Water at the nanoscale

One reason the nanoscale is home to some interesting physics and chemistry is that the nanometer is a typical scale for molecules.   When the size of your system becomes comparable to the molecular scale, you can reasonably expect something to happen, in the sense that it should no longer be possible to ignore the fact that your system is actually built out of molecules.

Consider water as an example.  Water molecules have finite size (on the order of 0.2 nm between the hydrogens), a definite angled shape, and have a bit of an electric dipole moment (the oxygen has a slight excess of electron density and the hydrogens have a slight deficit).  In the liquid state, the water molecules are basically jostling around and have a typical intermolecular distance comparable to the size of the molecule.  If you confine water down to a nanoscale volume, you know at some point the finite size and interactions (steric and otherwise) between the water molecules have to matter.  For example, squeeze water down to a few molecular layers between solid boundaries, and it starts to act more like an elastic solid than a viscous fluid.  

Another consequence of this confinement in water can be seen in measurements of its dielectric properties - how charge inside rearranges itself in response to an external electric field.  In bulk liquid water, there are two components to the dielectric response.  The electronic clouds in the individual molecules can polarize a bit, and the molecules themselves (with their electric dipole moments) can reorient.  This latter contribution ends up being very important for dc electric fields, and as a result the dc relative dielectric permittivity of water, \(\kappa\), is about 80 (compared with 1 for the vacuum, and around 3.9 for SiO₂).   At the nanoscale, however, the motion of the water molecules should be hindered, especially near a surface.  That should depress \(\kappa\) for nanoconfined water.

In a preprint on the arxiv this week, that is exactly what is found.  Using a clever design, water is confined in nanoscale channels defined by a graphite floor, hexagonal boron nitride (hBN) walls, and a hBN roof.  A conductive atomic force microscope tip is used as a top electrode, the graphite is used as a bottom electrode, and the investigators are able to see results consistent with \(\kappa\) falling to roughly 2.1 for layers about 0.6-0.9 nm thick adjacent to the channel floor and ceiling.  The result is neat, and it should provide a very interesting test case for attempts to model these confinement effects computationally.