Monomer, clusters, liquid: an integrated spectroscopic study of methanol condensation

Abstract

We have combined static pressure, spectroscopic temperature, Fourier transform infrared spectroscopy (FTIR), and small angle X-ray scattering (SAXS) measurements to develop a detailed picture of methanol condensing from a dilute vapor-carrier gas mixture under the highly supersaturated conditions present in a supersonic nozzle. In our experiments, methanol condensation can be divided into three stages as the gas mixture expands in the nozzle. In the first stage, as the temperature decreases rapidly, small methanol n-mers (clusters) form, increase in concentration, and evolve in size. In the second stage, the temperature decreases more slowly, and the n-mer concentrations continue to rise. Thermodynamic and FTIR experiments cannot, however, definitively establish if the average cluster size is constant or if it continues to increase. Finally, when the vapor becomes supersaturated enough, liquid droplets form via nucleation and growth, consuming more monomer and reducing the concentration of clusters. At the point where liquid first appears, cluster formation has already consumed up to 30% of the monomer. This is significantly more than is predicted by a model that describes the vapor phase as an equilibrium mixture of methanol monomer, dimer, and tetramer. An energy balance suggests that a significant fraction of the cluster population is larger than the tetramer, while preliminary SAXS measurements suggest that these clusters contain, on average, 6 monomers.

Nonisothermal Droplet Growth in the Free Molecular Regime

“…The experiments were conducted using the apparatus and techniques described in detail elsewhere (Laksmono et al 2011) and are summarized only briefly here. A figure of the flow system is available in the online supplementary material as Figure S1.…”

Comment on “Methanol dimer formation drastically enhances hydrogen abstraction from methanol by OH at low temperature” by W. Siebrand, Z. Smedarchina, E. Martínez-Núñez and A. Fernández-Ramos, Phys. Chem. Chem. Phys., 2016, 18, 22712

“…While one of the cited papers does involve a Laval nozzle apparatus, 19 the intention of that study is not to produce a medium suitable for kinetic measurements. Rather, the cited study by Laksmono et al 19 is primarily looking to deliberately induce condensation, and as such they operate their Laval nozzle at far higher pressures than used in more traditional Laval kinetic studies, and their expansion is clearly not stable, with the temperature observed to steadily decrease with distance upon exit of the Laval nozzle. Therefore, the experimental conditions in the work of Laksmono et al 19 are far removed from the ones used in the work of Shannon, 4 Gómez Martín et al 3 and Antiñolo et al 2 The other papers involve a free jet expansion and are in no way analogous to the thermal conditions within a Laval flow.…”

“…SSNR specifically question whether Laval nozzles themselves promote condensation and cite three principal papers as evidence for this phenomenon [17][18][19] . While one of the cited papers does involve a Laval nozzle apparatus, 19 the intention of that study is not to produce a medium suitable for kinetic measurements.…”

“…SSNR specifically question whether Laval nozzles themselves promote condensation and cite three principal papers as evidence for this phenomenon [17][18][19] . While one of the cited papers does involve a Laval nozzle apparatus, 19 the intention of that study is not to produce a medium suitable for kinetic measurements. Rather, the cited study by Laksmono et al 19 is primarily looking to deliberately induce condensation, and as such they operate their Laval nozzle at far higher pressures than used in more traditional Laval kinetic studies, and their expansion is clearly not stable, with the temperature observed to steadily decrease with distance upon exit of the Laval nozzle.…”

Formation and Growth of Clusters of Sulfur Dioxide

“…The large cross section of the sulfur 2p photoemission process makes XPS well suited for monitoring the mean cluster size. Different from the spatial or temporal resolution offered in some studies (Paci et al 2004;Laksmono et al 2011;Pathak et al 2013;Ezell et al 2014), the cluster beam is probed at a fixed distance about 10 cm downstream from the nozzle neck, i.e., at a mature particle population.…”