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

Abstract

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 (H3O+) 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 SO42− ions of different configurations reduce the icing temperature.

Heterogeneous Electrofreezing Triggered by CO2 on Pyroelectric Crystals: Qualitatively Different Icing on Hydrophilic and Hydrophobic Surfaces

“…This implies that ions concentrated at the charged surfaces, and the way they interact with the water molecules will determine their ability to enhance icing. A structural model that describes how ions of carbonic acid are responsible for electrofreezing is presented in the subsequent paper …”

Heroes of Stereochemistry and Crystals

“…Leslie and Meir (Figure 1) have been fortunate to work with many other great scientists and continue doing so. [7,8] An open discussion about their Passion and Chemistry with Prof. Ehud Keinan, President of the Israel Chemical Society and member of the Wolf Council, is available on YouTube. [9] Although not inclusive, they thank their collaborators and students for being part of their scientific journey.…”

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

“…Further work found that both planar trigonal negative nitrate and positive guanidinium ions elevate the freezing temperature of water, when accumulated at the interfacial water layers near the positive or negative charged surfaces, respectively. In contrast, the nontrigonal chloride ions reduce the freezing temperature …”

“…Despite comprehensive investigations during the years, the mechanism of this electrofreezing is still under debate. Some reports deny the existence of the effect, , whereas others maintain that electric fields can either reduce or elevate the icing temperature of SCW. − In order to gain additional insight into the electrofreezing mechanism, in particular, to disentangle electric and geometric effects, we capitalized on the unique properties of polar crystals. − Such crystals exhibit pyroelectricity, the derivative of the spontaneous polarization with temperature, and develop positive and negative charges at opposite hemihedral surfaces during a temperature change.…”

“…In order to disentangle the effects played by Bgdm from other effects, the following experiments were performed. The icing experiments were carried out comparatively with drops of pure water as in our previous work . In each experiment, using a wide field of vision, 30 μL droplets, six containing Bgdm HCl ions and six pure water droplets of pH 7, were simultaneously cooled at a rate of 2 °C/min on the two hemihedral (001) and (001̅) faces of the pyroelectric LiTaO 3 crystal, where positive and negative charges respectively are formed upon cooling.…”