Ionic gels - Physical Gels from Biological and Synthetic Polymers - page 133

Viebke et al.( 1998 ) used an electrostatic model to describe the speci c binding of

caesium and iodide ions to the -carrageenan helix. This modelling resulted in a semi-

quantitative description of the variation of the ion binding and the charge density in the
various salt mixtures. It was found that the model reproduces the relevant transition
enthalpies in the non-aggregated systems very well, while in the aggregated systems the
model predictions deviate markedly from experimental results. This deviation, inter-
preted in terms of an aggregation enthalpy, varies with the extent of thermal hysteresis.
The hysteresis occurs only when the charge density is lower than the charge density for
the bare

κ

-carrageenan helix (without bound ions). Another interesting observation is that
the width of the DSC peak obtained on heating increases drastically when the aggrega-
tion occurs. Finally, the

κ

ned as a maximum stress at a breaking
point determined by uniaxial compression, was found to depend linearly on the extent of
the hysteresis measured by DSC, and so re

' gel strength ' , here de

ects the degree of aggregation.
Gel formation of helix-forming anionic polysaccharides can be induced at a constant
temperature by increasing ionic strength. Using the dialysis technique mentioned above,
Piculell et al. placed a -carrageenan solution in LiI (all helical conformation), immersed

it in KCl salt solution and found a gel was formed (Piculell et al., 1993 ; Viebke et al.,
1994 ). From this evidence, they again concluded that the

κ

κ -carrageenan gel network is

created essentially by helix aggregation.
Much of the structuring effects noted above also apply to the gellans, but, for these
systems, it has been observed recently that the immersion of such gels in water or
electrolyte solution induces chain release, and this release is more noticeable for
shorter chains. Ultimately the gel becomes eroded and then disintegrates, and the
rate of collapse depends on polymer concentration, original molecular mass and
the initial salt content of the gels and the solvent. Salt diffusion from the gels into the
solution is faster than chain release; chains which lose condensed or bound ions cannot
retain a helical conformation, and so they then diffuse out into the solution. The storage
Young s modulus E 0 of gellan gels immersed in various solvents was observed as a

function of time ( Figure 5.6 ). For the

'

rst 3 h, E 0 increased both in water and in
Me 4 N + Cl − solution, a solution which inhibits the helix aggregation of gellan.
Subsequently, E 0 for a gel immersed in water decreased because of the release of chains
contributing to the network (Hossain and Nishinari, 2009 ). This process should be
studied further, but is consistent with the release of some of the sol fraction, as found in
lightly cross-linked chemical gels.

5.4.2 Effects of cations on the transition temperature

The gel sol transition temperature T m of polyelectrolyte gels depends on the salt con-

centration, and a linear relation between the inverse of the transition temperature and the
salt concentration has been found for many polysaccharide gels. Snoeren and Payens
( 1976 ) reported an equation for the salt dependence of the sol

-

-

gel transition temperature

for -carrageenan solutions from both optical rotation and light scattering measurements.

This showed a linear dependence on the logarithm of the salt concentration. However, the
slope of this linear relation is quite different from that expected from an earlier theoretical

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