Phase Transformations of Micron-Sized H2SO4/H2O Particles Studied by Infrared Spectroscopy

A new experimental technique has been developed for investigations of phase transitions of micron-sized particles through infrared spectroscopy. The particles are attached to the inner walls of a gold light pipe, enabling long observation times, temperature cycling, regulation of the particle composition, and statistically significant sampling (i.e., large particle populations). Phase transitions of 5−10 μm particles with compositions ranging from 10 to 60 wt % sulfuric acid in water have been studied with this technique. The results are in agreement with expectations in terms of (1) the dependency of light scattering upon particle size, (2) the extent of water uptake as monitored by the infrared spectra, (3) the formation of ice and sulfuric acid tetrahydrate (SAT), and (4) the equilibrium melting temperatures of these solids. In agreement with previous studies on the H 2 SO 4 /H 2 O system employing relatively large liquid volumes, the micron-sized droplets with compositions in the range from 40 to 60 wt % form glasses upon cooling; however, these droplets do not crystallize upon warming, in contrast to the observed behavior of the larger samples.