Multicomponent diffusion of sodium alginate solutions with added salt. II. Charged vs. Uncharged system

Abstract

AbstractDiffusion coefficients are reported for dilute aqueous solutions of sodium alginate, and for three compositions of the ternary system comprising of sodium alginate/sodium chloride/water at 20°C applying the conductimetric method. From binary transport data the ternary diffusion coefficients for alginate of added salt (NaCl) can be qualitatively calculated. Due to electrostatic interactions between the ions the diffusive flow of the alginate polyelectrolyte and added salt reveal strong coupling. At pH>5.0 (20°C) and ionic strength of 0.015 M NaCl each mole of diffusing alginate cotransports approximately 210 moles of NaCl. Coupled flow of NaCl ceased if the solution pH is approaching pH 5.5, close to the isoelectric point of alginate (pH 5.4). Between pH 4.5–5.0 the coupled transport of NaCl is governed by the diffusion of the protonated alginate concentration gradient due to the diffusion induced electric field along the neutral alginate. At acidic pH (3.5–4.0) diffusing alginate generates a counter flow of NaCl due to additional bound protons, yielding a net positive charge hence the D21‐value is large and positive.