Water is perhaps one of the most well studied substances due, in part, to its pervasiveness in the environment, the essential role it plays in biological systems and its remarkable physical properties. More solid forms of water have been documented than any other material yet, despite the best efforts of numerous researchers, the dipolar nature of the water molecule has not yet been observed unambiguously to induce ordering – creating a polar ice – when originating from a proton-disordered solid.
One approach to achieving a ferroelectric response in solid water is to place it in a confined geometry where spatial limitations mean the molecules are restricted to have a correlated, polar arrangement. The use of carbon and inorganic nanotubes has near-accomplished this where ferroelectric ordering has indeed been observed but only on cooling from a liquid state. Clearly, if ice is to be exploited in switchable devices, solid-solid transformations will be far more desirable.
We have investigated recently by single-crystal X-ray diffraction, solid 1D ice chains confined by columnar stacks of disc-shaped 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) molecules, arranged in a pseudo-square lattice, where hydrophilic hydroxyl groups on the edge of the HHTP molecules act as chemical anchors for the ice chains. At low temperatures the HHTP molecules mediate weak interactions (ca. 0.9 kJmol-1) between the chains, ultimately leading to the generation of an overall dipole moment, which solely originates from alignment of the water molecules. On raising the temperature above a modest 240 K, sufficient energy is supplied to disrupt the inter-chain communication, causing the system to adopt a paraelectric state. What remains now is to measure the strength of the ferroelectric response in the ordered, low-temperature phase.
The observation that very weak inter-chain coupling is the key to switchable behaviour in this system may open the door to the targeted design of commercially-feasible switchable devices that can finally make use of the abundant water molecule.