Ionic gels - Physical Gels from Biological and Synthetic Polymers - page 130

1 3.5

N = 78

0.8 3

N = 16

0.6 2.5

0.4 2

0.2 1.5

0 1

20 25 30 35

Temperature ( ° C)

40 45 50 55 60 0 0 0 0 0 0

Temperature ( ° C)

Left: helical content θ estimated for κ -carrageenan with various molar masses using the Zimm -
Bragg treatment: κ -carrageenan fractions with, from left to right, 16, 78 and 247 repeating
disaccharide units, at total concentrations of 13 (solid), 1.3 (dashed) and 0.13 (dot-dashed) mM.
Right: relative viscosity of 0.1% κ -carrageenan in 0.2M LiI as a function of temperature.
Reprinted with permission from Viebke et al.( 1994 ) © 1994 American Chemical Society.

Figure 5.2

gelation was also found to be reversible; dialysis of the gel against LiI again gave a clear
solution. A similar dissolution in LiI was also found for a cold-set gel under the same salt
conditions. Viebke et al.( 1994 ) suggested that this was a strong indication that branching
on the helical level is not required for the gelation of -carrageenan and that the network,

whether cold-set from initially randomly coiled molecules or salt-set from helical
molecules, is essentially formed on the superhelical level. The conclusion that the

κ

-
carrageenan gel network is not created by each chain forming double-helical junctions
with, on average, two or more partners is consistent with the so-called domain model
proposed by Morris et al.( 1980b ).
These workers showed that

κ

-carrageenan formed a helix in a solution, but not a gel,
when tetramethylammonium ions (Me 4 N + ) are the sole counterions, but did form a gel
when K + ions were added. Viebke et al.( 1998 ) showed that a non-gelling solution of
helical rods of

ι

-carrageenan could be reversibly transformed into a gel by dialysis
against an appropriate salt solution under conditions such that the carrageenan molecules
never pass through the coil state. This also implies that the branching of helical chain
portions caused by the presence of

κ

residues is not necessary for gel formation.
The model shown in Figure 5.3a is oversimpli

' kink '

ed, since the helical part should include
aggregated double helices. The difference between the models shown in Figure 5.3a and
Figure 5.3b seems to lie in the absence of

flexible chains connecting junction zones in the

latter. Model (b) resembles the ' 'fibrous model ' proposed from atomic force microscopy

(AFM) studies (Morris, V. J. et al., 1999 ).
Cryo-TEM observation was performed for

-carrageenan molecules without removing
water, and rigid superhelical rods and aggregates of such rods were found (Borgström
et al., 1996 ), in good agreement with observation for

κ

-carrageenan molecules deposited
on mica surface by electron microscopy (Hermansson et al., 1991 ) and by AFM (Ikeda
et al., 2001 ). However, this representation does not re

κ

ect the presence of the so-called