Ionic gels - Physical Gels from Biological and Synthetic Polymers - page 139

a homogeneous to an inhomogeneous gel structure. In order to make de nitive con-

clusions in this area, it is necessary to perform experiments using monodisperse samples
with narrow molar mass distribution and have them converted into the sodium cation
form. Few if any such studies appear in the literature.

5.4.4 Role of anions

In comparison with studies of the effects of cations on the gelation of anionic poly-
saccharides, the corresponding effects of anions have not so far been clari ed. Rochas

and Rinaudo ( 1980 ) and Piculell and Nilsson ( 1989 ) reported that the nature of the anions
had no in

uence on the gelation of carrageenans, and thus it was suggested that anions do
not take part in the gelling processes. As mentioned above, Grasdalen and Smidsrød
( 1981 ) showed the exceptional behaviour of iodide anions: from the observation of 127 I
NMR resonance in the presence of coil transition temper-

ature, they found the increase of the excess linewidth was proportional to the concen-
tration of

κ -carrageenan below the helix -

-carrageenan, which was attributed to the binding of a fraction of iodide
anions to the

κ

-carrageenan ( Figure 5.12 ).
Since the temperature dependences of the excess linewidth and of the speci

κ

c optical

rotation for tetramethylammonium (Me 4 N + )

κ

-carrageenate in the presence of Me 4 N
iodide resemble each other, they concluded that iodide binding to κ

-carrageenan gave rise
to the stabilization of its helical conformation. They also concluded that the conforma-
tional transition is an intramolecular event, i.e. a single-helix formation, based on the
observation that the transition temperature was independent of the concentration of

κ -

carrageenan, that the measured number average molecular mass of -carrageenan was

independent of anions (for I − and Cl − ) and that the intrinsic viscosity inMe 4 N + I − solution
is much greater than that in Me 4 N + Cl − solution.

κ

(c)

80

70

60

50

(a)

(b)

40

20 40 60 80

Temperature ( ° C)

Specific optical rotation of 1%w/v aqueous tetramethylammonium κ -carrageenate (M n = 150 000 g
mol − 1 ) at various temperatures: (a) in the absence of salt; (b) in the presence of 0.15 MMe 4 N + Cl − ;
(c) in the presence of 0.15 M Me 4 N + I − and 10 - 3 MNa 2 S 2 O 3 . Reprinted with permission from
Grasdalen and Smidsrød ( 1981 ) © 1981 American Chemical Society.

Figure 5.12