Ionic gels - Physical Gels from Biological and Synthetic Polymers - page 142

1988 ; Uedaira, 1990 ). Uedaira ( 1990 ) pointed out that the Hofmeister number is empiri-
cal and has no clear physical meaning. Instead he introduced the

' dynamic hydration

number n DHN to interpret the experimental data of Norton et al.( 1984 ), and showed

that both the DSC mid-point temperature (T m ) and the apparent number of effective
(active) disaccharide units n app are closely correlated with his new n DHN . The iodide ion
was most effective in increasing the temperature of helix formation and gave the smallest
n app , which is induced by the activated motion of macromolecular chains at higher
temperatures.
To understand the overall effect of charge on carrageenan gelation, Piculell and
Nilsson ( 1989 ) employed the neutral but structurally quite similar agarose ( Chapter 7 )
to study the effect of anions. Uedaira ( 1990 ) also estimated the concentration dependence
of the coil

'

helix transition temperature experimentally observed by Piculell and Nilsson,
and displayed it as a function of the dynamic hydration number of anions. The gelation
temperature shifts to lower temperatures with increasing concentration of the added salt.
The thiocyanate ion shows the strongest negative hydration and thus most strongly
depresses the gelation temperature.
What, then, is the overall conclusion regarding the structure of the carrageenan and
gellan gels formed in electrolyte solution? The consensus is that a double rather than a
single helix is formed, but that gelation is driven by the speci

-

cnatureofthecation.
The original Robinson domain model for carrageenan gelation ( Figure 5.1 ,right)
suggested that helix aggregation was induced by, say, (addition of extra) K + ions.

Indeed, by chemical treatment the so-called points (points of galactose-6-

sulphate substitution) which terminated a helix could be cleaved, leaving isolated
double-helical

' kink '

(Rees, 1969 ). One current picture ( Figure 5.3b ) suggests
that helix formation is more absolute and that a totally helical structure can be formed
which then aggregates into the superhelix, although this seems to be in direct contra-
diction with the existence of these kink points. However, it is what has been suggested
by, for example, AFM measurements (Ikeda et al., 2001 ) and by DSC results (Piculell
et al., 1993 ), and it is fair to say that almost no-one now accepts the original model of
Figure 5.3a .
Perhaps the best overall picture for the carrageenan gels is one which is somewhat less
perfect than Figure 5.3b , in that some more

' segments '

flexible regions remain. The problem in
resolving such issues is that perfect monodisperse chain length samples are not available,
and so almost all published work is on different materials, and under slightly different
conditions.
One exception to this, and a de nitive approach, is to adopt a similar strategy for the

carrageenans as was employed in the Japanese

' Round Robin ' exercise on gellan. From

issues were published. 1 Although monodisperse this work, three special journal

1

(a) Nishinari, K., Kajiwara, K., Ogino, K. (eds), 1993. Food Hydrocolloids 7 ,361 - 456. Special issue: Gellan Gum.

(b) Nishinari, K. (ed.) 1996. Carbohydr. Polym. 30 ,75 - 207. Special issue: Gellan Gum: Structures,

Properties and Functions.
(c) Nishinari, K. (ed.) 1999. Progr. Colloid Polym. Sci. 114 ,1 - 135. Special issue: Physical Chemistry and

Industrial Application of Gellan Gum.