The electrostatic expansion of linear polyelectrolytes: Effects of gegenions, co-ions, and hydrophobicity

The molar mass and ionic strength dependence of the dimensions of hydrophobically modified poly(vinylpyridinium) cations are demonstrated to be almost perfectly described by a theoretical expression derived on the basis of only excluded volume considerations. Generally, the effective charge density of the polyions decreases significantly with increasing hydrophobicity and with increasing polarizability of the gegenions. Unexpectedly, the intrinsic excluded volume effect which becomes dominating at high ionic strength not only depends on the hydrophobicity of the polyion but also significantly increases with decreasing polarizability of the gegenions (i.e., if the iodide gegenions are replaced by chloride gegenions). Measurements in different solvents reveal that the effective charge density does not linearly depend on the dielectric constant in the regime 20 less than or equal to epsilon less than or equal to 190. It is shown that the introduction of the electrostatic persistence length leads to less consistent results, although the experimental data could be fitted equally well.