Determination of the solid-vapor interfacial tension of nitric acid dihydrate crystals via experiments on the freezing of aqueous nitric acid droplets

We propose a method for determining the solid-vapor interfacial tension σ^sv of nitric acid dihydrate/trihydrate (NAD/NAT) crystals using experiments on crystal nucleation in aqueous nitric acid (NA) droplets. It exploits the dominance of the surface-stimulated mode of crystal nucleation in small droplets (of radii R ≤ 5 μm) and its negligibility in large ones (of radii R ≥ 20 μm). We show that, experimentally measuring the crystal nucleation rates in such large and small (but otherwise identical) droplets, one can determine the solid-vapor interfacial tension of that crystal facet which forms at the droplet surface in a surface-stimulated mode. A modified expression for the work of formation of NAD/NAT crystal nuclei in nonstoichiometric solutions is proposed; it more adequately takes account of the effects of NA vapors. Applying our method to existing experimental data, we estimate the σ^sv of NAD to be about 90 dyn/cm (in the temperature range from 190 to 204 K), weakly depending on temperature.