INFRARED-SPECTRA OF LARGE ACID HYDRATE CLUSTERS - FORMATION CONDITIONS OF SUBMICRON PARTICLES OF HNO3.2H2O AND HNO3.3H2O

Infrared spectra are reported for nitric acid hydrate clusters formed in a static cluster cell from mixtures containing water and l% HNO3 in N2(g) using temperatures in the 160-185 K range. The phase changes within the submicron hydrate clusters over periods up to 22 min have been monitored, and the nucleated phases identified through comparison with ''reference'' spectra for NAM, NAD, and NAT available from the studies of Koehler, Middlebrook, and Tolbert. For composition ratios ranging from 1.5 to approximately 4.0 the first crystalline phase formed is identified as NAD in each case, an observation consistent with vapor-phase evidence (Worsnop et al.) that NAD nucleates with a lower activation energy than other hydrates and, consequently, tends to have a ubiquitous presence as a metastable solid. However, clusters richer in water than 4:1 were observed to crystallize as NAT plus ice, with the beta polymorph of NAT apparently the preferred state at temperatures above approximately 180 K. In general, for the water-rich clusters, crystalline ice first nucleates. This results in clusters that contain a residual acid-enriched solution from which NAT crystallizes, with no evidence from the spectra that NAD is present at any time.