Chemistry Reference
In-Depth Information
This difference in chemical potential is seen to be a power series in the solute
concentration. Such equations are called
virial equations
and more is said about
them in
Section 3.1.4
.
It is evident that insertion of corresponding experimental values of
c
2
,
V
1
, and
ðμ
2
2
G
1
Þ
into
Eq. (3-10)
would provide a measure of the solute molecular weight
M
. We will show in
Section 3.1.2
how the difference in the value of a colligative
property in pure solvent and solution measures
G
1
Þ
and in
Section 3.1.3
that
the
M
measured by application of
Eq. (3-10)
is the
M
n
of the polymeric solute.
ðμ
2
2
3.1.2
Osmotic Pressure
Colligative properties reflect the chemical potential of the solvent in solution.
Alternatively, a colligative property is a measure of the depression of the activity
of the solvent in solution, compared to the pure state. Colligative properties
include vapor pressure lowering, boiling point elevation, freezing point depres-
sion, and membrane osmometry. The latter property is considered here, since it is
the most important of the group as far as synthetic polymers are concerned.
Figure 3.1
is a schematic of the apparatus used for the measurement of osmotic
pressure. A solution is separated from its pure solvent by a semipermeable mem-
brane, which allows solvent molecules to pass but blocks solute. Both components
are at the same temperature, and the hydrostatic pressure on each is recorded by
means of the heights of the corresponding fluids in capillary columns. The solute
cannot distribute itself on both sides of the membrane. The solvent flows initially,
Osmotic
head
Compartment 1
solvent
Compartment 2
solution
Membrane
FIGURE 3.1
System for demonstration of osmotic pressure.
