of 18
8/12/2019 Apresentao_SDS
1/18
1
4.4.2 Phase diagrams
ordered structures at high concentrations: lyotropic phases
interfacial curvaturemay be changed by varyingconcentrationbecause effective cross-sectional area of head group changes
normal structures:for surfactantshaving a head grouparea larger than thecross-section areaof the tail
8/12/2019 Apresentao_SDS
2/18
2
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. bccor rod-like micelles in hexagonal structure: HI
bilayers in lamellar structure: L
saddle-splay surfaces in bicontinuous phasese.g. gyroid phase: V1, three-fold connection nodes
two continuous channels of water,separated by bilayer of surfactant molecules
8/12/2019 Apresentao_SDS
3/18
3
surfactant concentration
L1 HI L HII L2a b c d
usual sequence of phases:
a-d: intermediate phases
a: often cubic micellar structureb: often bicontinuous cubic structure
inverse structureswhen solvent in minority phase:L2: inverse micellar solutionHII: inverse hexagonal phasec: often inverse bicontinuous phase V2
d: often inverse micellar cubic phase I2
8/12/2019 Apresentao_SDS
4/18
4
Phase diagram of SDS/water system
SDS: anionic surfactant
phase boundaries vertical as in prediction
Krafft pointquite highlarge regions of hydrated crystal phases
solubilitycurve
CMC
8/12/2019 Apresentao_SDS
5/18
5
Phase diagram of nonionic surfactants
phase diagrams of CmEn
constant hydrophobic chain length mincreasing EO chain length n(biphasic regions not indicated)
8/12/2019 Apresentao_SDS
6/18
6
for short E chains (C12E4):
preferred mean interfacial curvature = 0lamellar (L) and inverse micellar phases (L2) with Ns 1
longer E chains (C12E6, C12E8):increasing tendency for normal (L1, H1) phases
temperature plays important rolee.g. solubility of poly(oxyethlene)decreases with increasing temperature
phase boundaries not vertical
8/12/2019 Apresentao_SDS
7/18
7
4.4.3 Membranes
bilayers of surfactants formed for Ns 1e.g. for double-tailed surfactants:membranes formed right above CMCmean and Gaussian curvatures = 0
Lphase:strong thermal fluctuations at RTmay lead to sponge phase
stiffness may be controlled by charges
fluctuations entropic force, i.e. effective repulsion between bilayers
peristaltic mode:F~ d-5
spacing d
undulation mode:F~ d-3
8/12/2019 Apresentao_SDS
8/18
8
Applications of membranes for DNS delivery
J.O. Rdler et al., Science 275, 810 (2001)
transfer and expression of extracellular DNAto 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 carriersof DNA vectors for gene therapymade from cationic liposomes
liposomes change tobirefringent liquid-crystalline
condensed globulesmultilamellar structure with
alternating lipid bilayerand DNA monolayers
8/12/2019 Apresentao_SDS
9/18
9
Vesicles
vesicle:hollow aggregateshell: one or several bilayers
liposome:vesicle formed by lipidssimple model for cellcosmetics, drug delivery
unilamellarvesicle
MLV: multilamellar vesicleSUV: small unilamellar vesicleLUV: large unilamellar vesicle
optical micrograph
8/12/2019 Apresentao_SDS
10/18
10
Preparation of vesicles
vesicles not in thermodynamic equilibriumbut often kinetically stable
sonication of dilute lamellar phases/mechanical shearlamellae break upreassemble as vesiclessmall vesicles with broad size distribution
dissolution of dry phospholipids in watermultilamellar vesicles
dispersion of lamellar phase formedat high concentration by excess of water
dispersion of surfactant in organic solventthen addition of excess of water
8/12/2019 Apresentao_SDS
11/18
11
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: of liposome < 200 nm
cannot penetrateendothelial wall ofhealthy blood vesselsbut can penetratethe leaky vessels in tumors
liposomes are also of usefor oral delivery ofdietary/nutritional
supplements
8/12/2019 Apresentao_SDS
12/18
12
4.4.4. Liquid foams
coarse dispersion of gas in liquid
liquid is minority phase
usually not thermodynamically stable
surfactant: foaming agent
retard drainage of liquid from foam
prevent rupturemetastable foams
in vertices (plateau borders):liquid pressure lower than in channelsliguid flowrupture
gas
content
polyhedralcells
sphericalbubbles
8/12/2019 Apresentao_SDS
13/18
13
surfactants form lamellaeparallel 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 excessconcentration of surfactant
increase of surface tension (Gibbs effect)
opposes thinning
Marangoni effect:surfactant flows to regionsof reduced surface excessto restore original (lower) surface tension
(convection of surfactant along interface)
Gibbs and Marangoni effects
Gibbs and Marangonieffects oppose thedestabilizing influence
8/12/2019 Apresentao_SDS
14/18
14
4.4.5. Emulsions
two immiscible liquidsI and II
emulsion of phase IIdispersed in phase i
unstable emulsionseparates
surfactant positions itselfon interfaces betweenphase I and phase IIstabilizes emulsion
mixture of two or more
immiscible or partiallymiscible liquids
one liquid (the dispersed phase)is dispersed in the other phase(the continuous phase)
examples: vinaigrette, milk,technical fluids,
8/12/2019 Apresentao_SDS
15/18
15
free energyrequired
to disperse a liquid of volume Vinto drops of radius R:
R
VG
3=
lower interfacial tension reduction in free energy
stabilization of emulsion
emulsions: thermodynamically unstable
microemulsions:
thermodynamically stable
smaller droplet size than in emulsions
slow kinetics of exchange of moleculesin/out of stabilizing film
8/12/2019 Apresentao_SDS
16/18
16
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 invinegar or lemon juicesurfactant: lecithin
margarine is w/o emulsion
size of dispersed particles ~0.1-10 mscatter light emulsions appear cloudy
8/12/2019 Apresentao_SDS
17/18
17
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 onesOstwald ripening
Breaking up of emulsions
8/12/2019 Apresentao_SDS
18/18
18
usingemulsifiers, 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)
Stabilization of emulsions