Chemistry Reference
In-Depth Information
18
16
14
12
10
8
6
4
2
0
0
50
100
150
200
Segment number
Figure 6 The average distance of biopolymer segments from the surface at the transition
point (
o
R
i
4). The first monomer of the hydrophobic end of the diblock
copolymer chain is numbered i
¼
1. The dashed line represents the results for the
low-coverage surfactant phase, and the solid line that for the high-coverage
surfactant phases
along the chain backbone. The first monomer starting from the hydrophobic
end of the diblock copolymer is numbered as 1. Once again the solid and the
dashed lines represent the high- and low-coverage surfactant phases, respec-
tively. The results support a similar conclusion to that of Figure 5. In the low-
coverage phase, the hydrophobic part of the polymer lies completely flat on the
surface. But this is seen to be largely displaced by the surfactant, for the high
surface coverage phase at the transition point. A large proportion of hydro-
phobic monomers is now situated at an average distance of 5-6 monomer units
away from the surface, providing an explanation for the peak seen in Figure 5
at the same distance. The more extended nature of the interfacial layer is also
evident for the latter phase. At the transition, the average distance away from
the surface for the last hydrophilic segment on the chain makes a jump from a
value of
o
R
i
4
¼
10.7 to
o
R
i
4
¼
15.3 (in units of monomer size).
The current study has focused on a particular type of biopolymer structure,
roughly representative of a disordered amphiphilic protein such as b-casein.
However, the same phenomenon is also predicted for other types of polymer
architecture. For example, results not presented here show the occurrence of
the phase transition in mixed layers containing chains with a large number
of alternating hydrophobic and hydrophilic short blocks. These structures may
be rather closer representatives of globular-type proteins - at least in their
denatured state.
16.4 Conclusions
We have used SCF calculations to explore the behaviour of mixed surfactant +
biopolymer interfacial
layers for the case where there is some degree of
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