  The Donnan model describes the electric double layer structure inside carbon
micropores and is an essential element of larger-scale models for capacitive
porous carbon electrodes for energy storage and water desalination. The Donnan
model was not yet developed to describe the expansion of carbon electrodes upon
charging. Here we provide a simple theory which predicts that the
one-dimensional expansion of a carbon particle scales with electrode charge
density according to a 4/3$^\text{rd}$ power.
