Heterogeneous Electrofreezing of Super-Cooled Water on Surfaces of Pyroelectric Crystals is Triggered by Trigonal Planar Ions.

Electrofreezing experiments of super-cooled water (SCW) with different ions, performed directly on the charged hemihedral faces of pyroelectric LiTaO3 and AgI crystals, in the presence and in the absence of pyroelectric charge are reported. It is demonstrated that bicarbonate (HCO3 - ) ions elevate the icing temperature near the positively charged faces. In contrast, the hydronium (H3 O+ ) slightly reduces the icing temperature. Molecular dynamics simulations suggest that the hydrated trigonal planar HCO3 - ions self-assemble with water molecules near the surface of the AgI crystal as clusters of slightly different configuration from those of the ice-like hexagons. These clusters, however, have a tendency to serve as embryonic nuclei for ice crystallization. Consequently, we predicted and experimentally confirmed that the trigonal planar ions of NO3 - and guanidinium (Gdm+ ), at appropriate concentrations, elevate the icing temperature near the positive and negative charged surfaces, respectively. On the other hand, the Cl- and SO4 2- ions of different configurations reduce the icing temperature.