The behavior ofpolyampholytesis in some respects different. At a pH
far from its isoelectric point, a polyampholyte behaves roughly like a
polyacid (high pH) or a polybase (low pH), but near the isoelectric pH the
molecule has about equal numbers of positive and negative charges, and
these attract each other, thereby causing a less expanded conformation.
(Moreover, the solubility may be much decreased: Section 7.3.) The effect of
ionic strength on conformation will then be opposite to that of a simple
polyelectrolyte: at lower ionic strength, the oppositely charged groups
attract each other over a greater distance, leading to a more compact
conformation. In other words, near the isoelectric pH, salt screens
(diminishes) attraction, far away from this pH, it screens repulsion.
Note As mentioned already, the expose ́ given in this section is an
oversimplification. The most important point may be that at a high
(linear) charge density, i.e., at small nL=jzj, the Debye–Hu ̈ckel
theory on the screening of electric charge is no longer valid. At
some distance from the polyelectrolyte chain, the potential then is
nearly insensitive to (changes in) the charge density; the deviation
from the Debye–Hu ̈ckel theory is especially great at low ionic
strength. This factor also contributes to the downward trend of the
curves in Figure 6.10 at higha.6.3.3 The Donnan Effect
Principles. The condition of electroneutrality implies that a
polyelectrolyte molecule in solution is accompanied by counterions (small
ions of opposite charge). Unless a strong external electrical potential is
applied, the polyelectrolyte cannot be separated from its counterions. To be
sure, the counterions freely diffuse toward and away from the
polyelectrolyte molecule, but this only applies to individual ions. Each
polyelectrolyte molecule is always accompanied by the same number of
counterions, apart from statistical fluctuations around the average.
The polyelectrolyte also affects the distribution of coions (small ions
having a charge of the same sign). This is known as the Donnan effect,
which may be illustrated by envisaging a system with two compartments
that are separated by a semipermeable membrane. This membrane does not
allow passage of polymers but is permeable for small ions. Assume that one
compartment initially contains the polyelectrolyte ðPEzÞand sufficient
counterionsðNaþÞto neutralize the charge, while the other compartment
initially contains NaCl. At equilibrium, the activity of NaCl (which means
the free ion activitya+) must be equal in both compartments. This implies
that Clwill diffuse towards the other compartment, and the condition of