Water is the foundational element of life. Its high melting and boiling temperatures relative to analogs such as hydrogen sulfide are a precondition to life. The density maximum at 4°C makes ice float, ensuring that lakes and oceans do not freeze on the bottom. The high surface tension of water enables insects to walk on its surface. Its high dielectric constant makes it an excellent solvent, a key property for many geochemical, biological, and technological processes. These properties can be traced to the strong hydrogen bonding in liquid water (1–3). On page 1339 of this issue, Fumagalli et al. (4) report on another property of liquid water that relates to its hydrogen bonding: the dielectric constant in thin water layers.
The behavior of water near interfaces and in confined systems is even more complex than the properties of bulk water. Both biological and energy applications are affected by the structure and energetics of electrochemical double layers formed by adsorbed and solvated ions. But although the importance of dielectric phenomena in water on these length scales is well recognized, experimental data have remained elusive. Macroscopic electrochemical data provide only the gross characteristics of interface capacitance that cannot be partitioned into components. Measurements in nanoscale devices that allow fundamental studies of ferroelectric materials cannot be extended to liquid layers.
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