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 LiTaO ₃ and AgI crystals, in the presence and in the absence of pyroelectric charge are reported. It is demonstrated that bicarbonate (HCO ₃ ^- ) ions elevate the icing temperature near the positively charged faces. In contrast, the hydronium (H ₃ O ⁺ ) slightly reduces the icing temperature. Molecular dynamics simulations suggest that the hydrated trigonal planar HCO ₃ ^- 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 NO ₃ ^- 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 SO ₄ ^2- ions of different configurations reduce the icing temperature.
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