Amphiphiles are both polar and

non-polar – surface active

Lipids = Naturally occurring substances which are insoluble in water, and often with amphiphilic properties. Some examples:

Soap (’tvål’) – Soduim salts of fatty acidsSoft soap (’såpa’) – Potassium salts of fatty acidsPhosphoglycerides – Glycerol esterified with fatty acids and phosphorous acid

Polar (hydrophilic)headgroup

Non-polar (hydrophobic)tail

Synonyms:

Amphiphile SurfactantTensideDetergent Wetting agentEmulsifier

Classes of amphiphiles, with some examples(Named primarily after the charge of the polar headgroup)

N+

Br-

AnionicSodium dodecyl sulphate, SDS

(Sodium lauryl sulphate)

CationicCetyl pyridinium bromide

SO3-

ONa+

O O O O OH

Non-ionicPolyoxyethylene(4)lauryl ether (C12E5)

+_

+_

CO2-N+

Zwitterionic (ampholytic)Dodecyl betaine

Gemini (‘twin’)

PEOx PBOy PEOz

Block copolymersPluronic

Components in commercial

amphiphiles

Polar headgroups

Non-polar tails

Commercial use of amphiphiles

M€ ktonHousehold detergents 2800 4000 Industrial cleaning 420 530 Personal care 940 860 Crop protection 290 200 Oilfield 390 440 Paints and coatings 140 160 Textile 650 660 Construction 190 470 Emulsion polymerisation 240 290 Food 190 200 Leather 30 60 Ore/mineral 60 150 Plastic additives 60 40 Pulp and paper 100 120 Explosives 10 10 Other 630 380 Total 7140 8570

Typically n ≈ 3

”Sodium laureth sulphate”, SLS

SLS and SDS are among themost commonly usedamphiphiles in consumerhygiene products.

“Sodium cocoate sulphate”:Made from coconut oil, wherelauric acid is the primary component.

”Sodium coceth sulphate”:Cf. lauryl/laureth

Amphiphile aggregation

CAC/CMC

The free energy of solvation of the nonpolar parts in water is high, and aggregation occurs at the

Critical aggregation concentration (cac), orCritical micelle concentration (cmc)

Above CAC/CMC the concentration of freemonomers does not increase, just the numberof aggregates, and the surface is saturatedwith amphiphiles.

Increasing amphiphile concentration

Bul

k co

nc.

Surface conc.

Micelles

Monomers

CMC

What happens withthe surface tension?

Self-organization of amphiphiles

The aggregation is driven by…• Hydrophobic interaction between hydrocarbons• van der Waals-attraction between chains• Hydrogen bonds and electrostatic

attraction between polar groups

Steric and electrostatic repulsionbetween polar groups work againstaggregation!

Phase separation instead of micelle formation is possible (particularly for non-ionic surfactants), but is unfavourable for ionic amphiphiles due to repulsion between polar heads, and a considerable loss of entropy for the counterions.

Factors affecting CMC

The critical micelle concentration…… decreases if the hydrophobic tail is made longer… is moderately affected by temperature changes… can be reduced dramatically by addition of other

amphiphiles… decreases with increasing salinity for ionic

amphiphiles… is lower for non-ionic than for ionic amphiphiles… decreases if counterions with higher valency

are added… can vary considerably for molecules with small

differences in molecular structure

These factors also influence the structure (shape) of the aggregates!

Many measurable properties change their concentration dependence at the cmc...

This is used to determine cmc experimentally!

Micelles (and other aggregates)

are dynamic...

Simulation of micelle structure in aprimitive (coarse-grained) amphiphile.

This is not what micelles look like!

Smit, Langmuir 9, 9 (1993)

Properties of some common surfactants

Surfactant CMC(mM)

Molecules per micelle

% bound counterions

SDS 8.0 50 60

AOT 3.0 15 10

CTAB 0.8 55 85

Triton X-100 0.03 135 -

CMC-dependence on n and m in

CnEm-amphiphiles

Daful, J. Phys. Chem. B 115, 3434 (2011)

CMC-variation for different n

CnE3

CnE5

CnE7

CnE9

6

CMC-variation for different m

C6Em

C8Em

C10Em

C14Em

C16Em

C12Em

Circles = Experimental dataSquares = Calculated data

Synergy effects in mixed

amphiphilesC12Betaine

SDS

SDS

C12Betaine

50:50(experiment)

50:50 (ideal)

SO3-

O

Surface composition:

• Calculated (fromsurface tension)

• Experiment• Ideal mixture

CO2-N+

This system cannot be understood if it is considered as an ideal mixture!

Surface tension in the mixed system

Krafft-temperature

• At TKr the solubility is equal to that at the cmc.

• Below TKr the monomer solubulity is too low for micelle formation to be possible.

• Above TKr the solubility increases quickly since monomers form (easily soluble) micelles.

The Krafft-temperature is the lowest possible temperature for micelle formation.

Solubilization – Detergent (cleaning) action

A normally insoluble substance can be present at high concentration in surfactant solutions by being dissolved in the polar interor of micelles. This is the basic mechanism for the cleaning function of amphiphiles!

Nonpolarmolecule

Micelle

Invertedmicelle

Polarmolecule

Dirt

Molecular structure

Aerosol-OT

SDS

How does the molecular structure

affect the properties of amphiphiles?

n m o

Pluronic, EOnPOmEOo

The critical packing parameter (CPP)

The geometry of the amphiphile determines the shape of the aggregates it will form.

max 0

vCPP

l a=

⋅

Volume, v Area, a0

Chain length

The Critical Packing Parameter (CPP) gives an idea of which aggregates will minimize the free energy for a particular amphiphile.

Too coarse a model to permit detailed interpretation or predictions!

Not the geometric area of the molecule,but an effective area, where repulsion between polar heads is accounted for.

CPP and 3D-structure

CPP and aggregate structure

MicellesCPP < 1/3

Vesicles1/2 - 1

Lamellae,bilayersCPP ~ 1

Bicontinuous(”sponge”) phases

CPP > 1

InvertedmicellesCPP > 1

Cylindricmicelles1/3 – 1/2 ~

Lyotropic phases – Phase diagrams

Phase diagram for dodecyl trimethyl ammonium chloride.

Isotropic solution (micelles) at low concentrations, crystalline at low temperatures, and some liquid crystalline phases (cubic, hexagonal and lamellar).

The nearly vertical phase boundaries suggests a moderate temperature dependence.

Liquid crystallinephases

Lyotropic: Liquid crystalline form where the solvent concentration is most important for the properties.

Thermotropic: Temperature is the variable with greatest impact on the phase.

Two-dimensional systems

Insoluble amphiphiles on water surfaces form 2D-systems with unique phase properties. (…like e.g. layers in smectic phases (liquid crystals), lipid bilayers, and monolayers adsorbed on solid surfaces.)

Surface pressure (’yttryck’):π = γ0 − γ

γ γ0

π

π The decrease in surfacetension caused by the surfacefilm

γ0 Surface tension of water

γ Surface tension with the film

Air

Water

Langmuir’s surface balane

The surface film pressure can be varied by moving a barrier which restricts the film.

Surface pressure is measured using a hydrophilic plate immersed in the film (the Wilhelmy metod).

The amphiphiles must be insoluble in water,otherwise they pass under the barrier!

A droplet of e.g. stearic acid dissolved in hexane is injected at the surface; the solvent evaporates and leaves a monolayer of stearic acid at the surface.

2D-phases

Gas – Hydrocarbon chains in contact with the sub-phase, little interaction between the molecules.

Liquid – Chains are upright, but disordered.

Solid – chains are frozen in a crystalline state, cross sectional area ~20 Å2 per chain.

Gaseous

Liquid expanded

Liquid condensed Solid

Collapsing

π−Α –phase diagrams

Myristic acidC14COOH

C15COOH

Effects ofchain length,

(un)saturation,and temperature

Shorter chains, unsaturation, and

higher temperatures give greater chain mobility, and less

propensity for crystallization!

Oleic acid(unsaturated)

Brewster angle microscopy (BAM)

At the Brewster angle no p-polarized lightis reflected from a transparent medium.

θB

Water

Surface film

θB

A thin film on the surface changes the Brewster condition, and the surface willreflect some incoming light.

n2

n3

1

2tann

nB =θ

0

0.2

0.4

0.6

0.8

1

0 15 30 45 60 75 90

R p

R sθ B

Reflekterad intensitet

från luft-vatten gränsenAir n1

1.33arctan 53

1≈ �

Reflected intensity fromthe air-water boundary

BAM in practice

Dimyristoyl phosphatidylethanol amine (DMPE) onwater at 22 oC.a) Clean water surfaceb)-e) Increasing surface

pressuref) Fully covering film

Biological membranes

A bilayer membrane consists of a lipid bilayer with adsorbed or trans-membrane proteins, carbohydrates, glycolipids, etc.

Purpose of the membrane:

• Spatial confinement ofthe contents

• Control the transport ofions, molecules, energy,information...

• Solvent for membraneproteins

Phase behaviour in bilayers

Giocondi, Biochim. Biophys. Acta, 1798, 703 (2010)

Gel: Frozen phase with acyl chainspacked and extended, slow diffusion in the membrane. Unsuitable for biological membranes…

Fluid (Liquid disordered): Rapid diffusion and rotation of lipids, disorder (and thinning) of the non-polar region.

Liquid ordered: Ordered chains butstill relatively rapid diffusion.

For pure phospholipids there is a well-defined temperature, Tm, for the transition from gel to fluid phases. This transition can be eliminated by e.g. addition of cholesterol.

CMC/CAC by some diacyl PC-lipids

CMCs for somephosphatidyl-choline lipids with varying length in the nonpolar tails.

Remember: CMC is also approximately the bulk concentration of free molecules

Mesophases – Between liquid and

crystal

∆T

∆T

Liquid crystals

Translational order is lost, Orientational order is retained.

Plastic crystals

Translational order is retained,Orientational order is lost.

Liquid crystals

Thermotropic – Temperature is the variable which primarily determines the phase state.

Lyotropic – Solvent concentration determines the phase state (usually the case for amphiphiles).

Formes primarily by rodlike, but not too longmolecules, often with a hydrocarbon chain,whose tendency to disorder prevents crystallization as the temperature is lowered.

Rather, a series of phase changes withsuccessively increasing disorder are obtained!

Order parameter, S:

S = 1: Perfect orderS = 0: Disorder, isotropic

23cos 1

2S

θ −=

θ

Thermotropic phases

Isotropic

Completely disordered structure, normal liquid.

NematicLong-range orientational, but not positional order.

Smectic A or COrdered structure of two-dimensional layers.

(There are many different smectic phases with varying structure and order in the layers!)

Crystalline (Anisotropic)

Long-range orientational and positional order in all directions.

Foams, foam films and

surface films

Foam =

a material with gas trapped in cells in a liquid or solid matrix.

Fusion of bubbles is a spontaneous process, decreasing the total surface area.If this is prevented (e.g. with amphiphiles)a foam can be formed – pure liquids neverform any foams!

Surface active molecules at the air/waterinterfaces cause…

1) Repulsion between the interfacesElectrostatic repulsion between ionic or polar headgroups, orSteric repulsion between polymeric amphiphiles.

2) Surface elasticityHigh surface elasticity is necessary for foam stability(Gibbs-Marangoni effects).

Foaming

The Gibbs-Marangoni effect

Stretching a foam lamella (by e.g. mechanical vibrations) causes a local reduction in surfactant concentration.

This locally increases the surface tension, and the thus formed surface tension gradient drives molecules toward the disturbance (Gibbs).

The molecules diffusing along the interface will also sweep fluid along with them (Marangoni), and both effects act to stabilize the lamella.

dγ dγ

The Gibbs-Marangoni mechanism is most effective at intermediate surfactant concentrations (high surface elasticites):- Too low concentration gives a weak surface tension gradient.- Too high concentration (>cmc) restores the surface tension via diffusion from the interior of the film before the thickness has been recovered.

Surface active substances which do not aggregate do not form foams (e.g. ethanol in water)!

All foams are thermodynamically unstable!

Bubble zone

’Kugelschaum’ Low gas volume,thick liquid films

Polyhedral foamHigh gas volumeand thin liquid films

Flow ofbubbles

Drainageof liquid

Foam formation

Foams can be formed by, for example:

- Blowing gas/sparging intoa liquid

- Shaking- Nucleation of bubbles…

The anatomy of a foam

The Plateau borders form acontinuous network, throughwhich the foam is drained.

The amount of liquiddetermines if the foam is ”dry” or ”wet”.

Lower pressure in the junctionscause capillary suction whichincreases the foam drainage rate.

Foam drainage

AirAir

Water

R

The lamellae in a foam are drained by gravity andLaplace pressure.

The curvature at the intersection of the foam lamellae (the Plateau borders) induces a Laplace pressure difference between the gas cell and the Plateau border. The pressure in the border (PW) is lower than that in the flat lamellae (PF), drawing water out from the lamellae.

Diffusion between foam cells

Pressure differences drives the diffusion ofgas between the cells in the foam.

A two-dimensional foam consists ofcircular arcs whose curvature isdetermined by the pressuredifferences between adjacent cells.

Equilibrium structure in”dry” foams

In a three-dimensional foam (in equilibrium)the planes meet at an angle of 120° in thePlateau borders, and the angles betweenthe Plateau borders is 109,5°.

If this is not fulfilled, the dynamics of the foam will drive the foam toward this state.

3-sided cells are eliminated”4-way crossings” are eliminated

Topologicalchanges:

Cause Effect

Gravitation Drainage to the foam base

Laplace pressure between lamellae and Plateau borders

Drainage to Plateau borders

Pressure differences in foam cells ofdifferent sizes

Diffusion of gas from small to large cells through the foam films (lamellae)

Overlap between electrical double layers Foam stabilization(increasing the electrolyte concentration

reduces the repulsion)

In addition, the stability is also affected by the liquidviscosity, which can be increased by macromolecules(proteins, carbohydrates), thus reducing drainage.

Forces acting in a foam Naturalfoams

Cork

Foams fromSpittlebugs

Bone

Beer & champagne

Food (whipped egg white, whipped cream, bread, ice cream…)

Foam separation ofsurfactants (”foamfractionation”)

Flotation

Firefighting

Cleaning

Oil recovery

Uses offoams