In this paper is proved that the Gibbs-Tolman formula is universally valid for a class of interfaces larger than that first described by Tolman [1]. The starting assumption is that the interfaces between different phases can be modelled by nonmaterial bidimensional 2D-continua whose independent constitutive variables are the temperature and the interfacial mass density. Unfortunately the dependence of surface tension on curvature which is experimentally measured is inconsistent with Tolman formula. Our result implies that in order to supply theoretical forecasting consistent with experimental data it is useless to look for new constitutive equations for interfacial free energy. To account experimental evidence, it is necessary to construct 2D-continua endowed with more complex structure.