Homogeneous nucleation of a homologous series of n-alkanes (CiH2i+2, i=7–10) in a supersonic nozzle

Abstract

Homogeneous nucleation rates of the n-alkanes (C(i)H(2i+2); i=7-10) were determined by combining information from pressure trace measurements and small angle x-ray scattering (SAXS) experiments in a supersonic Laval nozzle. The condensible vapor pressure p(J max), the temperature T(J max), the characteristic time Deltat(J max), and supersaturation S(J max) corresponding to the peak nucleation rate J(max) were determined during the pressure trace measurements. These measurements also served as the basis for the subsequent SAXS experiments. Fitting the radially averaged SAXS spectrum yielded the mean droplet radius r, 5<r/nm<31, the width of the aerosol size distribution sigma, 2<sigma/nm<6, and the particle number density N, 7x10(10)<N/cm(-3)<2.2x10(12). The nucleation rates for the n-alkanes J(max), 4x10(15)<J(max)/cm(-3) s(-1)<2x10(18), vary by almost three orders of magnitude as the temperature T(Jmax) decreases from approximately 200 K to as low as 150 K. At the lowest temperatures, the supersaturations S(Jmax) are on the order of 10(5). In spite of these extreme operating conditions, we find good agreement between the current experimental results and those available in the literature using Hale's scaling formalism [Phys. Rev. A 33, 4156 (1986); Metall. Trans. A 23, 1863 (1992)] and the scaling parameters reported by Rusyniak and El-Shall [J. Phys. Chem. B 105, 11873 (2001)]. Comparing the experimental nucleation rates with the predictions of classical nucleation theory, we find that our experimental nucleation rates are 4.5-8 orders of magnitude higher than the predictions.