Transport of Probe Ions in Solutions of Biological Polyelectrolytes

Malgorzata Ciszkowska and Janet G Osteryoung
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204

Abstract:

Transport of monovalent cations was studied in solutions of biological polyelectrolytes, the sodium (or potassium) salts of anionic polysaccharides -, -, -carrageenan, dextran sulfate, and chondroitin sulfate, by steady-state voltammetric reduction of the probe ions Tl⁺ and H⁺ at mercury film and platinum disc microelectrodes, respectively. Diffusion coefficients of the electroactive probe ion are determined rapidly and precisely from steady-state, transport-limited current at microelectrodes in solutions with large excess and without supporting electrolyte over a wide range of polyelectrolyte and probe ion concentrations. Electrostatic interactions between polysaccharide anion and probe ion decrease the value of the diffusion coefficient of the probe ion with respect to the value without the polyelectrolyte, with the most pronounced effect in solutions without electrolyte. These interactions are quantified by the transport ratio, D/D₀, of the diffusion coefficient of the probe ion with polyelectrolyte, D, to that in solution without polyelectrolyte, D₀. The values of D/D₀ without supporting electrolyte for thallium ion are 0.59, 0.67, 0.50, 0.35, and 0.70, for -, -, -carrageenan, dextran sulfate, and chondroitin sulfate, respectively. For the hydrogen probe ion these transport ratios are 0.52, 0.79, 0.57, and 0.26, for -, -, -carrageenan and dextran sulfate, respectively. Experimental results are compared with theoretical predictions based on Manning's linear charge model and the Poisson-Bolzmann cylindrical cell model. According to both theories the dimensionless transport of the counterion in deionized solution is related to the charge separation in the polyelectrolyte: the higher the linear charge density, the greater the interaction and the smaller the transport ratio. Interactions in solutions of these biological polyelectrolytes are in accord with both theories, to a reasonable degree of accuracy, considering the uncertainty in the distance between charges.