Binary nucleation rates for ethanol/water mixtures in supersonic Laval nozzles.

Although the conditions corresponding to the onset of condensation of aqueous-alcohol mixtures have been measured in supersonic nozzles [B. E. Wyslouzil et al., J. Chem. Phys. 113, 7317 (2000)], the true nucleation rates have not. Here, we propose a new analytical method to estimate the temperature, the concentrations of condensable species in both the vapor and the liquid phases, and the amount of the condensate using only the measured static pressure profiles in the nozzle. We applied the method to ethanol/water (CH(3)CH(2)OH/D(2)O or CH(3)CH(2)OD/D(2)O) mixtures and confirmed that the aerosol volume fractions derived from pressure measurements and small angle neutron scattering measurements are in very good agreement when this method is used. Combining the results from the pressure measurements with the number densities of the condensed droplets, measured either by small angle neutron or small angle x-ray scattering, we determined the first quantitative ethanol/water binary nucleation rates in the supersonic nozzle at a temperature of 229±1 K.

Experimental evidence for internal structure in aqueous-organic nanodroplets.

The spatial distribution of species within an aerosol droplet influences how it interacts with its environment. Despite the ubiquity of multicomponent nanodroplets in natural and technological aerosols, there are no published measurements of their internal structure. Here, we report the first experimental results for structure in aqueous organic nanodroplets based on small angle neutron scattering by high number density aerosols. For H(2)O-n-butanol droplets, fitting of the diffraction patterns confirms the picture of an aqueous core containing approximately 3 mol% alcohol covered by a shell of densely packed alcohol molecules.

Homogeneous nucleation of n-propanol, n-butanol, and n-pentanol in a supersonic nozzle.

We have measured the nucleation conditions of n-propanol, n-butanol, and n-pentanol in a supersonic Laval nozzle, and estimated that the maximum nucleation rate J is 5 x 10(16) cm(-3) s(-1) with an uncertainty factor of 2. Plotting the vapor pressures p(J(max) ) and temperatures T(J(max) ) corresponding to the maximum nucleation rate as ln(p) versus 1T, produces a series of well separated straight lines. When these values are scaled by their respective critical parameters, p(c) and T(c), the data lie close to a single straight line. Comparing the experimental data to the predictions of classical nucleation theory reveals much higher experimental rates, and the deviation increases with increasing alcohol chain length and decreasing temperature. A scaling analysis in terms of Hale's scaled nucleation model [Phys. Rev. A 33, 4156 (1986); Metall. Trans. A 23, 1863 (1992)], clearly shows that our data are consistent with experimental nucleation rates measured using other devices that have characteristic rates many orders of magnitude lower.