Ionic gels - Physical Gels from Biological and Synthetic Polymers - page 141

relaxation time for topological entanglements among double-helical conformers but
not for the formation of a three-dimensionally percolated permanent network.
Again, the mid-point temperature of the disorder

order transitions increases system-
atically with the Hofmeister number for the anion through the lyotropic series, so that T m
values were in the order SO 4 2 − <F − <Cl − <Br − ≤

-

NO 3 − <I − < SCN − when salt
concentration and cation (Me 4 N + or K + ) are held constant. A corresponding increase
was observed in transition enthalpy ( helix aggregation

(as indicated by turbidity, gel formation and hysteresis between heating and cooling
scans) also shows a systematic dependence on the Hofmeister number for the anion, but
in the opposite sense. Thus, with Me 4 N + as the sole counterion present, clear solutions
with no thermal hysteresis in the order

Δ H cal ) and entropy ( Δ S cal ). Helix -

disorder transition are observed at all temper-
atures with I − ,Br − ,NO 3 − or Cl − as co-anion, whereas lower modulus turbid gels, with a
signi

-

cant thermal hysteresis between melting and re-forming, are created in the pres-
ence of SO 4 2 − , and to a lesser degree F − . With K + as counterion, a similar regular
progression is observed through the anion lyotropic series, from rapid formation of
very turbid gels in the presence of F − to very slow development of clear gels with I − or
SCN − ( Figure 5.14 ).
In agreement with previous studies, an increase in 127 I NMR linewidth was observed
on conformational ordering of -carrageenan (Me 4 N + salt form) in the presence of

Me 4 N + I − . However, closely similar behaviour was observed for 35 Cl and 81 Br, indicating
a simple charge-cloud interaction rather than the speci

κ

c site-binding of I − which has
previously been suggested. The change in enthalpy during (cooperative) helix formation
can be determined by Δ H app from a van ' t Hoff plot ( Chapter 3 ), and the corresponding

value H f for disaccharides observed by direct calorimetric measurement. Norton et al.

obtained the apparent number in app of disaccharides involved in the cooperative transition
of helix formation (given by the ratio

Δ

Δ H app / Δ H f ) (Norton et al., 1984 ; Austen et al.,

1.2

1.1

1.0

0.9

0.8

0.7

0.6

10 20 30 40 50 60 70 80 90

T ( ° C)

Conformational transition of κ -carrageenan (Me 4 N + salt form) in the presence of 0.167M ( ■ , □ ),
0.25M ( ● , ○ ) and 1M ( ▲ , Δ ) (Me 4 N) 2 SO 4 ; OR conditions 365 nm; 10 cm path length; polymer
concentration 0.5% w/v. Filled and open symbols are for heating and cooling, respectively.
Reprinted with permission from Norton et al.( 1984 ) © 1984 Elsevier.

Figure 5.14