Membrane Osmometry.pdf

Membrane osmometry Presentation
Osmometry is an advanced analytical method for determining the osmotic concentration of solutions. The osmotic – or solute – concentration of a colloidal system is expressed in osmoles (Osm) per unit of volume (Osm/L) or weight (Osm/kg).

In the membrane osmometry technique, a pure solvent and polymer solution are separated by a semipermeable membrane, due to the higher chemical potential of the solvent in the pure solvent than in polymer solution, the solvent starts moving towards the polymer solution
The upper limit of molecular weights to which the vapor phase osmometer can be applied is usually considered to be 20,000 g mol−1. Newer, more sensitive machines have extended this limit to 50,000 g mol−1 or higher.

1. Membrane Osmometry
Alfredo Clemente
CH 392N Prof. Grant Willson.
The University of Texas Austin

2. Membrane Osmometry
The University of Texas at Austin

Molecular Weight and polymer properties

Methods Used to determine Mn, Mw

Membrane Osmometry

Conclusions:
Advantages and disadvantages

Questions

Introduction and Theory

Measuring Mnby osmotic pressure

3. The University of Texas at Austin
Molecular weight
Relative fraction
Representative differential weight
distribution curves
1

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Relationship of polymer properties to
molecular weight.
1

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Typical Molecular Weight Determination Methods
1
Mol w Dist
To ∞ Relative SEC, with cdetector
To 4x10
4
Vapor phase osmometry
To ∞ Relative Viscometry
Mol w Dist
To ∞ SEC, with cand LS detectors
To ∞ Centrifugation
500-2500 Osmodialysis
To 2x10
4
End Group Analysis
To 5x10
4
Cryoscopy
To 4x10
4
Ebulliometry
Shape, dist
10
2
to ∞ Electron and X-ray microscopy
2x10
4
to 2x10
6
Membrane osmometry
Shape
To ∞ Light scattering (LS)
Info. Range
Type of mol.
wt. avg.
Method
n
M
n
M
zwn
M
,,
n
M
n
M
n
M
n
M
wz
M
,
wn
M
,
w
M

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Membrane Osmometry Introduction and Theory
Osmosis and Osmotic Pressure
Osmosis and Chemical Potential
h
0
1
m
1
m

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Equilibrium of Chemical Potential
2
( )
)(
)(
)(
)( ) ..(
)(
)(
1
0
1 1
1 0 1
0
1
1
,, 1 ,,
1
,,
,, 1 ,,
1
,,
,,
1
1
0
1
2
2 1
2 1
2 1 2 1
0
2 1
VI V
V VP P
IV V
n
G
P
III then volume solution the ei V
P
G
Since
P
G
n P
II that follows it
n
G
From
I dP
P
nPT nnT
nnT
nnT nnT
nTP i
i
P
P
nnT
j
p m m
m m
m
m
m
m
m m
-= -
- + =
=








¶
¶
= 





¶
¶
= 





¶
¶






¶
¶
¶
¶
= 





¶
¶








¶
¶
=






¶
¶
+ =∫

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Osmotic Pressure
( )
( )
) ( /
2
1
/
) ( / /
)(
)(
) (
)(
) ( /
2
1
/
) (
2
1
/
2
2 1
2
2
2 2
11
1
2
2 1
2
2
1 1
1
2
2 1 2
2
2 2
0
1 1
0
1 1
XIII VnVx T VnT
XII Vn Vx
XI Vn V
X
n
xn
xn n
xn
IX V V
VIII V T Vx T
VII T x T






- + =
=
»
»
+
=
=






- + =






- + -= -
- -
c p
f
f
fc f p
f c f m m
m m
R R
R R
R R
for expression Huggins Floury

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Osmotic Pressure and
) (
2
1 R R
) (
1
) ( /
) (
2
1 R R
) (
) (
2
2 1
2
2 1
1 2
2
1
1
2 2
2
XX c
V
T
M
T
c
XIX
M
V
M
xV
XVII VVx
XVI c
M
xV
V
T
M
T
c
XV
M
c
m
n
V
m
V
n
XIV
n
m
n
Mn
M
n
n n
n n
n
i
i i
n






-








+ =
= =
=














-








+ =
=












=
= =
∑
∑
c
r
p
r
c
p
n
M

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Osmotic Pressure and
) ( ...
1
R
) (
R
) (
R
2
3 2
0
XXIII cAcA
M
T
c
XXII
M
T
c
XXI
M
T
c
n
n c
n








+ + + =
= 





=





®
p
p
p
q
n
M

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Osmotic Pressure and
n
M
Typical plot of osmometry
experimental data
12
14
16
18
20
0 2 4 6 8 10 12
c/ g dm-3
(п/c) / J kg-1
2

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Measuring by osmotic pressure
n
M
Membrane osmometers used:

Static osmometer
2
p
µ
h
•Equilibrium by natural diffusion
•Large cell volumes
•Long equilibrium times

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Measuring by osmotic pressure
n
M
Membrane osmometers used:

Dynamic osometer
3
•Equilibrium by reducing
pressure on solution reservoir
•Small cell volumes
•Short equilibrium times

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Measuring by osmotic pressure
n
M
Membrane osmometers used:

Dynamic osometer
3

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Osmotic Pressure and
n
M
5
20.0°C:Slope = A
2
= -2.0x10
-4
cm
3
mol/g
2
1/intercept = M
n
= 1.97x10
4
g/mol
34.5°C:Slope = A
2
= 1.7x10
-5
cm
3
mol/g
2
1/intercept = M
n
= 2.02x10
4
g/mol
50.0°C:Slope = A
2
= 1.7x10
-5
cm
3
mol/g
2
1/intercept = M
n
= 2.00x10
4
g/mol
Polystyrene in cyclohexane
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.000 0.010 0.020 0.030 0.040 0.050
c, g/mL
Π, atm
20.0°C 34.5°C 50.0°C
Polystyrene in cyclohexane
4.0E-05
4.5E-05
5.0E-05
5.5E-05
6.0E-05
0.000 0.010 0.020 0.030 0.040 0.050
c, g/mL
Π/ cRT, mol/g
20.0°C 34.5°C 50.0°C

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Conclusion: Advantages and disadvantages
Disadvantages
Membrane problems: leakage, asymmetry and ballooning

Overestimation of molecular due low molecular weight
molecules

Not suitable for electrolytes
Advantages
Absolute value of Mn

No calibration with standards required

Independent of chemical heterogeneity

Applicable to polymers with broad range of molecular
weights

Measurement of Mn within 10,000 to 2x10
6

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Questions?

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References
1. Carraher, C. E., JrPolymer Chemistry: An Introduction, 4th Ed., Marcel
Dekker, NY: 1996.
2. Young, R. J.; Lovell, P. A. Introduction to Polymers, 2nd Ed., Chapman &
Hall, New York: 1991.
3. Lipták B. G.; Brodgesell, A . Instrument Engineers' Handbook, Process
measurement and analysis.CRC Press, Florida: 1995
4. W. R. Krigbaum and L. H. Sperling, J. Phys. Chem., 64, 99 (1960)
5. Hiemenz, Paul C., Lodge, Timothy P.; Polymer Chemistry, 2nd Ed., CRC
Press, Boca Raton: 2007.
6.
http://www.engga.uwo.ca/people/pcharpentier/392-
2004/MW%20Measurement.pdf 7.
http://www.chem.ufl.edu/~polymer/instrumentation/vp o.html
8.
http://www.chem.ufl.edu/~polymer/instrumentation/vp o.html
9. http://www.eng.uq.edu.au/files/course/files/CHEE200 6/CHEE2006%20Week% 2012_2.pdf

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References
10.
http://www.humancorp.co.kr/catalog/272-277.pdf
11. http://www.gonotec.com/content.OSMO_090.PRODUCTS_CHEM.OSMO_090 .USA.ENG.html 12.
http://www.princeton.edu/~pccm/facilities-polymersy nth-eq.htm
13 Chalmer, John M.; Meier, Robert J., Molecular Characterization and
Analysis of polymers, Elsevier Science, Burlington: 2008

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