Use of Ion Exchange To Regulate the Heterogeneous Ice Nucleation Efficiency of Mica

Heterogeneous ice nucleation (HIN) triggered by mineral surfaces typically exposed to various ions can have a significant impact on the regional atmosphere and climate. However, the dependence of HIN on the nature of the mineral surface ions is still largely unexplored due to the complexity of mineral surfaces. Because K⁺ on the atomically flat (001) surface of mica can be readily replaced by different cations through ion exchange, muscovite mica was selected; its simple nature provides a very straightforward system that can serve as the model for investigating the effects of mineral surface ions on HIN. Our experiments show that the surface (001) of H⁺-exchanged mica displays markedly higher HIN efficiencies than that of Na-/K-mica. Vibrational sum-frequency generation spectroscopy reveals that H-mica induces substantially less orientation ordering than Na-/K-mica within the contact water layer at the interface. Molecular dynamics simulations suggest that the HIN efficiency of mica depends on the positional arrangement and orientation of the interfacial water. The formation of the hexagonal ice Ih basal-type structure in the first water layer atop the mica surface facilitates HIN, which is determined by the size of the protruding ions atop the mica surface and by the surface adsorption energy. The orientational distribution is optimal for HIN when 25% of the water molecules in the first water layer atop the mica surface have one OH group pointing up and 25% have one OH group pointing down, which, in turn, is determined by the surface charge distribution.