Ionic gels - Physical Gels from Biological and Synthetic Polymers - page 137

10

8

Heating
Cooling

6

4

2

0

− 2

20 40 60

Temperature ( ° C)

Temperature dependence of 133 Cs NMR line shift for the sample defined in Figure 5.9 . The gure
shows the effect of cooling and heating through the sol - gel transition, allowing 10 min for
equilibration at each temperature. The gel formation temperature is just below 40°C. Reprinted
with permission from Grasdalen and Smidsrød ( 1981 ) © 1981 American Chemical Society.

Figure 5.10

compared to ~2 - 3 for ι -carrageenan gels (Smidsrød et al., 1980 ). This finding suggests

the formation of ion-selective , as proposed by Bayley ( 1955 ).

NMR peak intensity, peak width and chemical shift for

' salt bridges '

-carrageenan solutions in the
presence of Na + ,Rb + ,Cs + and K + ions suggest that gel-forming cations (Rb + ,Cs + and K + )are
bound to

κ

-carrageenan in the gel state, but that the Na + ion is not (Tokita et al., 1989 ). In the
domain model (Morris et al., 1980a , 1980b ), gelation of

κ

helix
transition (non-gelling) followed by the formation of cation (K + ,Rb + and Cs + ) -mediated
side-by-side aggregates of double helices (formation of junction zones = gelling).
Several workers, however, have argued for a single-helix model for

κ -carrageenan involves a coil -

-carrageenan
gelation (Smidsrød et al., 1980 ; Smidsrød and Grasdalen, 1984 ). In this model, it is also
assumed that K + ,Rb + and Cs + are bound to the junction zone but that this is formed by
cation-mediated side-by-side aggregation of single helices. In either model, the line
shape of NMR spectra of gel-forming cations becomes broad when a gel is formed
because of the restriction of the molecular motion of these cations in the formation of
junction zones. These NMR results are direct evidence of the very highly selective
interaction between the gel-forming group I cations and

κ

κ

-carrageenan. However, for

κ

-carrageenan the ionic selectivity for divalent counterions is low.
The mid-point temperatures (T m ) of the disorder order transitions were determined at

a constant ionic concentration, and in the presence of monovalent counterions they varied
according to the following sequence: Rb + >K + ,Cs + >Na + >Li + ,NH 4 + >R 4 N + .No
selectivity was observed for monovalent anions with the exception of I − , which stabilizes
the helix but prevents gelation (Rochas and Rinaudo, 1980 ; Borgström et al., 1996 ;
Rinaudo, 2008 ). Rochas and Rinaudo ( 1980 ) measured the activity coef

-

γ M + of

cient

sodium and potassium counterions as a function of the square root of κ -carrageenan

γ Na + and γ K + extrapolated to a value of 0.72 at concentration. Results showed that both

in nite dilution ( Figure 5.11 ).