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Water is one of the most studied substances on Earth, owing to its unique properties and ubiquity, yet it never ceases to surprise scientists by defying expectations. Writing in Nature, Wang et al. report that the electrical properties of water undergo a remarkable transformation when it is confined between atomically flat crystals in spaces only a few nanometres wide. The findings open up new frontiers in understanding how water behaves at its interfaces with other materials, with wide-reaching implications for many fields.
In its bulk form, liquid water allows only a small flow of electric current but effectively screens out the electrical forces between ions or molecules (the screening effect is quantified by the dielectric constant, the value of which is about 80 for bulk water at room temperature). These electrical properties are closely associated with the dense network of hydrogen bonds in bulk water and underpin the liquid's exceptional ability to dissolve salts, stabilize biomolecules and enable the complex chemistry required for living systems to function. Yet much of the water in nature does not exist as a bulk fluid. In biological cells, soils, membranes and porous rocks, water can be confined to nanoscale spaces, where its molecular arrangement deviates from that observed in its bulk form.
Liquid water has a high polarizability, which means that it is easy to align the electric dipoles of its molecules in an electric field. However, an earlier study showed that, under nanoscale confinement, thin layers of water molecules between surfaces lose their ability to respond to electric fields and to orient their dipoles perpendicular to those surfaces. The water molecules in these interfacial layers therefore exhibit reduced perpendicular polarization and rotational freedom compared with those in the bulk liquid, and the layer becomes nearly 'electrically inactive', with a dielectric constant of about 2 perpendicular to the surface.
By contrast, what happens in the in-plane direction (parallel to the confining surfaces) has remained largely unknown, because of the difficulty of measuring local dielectric properties in restricted spaces. Wang et al. overcame this problem by creating slit-like channels a few nanometres thick and hundreds of nanometres wide, between layers of hexagonal boron nitride stacked on graphite (Fig. 1). The authors filled the channels with ultrapure water and used the extremely sharp metal tip of an atomic force microscope to probe the liquid's behaviour. By detecting the confined water's response to many frequencies of an alternating current in the tip, the team could disentangle its dielectric constant and in-plane conductivity.
The authors observed that, in moderate confinement (channel thicknesses greater than about 3 nanometres), water behaves similarly to its bulk counterpart, with only a small increase in conductivity owing to the influence of electric charges on the channel surfaces. But at thicknesses approaching just 1-2 nanometres, equivalent to only four or five layers of water molecules, the liquid undergoes a dramatic transformation: its in-plane dielectric constant shoots up to values above 1,000. This giant polarizability is reminiscent of the dielectric constants of technologically useful materials called ferroelectric crystals, which exhibit a spontaneous electrical polarization that can be reversed by the application of an electric field. Moreover, the in-plane conductivity of the confined water rises by several orders of magnitude, peaking at about 3 siemens per metre before decreasing again at the smallest thicknesses.
How can a liquid become so polarizable under such constraints? The authors suggest that extreme confinement disrupts the network of hydrogen bonds that usually forms between the molecules of bulk water. The resulting disordered arrangement allows the molecular dipoles to reorient more collectively in-plane than they do in unconfined systems, leading to giant polarizability. The same disruption enhances the transport of protons (hydrogen ions, H) between water molecules, producing conductivities approaching those of superionic crystals -- which are some of the most conductive materials that use ions for charge transport.
In effect, confining water to two dimensions creates a state that is neither bulk liquid nor interfacial layer, but something altogether new: quasi-2D ferroelectric-like water. Taken together with the observation this year of a surface ferroelectric transition in ice at low temperatures, Wang and colleagues' results reveal that water, in both its liquid and solid phases, can display electrical order under extreme constraints -- behaviour that was long thought impossible for this substance.
The consequences of these findings reach well beyond fundamental physics. In biology, many essential processes involve water transport through nanoscale channels (aquaporins) in cell membranes, and nanoscale water layers also form around proteins and lipid membranes. Wang and colleagues' study provides important insights into how proton transport is intrinsically modified by water confinement in such environments. Understanding how confinement alters dielectric screening and proton transport could shed new light on ion-channel function, bioenergetics (how energy flows through living systems) and even on how enzymes use water's ability to rapidly rearrange its hydrogen-bond network to stabilize reactions.
The team's findings could also have implications for atmospheric science. In the upper atmosphere, water freezes on tiny aerosol particles, forming ice particles -- a process known as ice nucleation. These ice particles affect the properties of clouds and, if the particles grow in size, can initiate precipitation. Ice nucleation involves the freezing of water in the nanoscale pores of aerosol particles, and such nanoscale confinement is known to substantially modify the conditions under which freezing occurs. The extraordinary dielectric and conductive behaviour of water under confinement must now be considered in models of ice nucleation and cloud formation, because the way in which water screens charges and transports protons in pores might strongly influence how the first crucial ice clusters form on surfaces or around atmospheric nanoparticles -- freezing processes that directly affect the climate.
Technologically, the results could open up ways to improve the designs of membranes that control ion and water flow in desalination, fuel cells and batteries. The discovery that water can exhibit a colossal dielectric constant and mimic superionic conductors under confinement suggests that confined liquids could be used as active components in nanoscale devices.
Although the results are striking, several questions remain. The molecular mechanisms underlying the collective dipole response under confinement are not fully understood. The observed conductivities approach those of engineered proton-conducting polymers such as Nafion, yet it is unclear whether the confinement effect occurs with different confining materials and surface chemistries -- would water molecules confined in tiny channels made of oxide crystals or 2D materials, such as graphene, behave in the same way? What is the role of the hydrophobicity of the channel surface? The reproducibility of such extreme electrical states in less well-controlled, real-world environments also remains to be tested.
Future work could explore how charges, hydrophobic properties or chemical groups on surfaces tune water's behaviour, and whether similar effects occur in other polar liquids (those consisting of molecules that have a non-symmetrical distribution of electric charge). High-resolution simulations and complementary experimental probes will be crucial for building a theory of the microscopic origins of the observed phenomenon.
Water has long been described as anomalous, a liquid with properties that resist simple categorization. Wang and colleagues now add yet another anomaly: in quasi-2D confinement, the behaviour of bulk water transforms to reveal hidden electrical attributes. Far from being a passive solvent, water under extreme conditions becomes an electrically active liquid, reshaping how we must think about processes as diverse as protein folding, cloud formation and electrochemical energy storage.