Na- and K-alginate;: effect on Ca2+-gelation

The effect of two different monovalent alginate counterions has been studied in Ca2+-mediated sol/gel transitions and in Ca2+-gels at apparent equilibrium for alginates with different chemical composition and sequence. K-alginates were generally found to undergo faster sol/gel transitions compared to the Na-alginate. At apparent equilibrium and at low Ca2+ concentration, Na-alginate gels showed reduced elastic moduli compared to the K-alginates. The relative differences both in sol/gel transition kinetics acid modulus at apparent equilibrium increased with increased content of guluronic acid residues in the alginate sample. Increasing the amount of internally released Ca2+ from 15 to 30 mM resulted in equal elastic properties at apparent equilibrium, suggesting that dissimilarities in ion exchange reactions between Ca2+/Na+ and Ca2+/K+ is the main driving force. The specific effect of one of the counterion could be reversed by adding increasing amounts of the other. Selectivity coefficient determinations for alginates revealed a slight enhanced binding of Na+ compared to K+, most probably due to the guluronate residues. (C) 1998 Elsevier Science Limited. All rights reserved.