Chemistry Reference
In-Depth Information
Generally, if the intermolecular interactions favor mixing, the mutual diffusion
coefficient will be higher than that predicted by the linear rule of mixing.
Otherwise, the opposite behavior would be observed.
Figure 6.2
shows the con-
centration dependency of
the mutual diffusion coefficient
for such binary
systems.
In the case of simple liquids, mutual diffusion is affected more or less equally
by the entropic and enthalpic factors. But in mixtures containing polymer mole-
cules, the entropic contribution to the mutual diffusion is negligible as the entropy
of mixing is scaled with the inverse of the number of polymer segments (the
Flory
Huggins theory). This in turn makes the enthalpic contribution to the
mutual diffusion more crucial for polymer mixtures.
10
−
14
10
−
15
0
0.2
0.4
0.6
0.8
1
(a)
ϕ
d-ps
10
−
14
10
−
15
0
0.2
0.4
0.6
0.8
1
(b)
x
FIGURE 6.2
The compositional dependence of the mutual diffusion coefficient for (a) polymer-polymer,
polystyrene-deuterated polystyrene; (b) a metallic alloy, iron-palladium
[10]
.
