Chemistry Reference
In-Depth Information
100 000
10 000
1000
100
10
1
0
20
40
Guluronate (%)
60
80
100
Fig. 6.7
Effect of guluronic acid content on the Young's Modulus of alginic acid gels at
apparent equilibrium (squares) in comparison to predicted results for ionically cross-linked
alginate gels (circles). (Adapted from Draget et al., 2006b).
where measurements of dynamic moduli show acid gels prepared from
a high G alginate have values that are about 2 orders of magnitude
greater than those of intermediate G alginate (Draget
et al
., 2003). Acid
gels, however, do show some variations from ionic alginate gels in the
nature of gel network formation and subsequently in rheology. These
differences are summarised in detail by Draget
et al
. (2006a), where it is
explained that M blocks provide stable intermolecular bonds which are
not apparent in ionically cross-linked alginate (Fig. 6.7). Furthermore,
alginic acid gels seem to be of an equilibrium nature, and the largest
junction zones are of higher multiplicity than the largest junction zones
in the ionically cross-linked gels (Draget
et al.
, 2006a).
6.4
ALGINATE SOLUTIONS
Generally, solutions of alginate tend to be highly viscous. Concen-
trated, entangled solutions of alginate, however, exhibit shear-thinning
behaviour. This effect is shown by Rezende
et al.
(2009), where appar-
ent viscosity is reduced in various alginate concentrations on increasing
rate of deformation. Like most polysaccharides, the viscosity of con-
centrated alginate solutions is highly dependent upon the molecular
weight, with viscosity increasing with increased molecular weight. In
alginates, other factors such as G:M ratio, pH, ionic strength and, in
particular, presence of divalent cations can all cause variations in vis-
cosity (Onsøyen, 2001). Solutions of alginate also obey the cox-merz
rule, where steady-state viscosity as a function of shear rate is highly
comparable to the complex viscosity as a function of frequency. There-
fore, when measuring rheological properties by oscillatory deformation,
