When trapped, water becomes less polar

Water is a versatile solvent that plays a significant role in many chemical systems, whether you’re studying a protein or a fuel cell. This versatility is rooted in water’s relatively high polarizability. Water molecules’ uneven charge distribution allows them to dissolve all sorts of charged and polar molecules. As a bulk solvent, water’s dielectric constant—a measure of its polarizability—is about 80. New experiments show that this value becomes dramatically smaller when water is trapped in small spaces (Science 2018, DOI:10.1126/science.aat4191).

The new measurement will improve modeling of molecular interactions in biology, energy systems, and elsewhere, writes Sergei V. Kalinin, a materials scientist at Oak Ridge National Laboratory, in a perspective accompanying the research paper.

When water is confined in small spaces or forms a film on a surface, the molecules near the solid surfaces arrange themselves into layers and resist the changes in orientation that normally lead to polarization. As a result, the solvent’s dielectric constant drops. But attempts to measure how much the constant decreases, either through experiments or through computer simulations, have yielded imprecise results because of the many variables in these systems.

A team led by Laura Fumagalli and Andre Geim of the University of Manchester used atomic force microscopy and a carefully controlled experimental setup to make new measurements on confined water. The group etched channels of varying heights into hexagonal boron nitride (hBN) crystal and filled them with water. They applied a voltage between a graphite backing plate and the tip of a scanning atomic force microscope, allowing them to calculate water’s dielectric constant.

In 1-nm-tall channels, water’s dielectric constant is just 2, smaller than previously predicted values of around 10. Essentially, water inside these nanochannels is electrically dead, with its dipole moments immobilized due to the molecules’ confinement, Fumagalli says.

Roland Netz of the Free University of Berlin, who has explored water’s dielectric constant using molecular dynamics simulations, says the study reports “a really exciting effect that is important whenever two surfaces, be they biological or solid, come really close to each other.”