- The polyelectrolyte itselfcontributes to the ionic strength. Recall
thatI¼ð 1 = 2 ÞSðmiz^2 iÞ. For a high polymermis very small, butjzj
may be very high. For example, a 10^5 molar concentration of a
polyelectrolyte withz¼100 (which is not exceptional) would
yield an ionic strength of at least ð 1 = 2 Þð 100610 ^5612 þ
10 ^561002 Þ¼ 0 :05 molar; the first term between parentheses is
due to the counterions, the second to the polyelectrolyte.
Although the Debye–Hu ̈ckel theory as discussed in Section 2.3.2
cannot be precisely applied, the high ionic strength of a
polyelectrolyte solution does have large effects on several proper-
ties. Some examples are given later on. - The electric charge on the molecule strongly affects itsconforma-
tion, causing it to be more expanded than a neutral polymer. The
extent of expansion can be very high ifzis high, but it markedly
decreases with increasing ionic strength. The change in conforma-
tion affects several properties, for instance the viscosity of the
solution; see Section 6.3.2. Other properties affected are diffusion
coefficient, sedimentation rate (in an ultracentrifuge), light
scattering, the second virial coefficient (Section 6.4.1), and the
chain overlap concentration (Section 6.4.2). - Polyelectrolyte molecules of like chargerepel each other. Unless
the concentration is very small, this may affect the distribution of
molecules over the available space, hence light scattering and the
second virial coefficient. It also causes the conformation of the
molecules to be somewhat less expanded at a higher concentra-
tion. - If polyelectrolytesadsorbonto (uncharged) particles, this gives the
particles an electric charge, causing interparticle repulsion. This is
discussed in Chapter 12. - If an electric potential gradient is applied in the solution,
polyelectrolyte molecules will move in the direction of the
electrode of opposite charge. This is calledelectrophoresis. If the
polyelectrolyte is immobilized, the solvent will move in the electric
field, a process called electroosmosis. These principles are applied
in several laboratory techniques but will not be discussed here. - Solubilitymay strongly depend on pH and ionic strength. This is
especially important for proteins and is discussed in Section 7.3. - Specific interactions may be caused by the presence of polyvalent
counterions or oppositely charged polyelectrolytes.
singke
(singke)
#1