Structural Growth and Viscoelastic Properties of Adsorbed Alginate Layers in Monovalent and Divalent Salts

The growth and evolution of the viscoelastic properties of an alginate layer as a function of ionic strength and the presence of divalent calcium ions are monitored by quartz crystal microbalance with dissipation and atomic force microscopy. The properties of the alginate layer are investigated by combining results of incremental layer thickness or adsorbed areal mass, viscoelastic properties of the film, adhesion forces between adsorbing polysaccharides, and pull-off distances of the adsorbed molecules. In the absence of calcium, alginate adsorption behavior is strongly influenced by charge screening of the negatively charged carboxyl moieties of the alginate molecules. Reduction in alginate layer swelling is observed as ionic strength is increased, most likely due to Donnan equilibrium effects. The presence of calcium ions induces the formation of a thick and fluid gel layer via the complexation of alginate molecules. As the monovalent salt concentration is increased, a displacement of the ion-exchange equilibrium takes place in response to the competition between monovalent (potassium) and divalent (calcium) cations. The resulting impact of this phenomenon on the structure and viscoelastic properties of the alginate layer is discussed.