In order to develop a consistent nucleation theory, the main assumptions of the theory should be revised. One of the questionable problems is the role of the carrier gas in nucleation and the surface tension for the critical embryo as a function of cluster size. Using a flow diffusion chamber, the vapor nucleation rates were measured with high precision and phase transitions in critical embryos containing two and more dozen molecules were detected. Phase transitions in critical embryos were used as markers to detect that the new phase critical embryos contain two components. Phase transitions of the first order related with critical point second-order phase transitions in the pure CO2 carrier-gas were used as markers to demonstrate the presence of CO2 in critical embryos of condensate. Results of this research, in our opinion, very clearly demonstrate that vapor nucleation in a gaseous atmosphere is a binary process and must be interpreted from the point of view of nucleation theory within a binary system. “Supercritical” nucleation is a virtual term born by interpretation of binary vapor–gas nucleation by using the nucleation model of a single component. A critical condition for the binary system could be a higher level for the single component critical pressure and/or temperature, which can produce the illusion of supercritical nucleation. One component interpretation can be used far from the critical condition. On the other hand, the Laplace pressure practically always is able to approach the nucleation condition to the critical pressure. This level of detail is a problem for future studies. The traditional application of classical nucleation theory for vapor–gas nucleation should be modified to consider the nucleation conditions in pressure-temperature-composition space.

Nucleation rates of supersaturated vapors near the conditions for the critical line for each pair of 1,2- or 1,3-propanediol and carbon dioxide or sulfur hexafluoride at total system pressures of P=0.10, 0.20, and 0.30 MPa have been measured in a flow diffusion chamber. Critical parameters, i.e., temperatures and pressures, of the binary systems versus compositions were semiempirically evaluated. It was found experimentally that there is an area of parameters in the PTx phase diagram, for each pair of vapor and carrier gas investigated in the present research, for which critical embryo phase transitions of the first order take place (where P, T, and x are pressure, temperature, and embryo composition, respectively). The nucleation rate surface singularity and a gap in the number of molecules in critical embryos reflect this area. Shifts of the phase transition temperatures can be initiated by increasing the pressure (or concentration) of the carrier gas. This behavior is peculiar for binary systems. It is well-known that no liquid–liquid phase transitions can exist for a one-component system. At least a binary solution is required for liquid–liquid phase transitions. This means that vapor nucleation of the investigated substances in the carrier gas atmosphere can be considered as nucleation of a binary vapor–gas system. A schematic diagram for the possible metastable vapor conditions is presented. On the basis of our experimental results, one can assume that there exists a surface describing the parameters related with a phase transition of the first order in critical embryos. This surface is located within the volume of the possible vapor metastable conditions.

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