Problem: Location of a surface at a liquid/vapor interface?

Liquid surface: interfacial region a few molecular diameters thick (nm)Solid surface: interfacial region on a Å scale

@ liquid surface, surface and subphase are in equilibrium:exchange possible

GibbsGibbs adsorptionadsorption isothermisotherm

liq

vap

c(z) c(z) c

c(z) c(z) c

Δ = −

Δ = −

SDSddγμ= −Γ

0

2 20

0

0

z

1 H O H O,v H O,liqz

z

2 SDS SDS,v SDS,liqz

(c(z) c )dz (c(z) c )dz

(c(z) c )dz (c(z) c )dz

∞

−∞

∞

−∞

Γ = Γ = − + −

Γ = Γ = − + −

∫ ∫

∫ ∫

2

vap liq

c(z)dz

(c(z) c )dz (c(z) c )dz

∞

−∞

∞ ∞

−∞ −∞

Γ = Δ

Γ = − + −

∫∫ ∫

= 0

SDSSDS

1 dRT d ln c

γΓ = −

GibbsGibbs adsorptionadsorption equationequation

cvap~ 10-2(M)

SurfaceSurface tensiontension of of surfactantsurfactant solutionssolutions

c << cmc c < cmc c > cmc

saturationbelow cmc

Slopecorresponds tosurface density

Surface tension of polyelectrolyt/surfactant solutions

PSS/CPSS/C1212TABTAB+-

- -

10-5 10-4 10-3 10-2 10-130

40

50

60

70 C12TAB

5*10-3

surfa

ce te

nsio

n / m

N/m

C12TAB concentration / mol/l

A. Asnacios, R. v. K., D. Langevin, Coll. Surfaces A (2001)cac

SO3

z

Na+

OH OCH2OH

OHOH O

2

N+

**

CH3CH3

Cl

m

N+

CH3

CH3

CH3

Br

A) Polyelectrolytes B) Surfactants (c << cmc)

C12G2: nonionic surfactant

CnTAB: cationic surfactant

PDADMAC: polycation

PSS: polyanion

(W. Jaeger, FHI Golm)

MaterialsMaterials

CCnnTABTAB / PAMPS/ PAMPS+ -

C12TAB / PAMPS C16TAB / PAMPS

C12TAB

C16TAB

C12TAB / PAMPS(75 – 750 ppm) C16TAB / PAMPS

(75 ppm)

A. Asnacios, D. Langevin, J.-F. Argillier, Macromolecules (1996)A. Asnacios, R. v. Klitzing, D. Langevin Coll. Surf. A (2000)

CMC (C12TAB) = 15 mM

CMC (C16TAB) = 1 mM

cac caccmc

cmc cmc‘

PAMPSC12TAB / PAMPS

KBr / PAMPS

Stretched chains

P(DADMACP(DADMAC--statstat--NMVA)/SDSNMVA)/SDS+ -

0 20 40 60 80 100

45

50

55

60

65

70

75

su

rface

tens

ion

/ mN

/m

polymer charge density / %

Maximum in density at 50 %- - -

N+

CH3CH3

*

Cl

f

*NCH3

n

OCH3

P(DADMAC-stat-NMVA)

Coiled chainsLow densityof binding sites

Langmuir films

Preparation: dissolve insoluble amphiphiles in a volatile organic solvent and deposit drops of solution onto the air/water interface

S>0 => spreading, evaporation of solvent => monolayer of amphiphiles

Pressure is needed to prevent film from spreading:

0s γ γΠ = −

G: gas phaseL1: liquid expanded phase

(e.g. saturated unbranchedcarbon chains: a0≈30-50Å2)

L2: liquid condensed phase (stronger molecular interactions,lower compressibilty

S: solid (e.g. alcohols, esters: a0≈19 Å2)

G->L1: typical gas liquid transitonlike in 3D

L1->L2: transition not finally explained.

Collapse

Langmuir films

Effect of polymer charge on lipid/polyelectrolyte complexes @ air/water interface:DPPA / PDADMAC–co-polymer

Thickness: EllipsometrynCP-47=nCP-73=1.35dCP-73=7.5 nmdCP-47=9.0 nm

Kerstin de Meijere et al. Macromolecules 1997

GID (grazing incidence diffraction)

Kerstin de Meijere et al. Macromolecules 1997

Effect of polymer charge on lipid/polyelectrolyte complexes @ air/water interface:DPPA / PDADMAC–co-polymer

DPPA

PDADMAC

With PDADMAC

Domains of tilted chains

WithoutPDADMACGID:

αi=0.85αc

in plane diffraction:dhk=2π/Qxy