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

2

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

4

Phase diagram of nonionic surfactants phase diagrams of CmEn constant hydrophobic chain length m increasing EO chain length n (biphasic regions not indicated)

5

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 7

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 9

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 … 10

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 11

• 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 12

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 13

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 14

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 15

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 16

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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