Two-pathway homogeneous nucleation in supersaturated water - n-nonane vapor mixtures

We report an experimental study of homogeneous nucleation in supersaturated vapor mixtures of n-nonane and water exhibiting a pronounced miscibility g ap in the liquid state. Using the expansion Pulse method, we measured homog eneous nucleation rates J for unary water and n-nonane vapors as well as fo r vapor mixtures with six different ratios of the partial vapor phase activ ities a(1) and a(2). All measurements were performed at the same constant n ucleation temperature of 230 K. The measured nucleation rates shape a nucle ation rate surface in the (In J)-a(1)-a(2) space which is found to be quali tatively different from previously observed nucleation rate surfaces in tha t it exhibits two rather flat parts separated by a limited region with comp aratively strong curvature. There is strong indication that the nucleation in water-n-nonane vapor mixtures can be viewed as a superposition of two si multaneous unary nucleation processes. In the flat parts of the nucleation rate surface the nucleation rates are dominated by the nucleation of one va por component and are thus practically independent of the vapor-phase activ ity of the other component. On the other hand, in the region of strong curv ature it appears that we have observed simultaneous homogeneous nucleation via two kinds of critical nuclei and thus via two separate pathways in the free energy surface. For comparatively low n-nonane vapor content, water va por was found to nucleate, while the Mie-scattering pattern indicates mainl y n-nonane growth at later stages. Thus, the water droplets apparently serv e as heterogeneous nuclei and "catalyze" the subsequent condensation of n-n onane onto the droplets.