Ionic gels - Physical Gels from Biological and Synthetic Polymers - page 129

-
carrageenan, the Group I cations, potassium, caesium and rubidium were reported in
early work to bind only to helical

Illustrating the ion speci city of the salt-induced conformational transition of κ

-carrageenan and not to coils (Rinaudo et al., 1979 ;
Smidsrød and Grasdalen, 1984 ; Belton et al., 1985 ), while in other work, based on optical
rotation and intrinsic viscosity measurements, it was reported that sodium and potassium
ions also bind to disordered

κ

-carrageenans (Norton et al., 1983 ). In a more recent study,
Nilsson and Piculell estimated cation binding using a self-consistent treatment (Poisson

κ

-
Boltzmann equation) and compared thermodynamic data obtained by optical rotation
measurements with counterion NMR data on the binding of ions to the helix. They found
good agreement, and concluded that cations bind to -carrageenan in the helical con-

formation and not to the same polymers in the disordered (coil) conformation (Nilsson
and Piculell, 1991 ; Zhang et al., 1994 ).
A number of studies have been carried out using the specialized but powerful
techniques of speci

κ

c nucleus NMR, especially illustrated by 133 Cs resonance. Such
investigations showed no evidence of speci cCs + interaction with -carrageenan in the

coil form (Grasdalen and Smidsrød, 1981 ), whereas, with the formation of helices,
signi

κ

cant line broadening of the 133 Cs signals occurs. Under these conditions, the
peak changes from a single Lorentzian to a multi-Lorentzian line shape, and changes
in the chemical shift are observed, indicating a speci c ion binding to the helix. Using

this method, Grasdalen and Smidsrød also showed that iodide ions promote helix
formation but prevent the aggregation of κ -carrageenan molecules.

flexible chains tend to disentangle simply by reptation (Doi and Edwards,
1999 ), while aggregated helices, because of their geometry, have a far longer lifetime.
Using small-deformation dynamic viscoelastic measurements ( Chapter 2 ), Grasdalen,
and Smidsrød ( 1981 ) also showed both the storage and loss moduli to be almost
frequency-independent at lower temperatures when these polymers are in a helical
conformation, while both moduli showed strong frequency dependencies at higher
temperatures.
Viebke et al.( 1994 )

Entangled

fitted the experimental data reported by Rochas et al.( 1990 ) for
helix fraction as a function of temperature, using the Zimm

coil
transition ( Figure 5.2 ), and found good agreement with a viscometric experiment per-
formed under non-aggregating conditions (in 0.2 M LiI), and so showed that the relative
viscosity of a

- Bragg theory on helix -

κ -carrageenan solution was a monotonic function of the helix fraction in the

system.
To separate helix formation and aggregation, they

first prepared a non-gelling solution of

all-helical -carrageenan in a LiI solution. This solution was subsequently dialyzed against

KCl; i.e. it was exposed to salt conditions that induce aggregation. (With a normal cold-
setting procedure, such a combination gives a strong gel.) They found that a gel, visibly
indistinguishable from the cold-set gel, was also formed by the dialysis procedure.
It should be noted that the dialysis conditions were such that the coil content should
have been insigni

κ

cant at all stages. This conclusion is based on studies of the helix

formation of -carrageenan in solutions of mixed salts of a gelling cation and a helix-

inducing anion, which showed that the helix is, in fact, more stable in the mixed salts at a
constant ionic strength (Zhang et al., 1992 ; Viebke et al., 1998 ). The salt-induced

κ