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Nucleation rate data, obtained from expansion wave tube experiments, are reported for several vapor-gas mixtures at high pressure. Results are given for water-vapor in the presence of helium and nitrogen gas, and for n-nonane in helium and methane. For all these mixtures, carrier gas pressures of 10, 25, and 40 bar have been applied, with temperatures ranging from 230 to 250 K. An extended form of the nucleation theorem ͑in terms of the derivative of the nucleation rate with respect to carrier gas pressure͒ is derived, which appears to be very helpful in the interpretation of high pressure data. It can be used to obtain the carrier gas content of the critical nucleus directly from the pressure dependence of experimental nucleation rates. Combining this method with the theoretical considerations of part I of this paper ͓J. Chem. Phys. 111, 8524 ͑1999͒, preceding paper͔: the nucleation behavior of water at high pressures of both helium and nitrogen can quantitatively be understood. For n-nonane in helium our ''pressure perturbation approach'' is also valid. For n-nonane in methane, however, this approach fails because of the high methane solubility in the liquid phase.

A new semiphenomenological theory of homogeneous vapor–liquid nucleation is proposed. It is based on the Fisher droplet model applied at the saturation point within the framework of the kinetic approach. The microscopic surface tension of a droplet is supposed to have the Tolman form. The unknown Tolman length is naturally identified by equating a known empirical value of saturation pressure psat to the sum of the series over all droplet sizes for psat emerging from the Fisher model. The theory contains no adjustable parameters. Predictions of the new theory for various substances are compared with available experimental data and with the three other widely used theoretical models: classical nucleation theory, corrected Dillmann–Meier and Delale–Meier theory.

Homogeneous nucleation rates of water droplets were measured at a nucleation temperature close to 240 K in a Pulse-Expansion Wave Tube (PEWT). Several measures were taken to improve the data obtained with the PEWT. For instance, the molar water vapor fraction was determined with three independent techniques. The resulting standard uncertainty of the supersaturation was within 1.8%. Results are given for water nucleation in helium at 100 kPa and at 1000 kPa and in nitrogen at 1000 kPa. Two trends were observed: (i) the values of the nucleation rate of water in helium at 1000 kPa are slightly but significantly higher (factor 3) than its values at 100 kPa and (ii) nucleation rates of water in nitrogen at 1000 kPa are clearly higher (factor 10) than in helium at the same pressure. It is argued that the explanation of the two observed trends is different. For case (i), it is the insufficient thermalization of the growing water clusters in helium at the lowest pressure that has a reducing effect on the nucleation rate, although a full quantitative agreement has not yet been reached. For case (ii), thermal effects being negligible, it is the pressure dependency of the surface tension, much stronger for nitrogen than for helium, that explains the trends observed, although also here a full quantitative agreement has not yet been achieved.

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Homogeneous nucleation of water is investigated in argon and in nitrogen at about 240 K and 0.1 MPa, 1 MPa, and 2 MPa by means of a pulse expansion wave tube. The surface tension reduction at high pressure qualitatively explains the observed enhancement of the nucleation rate of water in argon as well as in nitrogen. The differences in nucleation rates for the two mixtures at high pressure are consistent with the differences in adsorption behavior of the different carrier gas molecules. At low pressure, there is not enough carrier gas available to ensure the growing clusters are adequately thermalized by collisions with carrier gas molecules so that the nucleation rate is lower than under isothermal conditions. This reduction depends on the carrier gas, pressure, and temperature. A qualitative agreement between experiments and theory is found for argon and nitrogen as carrier gases. As expected, the reduction in the nucleation rates is more pronounced at higher temperatures. For helium as the carrier gas, non-isothermal effects appear to be substantially stronger than predicted by theory. The critical cluster sizes are determined experimentally and theoretically according to the Gibbs–Thomson equation, showing a reasonable agreement as documented in the literature. Finally, we propose an empirical correction of the classical nucleation theory for the nucleation rate calculation. The empirical expression is in agreement with the experimental data for the analyzed mixtures (water–helium, water–argon, and water–nitrogen) and thermodynamic conditions (0.06 MPa–2 MPa and 220 K–260 K).

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