An iterative procedure based on donnan equilibrium for calculating the polymer-subphase volume of alginic acid

Abstract

A numerical iterative procedure is presented to predict the polymer-subphase volume that is formed when anionic polysaccharides such as alginic acid (polyuronic acid from kelp) are suspended in an aqueous solution. (The aqueous region surrounding the polymer chain where a strong electrostatic attractive force for counterions exists is defined as the separate polymer subphase within the colloidal phase enclosed by the polymer coil.) Based on the phase-partition model of Marinsky et al.5 and Donnan equilibrium theory, this iterative procedure utilizes the base titration data of the acidic polysaccharide at different ionic strengths as well as the osmotic properties of the sodium form of the polysaccharide. No detailed structural information of alginic acid is required. The resulting calculations show that the polymer subphase, which accounts for a small fraction of the total solution volume, should be regarded as the reaction zone for acid dissociation and metal binding reactions. The volume of polymer subphase thus calculated may also serve as an excellent index for the morphology of the polymer molecule at different ionic strengths, degrees of ionization, polymer concentrations, and extent of polymer-metal binding.