This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Using an advanced new mathematical model, a pair of physicists in Belgium has revealed the origins of an unusual, direction-dependent effect found in water confined to a narrow gap between two plates. Through research published in Physical Review Letters, Mehdi Neek-Amal and Francois Peeters at the University of Antwerp showed how two drastically different dielectric responses in this water emerge from the natural bonding behavior of its molecules.
Dielectric materials are made up of molecules with built-in electric dipoles: a positive charge at one end and a negative charge at the other. When exposed to an electric field, these dipoles rotate so that their positive ends point in the direction of the applied field and their negative ends in the opposite direction. This polarization creates an internal electric field within the material that partially cancels out the applied field.
In pure water, the dielectric effect emerges due to the tendency of oxygen atoms to pull electrons away from the hydrogen atoms they bond to covalently, creating a dipole across the chevron-shaped molecule. Under ordinary conditions, this creates hydrogen-bonding networks, where the positive hydrogen atoms of one molecule are attracted electrostatically to the negative oxygen atom at the tip of a neighboring chevron. Overall, this means that the molecular dipoles point in random directions—but will readily polarize when an electric field is applied.
However, a strange effect emerges when water is sandwiched between two flat plates just a few nanometers apart. "For decades, the dielectric properties of water confined to nanometer-scale spaces have challenged conventional electrostatics," Neek-Amal explains. "In our previous work, some effects of interfacial water were explained by capacitor-based models." These models treated confined water as stacked layers of capacitors with differing dielectric constants.
Strangely, however, confined water's dielectric response strongly depends on the direction of the applied field. When the field is applied parallel to the plane of the confined water, the response is over 10 times stronger than in ordinary water, but the response almost disappears when the field is applied perpendicular to the plane. Despite their successes, earlier models couldn't account for the correlations between dipoles induced by the hydrogen-bond network, through which groups of molecules can reorient themselves collectively in the same direction.
These correlations are crucial to understanding fluctuations in water's polarization: the constant, random jostling of its molecular dipoles, which determines how readily the dipoles reorient in response to an external field. As a result, they are crucial to understanding the strength of water's dielectric response. In their latest study, Neek-Amal and Peeters aimed to capture this behavior with an advanced mathematical approach.
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