Electrostatics in Macromolecular Solutions - Food Colloids: Self-Assembly and Material Science - page 148

charged chain - a chain with large separations between the charged monomers
- will lead to a more long-ranged but weaker attraction. In general, one finds
that highly charged systems give rise to fewer, but stronger 'bridges', and there
is an optimal choice of polyelectrolyte structure for maximizing the attraction
between the colloids.
The interaction between charged macromolecules is, from an electrostatic
point of view, rather insensitive to the addition of neutral random poly-
ampholytes. It is only with block polyampholytes that the normal double-layer
repulsion can be reduced in the same way as with oppositely charged polyelec-
trolytes. The oppositely charged block acts like an oppositely charged poly-
electrolyte. The only complication or constraint is that the equally charged
blocks should avoid the aggregates. If the polyampholyte has a net charge, then
it behaves qualitatively like a weakly charged polyelectrolyte. A mixing of
positively and negatively charged monomers allows a tailoring of the range and
magnitude of the attraction. Figure 12(b) shows the free energy of interaction
between two charged macromolecules with different types of polyampholytes.
A naı¨ ve picture of a triblock between two adsorbing macromolecules, which
seems to be true for neutral block copolymers, is one where the two ends of the
polyampholyte chain adsorb to one aggregate each and 'pull' them together.
Such a structure is quite common in a simulation, but it does not lead to a
significant 'pulling' force due to the weak force constant of a long segment of
negatively charged monomers. Another way to express this is that the free
energy gain of adsorbing a polyampholyte chain is approximately distance-
independent for a tri-block of the type ( 10,+20, 10). Figure 13 is a snapshot
from a simulation illustrating this conformation.
9.5 Protein-Polyelectrolyte Complexation
The complexation of polyelectrolytes and proteins is extensively exploited in
pharmaceutics, foods and cosmetics. 26-33 The subject has been addressed by a
Figure 13 Simulated picture of a triblock copolymer (+10, 20,+10) adsorbing onto
two negatively charged macroions (Z ¼ 20). Counter-ions and positively
charged monomers are shown in grey and negatively charged monomers in
white