Binary nucleation at low temperatures

The onset of homogeneous condensation of binary vapors in the supersaturated state is studied by means of the unsteady expansion in a shock tube. The experiments are concerned with low temperatures down to 220 K which are of particular interest for atmospheric conditions. Two representative systems, ethanol/n-propanol and water/ethanol, are investigated since they allow a sufficient theoretical description. The first system forms a nearly ideal solution, whereas the latter one is an example of a strongly nonideal mixture. For either system vapor mixtures of various compositions are diluted in dry air at small mole fractions and expanded in the driver section from room temperature. The onset of homogeneous condensation is detected optically and the corresponding thermodynamic state is evaluated. The experimental results are compared with the binary nucleation theory and the particular problems of the theoretical evaluation at the low temperatures are discussed. The use of surface tension values derived for small binary droplets1 in the nucleation theory is mandatory to restore agreement between theory and experiment for the surface active system water/ethanol.