THERMODYNAMICS, GAS-LIQUID NUCLEATION, AND SIZE-DEPENDENT SURFACE-TENSION

Phenomenological nucleation theories are considered from the viewpoint of Gibbs' surface thermodynamics. We point out, in defining the criti cal nucleus, that it is important to make a distinction between the nu mber of molecules enclosed by the surface of tension and the excess nu mber of molecules over the uniform vapor phase. We show that, the Kelv in equation should be employed in determining the size of the critical nucleus even if the nucleus free energy contains a size-dependent sur face energy term. Furthermore, we make use of the fact that the classi cal form of Kelvin equation (containing the surface tension of a flat interface) predicts the equimolar radius of the critical nucleus well down to nuclei of about 40 molecules, and derive a new equation for th e size-dependent surface tension that differs from the Tolman relation . Density functional calculations support the new formula.