Biguanide, an Efficient Electrofreezing “Ice-maker” Ion of Supercooled Water

Following our previous investigations on the electrofreezing mechanism of supercooled water on pyroelectric crystal surfaces, we discovered that electrofreezing is a process involving the attraction and arrangement of specific ionic charges by an electric field. We found two classes of ions: the trigonal planar ions that raise the icing temperature, or “ice-makers”, and ions of different structures that reduce the icing temperature, or “ice-breakers”. In the search for more efficient promoters for electrofreezing, we anticipated that molecules that have the propensity to self-assemble with water to form hexagonal clusters might be better ice nucleators. Through icing experiments performed directly on the hemihedral faces of pyroelectric crystals of LiTaO₃, we found that ions of biguanide elevate the icing temperature of supercooled water when concentrated near the negatively charged crystal’s interfacial water layer, either upon cooling or upon heating. On the other hand, the analogous guanylurea ions, which presumably assume configurations with deviations from planarity, operate as “ice-breakers”.