Studies of Polyelectrolyte Solutions VI. Effects of Counterion Binding by Dextran Sulfate and Dextran Phosphate in Aqueous/Organic Solvents

Abstract

The influence of the solvent on the degree of counterion binding of dextran sulfate and dextran phosphate is studied by activity measurements, dye-spectroscopy, and static light scattering. The solvents used are binary mixtures of water and organic solvents such as acetone, alcohol, and formamide. The mixing ratio is varied over a wide range in order to obtain different solvent dielectric constants. The results of investigations are as follows. First of all, in pure water the alkali ions are bound by dextran sulfate in the order Cs+>K+>Na+>Li+, i.e., the degree of counterion binding for Li+ is lowest while that for Cs+ is largest. The inverse ion sequence is observed for dextran phosphate. In mixed solvents the ion sequence depends on the mixture used. Secondly, there exists no universal correlation between the dielectric constant of the solvent and the degree of counterion binding. However, it is common to all aqueous/organic solvent mixtures that the degree of counterion binding decreases as the fraction of the organic compound in the solvent increases. Thirdly, condensation theory is not valid in mixed solvents, while the Poisson-Boltzmann approach may be applied. Taking into account the size of the solvated small ions we find that the type of counterion binding of a divalent counterion, such as Mg2+, depends on the kind of the univalent counterions added in excess. For water/n-propanol it seems that in the presence of Li+, Mg2+ is bound delocalized, while in the presence of K+, Mg2+ is bound by site binding. Fourthly, in water/formamide and water/dimethyl sulfoxide (DMSO) the organic solvents are adsorbed preferably by dextran sulfate. As a consequence, the dielectric constant at the polyion surface differs from that of the bulk solvent. Nevertheless, if these changes are taken into account, the predictions of condensation theory remain contradictory to the results of measurements.