Ionic gels - Physical Gels from Biological and Synthetic Polymers - page 127

where l pe 0. Conversely a very stiff chain polymer, i.e. one where l pi is high, say

>50 nm, shows relatively little

' polyelectrolyte effect ' , since the maximum value of l pe at

low ionic strength is typically <10 nm.
In practice, physical gels as we recognize them are never formed from dilute solution

of flexible polymers such as poly(ethylene oxide), which entangle but do not form

junction zones. Such a system cannot retain solvent (water) and has no

' solid ' character,

so the system flows even when the concentration is high. At one limit, the minimum

concentration necessary for gelation ought to be related to the

flexibility (persistence

length) of the chain.
Both structure formation and solubility of polyelectrolytes are strongly in

uenced by
the counterion species and the ionic strength. An increase in ionic strength changes the
conformation of polyelectrolytes via the screening effect and also because of the ther-
modynamically controlled

effect which makes the solvent quality poorer.
Coulombic interactions play the dominant role, while other intermolecular forces
(including van der Waals forces, hydrogen bonding and hydrophobic interactions) also
modify structure formation. More particularly, hydration effects and the Coulombic
interaction are important for monovalent cations, while divalent and multivalent cations
can

' salting out '

polyelectrolytes by sharing charges between different chains.
Most gel-forming polymers are stiff, at least in their ordered (e.g. helical) form, and the
effect of salt is different from that for

' cross-link '

flexible polymers. As will be described later, in
most cases salts also promote the formation of these ordered structures, including multi-
ple helices - double ( κ - and ι -carrageenan, gellan, DNA) or triple (gelatin; see

Chapter 7 ) and lead to the formation of more complex structures. In this chapter we

restrict ourselves to certain gelling polysaccharides including the carrageenans, pectins,
alginates and gellans. A number of other systems, including the liquid crystal

-

'
which can be formed from such persistent ordered structures, are discussed in Chapters 3
and 9 .

' gels

5.4 Gelation of carrageenans and gellans

Two of the most important classes of gelling polysaccharides are the carrageenans, from
marine algae, and the gellans, which are microbial fermentation products. Both are of
importance in food, cosmetic and personal products, although the use of gellans in food
products has been restricted in some jurisdictions (only cleared for food use in Europe in
1994, for example). More recently, gellan has been suggested as an injectable formula-
tion in tissue engineering ( Chapter 11 ). A comprehensive review on gelation of gellan has
been published recently (Morris et al., 2012 ).
The behaviour of polyelectrolyte solutions depends strongly on temperature, shear
rate, stiffness, branching and chemical structure. Certain ionic polysaccharides, such as
some marine polysaccharides (carrageenans, alginates), plant polysaccharides (pectins)
and microbial polysaccharides (gellans) form thermoreversible gels by cooperative
Coulombic interactions with counterions, by hydrogen bonding and by hydrophobic