Electro-Freezing of Supercooled Water Is Induced by Hydrated Al³⁺ and Mg²⁺ Ions: Experimental and Theoretical Studies

This work reports that the octahedral hydrated Al³⁺ and Mg²⁺ ions operate within electrolytic cells as kosmotropic (long-range order-making) "ice makers" of supercooled water (SCW). 10^-5 M solutions of hydrated Al³⁺ and Mg²⁺ ions each trigger, near the cathode (-20 ± 5 V), electro-freezing of SCW at -4 °C. The hydrated Al³⁺ ions do so with 100% efficiency, whereas the Mg²⁺ ions induce icing with 40% efficiency. In contrast, hydrated Na⁺ ions, under the same experimental conditions, do not induce icing differently than pure water. As such, our study shows that the role played by Al³⁺ and Mg²⁺ ions in water electro-freezing is impacted by two synchronous effects: (1) a geometric effect due to the octahedral packing of the coordinated water molecules around the metallic ions, and (2) the degree of polarization which these two ions induce and thereby acidify the coordinated water molecules, which in turn imparts them with an ice-like structure. Long-duration molecular dynamics (MD) simulations of the Al³⁺ and Mg²⁺ indeed reveal the formation of "ice-like" hexagons in the vicinity of these ions. Furthermore, the MD shows that these hexagons and the electric fields of the coordinate water molecules give rise to ultimate icing. As such, the MD simulations provide a rational explanation for the order-making properties of these ions during electro-freezing.