Numerous physicochemical studies have been per-formed in order to obtain information regarding the mechanismsand the structural features involved in the gelation process.Morris et al.6,7have shown that calcium ions induce chain-chain association and proposed a model for the junction zone,popularly known as the “egg-box model” (Figure 1).
The egg-box model is based on X-ray fiber diffraction and dialysis ofCa2+ion.6-10However because calcium alginate fibers give avery poor diffraction pattern, it is impossible to deduce crystal structure.8-10The 2/1 zigzag conformation in the egg-box model was deduced from the similarity of acid gel, whichgives a clear scattering pattern corresponding to 2/1 helicalconformation.9,10The dialysis experiment performed by Morrisand Rees showed that a ratio of 4:1 between G units and Ca2+ions exist in Ca-alginate gel, which fits well with the egg-boxmodel (dimer). Nevertheless, the dialysis result is an indirectevidence for the structure model, and the egg-box model maynot be the only possible interpretation. Though the egg-boxmodel is popularly used by many researchers in this field, ithas been questioned by many authors. On the basis of calculatedfree energy, a parallel and an antiparallel model seem morestable than the egg-box model.11,12Recently Donati et al.13suggested that not only G blocks but also random GM blockscontribute to the junction zone. The egg-box model with a dimerstructure is not supported by the in situ observation of small-angle X-ray scattering during gelation of Ca-alginate.14,15Evenat the beginning of gelling, the width of the junction zone isalready much larger than that for the egg-box dimer.