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1.2.20. Homogeneous gels – controlled release of calcium ions
(in situ gelation)
When Na-alginate drops into a Ca++ solution gelation occurs instantaneously
at the interface, whereas the inner part of the drop remains a solution
because it takes time before Ca++ diffuses into the bead. Thus the gel
becomes inhomogeneous. This applies in particular to gelation of larges
volumes, for example when an alginate solution is mixed with a Ca++ solution.
Small alginate beads (d < 1 mm) may be made homogeneous by gelation in
the presence of NaCl or an osmolyte (e.g. mannitol).
A more common approach suitable for both small beads and large gels is the
CaCO3 /GDL method. GDL refers to glucono-δ-lactone:
Figure 27. GDL: Structure and hydrolysis to form gluconic acid
In this case insoluble CaCO3 in the form of a finely ground powder is mixed
thoroughly with the alginate, forming a homogeneous suspension. In contrast
to CaCl2, the CaCO3 does not dissolve and release calcium ions immediately.
When adding GDL (glucono-δ-lactone) the following happens:
a) GDL (a cyclic ester) slowly hydrolyses, forming the parent gluconic
acid.
b) The acid dissociates and pH consequently drops (slowly)
c) H+ reacts with CaCO3 , releasing Ca++ homogeneously into the solution:
CaCO 3 + 2H+ = Ca++ + 2 HCO3 -
d) Ca++ reacts with the alginate leading to slow and homogeneous
gelation
The rate of gelation depends, among others, on the particle size of the
CaCO3 . Smaller particles have a larger specific surface (m2 /g) than larger
particles, leading to a more rapid reaction with acid, and consequently faster
release of Ca++ ions. This in turn leads to more rapid gelation (time to reach a
given elastic modulus).
COOH
OH H
H HO
OH H
OH H
CH2 OH
C
OH H
H HO
OH H
OH H
H2C
O
O
O O
HO
OH
OH
OH
Glucono-δ-lactone (GDL) Gluconic acid
Spontaneous
hydrolysis (slow)