# Poisson-Boltzmann equation

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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>