The Van der Waals-Cahn-Hilliard gradient theory ͑GT͒ is applied to determine the structure and the work of formation of clusters in supersaturated n-nonane vapor. The results are analyzed as functions of the difference of pressures of the liquid phase and vapor phase in chemical equilibrium, which is a measure for the supersaturation. The surface tension as a function of pressure difference shows first a weak maximum and then decreases monotonically. The computed Tolman length is in agreement with earlier results ͓L. Gránásy, J. Chem. Phys. 109, 9660 ͑1998͔͒ obtained with a different equation of state. A method based on the Gibbs adsorption equation is developed to check the consistency of GT results ͑or other simulation techniques providing the work of formation and excess number of molecules͒, and to enable an efficient interpolation. A cluster model is devised based on the density profile of the planar phase interface. Using this model we analyze the dependency of the surface tension on the pressure difference. We find three major contributions: ͑i͒ the effect of asymmetry of the density profile resulting into a linear increase of the surface tension, ͑ii͒ the effect of finite thickness of the phase interface resulting into a negative quadratic term, and ͑iii͒ the effect of buildup of a low-density tail of the density profile, also contributing as a negative quadratic term. Contributions ͑i͒-͑iii͒ fully explain the dependency of the surface tension on the pressure difference, including the range relevant to nucleation experiments. Contributions ͑i͒ and ͑ii͒ can be predicted from the planar density profile. The work of formation of noncritical clusters is derived and the nucleation rate is computed. The computed nucleation rates are closer to the experimental nucleation rate results than the classical Becker-Döring theory, and also the dependence on supersaturation is better predicted.
Nucleation experiments in binary (a-b) mixtures, when component a is supersaturated and b (carrier gas) is undersaturated, reveal that for some mixtures at high pressures the a content of the critical cluster dramatically decreases with pressure contrary to expectations based on classical nucleation theory. We show that this phenomenon is a manifestation of the dominant role of the unlike interactions at high pressures resulting in the negative partial molar volume of component a in the vapor phase beyond the compensation pressure. The analysis is based on the pressure nucleation theorem for multicomponent systems which is invariant to a nucleation model.
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