Session: Molecular Processes at Mineral-Water Interfaces:

Ice clouds play an important role in the Earth’s radiative budget and hence climate. Heterogeneous ice nucleation (IN), a major pathway for ice formation in cirrus and mixed-phase clouds, is induced by active sites present on atmospheric aerosol particles termed as ice-nucleating particles. Despite the importance of mineral dusts for IN, the role of atmospheric aging (e.g. processing due to chemical species) on their IN efficiency is largely unknown. This is primarily due to the lack of obtaining microscopic level insight into nucleation from laboratory/field-based experiments due to the inability to experimentally access small spatial and temporal scale at which nucleation process occurs – a problem that can be potentially tackled with computer simulations. We utilize direct Molecular Dynamics simulations (GROMACS 5.1.4) to investigate the interactions of monovalent cations (H₃O⁺, (NH₄)⁺, Li⁺, K⁺, Cs⁺) with the (001), (010) and (100) surfaces of potassium feldspar mineral (microcline) at 300 K, and the corresponding interfacial water structure at 230 K at a microscopic level. Feldspar is modeled as semi-rigid (lattice atoms fixed expect K⁺ and H of surface OH) and as fully flexible (lattice atoms free to move) with the CLAYFF force field, and the TIP4P/Ice model is employed for water. Results show that, on simulation timescales, lattice vibration is necessary for ion exchange between added cation and lattice K⁺, albeit at different exchange rates for the 3 planes. For both semi-rigid and flexible surfaces, the adsorption density of molecular cations (H₃O⁺ and (NH₄)⁺) is larger than that of the simple spherical cations. In addition, we do not observe ice nucleation on modified microcline surfaces (both semi-rigid and flexible) at 230 K within the simulation timescales. The current work provides an improved understanding of the processes modifying feldspar surfaces in water and aqueous solutions and its possible relevance on ice formation.

Allan Bertram

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