Temperature of critical clusters in nucleation theory: generalized Gibbs' approach.

According to the classical Gibbs' approach to the description of thermodynamically heterogeneous systems, the temperature of the critical clusters in nucleation is the same as the temperature of the ambient phase, i.e., with respect to temperature the conventional macroscopic equilibrium conditions are assumed to be fulfilled. In contrast, the generalized Gibbs' approach [J. W. P. Schmelzer, G. Sh. Boltachev, and V. G. Baidakov, J. Chem. Phys. 119, 6166 (2003); and ibid. 124, 194503 (2006)] predicts that critical clusters (having commonly spatial dimensions in the nanometer range) have, as a rule, a different temperature as compared with the ambient phase. The existence of a curved interface may lead, consequently, to an equilibrium coexistence of different phases with different temperatures similar to differences in pressure as expressed by the well-known Laplace equation. Employing the generalized Gibbs' approach, it is demonstrated that, for the case of formation of droplets in a one-component vapor, the temperature of the critical droplets can be shown to be higher as compared to the vapor. In this way, temperature differences between critically sized droplets and ambient vapor phase, observed in recent molecular dynamics simulations of argon condensation by Wedekind et al. [J. Chem. Phys. 127, 064501 (2007)], can be given a straightforward theoretical interpretation. It is shown as well that - employing the same model assumptions concerning bulk and interfacial properties of the system under consideration - the temperature of critical bubbles in boiling is lower as compared to the bulk liquid.