Abstract
The counterion condensation of mono- and divalent cations on the polyanion dextran sulphate (DS) was studied by conductometry and dye spectroscopy. The measurements were analysed using the limiting law for the conductance of polyelectrolytes and the two-variable theory of G. S. Manning. There are significant deviations between the experimental results and the predictions of the theories. Conductivity measurements show only at high temperatures and low polyion concentrations according to Manning's limiting law. The condensation of Ca 2+-ions, measured by dye spectroscopy, is affected by the species of monovalent counterion and the bulk solvent permittivity. We used Manning's formulae to adjust the apparent permittivity of the condensation volume of the polyelectrolyte and the charge-density parameter. The apparent permittivity ranges from 80 (ε r of pure water) to over 200. Counterion condensation only takes place if the permittivity is lower than 160. According to theory the critical charge-density parameter is 1, experimentally it is found that ξ c is 1.4.
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