# 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

$\epsilon_0 \epsilon_r \nabla^2 \phi = \rho_{(free ions)}$

where the charge distribution is

$\rho_{(free ions)} = e \sum_i z_i c_i$.