Physico-chemical properties of polycarboxylate gel phase and their incidence on the retention/release of solutes

Numerous applications of synthetic or biological polyelectrolytes are based upon the control of their interactions with multivalent ions. In the phase diagram of natural polycarboxylates obtained by addition of divalent catio ns in aqueous solutions, three major types of behavior were identified on t he basis of the position and shape of the demixing line depending on the ch arge density and the cation/polyion affinity. A model was proposed for quan titative prediction of the demixing and sol-gel transition. The local confi guration of the backbone promote intermolecular cross-links, leading to lar ge gel phase. In the field of bioencapsulation, many applications are based on the diffus ion properties of various solutes into or out of gelled networks. Polycarbo xylate gel beads are often used as a matrix to entrap molecules of biologic al significance. The elasticity of the beads increases with the cation/poly ion affinity and this parameter is also responsible for their syneresis beh avior. The cation's concentration, the ionic strength and the pH determine the kinetics of the gel formation as well as the volume and the stability o f the beads. The pore size distributions vary and influence the diffusion o f molecules. The partition coefficients are affected by the pH and/or the i onic strength of the medium.