Abstract
A re-examination of measurements of heterogeneous nucleation of water vapor on silver nanoparticles is presented here using a model-free framework that derives the energy of critical cluster formation directly from measurements of nucleation probability. Temperature dependence is correlated with cluster stabilization by the nanoparticle seed and previously found cases of unusual increasing nucleation onset saturation ratio with increasing temperature are explained. A necessary condition for the unusual positive temperature dependence is identified, namely that the critical cluster be more stable, on a per molecule basis, than the bulk liquid to exhibit the effect. Temperature dependence is next examined in the classical Fletcher model, modified here to make the energy of cluster formation explicit in the model. The contact angle used in the Fletcher model is identified as the microscopic contact angle, which can be directly obtained from heterogeneous nucleation experimental data by a recently developed analysis method. Here an equivalent condition, increasing contact angle with temperature, is found necessary for occurrence of unusual temperature dependence. Our findings have immediate applications to atmospheric particle formation and nanoparticle detection in condensation particle counters (CPCs).
Highlights
For the case of heterogeneous nucleation from vapor, new interactions come into play as molecular clusters of condensate are adsorbed on or otherwise stabilized by the seed, which itself may be a single large molecule or molecular cluster of a composition different from that of the condensing vapor
Consider first the homogeneous nucleation case where there are no seed interactions to consider and a “second nucleation theorem” has been developed that provides a molecular bases for temperature dependence of nucleation rate[12]:
In this paper we demonstrated how the first and second nucleation theorems can be combined with measurements of nucleation probability at several different temperatures to provide a direct, model-free, determination of the energy of critical cluster formation
Summary
For the case of heterogeneous nucleation from vapor, new interactions come into play as molecular clusters of condensate are adsorbed on or otherwise stabilized by the seed, which itself may be a single large molecule or molecular cluster of a composition different from that of the condensing vapor. While for homogeneous nucleation a comparatively small correction to CNT was found sufficient to restore agreement with measurement, for heterogeneous nucleation even the sign of the temperature dependence can differ from the expectations of CNT6. Resolution of this puzzle and estimation of critical cluster energies and heats of wetting at near molecular scale are objectives of the present study. Measurements using the size analyzing nucleus counter (SANC) reveal that the critical vapor saturation ratio for onset of heterogeneous nucleation can either increase or decrease with increasing temperature[6,8].
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