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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
motivationmotivation
statestate
colloids colloids
PEOPEO
theorytheory
directionsdirections
What is the Soret effect ?
( )lnqX T= −∇r r 1
mX Tµ= − ∇
r r
motivationmotivation
statestate
colloids colloids
PEOPEO
theorytheory
directionsdirections
Gedankenexperiment
Thermodiffusion behavior, so what ?
motivationmotivation
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colloids colloids
PEOPEO
theorytheory
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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
motivationmotivation
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colloids colloids
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theorytheory
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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 !!!
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
=
motivationmotivation
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colloids colloids
PEOPEO
theorytheory
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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
<∅ > ∅<
motivationmotivation
statestate
colloids colloids
PEOPEO
theorytheory
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Classical Experiments
Gravitational column
Thermal diffusion cell
motivationmotivation
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colloids colloids
PEOPEO
theorytheory
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Principle Interference of two laser beams creates a transientthermal grating.
motivationmotivation
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colloids colloids
PEOPEO
theorytheory
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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
motivationmotivation
statestate
colloids colloids
PEOPEO
theorytheory
directionsdirections
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
motivationmotivation
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colloids colloids
PEOPEO
theorytheory
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SetSet--up: up: TDFRSTDFRS
motivationmotivation
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colloidscolloids
PEOPEO
theorytheory
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Colloidal system Colloidal system
Properties:average length = 251 nm (TEM)polydispersity = 40 %)average thickness = 8 nm
boehmite (g-AlOOH) rods
motivationmotivation
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colloidscolloids
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theorytheory
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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
motivationmotivation
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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 %
motivationmotivation
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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
motivationmotivation
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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
motivationmotivation
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theorytheory
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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)
motivationmotivation
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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
motivationmotivation
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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
motivationmotivation
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Preferential adsorptionPreferential adsorption
Preliminary results for a chain with N=15 monomers
For low water contents ethanol is preferentially adsorbed.
motivationmotivation
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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
motivationmotivation
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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
motivationmotivation
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