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arXiv · 1702.02500

Free energy of formation of a crystal nucleus in incongruent solidification: Implication for modeling the crystallization of aqueous nitric acid droplets in type 1 polar stratospheric clouds

Yuri S. Djikaev·Eli Ruckenstein
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Abstract

Using the formalism of the classical nucleation theory, we derive an expression for the reversible work $W_*$ of formation of a binary crystal nucleus in a liquid binary solution of non-stoichiometric composition (incongruent crystallization). Applied to the crystallization of aqueous nitric acid (NA) droplets, the new expression more adequately takes account of the effect of nitric acid vapor compared to the conventional expression of MacKenzie, Kulmala, Laaksonen, and Vesala (MKLV) [J.Geophys.Res. 102, 19729 (1997)]. The predictions of both MKLV and modified expressions for the average liquid-solid interfacial tension $\sigma^{ls}$ of nitric acid dihydrate (NAD) crystals are compared by using existing experimental data on the incongruent crystallization of aqueous NA droplets of composition relevant to polar stratospheric clouds (PSCs). The predictions based on the MKLV expression are higher by about 5% compared to predictions based on our modified expression. This results in similar differences between the predictions of both expressions for the solid-vapor interfacial tension $\sigma^{sv}$ of NAD crystal nuclei. The latter can be obtained by analyzing of experimental data on crystal nucleation rates in aqueous NA droplets and exploiting the dominance of the surface-stimulated mode of crystal nucleation in small droplets and its negligibility in large ones. Applying that method, our expression for $W_*$ provides an estimate for $\sigma^{sv}$ of NAD in the range from 92 dyn/cm to 100 dyn/cm, while the MKLV expression predicts it in the range from 95 dyn/cm to 105 dyn/cm. The predictions of both expressions for $W_*$ become identical in the case of congruent crystallization; this was also demonstrated by applying our method to the nucleation of nitric acid trihydrate (NAT) crystals in PSC droplets of stoichiometric composition.

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Yuri S. Djikaev, Eli Ruckenstein. 2017-02-08. Free energy of formation of a crystal nucleus in incongruent solidification: Implication for modeling the crystallization of aqueous nitric acid droplets in type 1 polar stratospheric clouds. https://doi.org/10.1063/1.4979069

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