The use of alginate as an immobilizing agent in most applications rests in its ability to form heat-stable strong gels which can develop and set at room temperatures.
It is the alginate gel formation with calcium ions which has been of interest in most applications.
However, alginate forms gels with most di- and multivalent cations. Monovalent cations and Mg2+ ions do not induce gelation while ions like Ba2+ and Sr2+ will produce stronger alginate gels than Ca2+. The gel strength will depend upon the guluronic content and also of the average number of G-units in the G-blocks. Gelling of alginate occur when divalent cations takes part in the interchain binding between G-blocks giving rise to a three-dimensional network in the form of a gel. The binding zone between the G-blocks is often described by the so-called "egg-box model" (figure).
During gelling the concentration of divalent cations have large impact on the gel network and gel homogeneity.
When the gelling takes place in presence of excess calcium, a modified egg-box model has been suggested, involving more than two alginate chains in the gelling zone. This may have an impact on the porosity of the gel.
If no shrinking of the gel occurs, there may be more space in between the chains, leading to an increased porosity.