Chemistry Reference
In-Depth Information
CO
2
+H
2
O
CaCO
3
Ca
2+
2H
+
Ca
2+
Cross-
linked alginate
Alginate
GDL
Fig. 6.2
Schematic diagram of internal gelation method.
Calcium can be introduced to the polymer in a variety of ways and
forms. Indeed, the method used for introducing the calcium ions can
produce different types of gels described as homogeneous and inhomo-
geneous gels. When calcium chloride is added to a solution of alginate,
localised instantaneous gelation or precipitation occurs, which makes it
extremely difficult to attain homogeneous gels.
To overcome this problem, an internal gelation mechanism developed
by Draget
et al.
(1990) can be used. This method utilises calcium in an
insoluble form (such as calcium carbonate) that can be mixed through-
out the alginate solution. The calcium carbonate is then dissociated by
slowly reducing the pH by the addition of glucono-δ-lactone (GDL),
which hydrolyses over a period of 40-60 minutes. This controlled re-
lease of the Ca
2
+
is known as internal gelation (Fig. 6.2).
The acidity, gelation time and strength of the resulting gels can be
regulated by carefully adjusting the ratio of CaCO
3
to GDL. Rheo-
logically, an example of the typical mechanical spectra exhibited by a
2% (w/v) alginate sample prior to and following cross-linking by inter-
nal gelation is shown in Fig. 6.3a and b, respectively. These examples
show spectra as expected for a typical entangled polymer solution and a
cross-linked gel network. Gelation rate and final gel strength, however,
are also affected by alginate concentration, molecular weight and MG
sequence (Draget
et al.
, 1990). Particle size of the insoluble calcium
salt can also be used to control gelation rate without affecting the final
gel strength, with smaller particle size favouring a more rapid gelation
(Fig. 6.4) (Draget
et al.
, 2000, 2006b).
When calcium is introduced to the alginate in the soluble form (e.g.
CaCl
2
), the gels produced are usually inhomogeneous. The junctions
formed when gelling alginate in this manner are more concentrated at the
surface of the gel and require diffusion of calcium ions through the gel,
resulting in a structure that is stronger at the exterior than at the interior
