Physical Chemistry of Foods

at the prevailing ionic strength; pKavis the average pKaof all groups andfis
a constant>1.
It is seen in Figure 6.7a that the curve does not precisely follow
Eq. (6.9); this is due to the PMA undergoing a conformational transition at
a certain pH, which leads to a different distance between charged groups. It
is generally observed that the conformation of a polyelectrolyte (somewhat)
affects its titration curve.
The latter is part of the explanation of the third point to be mentioned
about Figure 6.7a, viz., the dependence of the titration curve onionic
strength. Both pKavandfare smaller at higher ionic strength. This is partly
clear from Eq. (6.8); at higher ionic strength, the activity coefficientgis
smaller, the more so for a highera, according to the Debye–Hu ̈ckel theory
(Section 2.3.2). Moreover, the shielding of charges is stronger at higher ionic
strength, causing the removal of a proton from a charged molecule to be
easier, as the proton senses a smaller attractive electrostatic potential.
Altogether, it is mostly impossible to calculate the titration curve from
the molecular structure of the polyelectrolyte, although the explanations
given are useful in a semiquantitative sense. In practice, one just determines
a titration curve. An example is in Figure 6.7b for a protein, i.e., a
polyampholyte. Such a molecule has an isoelectric pH, i.e., a pH at which
the net charge is zero. For a polyacid (or a polybase), net zero charge means
no charge, but this is not so for a polyampholyte, which will have several
positive as well as (an equal number of) negative charges at its isoelectric
pH.

Note Even the net charge of most individual molecules will not be

zero, since the charge distribution shows statistical variation.

This has some consequences for the behavior of these molecules, which will
be discussed further on. Quite in general, the presence of charged groups
causes greater heterogeneity of the polymer, and polyelectrolytes are
virtually never true homopolymers.
The presence of charges on a polymer in solution has several
importantconsequences:

1. The solution must beelectrically neutral(unless a large external
   electrostatic potential gradient is applied). If the molar polyelec-
   trolyte concentration ism, and the average valence isz, there must
   bemzcounterions (of unit valence) in solution, for instance Naþ
   ions for a polyacid.


2. The presence of counterions causes theDonnan effect, discussed in
   Section 6.3.3.