Hydration Structure on Ice Nucleating Mineral Surfaces

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Heterogeneous ice nucleation is omnipresent in Nature and pivotal for many processes such as cloud formation and precipitation. Although ice nucleation particles have been studied since decades, it remains unclear at present why some particles exhibit efficient ice nucleation while others do not. In this project, two important classes of ice nucleating minerals shall be investigated. We will study feldspar minerals of different chemical composition (orthoclase, microcline and others) as well as the thermodynamically most stable modification of silver iodide (iodargyrite). The goal of this project is to elucidate the atomic structure of the surface in the presence of water and to map the room temperature hydration structure at the interface. The high ice nucleating ability of potassium-rich feldspar samples has been assigned to the high-energy (100) plane, which might be exposed at defect sites and surface cracks. To this end, we will address the question whether or not the hydration structure is significantly different atop the natural cleavage planes (001), (010) as compared to the high-energy (100) surface. Moreover, we will study feldspars different from potassium-rich orthoclase for elucidating the question whether or not differences in the hydration structure can be found as a function of the chemical composition of the mineral. Regarding silver iodide, the primary and pivotal question to be answered concerns the stabilization mechanism of the polar (0001) cleavage plane. So far, all theoretical studies are performed with the bulk-truncated surface in mind while no experimental evidence exists for the stabilization mechanism in action. Moreover, significant differences in the ice nucleating ability of the silver- as compared to the iodide-terminated surface have been predicted by theory. Consequently, we will investigate both terminations to shed light onto the atomic-scale surface structure and to identify possible surface reconstructions. We will investigate whether the structure of the hydration layers on the two terminations provides an indication for the different ice nucleating ability. In summary, we will gather direct-space information of the mineral-water interface at the molecular level. With this information we hope to contribute to our understanding of the fundamental mechanisms governing heterogeneous ice nucleation by mineral particles.