Impact of metallic ions on electrohydrodynamics of soft colloidal polysaccharides

Abstract

Electrostatic and hydrodynamic properties of carboxymethyldextran (CMD), a charged and well-defined polysaccharide, were evaluated in the presence and in the absence of divalent cations (calcium and cadmium) in aqueous NaNO3 solution. For that purpose, size, electrophoretic mobility and protolytic features of CMD macromolecules were derived from dynamic light scattering, electrophoresis and proton titration measurements. Electrokinetic properties evaluated in the absence of metal ions clearly reflect the permeable nature of CMD particles to ions and water. Size measurements further reveal CMD swelling with decreasing solution ionic strength. Binding of divalent ions with CMD was examined by measuring adsorption isotherm and protolytic titrations under conditions where particle charge is entirely screened. Results evidenced that complexation is the main mechanism underlying the interaction of Cd2+ and Ca2+ with CMD colloidal ligand. Finally, we investigated the effect of such metal ions on the intertwined conformational and electrostatic properties of CMD macromolecule. At high electrolyte background concentration where electrostatic effects are minimized, the presence of metal ions leads to a slight shrinking of the coiled macromolecules. This collapse of the CMD structure is accompanied by an increase in volume charge density and a decrease in particle permeability, as found from analysis of the decrease of CMD electrophoretic mobility in the presence of divalent ions. This result points out a structural modification and stiffening of CMD polymer segments following complexation of metal ions with CMD carboxylic reactive sites.