To the warm or to the cold ? Unusual thermodiffusion...

To the warm or to the cold ? Unusual thermodiffusion behavior of

polymers and colloids in solvent mixtures

Berend-Jan de Gans, MPIP

Jutta Luettmer-Strathmann, University Akron

Simone Wiegand, MPIP

Ak-Wegner Seminar, 27. May 2002

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What is the Soret effect ?

( )lnqX T= −∇r r 1

mX Tµ= − ∇

r r

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Gedankenexperiment

Thermodiffusion behavior, so what ?

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Technical application:

� understanding of the mechanics of crude oil extractionand reservoir characterisation (Petrol industry)

� Thermal field flow fractionation (polymer characterization)

� Separation techniques in organic chemistry

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Theoretical background

� gases: Chapman-Enskog theory some rules of the thumb

(i) heavier species goes to the cold side(ii) larger species move to the cold (m1 ~ m2)

� fluids: thermodynamic description (Onsager equations)no microscopic theory !!!

qq

mq T2 2

th2

m

mm

q

theoretical treatment experimental description1 (1 )

1

m

q

L Dcj c c c TT T t

Tj L TT T

L

L D

D

t

µ

µ

∂ = − ∇ + ∇ ↔ = ∇ + − ∇ ∂ ∂ = − ∇ + ∇ ↔ = ∇ ∂

r

r

th

: mutual diffusion coefficientSoret coeffient: : thermal diffusion coefficient

: thermal diffusivity

TT T

DD

DD

DS

=

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SimulationSimulation of Lennardof Lennard--Jones fluidsJones fluids

Dirk Reith and Florian Müller-Plathe, Journal of Chemical Physics, 112, (2000) 2436

1 2

1 2

1 2

M M

ε ε

<∅ > ∅<

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Classical Experiments

Gravitational column

Thermal diffusion cell

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Principle Interference of two laser beams creates a transientthermal grating.

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Principle

nC>n

H

nC<nH

Temperature

refractive index grating caused by T

Thermal diffusion

refractive index grating caused by migration

nT+nc

nT+nc

nT

T

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Working equation Working equation

temperature

concentration

Heterodyne signal:(Switching on the grating)

1E-3 0.01 0.1 1 10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

PS in ethylacetate

She

t / S

het,t

h

time / ms

( ) ( )( ) ( )

( ) ( )( ) ( )22

,/

,

//

wi 1t /h 1/ ,

/ 11 1/

1 1

th th

th

th

Tp Tt

hetthp c

ttth

q D q D

n cS e c c

n T

e e

S

τ

τ

τ

τ τ

τ τ

ττ

−

−−

= =

∂ ∂= − − −

∂ ∂ −

− − −

0 50 100 150 200 250 300 350 400 450 500 550 600

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

PS in ethylacetate

S het /

She

t,th

time / ms

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SetSet--up: up: TDFRSTDFRS

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Colloidal system Colloidal system

Properties:average length = 251 nm (TEM)polydispersity = 40 %)average thickness = 8 nm

boehmite (g-AlOOH) rods

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ResultsResults:: colloidal system colloidal system

boehmite in ethanol/water mixtures

H2O: 4.1 wt %

H2O: 5.5 wt %

0 2 4 6-1.5x105

-7.5x104

0.0

7.5x104

1.5x105

0 2 4 6-3.0x105

-1.5x105

0.0

1.5x105

3.0x105

ζ het /

a.u.

t / s

ζ het /

a.u.

t / s

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PEOPEO inin ethanolethanol//water water

0.0 0.5 1.0 1.5 2.0-4.0x105

-2.0x105

0.0

2.0x105

4.0x105

ζ het /

a.u.

t / s

0 50 100 150-3.0x105

-1.5x105

0.0

1.5x105

3.0x105

ζ het /

a.u.

t / ms

Cwater = 15.02 %

W/E=18 %

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PEOPEO inin ethanolethanol//water water

0.0 0.1 0.2 0.30.0

5.0x10-8

1.0x10-7

1.5x10-7

2.0x10-7

0.0 0.1 0.2 0.3-0.15

-0.10

-0.05

0.00

0.0 0.1 0.2 0.3-1.5x10-8

-1.0x10-8

-5.0x10-9

0.0

D /

cm-2. s

-1

φm, H

2O

S T / K

-1

φm,H

2O

DT /

cm

2 .s-1

φm, H

2O

with increasing water content the Soret coefficient ST decreases

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Dynamic and static light scattering

( )( ) ( )

0

1 2 22

22 40

1/ 2,

, ,, , , , `0

11 23

optical constant: 4 / /

1j j

w G

solv LT

w appw

PEO w PEO wp T cp T p T c

Kc M q R A cR q

K n n c N

Mc n nc c c M

µ

π λ

−

≠

= + + ∆

= ∂ ∂

∂ ∂ ∂ = − ∂ ∂ ∂

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light scattering results

The addition of water expands the chains

5 10 15 20 2522

24

26

28

30

32

34

36

RG

2

Water content g/L

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refractive index increment

0.00 0.05 0.10 0.15 0.20 0.251.363

1.364

1.365

1.366

-0.04

-0.02

0.00

0.02

0.04

ρ (dn/dc1 )p,T,c

0

n

φm , H2O

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assumptions for a simple model assumptions for a simple model

A) PEO/EthanolEthanol is a bad solvent at room temperaturePEO chains are collapsedUCST

B) PEO/waterWater is a good solvent at room temperaturePEO chains are extendedinteractions between PEO and water are specific, they areattractive if the water molecules come from the �right� side

C) water/ethanol highly miscible

D) PEO in a mixture ethanol/waterfor low water content, ethanol is enriched in the vicinity of the polymer (higher water concentration in the bulk than in the vicinity of the polymer)

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the lattice model the lattice model

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Results in a simple lattice model Results in a simple lattice model

1. A polymer in a good solvent typically moves to the cold side.

2. A polymer in a bad solvent typically moves to the warm side.

3. In the case of a specific interactions between one of the solvents and the polymer. Part of the molecules are attached to the chain which improves the solubility at some points of thechains which keeps the polymer in solution.

Work in progress (Jutta Luettmer-Strathmann)

0 5 10 15 20 25 303.25

3.30

3.35

3.40

3.45

RG

2 in la

ttice

uni

ts s

quar

ed

Water content g/L

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Mean squared radius as function of water contentMean squared radius as function of water content

Preliminary results for a chain with N=15 monomers

The addition of water expands the chain for all concentration

5 10 15 20 25 30 35 400.95

1.00

1.05

wat

er R

atio

: nea

r pol

ymer

/ bu

lk

Water content g/L

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Preferential adsorptionPreferential adsorption

Preliminary results for a chain with N=15 monomers

For low water contents ethanol is preferentially adsorbed.

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Excess probability to find the polymer in the warm boxExcess probability to find the polymer in the warm box

5 10 15 20 25 30 35

1.3x10-4

1.3x10-4

1.3x10-4

1.3x10-4

1.3x10-4

1.3x10-4

0 200 400 600 800 1000

-1.5x10-4

-1.0x10-4

-5.0x10-5

0.0

5.0x10-5

1.0x10-4

1.5x10-4

Exce

ss p

roba

bilit

y (p

- 0.

5)

Water content g/L

Water content g/L

For high water contents PEO moves to the cold side.

Preliminary results for a chain with N=15 monomers

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Phase Phase diagram and many open questionsdiagram and many open questions

ProjectsProjects inin progress progress

polystyrene in ethyacetate

DT is independent of the molecular mass (up to 7 monomers)the mass of the monomer is an important parameter.

TFFF

M.Schimpf in Köhler&Wiegand, LNP, 2002 P. Rossmanith, W. Köhler, MM 29,3203(1996)

Thermodiffusion - monomer to polymermotivationmotivation

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AcknowledgementAcknowledgement

Jutta Luettmer Strathmann

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