On the formulation of the material equilibrium condition for a dissolving solid nanoparticle.

Widely known Gibbs' relationship predicting a difference of chemical potentials in a small solid particle and in a surrounding fluid phase at equilibrium is shown to refer to a real or imaginary bulk phase inside the particle. A similar relationship derived for the real surface monolayer of a nanoparticle exhibits the equality of chemical potentials at equilibrium, which allows for experimental measurement of the surface chemical potential of a dissolving solid nanoparticle.