View source for Poisson-Boltzmann equation

The Poisson-Boltzmann equation describes the ion distribution in an electrolyte solution outside a charged interface.  It relates the mean-field potential to the concentration of electrolyte.
==Short derivation==
The Poisson equation reads
<math>\epsilon_0 \epsilon_r \nabla^2 \phi = \rho_{free ions)</math>
where the charge distribution is
<math>\rho_{free ions) = e \sum_i z_i c_i</math>