gibbs and Marangoni
effects oppose the
destabilizing influence
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4.4.5. Emulsions
two immiscible liquids
I and II
emulsion of phase II
dispersed in phase i
unstable emulsion
separates
• mixture of two or more
immiscible or partially
miscible liquids
• one liquid (the dispersed phase)
is dispersed in the other phase
(the continuous phase)
• examples: vinaigrette, milk,
technical fluids, …
surfactant positions itself
on interfaces between
phase I and phase II
→ stabilizes emulsion
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free energy required
to disperse a liquid of volume V
into drops of radius R:
∆G = γ
3V
R
lower interfacial tension γ
→ reduction in free energy
→ stabilization of emulsion
emulsions:
• thermodynamically unstable
microemulsions:
• thermodynamically stable
• smaller droplet size than in emulsions
• slow kinetics of exchange of molecules
in/out of stabilizing film
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Emulsions
two types:
• water-in-oil (w/o)
• oil-in-water (o/w) emulsions
milk is /w emulsion:
fat droplets in aqueous phase
mayonnaise is o/w emulsion:
vegetable oil in
vinegar or lemon juice
surfactant: lecithin
margarine is w/o emulsion
size of dispersed particles ~0.1-10 µm
→ scatter light → emulsions appear cloudy
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Breaking up of emulsions
flocculation
due to net attractive forces between dispersed droplets
coagulation
droplets aggregate irreversibly
creaming/sedimentation
for unaggregated droplets
coalescence
droplets merge
→ large droplets grow at expense of small ones
„Ostwald ripening“
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Stabilization of emulsions
using emulsifiers, e.g surface-active agents
→ reduction of interfacial tension
→ increasing long-term kinetic stability
activity of surfactant emulsifier:
measured by hydrophile-lipophile balance (HLB)
which runs from 1 (hydrophobic surfactant)
to 20 (hydrophilic surfactant)
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