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.

Nonisothermal Droplet Growth in the Free Molecular Regime

“…Figure 1c illustrates the changes in mass fraction of condensate g (left axis) and the supersaturation S of nonane in the vapor phase (right axis). The slightly negative values of g prior to onset are thought to arise from nonane vapor phase heat capacities that are slightly lower than those predicted by available correlations (Ghosh et al 2010). Shortly after onset, the estimate for g based on PTM alone are significantly lower those based on SAXS.…”

“…Liquid N 2 (Praxair or Airgas) had a minimum purity of 99.99%, D 2 O (Cambridge Isotopes Labs) had more than 99.9% D substitution, and n-nonane (Sigma Aldrich and Chemsampco) was more than 99% pure. The thermophysical properties used in the analyses are reported in Wölk and Strey (2001) and Sinha et al (2009) for D 2 O, and Ghosh et al (2010) for n-nonane. The compressibility for liquid nonane over the temperature range of interest was estimated as 7 × 10 −10 Pa −1 (NIST 2002).…”

“…Here, t = 0 corresponds to the onset of condensation that we define as the point in the flow where the nucleation rate is maximized. In Figure 1d the normalized nucleation rates were calculated using Classical Nucleation Theory using the expressions and physical property data presented by Ghosh et al (2010). As illustrated in Figure 1a, the pressure and temperature decrease isentropically up to the onset of condensation.…”

Overview: Homogeneous nucleation from the vapor phase—The experimental science

“…More significantly, Figure 10 summarizes water nucleation rates measured using diffusion and expansion based methods that now span more than 20 orders of magnitude and cover a temperature range of over 100 K. Other materials that have been investigated over comparable ranges include the alkanes 60 and the lower n-alcohols ethanol through pentanol. Although n-pentanol was chosen by the experimental FIG.…”

“…To do so they adopted the analysis methods used for LFDDs to estimate the characteristic time and used small angle neutron scattering to determine the aerosol number density. Small angle X-ray scattering extended the reach of these experiments to nondeuterated compounds, 60,78,79,81,[169][170][171] and Fourier transform infra-red spectroscopy (FTIR) enabled both multicomponent nucleation rate measurements 171 and freezing studies. [172][173][174] Tunable diode laser absorption spectroscopy (TDLAS) measurements were used to measure the temperature of the flowing gas mixture 175,176 and the concentrations of small condensable molecules in the gas phase.…”