Heterogeneous Freezing Starts with Surface Interactions

Why does water freeze when and where it does in Earth's atmosphere? For temperatures above about -34 C, where the homogeneous nucleation rate becomes appreciable, freezing proceeds only if catalyzed by the presence of an ice nucleating particle. The foreign surface can act as a template, ordering water molecules in the liquid and/or stabilizing the nascent ice embryo. In either case, it is the interaction of water molecules with the surface that ultimately determines the surface's efficacy as an ice nucleator.

Understanding such interactions is at the core of an understanding of heterogeneous nucleation. However, we do not understand such interactions at a fundamental level, which is why there is currently no framework to predict the ice nucleating ability of a surface based on its characteristics. The primary challenge is the inability to access the relevant length and time scales. Nucleation occurs on nano-to-micro second timescales and lengthscales of few hundred-to-thousand molecules. These are difficult, if not currently impossible, to probe in experiments. While, in principle, such scales are ideally suited for molecular simulations, ice nucleation is a rare event – i.e., an event which occurs at frequencies lower than the simulation sampling time, which makes it challenging to model. Comparison of simulation and experiment is even more rare because the complexities inherent in natural systems are not captured in simulations.

In an effort to facilitate a correlation between results from experiments and simulations, we have chosen muscovite mica as a model substrate. Mica is atomically smooth when cleaved, reducing the conflating effects of surface defects. When cleaved, K+ ions are exposed on mica's basal plane. These ions can be replaced by, H+, Ca++ and Mg++ for example, by rinsing the surface with an appropriate salt solution (or water in the case of H+) [1]. Ion exchange allows both experiment and simulation to explore the effect of molecular level changes on the structure of water adsorbed to mica.

We have measured water adsorption isotherms on mica (K+ as well as ion exchanged) using infrared spectroscopy. Mica treated to exchange potassium ions for magnesium show the highest affinity for water, with an estimated layer thickness of 1 nm at ~ 80% relative humidity. The spectra are red shifted, relative to bulk water, indicating that the hydrogen bonding network is more rigid and/or more structured. Guided by the results from simulations, we will discuss these results in the context of how water adsorbs at specific sites on mica and how the resulting water structure may influence the nucleation of ice.

1. Xu, L., & Salmeron, M. (1998). An XPS and scanning polarization force microscopy study of the exchange and mobility of surface ions on mica. Langmuir, 14(20), 5841-5844.