Homogeneous nucleation rates for n-alcohol vapors measured in a two-piston expansion chamber

Homogeneous nucleation rates J in supersaturated n-alcohol vapors (methanol through n-hexanol) were measured in a two-piston expansion chamber as functions of supersaturation S and temperature T. The measured nucleation rates were compared with the classical nucleation theory. We found that the slopes of the J–S curves are in good agreement with the theoretical prediction. However, the actual values of experimental and theoretical rates were generally found to differ significantly, Jexptl/Jtheor ranging from about 10−10 (methanol, 273 K) to 107 (n-hexanol, 257 K). In particular, the experimental nucleation rates show a significantly weaker temperature dependence as compared to the classical nucleation theory, the difference being more pronounced for the higher alcohols. Thus, a temperature dependent correction to the classical nucleation theory is needed. For the lower alcohols, in particular methanol, we found that the change of the cluster distribution during the expansion strongly influences the nucleation process. The heat released during the formation of oligomers causes a significant temperature increase in the system and thus alters the nucleation conditions dramatically. For methanol vapor under the considered experimental conditions this effect leads to a reduction of the theoretically predicted nucleation rates by factors up to about 108. Since oligomer formation is prerequisite to nucleation, this effect has to be taken into account generally for nucleation experiments in adiabatically closed systems.