ON THE TEMPERATURE DEPENDENCE OF HETEROGENEOUS NUCLEATION
OF N-BUTANOL VAPOR ON SILVER AND SODIUM CHLORIDE NANOPARTICLES
C. TAUBER1, G. STEINER1,2 and P.M. WINKLER1
1
Faculty of Physics, University of Vienna,
Boltzmanngasse 5, 1090 Vienna, Austria.
2
Institute for Ion Physics and Applied Physics, University of Innsbruck,
Technikerstraße 25/3, 6020 Innsbruck, Austria.
Keywords: TEMPERATURE DEPENDENCE, HETEROGENEOUS NUCLEATION, N-BUTANOL,
SILVER, SODIUM CHLORIDE.
INTRODUCTION
Commercial Condensation Particle Counters (CPCs) are mainly used to measure the number concentration
of airborne particles (McMurry, 2000). Most ultrafine continuous flow CPCs use n-butanol as working fluid
and have typical lower particle detection limits in the range between 2.5 and 10 nm (Stolzenburg and
McMurry, 1991). Given the fact that n-butanol CPCs are operated at fixed temperatures, the performance
towards the detection of even smaller particles may be optimized by choosing appropriate temperature
settings.
Until now the temperature dependence of heterogeneous nucleation of n-butanol vapor on nano-particles in
a size-range down to 2.5 nm was not investigated. A topic, however, that is worth examining since it gives
information on the saturation ratio which is needed to activate particles in this size range. Results of earlier
experiments with water vapor nucleating on silver particles have shown a theoretically unpredicted
maximum in the onset saturation ratio as a function of temperature (Kupc et al., 2013). Further studies on
the nucleation of n-propanol on sodium chloride particles in the temperature range from 262K to 287K
indicate a reversed trend of the onset saturation ratio compared to the Kelvin equation (Schobesberger et al.,
2010). Due to the close chemical similarity of n-propanol and n-butanol, and the common use of n-butanol
as a working fluid in commercial CPCs, the temperature dependence of heterogeneous nucleation of nbutanol on Ag and NaCl particles is investigated in this study.
METHODS
Here we report measurements of heterogeneous nucleation of n-butanol at nucleation temperatures ranging
from 269 K to 289 K. To this end, we generated monodisperse NaCl and Ag particles using a tube furnace
particle generator (Scheibel and Porstendörfer, 1983) in combination with a Vienna type differential
mobility analyzer for size selection. Subsequently, particles were mixed with n-butanol vapor and led into
the expansion chamber of the Size Analyzing Nuclei Counter (SANC) (Wagner et al., 2003). N-butanol
vapor was added to the system by controlled injection from a syringe pump, followed by quantitative
evaporation of the liquid beam in a heating unit (Winkler et al., 2008). Thereby, a well-defined and nearly
saturated binary vapor-air mixture together with size selected, neutralized monodisperse seed particles from
the DMA were passed into the temperature controlled expansion chamber of the SANC. A schematic
diagram showing the experimental setup is illustrated in figure 1. Vapor supersaturation was achieved by
adiabatic expansion and the number concentration of droplets nucleated on the seeds was measured with the
Constant Angle Mie Scattering (CAMS) method (Wagner, 1985). Therewith the radius and the number
concentration of the growing droplets can be determined simultaneously. By varying the chamber
temperature and the pressure drop in the expansion chamber, different nucleation conditions were analyzed.
The nucleation or activation probabilities (shown in figure 2) using the SANC/CAMS method can be
expressed as:
𝑃=
𝑁𝑎𝑐𝑡𝑖𝑣𝑎𝑡𝑒𝑑
= 1 − 𝑒𝑥𝑝(−𝐽𝑡).
𝑁𝑡𝑜𝑡𝑎𝑙
Here, 𝐽 is the heterogeneous nucleation rate and t is the time for activation. We define the onset conditions
as the saturation ratio S0 when the nucleation probability reaches the value 𝑃 = 0.5.
The nucleation probability 𝑃(𝑆) at 𝑆0 has the form of a cumulative Gumbel distribution (Winkler et al.,
2016)
𝑃(𝑆) = 1 − 𝑒𝑥𝑝{ −𝑒𝑥𝑝[ 𝑙𝑛(𝑙𝑛 2) + (𝑛∗ + 1)(𝑙𝑛 𝑆 − 𝑙𝑛 𝑆0 )] }.
By applying this form of distribution as a two-parameter fit function to the experimental data the parameters
𝑛∗ (number of molecules in the critical cluster) and 𝑆0 can be evaluated. The resulting onset saturation ratio
depending on the nucleation temperature can be compared to Kelvin prediction.
As a result, heterogeneous nucleation of n-butanol vapor on NaCl and Ag aerosol particles shows different
behavior depending on the nucleation temperature. An inverse temperature trend for NaCl seeds was
identified when compared to the Kelvin prediction. No such trend could be found for silver nano-particles.
In addition to the SANC measurements, the counting efficiency of an ultrafine continuous flow CPC (Model
UCPC 3776, TSI Inc., Minneapolis, USA), whose temperature settings were changed over a range of 20
degrees, was measured. The particles were size selected with a Vienna type UDMA (Steiner et al., 2010)
and the counting efficiency was determined relative to a Faraday cup electrometer operated in parallel as
shown in figure 3.
Figure 1. Schematic diagram for the experimental setup of the SANC.
At the standard temperature settings of the UCPC 3776 the measured counting efficiencies of Ag particles
agree nicely with the data published in the manual. However, for the same temperature settings the activation
behavior of NaCl particles looks considerably different. The cut-off diameter has moved to a larger size and
the slope is not as steep as for Ag particles. These findings are consistent with data reported in the literature
(Ankilov et al., 2002; Hermann et al., 2007) and would conventionally be explained by a composition
dependent CPC response. Therefore it is notable that by reducing the nucleation temperature by only a few
K, we find counting efficiencies of the NaCl particles exceeding those for Ag particles.
The heterogeneous nucleation of butanol on NaCl seed particles thus indicates a clear dependence on
nucleation temperature.
Figure 2. Heterogeneous nucleation of n-butanol on silver seeds at different nucleation temperatures. In
this case, lower nucleation temperature coincides with higher saturation ratios needed for particle
activation.
CONCLUSION
We have measured the onset saturation ratio of n-butanol in dependence of nucleation temperature with the
SANC and determined the cut-off diameter at variable condenser temperature, but constant Δ𝑇 between
Saturator and Condenser, for a commercial n-butanol CPC. NaCl and Ag seeds in the size range from 2.5 to
9.0 nm have been used as condensation nuclei. An inverse temperature dependence of heterogeneous
nucleation of n-butanol on sodium chloride particles was found, resulting in a lower cut-off diameter for
NaCl particles at reduced temperatures. Accordingly, by lowering the condenser temperature and hence
nucleation temperature of a butanol CPC the counting efficiency can be increased significantly for both
NaCl and Ag seeds, with a stronger response for NaCl particles.
This finding is of immediate relevance for nanoparticle detection in CPCs and raises questions on the
fundamental mechanisms leading to this behavior.
Figure 3. Schematic diagram of the experimental setup for counting efficiency measurements.
ACKNOWLEDGEMENT
This work was supported by the European Research Council under the European Community's Seventh
Framework Program (FP7/2007-2013) ERC grant agreement No. 616075.
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