The interaction of the alkali ions with some linear polyelectrolytes

Small ions in solutions of charged high polymers are immobilized to some extent by the electrical fields surrounding the large molecule. This immobilization, "binding," is readily shown by conductance studies and accurately measured by equilibrium dialysis experiments. Conductimetric titration curves are particularly revealing. Studies were made of the interaction of sodium and potassium ions with arabic acid, agar acid and nucleic acid. In unbuffered arabic acid systems at 0°, it was found that binding was a function of the free cation concentration. Binding increased rapidly for both cations in the range from 5 × 10^-4 to 3 × 10^-3 N salt and then leveled off to a constant value. Within experimental error sodium and potassium ion binding were identical at high salt concentrations. The maximum amount bound in each case, expressed as equivalents of cation bound per mole of arabic acid was 78 ± 8. In terms of equivalents of polymer acid the binding quotient was 0.26 ± 0.02. Qualitatively similar results were obtained with agar and nucleic acids. Alkali ion, nucleic acid binding increased with increasing pH. Donnan effect calculations for arabic acid systems showed that binding could be qualitatively explained in terms of that theory. Osmotic pressure measurements, however, indicated that simple Donnan calculations were not adequate. The interaction is discussed in terms of the polyelectrolyte configuration.