Ionic gels - Physical Gels from Biological and Synthetic Polymers - page 125

5.2 Molecular characteristics of polyelectrolytes

In principle, any macromolecule can be transformed into a polyelectrolyte by covalent
attachment of ionic groups to the polymer backbone. For example, certain essentially
non-ionic polysaccharides such as cellulose and starch can be transformed into poly-
electrolytes by chemical modi

cation (functionalization).
Using the intrinsic equilibrium constant pK, ionic groups are classi

' '

ed as weak or

' strong ' , depending respectively on whether they do or do not have a pK in the range

0 14, so that they can undergo proton exchange reactions at experimentally relevant

pH values. Polymers with weak ionizable groups (0 < pK < 14) can adjust their
average degree of dissociation to the physicochemical conditions, and the charge on
individual groups can

-

fluctuate. Polymers with strong groups (pK < 0; pK > 14) have

fixed charges and charge distributions along the chain, whatever the pH (Stuart et al.,
2005 ).
A polyampholyte is a polyelectrolyte which has both anionic and cationic groups
covalently bound to the macromolecule, and proteins having both - NH 2 and - COOH

NH 3 + at low pH (high

groups belong to this category. The - NH 2 groups are converted to -

COO - at high pH
(low hydrogen ion concentrations). At an intermediate condition, these two ions are
present in equal amounts. This is the so-called isoelectric point (pI), where the viscosity
of the solution shows a minimum.
The conformation of such charged polymers in solution depends strongly on the
distribution of charged monomers along the polymer backbone and their environment.
If these groups are weak acids or bases, the net charge of a polyampholyte in aqueous
solution can be changed by varying the pH. In the vicinity of the isoelectric pH, the
polymers are nearly charge-balanced and exhibit the unusual properties of polyampho-
lytes. At high charge asymmetry (far above or far below the isoelectric pH), these
polymers demonstrate polyelectrolyte-like behaviour. Dobrynin et al.( 2004 ) have sum-
marized solution properties of such amphoteric polymers and their interactions with
surfaces and polyelectrolytes.
In addition to the acid and base strength of the ionic site, the average distance between
adjacent anionic or cationic charges along the chain contour is an in

hydrogen ion concentrations), while - COOH groups are coverted to -

uential parameter
governing overall polyelectrolyte behaviour. The charge density represents the spatial
distribution of electric charge, and is usually de

ned as the average number of ionic sites
per monomer unit. The regularity of the distribution of ionic sites along the chain is also
important in determining solubility. Besides the acid and base strength and the charge
density, the location of charged sites within the macroion is also an important factor in
determining its behaviour.
Speci uence, especially on

solubility and structure formation. The counterion is not just an anonymous particle
maintaining the electroneutrality, but has speci

c low molecular mass counterions have an important in

city (Dautzenberg et al., 1994 ).

Consequently the final properties of gels discussed in this chapter often depend on the

cation(s) employed.