Classical thermodynamics is applied to the melting of nanometer-sized Sn particles with radii in the range 5-50 nm. Such particles display a depression of both the melting point and the latent heat of fusion depending on the particle size. The size dependence can be explained with the formation of a structurally perturbed layer at the particle surface. The experimental measurement of both melting temperatures and latent heats of fusion allowed for estimation of the thickness of the perturbed layer. This permitted in turn the evaluation of the excess Gibbs free energy associated with the perturbed layer at melting and the determination of its variation with particle size and temperature.