The condition of the phase equilibrium of an amorphous nanoparticle and a nanocrystal is derived with allowance for a possible presence of movable components in a solid body and the dependence of thermodynamic surface tension on the surface curvature. With the use of dispersion forces as an example, it is demonstrated by direct calculation that a correction to the Gibbs formula for the variations of surface tension can be substantial for nanoparticles that approach molecular sizes.