Self-assembly and phase behavior

� Amphiphiles and surface tension

� Lyotropic phases

� Micelles

� Key parameters for micellisation

� Critical packing parameter

� Other lyotropic phases

� Special lyotropic phases: vesicles

o Surface tension of a liquid = a measure of the cohesive forces betweenthe molecules at a surface liquid/air

-Molecules inside the liquid

Amphiphiles and surface tension

0==∑ iFF

-Molecules at the surface of the liquid

0≠=∑ iFF

γ mN /072.0=γ

o Surface tension of a film = the increase in free energy/area (A)

Amphiphiles and surface tension

y

F

ydx

Fdx

dA

dw ===γ Fy

xxw = work done to increase the area Ax-y = rectangular frame to create a thin

film of fluid

o Surfactants: decrease the surface tension when they localize at thesurface → monolayer of surfactants

Application of the surfactants:

Amphiphiles and surface tension

o low c(surfactant) → surfactants molecules at the surface (surface excess,Γ) + surfactants molecules inside the liquid

Γ−= RTcd

d

ln

γ

o c(surfactant) Γ monolayer of surfactant

o c(surfactant) micelles

Gibbs isotherm:

c = concentration of surfactant

R = ideal gas constant

T = temperature

Γ = surface excess of surfactant

Amphiphiles and surface tension

Γ−= RTcd

d

ln

γGibbs isotherm:

c = concentration of surfactant

R = ideal gas constant

T = temperature

Γ = surface excess of surfactant

o Surface excess depends on the affinity of the surfactant molecules for the o Surface excess depends on the affinity of the surfactant molecules for the surface

Langmuir equation:ck

cka

ad

ad

+=Γ

10

c = concentration of surfactant

kad = rate constant for surfactant

adsorption to the interface

Γ= surface excess of surfactant

( ) 00

1ln γγ ++−= cka

RTad

=0γ surface tension of the

solvent

=0a Surfactant head group area

o Lyotropic phases = phases that are formed when the concentration ofthe amphiphilic molecules is increasing

Lyotropic phases change their architecture as function of the amphiphilicconcentration

Lyotropic phases

Lyotropic phases:

-Micelles

-Lattice-like arrangements

-Lamellar phases

-Inverse phases

Micelles

o Micelles = supramolecular assembly (normally spherical as shape) basedon surfactants/amphiphilic molecules that are formed above a certainconcetration of the surfactant/amphiphilic molecules

o Micelle architecture: a core of the hydrophobic chains of thesurfactant/amphiphilic molecuels surrounded by the hydrophilic headgroups/corrona of the hydrophilic chains:

Micelles form at a low concentration of the surfactant/amphiphilic molecules

TEM image of PEG-SS-PLA-SS-PEI micelles

C. He, et al, Polym. Chem, 2016, 7, 4352-4366

Micelles

o Critical micellar concentration, CMC = concentration of thesurfactants/amphiphilic molecules where a transition from a disperse of thesurfactant/amphiphilic moleculea to a mielle phase occurs.

c < CMC c > CMC

� Micelle solution is highly dynamic > molecules leave/rejoin micellearchitecture

� Micelle dynamics = f (T)

Micelles: CMC

o Various macroscoppic properties = f (CMC) :

- Surface tension

- Viscosity

- Optical scattering properties

Example: Changes in some physical properties for

an aqueous solution of sodium dodecyl sulfate

(SDS) in the neighborhood of the CMC

Various methods to detemine CMC use the change of the macroscopic propertyx, x = f(c ) : CMC is the discontinuity point/region

CMC importance

Example. Changes in surface tension

serve for the determination of the

CMC

CMC importanceExample. sodium dodecyl sulfate (SDS)

Electric conductivity

TurbidityTurbidity

Surface tension

In most cases there is a small range of concentration where changes inmacroscopic properties appear: CMC range !

CMC examples

CMC examples

Types of surfactants/amphiphiles:- Ionic- Cationic- Non-ionic- Zwitterionic

Micelles: Aggregation number

o Aggregation number, Nagg = number of surfactant molecules/micelle

o Nagg ranges from 50 up to 100 for spherical micelles, depending on thesurfactant type.

surfact

micelle

surfact

micelleagg A

A

V

VN ==

surfactsurfact AV

� Nagg when CMC

Nagg of non-ionic polyethylene oxide amphiphiles.

� Nagg when CMC

Micelles: Aggregation number

Key parameters for micellisation

o CMC = f (length of the hydrocarbon chains)

o CMC = f (head groups charge)

� CMC when chain length

o CMC = f (addition of salts in the solution)

o CMC = f (T) – complex behavior

� CMC for anionic/cationicamphiphilic molecules

� CMC by addition of counterions

CMC decreases with increasing

the chain length of the

hydrophobic tail, more

pronounced for non-ionic

surfactants/amphiphiles.

Key parameters for micellisation

Key parameters for micellisation

o Condition for an efficient packing into a shperical micelle → surfacearea occupied by the hydrophilic head groups/domains must shield thevolume ocupied by the hydrophhobic tails/domains.

optRR >optRR = optRR <

Key parameters for micellisation

o CMC = f (T) – complex behavior butdoes not vary significantly with T

surfactants C4-Azo-OCnTMAB in pure

water

- Ionic surfactants:

Exception SDS

water

� CMC when T

Temperature-

dependent CMC of SDS

Key parameters for micellisation

� CMC when T

- Non-ionic surfactants:

CMC allows estimation of TD functions:CMC allows estimation of TD functions:

( )CMCTRG ig ln=∆

( )CMCRdT

CMCdTR

dT

dGS igig ln

)ln( −−=−=∆

( )dT

CMCdTRH ig

ln2−=∆

Rig = ideal gas constant

T = temperature

Critical packing parameter

Important:

- Low concentrations of surfactant → spherical micelles

- High concentrations of surfactant → other phases

Critical packing parameter, CPP = geometric parameter of asurfactant defined as :

c

surfact

la

VCPP

0

=

surfactant defined as :

=surfactV

=0a

=cl

Volume of the tail

chain

Area of the head group

at the head-tail

interface

Critical length of the

tail chain

CPP can be used to predict the likely phase of a particular surfactantsystem

Critical packing parameter

CPP can be estimated using empirical values for Vsurfact and lc.

( )mnVsurfact 0269.00274.0 +=

nlc 1265.0154.0 +=

=n

=m

Number of C in the

hydrophobic chain

Number of

hydrocarbon chains

Estimation of CPP for spherical micelles

3

3

4RMVsurfact π=

20 4 RM πα = 3

1

0

=R

Vsurfact

α

Rlc ≥3

1≤CPP

CPP for other shapes of micelles: 3

1>CPP

Critical packing parameter

Polybutadiene- block -poly(1-methyl-2-vinyl pyridinium)- block

-poly(sodium methacrylate) (BVqMANa) micelles with a rather

thin corona

Anionic/zwitterionic surfactant solution (SDS/ TPS) in

the presence of Ca(NO3)2 in which viscoelastic

wormlike micelles are formed

Other lyotropic phases

Other lyotropic phases are formed as function of: molecular geometry of the surfactant molecules + concentration of surfactant molecules:

� Lattice-like arrangement: - Cubic phase → micelles packed closely and interacting- Hexagonal phase → closely packed arrangement of cylindrical micelles

� Other architectures can be found in specific conditions (different surfactant concentrations) → hollow disks, tubules, vesicles

� Lamellar phase → bilayer sheets

� Inverse haxagonal phase → cylinders of water surrounded by surfactant phase

� Inverse micelles → water spherical domains surrounded by surfactant phase

Other lyotropic phases

Micelle

Hexagonal

phaseLamellar

phase

Inverse

hexagonal

phase

Inverse

micelle

Critical packing parameter

Other lyotropic phases

Example: Lipids CPP Shape Structures

Special lyotropic phases

scale bar = 100 nm)

scale bar = 10000 nm)

Vesicles/polymersomes Giant Unilemellar NanotubesVesicles/polymersomes Giant Unilemellar

vesicles (GUVs)

Nanotubes

Complex phases

Vesicles: mechanism of formation

From disk-like bilayer structures to closed vesicles

Line energyLine energy

Membrane bending energy

Minimal vesicle size

ac → critical scaling parameter

(associated with nonlinear elasticity)

d → thickness of membrane

d0 → the length of the hydrophobic core

of the membraneof the membrane

Minimal vesicle size

membrane stiffness

membrane thickness

formation of larger vesicles

Giants formed by PEO-b-PCL-b-PMOXAPure egg lecithin (black) and egg

lecithin/cholesterol (red) vesicles

Thermodynamic stability

Most of vesicles are non-equilibrium structures. The molecules are kinetically trapped during preparation.

� Size regulation by extrusion

� Vesicle formation with various shapes

Vesicles: types and applications

Liposomes (phopholipid)

Polymersomes (amphiphilic block copolymers)

� Drug delivery

� Artificialorganelles

Classification based on materials

Collioidsome

� Sensors

• P. Tanner, V. Balasubramanian, C.G. Palivan, Ading Nature’s organelles: Artificial peroxisomes play their role, Nano Letters, 2013, 13(6), 2875-2883.• X. Zhang, M. Lomora, T. Einfalt, W. Meier,N. Klein, D. Schneider, C. G. Palivan, Active surfaces engineered by immobilizing protein-polymer nanoreactors for selectively detecting sugar alcohols, Biomaterials, 2016, 89, 79-88.• J. Liu, V. Postupalenko, S. Lörcher, D. Wu, M. Chami, W. Meier, C. G. Palivan, DNA-mediated self-organization of polymeric nano-compartments leading to interconnected artificial organelles, Nano Letters, 2016, 16, 7128-7136.

� New materials

Giant unilamellar vesicles: types and applications

giant unilamellar vesicles� Artificial cells and synthetic cells

Janus giant vesicles multicompartment vesicles

� synthetic tissues

� G. M. Kontogeorgis, S. Kill, Introduction to applied colloid andsurface chemistry, Wiley-VCH, 2016

� L.S. Hirt, Fundamentals of soft matter science, CRC Press,2013.

� D. F. Evans, H. Wennerstrom, The colloidal domain, Wiley-

References:

� D. F. Evans, H. Wennerstrom, The colloidal domain, Wiley-VCH, second edition, 2014.

� M. Antonietti, S. Förster, Vesicles and liposomes: A self-assembly principle beyond lipids, Advanced Materials, 2003, 15,1323.

� C.G. Palivan, R. Goers, A. Najer, X. Zhang, W. Meier,Bioinspired polymer vesicles and membranes for biological andmedical applications, Chem. Soc. Rev, 2016, 45, 377.

Self-assembly and phase behavior

� Amphiphiles and surface tension

� Lyotropic phases

� Micelles

� Key parameters for micellisation

� Critical packing parameter

� Other lyotropic phases

� Special lyotropic phases: vesicles