Chemical Nature of Heterogeneous Electrofreezing of Supercooled Water Revealed on Polar (Pyroelectric) Surfaces

Abstract

Conspectus The ability to control the icing temperature of supercooled water (SCW) is of supreme importance in subfields of pure and applied sciences. The ice freezing of SCW can be influenced heterogeneously by electric effects, a process known as electrofreezing. This effect was first discovered during the 19th century; however, its mechanism is still under debate. In this Account we demonstrate, by capitalizing on the properties of polar crystals, that heterogeneous electrofreezing of SCW is a chemical process influenced by an electric field and specific ions. Polar crystals possess a net dipole moment. In addition, they are pyroelectric, displaying short-lived surface charges at their hemihedral faces at the two poles of the crystals as a result of temperature changes. Accordingly, during cooling or heating, an electric field is created, which is negated by the attraction of compensating charges from the environment. This process had an impact in the following experiments. The icing temperatures of SCW within crevices of polar crystals are higher in comparison to icing temperatures within crevices of nonpolar analogs. The role played by the electric effect was extricated from other effects by the performance of icing experiments on the surfaces of pyroelectric quasi-amorphous SrTiO 3 . During those studies it was found that on positively charged surfaces the icing temperature of SCW is elevated, whereas on negatively charged surfaces it is reduced. Following investigations discovered that the icing temperature of SCW is impacted by an ionic current created within a hydrated layer on top of hydrophilic faces residing parallel to the polar axes of the crystals. In the absence of such current on analogous hydrophobic surfaces, the pyroelectric effect does not influence the icing temperature of SCW. Those results implied that electrofreezing of SCW is a process influenced by specific compensating ions attracted by the pyroelectric field from the aqueous solution. When freezing experiments are performed in an open atmosphere, bicarbonate and hydronium ions, created by the dissolution of atmospheric CO 2 in water, influence the icing temperature. The bicarbonate ions, when attracted by positively charged pyroelectric surfaces, elevate the icing temperature, whereas their counterparts, hydronium ions, when attracted by the negatively charged surfaces reduce the icing temperature. Molecular dynamic simulations suggested that bicarbonate ions, concentrated within the near positively charged interfacial layer, self-assemble with water molecules to create stabilized slightly distorted “ice-like” hexagonal assemblies which mimic the hexagons of the crystals of ice. This occurs by replacing, within those ice-like hexagons, two hydrogen bonds of water by C–O bonds of the HCO 3 – ion. On the basis of these simulations, it was predicted and experimentally confirmed that other trigonal planar ions such as NO 3 ...

Ice interfaces: general discussion

“…We next look at the accuracy of the ML model trained on the revPBE0-D3 functional. We and others [3][4][5][6] have extensively used these revPBE0-D3 ML models to study the properties of bulk, interfacial and nanoconned water, and we observe high delity in predicting the thermodynamic and dynamical properties of revPBE0-D3 water.…”

“…I would also like to mention the work of two other participants attending this Faraday Discussions meeting, Leah Javitt and Prof. Meir Lahav, who have shown that various dissolved substances, including guanidinium chloride, aluminium ions and magnesium ions can promote ice nucleation on pyroelectric surfaces. 6 It is my view that this is probably a rather different ice nucleating system from feldspar and other atmospherically relevant substances, but that comparison between the different systems may at some stage prove revealing.…”

“…This surmise was conrmed with the hydrated ions of Al 3+ and Mg 2+ , which trigger icing at −4 °C. 6 According to your interesting results, one might anticipate that the hydrated NH 4…”

Microstructure Enhances the Local Electric Field and Promotes Water Freezing

“…Last but not the least, we also compare the studies performed in recent years on promoting water freezing with an electric field to guide follow-up research or industrial design. Pioneering research conducted by Lahav et al 51 demonstrates that electrofreezing on the polar crystal is a chemical process affected by the electric field and specific ions. Another significant research by Peleg et al 52 shows that electrofreezing is also influenced by the surface charge and electric field.…”