The series on 'Charged and Reactive Polymers' was set forth in two
volumes concern ing the fundamentals and applications of
polyelectrolytes. A follow-up on 'Charged Gels and Membranes' would
therefore seem appropriate, necessitating, however, some
explanation for non-specialists. Theories of the most dilute gels
originate in that of concentrated polyelectrolytes: the methods and
problems are similar in structural, spectroscopic or thermodynamic
properties. The borderline can be situated in dialysis conducted
with a 'bag' imper meable to polyelectrolytes but not to small
ions, solutes and water. One may recall Donnan's use of such a
system to experiment and discover his famous law of unequal
distribution of ions of different charge inside and out. Remark
ably so, it is the difference in scale which characterizes the
difference between poly electrolyte solutions and gels and
membranes: the colloidal solution of macro molecules is
heterogeneous only on the microscopic level, whereas the
gel-solution system is a macroscopically heterogeneous one. A gel
is formed when weak or strong cohesive forces counterbalance the
dispersing ones (usually by crosslinking) without inhibiting the
penetration of solvent and of small solutes into the polymeric
network. The solvophile macromolecules cannot invade the total
volume of liquid. As a result of phase-segregation excess solution
and gel coexist and interact. The macroscopic swelling depends on
gel cross-linking as well as on ionic concentration and type and
ion-selectivities are observed."
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