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 spectrosc opy. The particles are attached to the inner walls of a gold light pip e, 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 mu m particl es with compositions ranging from 10 to 60 wt % sulfuric acid in water have been studied with this technique. The results are in agreement w ith expectations in terms of (1) the dependency of light scattering up on particle size, (2) the extent of water uptake as monitored by the i nfrared spectra, (3) the formation of ice and sulfuric acid tetrahydra te (SAT), and (4) the equilibrium melting temperatures of these solids , In agreement with previous studies on the H2SO4/H2O system employing relatively large liquid volumes, the micron-sized droplets with compo sitions in the range from 40 to 60 wt % form glasses upon cooling; how ever, these droplets do not crystallize upon warming, in contrast to t he observed behavior of the larger samples.