Following heterogeneous nucleation of CO2 on H2O ice nanoparticles with microsecond resolution

Abstract

Heterogeneous nucleation of CO₂ onto H₂O ice particles may play an important role in proposed innovative CO₂ capture technologies, as well as in the formation of Martian clouds. In this work we follow the nucleation/condensation of CO₂/H₂O gas mixtures with microsecond resolution in supersonic Laval nozzles using pressure trace measurement (PTM) and small angle X-ray scattering (SAXS). The latent heat release detected by the PTM reveals that the first phase transition in the expanding CO₂/H₂O mixture is the formation of H₂O ice particles by the homogeneous nucleation/condensation and freezing of H₂O. This is followed by the heterogeneous nucleation and growth of CO₂ on the H₂O ice particles. The onset conditions for heterogeneous nucleation, i.e. the partial pressure of CO₂ and temperature from PTM and the radius of gyration of the H₂O ice particles from SAXS, were determined in the temperature range 124 to 146 K and for particles with radii of gyration in the range of 2.1 to 4.3 nm. The onset conditions suggest that the heterogeneous nucleation of CO₂ may start from the supercooled liquid phase under our conditions. Downstream of the onset point, the partial pressure of CO₂ and temperature rapidly approach the vapor–solid equilibrium line of CO₂, demonstrating that even if CO₂ condensation is initiated by heterogeneous nucleation of the liquid phase, it proceeds via growth of the solid.