Ice Nucleation by the Primary Prism Face of Silver Iodide
Abhishek Soni, G. N. Patey
Abstract
Silver iodide is one of the most effective ice nucleating agents known. Silver iodide particles induce heterogeneous ice nucleation at temperatures as warm as −3 °C. Consequently, silver iodide, particularly the hexagonal polymorph (β-AgI), has been the focus of recent research aimed at understanding the microscopic mechanism of ice nucleation. Molecular simulations have shown that the basal (0001) plane of β-AgI nucleates the basal plane of ice due to a good lattice match combined with favorable atomistic surface morphology. However, ice nucleation has not been previously observed for the primary prism (101̅0) face of β-AgI, despite its close lattice match to the primary prism plane of hexagonal ice (Ih). Here we report molecular dynamics simulations employing the TIP4P/Ice model at a lower temperature (230 K) than those considered in earlier simulations. Our simulations show spontaneous nucleation of Ih by the primary prism face of β-AgI. Nucleation occurs via the primary prism face of Ih. We show how the bilayer characteristic of the primary prism face of Ih maps onto the surface morphology of β-AgI (101̅0). The primary prism face of β-AgI differs from the basal plane in that it has no electrical dipole perpendicular to the surface, which reduces the likelihood that an exposed β-AgI (101̅0) surface will undergo reconstruction. Thus, the primary prism plane might be a more viable possibility for ice nucleation by real β-AgI particles. Simulations were also performed for the secondary prism (112̅0) face of β-AgI, but ice nucleation was not observed on simulation time scales. Nevertheless, detailed geometric analysis shows that the atomistic morphology of this surface could provide a good match to the secondary prism plane of Ih.