Biomedical Engineering Reference
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The absence of turbidity and thermal hysteresis in the temperature dependences of the
speci
c optical rotation for Me
4
N
+
3% w/v) in the presence of
Me
4
N
+
I
−
also led Grasdalen and Smidsrød (
1981
) to propose that I
−
ions promote
helix formation but prevent the aggregation of helices. Although they did not
κ
-carrageenate (0.1
-
nd a
c optical rotation in the presence of Me
4
N
+
Cl
−
, but only in
Me
4
N
+
I
−
, Norton et al.(
1984
) found an analogous transition in the presence of Br
−
and
Cl
−
co-anions. At all experimentally accessible concentrations of Me
4
N
+
Cl
−
,Br
−
,I
−
and NO
3
−
, the results obtained on heating and on cooling were closely superimposable,
no gel formation was observed and the solutions remained optically clear at low temper-
ature. In the presence of Me
4
N
+
sulphate (and to a lesser extent
steep increase in the speci
fluoride), by contrast,
signi
cant hysteresis was observed between heating and cooling scans, conformational
ordering was accompanied by an increase in turbidity and a weak but cohesive gel
structure was formed. These effects increased with increasing concentration of salt,
and became undetectable at low levels of salt.
The mechanical spectra of
-carrageenan helices dispersed in KCl and NaI aqueous
mediawerecompared(IkedaandNishinari,
2001
). A dispersion of 1.5% w/w non-
aggregated
κ
-carrageenan in 0.2 M NaI solution (which prevents aggregation of
helices) exhibited a
κ
type dynamic mechanical spectrum at 20°C.
In other words, the storage modulus G
0
predominated over the loss modulus G" in the
entire frequency range examined (0.5
'
structured liquid
'
100 rad s
−
1
), both moduli showed a slight
-
frequency dependence and tan
δ
was not so small (>0.1) (
Figure 5.13
). On the other
hand, 0.15% w/w
-carrageenan in an aggregating condition, i.e. in 0.2 M KCl
solution, at 20°C showed elastic gel behaviour. However, under large deformation
κ
-
large enough for conventional gels to rupture
-
the structured liquid systems
owed but
never ruptured, suggesting that the material (sometimes called
'
weak gel
'
) rheological
properties of the
κ
-carrageenan dispersions were the result of a suf
ciently long
10
2
10
1
1
10
Frequency (rad s
−
1
)
100
Frequency dependence of G
0
(solid symbols) and G" (open symbols) of 1.5%w/w
κ
-carrageenan in
0.2 M NaI (circles) and 0.15% w/w
κ
-carrageenan in 0.2 M KCl (squares) at 20°C. Reprinted with
permission from Ikeda and Nishinari (
2001
) © 2001 American Chemical Society.
Figure 5.13
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