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146
3.3. OSMOMETRY
3.3.1. General
Osmometry is experimental method to determine M
n
and A
2
of
macromolecules. The instrument is in principle simple. It consists of a sample
(dissolved polymer) (
α
-side) which is separated from the pure solvent (
β
-side)
by a semi-permeable membrane. The solvent (water and salts) may pass
freely through the membrane, but it is impermeable for the polymer.
When equilibrium is reached (hours to days) the difference in pressure is
recorded (see figure). At equilibrium the chemical potential of diffusible
molecules (solvent, salts) is by definition equal on the two sides. Hence, for
the solvent:
0
1
1
1
( )
( )
µ
α
µ
β
µ
=
=
The difference in chemical potential due to different pressures (P
0
and P
0
+P)
is generally given by:
Δ
µ
i
P
=
∂
µ
1
∂
P
⎛
⎝⎜
⎞
⎠⎟
P
0
P
0
+
Π
∫
T
∂
P
=
V
1
P
0
P
0
+
Π
∫
∂
P
≈
V
1
0
P
0
P
0
+
Π
∫
∂
P
=
V
1
0
Π
Combined with Eq. 10.60 this leads to the main equation:
Solvent
(component 1)
+
Dissolved polymer
(component 2)
Pure
solvent
(component 1)
α
side:
P
0
+
Π
β
side:
P
0
Π