\(\left.\ \lambda_{D}\frac{e_{d}}{e_{f}}n\bullet\nabla\Phi_{d}\right|_{n=0}=2\frac{K_{B}T}{\text{ze}}\sinh\left[\frac{ze(\left.\ \Phi_{f}\right|_{n=\lambda_{D}}-\left.\ \Phi_{d}\right|_{n=0})}{2K_{B}T}\right]+\zeta_{d0}\) (5)