The effect of the heat of association on homogeneous nucleation rates in methanol vapor

Homogeneous nucleation rates in methanol vapor were measured using the expansion pulse technique. Our results show an apparently inverted temperature dependence. The obtained data are consistent with earlier expansion chamber investigations, reported in the literature, however, strong deviations from corresponding diffusion chamber data are observed. In the present paper we show that these deviations can be explained by considering vapor association during adiabatic expansion. The homogeneous nucleation rates are found to be drastically reduced due to the latent heat released during the formation of subcritical clusters. Thereby the severe discrepancies between expansion chamber experiments for methanol and the corresponding diffusion chamber data are now essentially resolved. Furthermore, an apparently anomalous behavior of ethanol vapor, observed in expansion chambers at higher temperatures, is now qualitatively understood.