The length characterizing the decrease with distance of the potential in the double layer = characteristic @[email protected] length in the corresponding electrolyte solution = \(\kappa^{-1}\):

\[\frac{1}{\kappa } = \sqrt{\frac{\varepsilon_{\text{r}}\varepsilon_{0}RT}{F^{2}\sum _{i}c_{i}z_{i}^{2} }}\]

(rationalized four-quantity system)

\[\frac{1}{\kappa } = \sqrt{\frac{\varepsilon_{\text{r}}RT}{4\pi F^{2}\sum _{i}c_{i}z_{i}^{2} }}\]

(three-quantity electrostatic system)

where \(\varepsilon\) = static @[email protected] = \(\varepsilon_{\text{r}}\varepsilon_{0}\), \(\varepsilon_{\text{r}}\) = relative static @[email protected] of solution; \(\varepsilon_{0}\) = @[email protected], \(R\) = @[email protected], \(T\) = @[email protected], \(F\) = @[email protected], \(z_{\text{i}}\) = concentration of species \(i\), \(z_{\text{i}}\) = ionic charge on species \(i\).