We might think we know how water behaves, but we’ve been overlooking what happens when it gets into spaces so small it’s just a single molecule thick. The outcomes turn out to be very different from water in bulk.
Water’s three familiar phases of matter – solid, liquid, and gas – can lead to icebergs when they meet. Like the metaphorical counterpart, however, there is much more going on where we can’t see, including numerous phases of matter that only exist under extreme conditions. More of these are discovered frequently as we learn how to manipulate and measure matter in new ways.
The latest examples occur when water is trapped under conditions where it forms a sheet a single molecule thick, at which point several phases can occur in sequence. Unlike some other phases of matter, not all of these need immense amounts of energy or pressure to make. Instead, the challenge lay in observing their behavior and understanding the structures, which has now been addressed in a paper published in Nature.
Using computational modeling of water within a tiny graphene channel, Dr Venkat Kapil of the University of Cambridge and colleagues found at low pressure, the water forms a phase matching bulk ice – other than having a melting point approximately 100° C lower than the three-dimensional version – with molecules arranged in a hexagonal fashion.
As pressure or temperature increases, the molecules rearrange themselves into first pentagonal and then rhombic forms. At pressures the authors call “intermediate” (about 8,000 atmospheres) the water enters what the authors call the “hexatic” phase. Here water behaves in a way somewhere between solid and liquid, with fixed but rotating molecules, until temperatures rise above about 70°C (158°F).
When pressures increase further, the water becomes superionic. It more closely resembles ice than water, but is highly electrically conductive. However, the current is carried not by protons, not electrons. Bulk water can have a superionic phase as well, but you need vastly higher pressures to achieve it.
Other phases of matter, such as those that occur at very cold temperatures or under extreme pressures, are sometimes of more curiosity value than practical application, but that’s not necessarily the case here. Tiny cavities exist in all sorts of porous materials, and single-molecule thick layers can occur between membranes whether we need them or not.
The authors argue our own bodies contain some of these phases when spaces inside are too small for bulk water, and its response could affect the effectiveness of medical treatments. Likewise, batteries and water desalination projects have included water in these phases for a long time, we just haven’t known how it influences production.
"For all of these areas, understanding the behavior of water is the foundational question," Kapil said in a statement. "Our approach allows the study of a single layer of water in a graphene-like channel with unprecedented predictive accuracy."
The hexatic phase behavior the authors report largely matches previous predictions, but the superionic takes us further into unknown territory.
"The existence of the superionic phase at easily accessible conditions is peculiar, as this phase is generally found in extreme conditions like the core of Uranus and Neptune. One way to visualize this phase is that the oxygen atoms form a solid lattice, and protons flow like a liquid through the lattice, like kids running through a maze,” Kapil said.
The authors hope to harness the exceptional conductivity of the superionic phase to improve battery design.