Molecular Simulations of Feldspar Surfaces Interacting with Aqueous Inorganic Solutions: Interfacial Water/Iion Structure and Implications for Ice Nucleation

Recent studies have established the superior ice-nucleating abilities of feldspars and the varying effects of inorganic solutes on their ice-nucleating abilities. However, little is known about the mechanism of ice nucleation by feldspar at the microscopic level as well as how the presence of ionic solutes might alter feldspar surfaces and hence influence ice nucleation. To explore these questions, we use molecular dynamics simulations to examine the interactions of monovalent cations (NH4⁺, H3O⁺, Li⁺, K⁺, and Cs⁺) with the (001), (010), and (100) surfaces of potassium feldspar (microcline phase) at 300 K and the corresponding interfacial water structure in supercooled solutions (230 K). Both semi-rigid (only lattice K⁺ free to move) and fully flexible (all lattice atoms free to move) microcline slabs are considered. On simulation timescales, ion exchange between solution cations and lattice K⁺ is observed only for fully flexible slabs, where the release of K⁺ is facilitated by lattice vibrations. The exchange rates are strongly surface dependent. For both semi-rigid and flexible surfaces, the surface densities of adsorbed polyatomic cations are, in general, larger than those of the monoatomic spherical cations. We do not observe any sign of ice nucleation on pristine or NH4⁺-adsorbed/exchanged microcline surfaces (both semi-rigid and flexible) at 230 K within the simulation timescales. This contrasts with the laboratory experiments and strongly suggests that simple, unreconstructed, planar surfaces are not responsible for the excellent ice-nucleating ability of potassium feldspar.