  Although the volume transition of the polyelectrolyte gel has been studied
for decades, little research on the effects of size of the mobile ions has been
conducted. In the present paper, Tanaka classical theory of polyelectrolyte gel
is extended to the cases of mobile ions of finite volume. In the salt free
limit, the theoretical results show that the discontinuous volume transition of
the polyelectrolyte gel will become a continuous one with an increase of the
counter-ionic size. An increase in salt concentration can also make the
polyelectrolyte gel in poor solvent collapse. Poorer solvent is needed to
trigger the salt-induced collapse in polyelectrolyte gel with larger mobile
ions than that with smaller ones. The effects of ionic size on the critical
points and phase diagram of the volume transition are also discussed. The
theoretical results suggest that the swelling behavior of polyelectrolyte gel
might be tuned with salt of different volumes.
