Ionic gels - Physical Gels from Biological and Synthetic Polymers - page 124

5 Ionic gels

5.1 Introduction

In this chapter, we consider ionic polysaccharides and the gels formed from them, since
these form by far the majority of such physical gels of commercial and practical interest.
A polyelectrolyte consists of a macroion (a macromolecule carrying covalently bound
anionic or cationic groups) and suf

cient low molecular mass counterions to assure

electroneutrality.
The term

is, of course, well known from those chemically cross-linked
polyelectrolytes which form superabsorbent gels (particularly of acrylamides, acrylic
acid etc.), and the swelling of these was examined in detail by the late Toyoichi Tanaka
and co-workers, who quanti

' ionic gels '

ed the very spectacular degree of swelling of the networks
in pure water, their lesser swelling in electrolyte solutions and their de-swelling in poor
solvents (see Chapter 4 ). More detailed discussion of these systems lies outside the scope
of this volume.
However, of direct relevance here are the major anionic polysaccharide gels formed
from the carrageenans, gellans, alginates and pectins. These are well known in a number of
applications (including food, cosmetics, personal products and biomedical applications).
They exhibit considerable variability in their properties, particularly in relation to the ionic
content of the aqueous solution, and have been examined in the numerous investigations
discussed here. The choice of ions allows changes in the mechanism and the structure of
the network, thus allowing a tuning of the gel and/or temperature of gelation.

In these gels, in addition to hydrogen bonds and hydrophobic interactions, electrostatic
forces and ordered secondary structures play an important role in gelation. These are the
main features distinguishing these ionic gels from other physical gels. In this chapter we
also introduce the cation polysaccharide chitosan. On its own, this system does not gel, but
it can do so by interacting with other, particularly anionic, polymers; such systems are
discussed in Chapter 11 .
For all such naturally occurring polymers, changes in the molecular structure and
composition according to the origin of the raw material

' stiffness '

-

and, just as crucially, in the

-

extraction regime not only are sources of difference in the properties observed but also

introduce extra complexity into their study. This chapter first presents a brief survey of

overall polyelectrolyte behaviour, before going on to discuss the properties and mecha-
nisms of gelation of these polymers.