Debye length dependence of the anomalous dynamics of ionic double layers in a parallel plate capacitor

The electrical impedance spectrum of simple ionic solutions is measured in a parallel plate capacitor at small applied ac voltage. The influence of the ionic strength is investigated using several electrolytes at different concentrations in solvents of different dielectric constants. The electric double layers that appear at the electrodes at low frequencies are not perfectly capacitive. At moderate ionic strength, ion transport agrees with a model based on the Poisson-Nernst-Planck (PNP) equations. At low ionic strength, double layer dynamics deviate from the PNP model, and the deviation is well described by an empirical function with only one fit parameter. This deviation from the PNP equations increases systematically with increasing Debye length, possibly caused by the long-range Coulomb interaction.