[Paper Review] In-plane dielectric constant and conductivity of confined water

Bing‐Zhong Wang, M. Souilamas+11 more|arXiv (Cornell University)|Jul 31, 2024

Electrostatics and Colloid Interactions6 citations

TL;DR

The paper uses scanning dielectric microscopy to measure in-plane dielectric constant and proton conductivity of water confined between atomically flat surfaces down to 1 nm, revealing giant dielectric constants and high conductivity in few-molecule-thick water due to disordered hydrogen bonding.

ABSTRACT

Water is essential for almost every aspect of life on our planet and, unsurprisingly, its properties have been studied in great detail. However, disproportionately little remains known about the electrical properties of interfacial and strongly confined water where its structure deviates from that of bulk water, becoming distinctly layered. The structural change is expected to affect water's conductivity and particularly its polarizability, which in turn modifies intermolecular forces that play a crucial role in many physical and chemical processes. Here we use scanning dielectric microscopy to probe the in-plane electrical properties of water confined between atomically flat surfaces separated by distances down to 1 nm. For confinement exceeding a few nm, water exhibits an in-plane dielectric constant close to that of bulk water and its proton conductivity is notably enhanced, gradually increasing with decreasing water thickness. This trend abruptly changes when the confined water becomes only a few molecules thick. Its in-plane dielectric constant reaches giant, ferroelectric-like values of about 1,000 whereas the conductivity peaks at a few S/m, close to values characteristic of superionic liquids. We attribute the enhancement to strongly disordered hydrogen bonding induced by the few-layer confinement, which facilitates both easier in-plane polarization of molecular dipoles and faster proton exchange. This insight into the electrical properties of nanoconfined water is important for understanding many phenomena that occur at aqueous interfaces and in nanoscale pores.

Motivation & Objective

• Investigate how strong confinement alters water's interfacial electrical properties compared to bulk water.
• Characterize the in-plane dielectric constant and proton conductivity of water confined between atomically flat surfaces at sub-nanometer to several nanometer thicknesses.
• Identify structural reasons for observed electrical property changes under extreme confinement.

Proposed method

• Employ scanning dielectric microscopy to probe in-plane electrical properties of confined water.
• Systematically vary confinement thickness down to 1 nm between atomically flat surfaces.
• Measure in-plane dielectric constant and proton conductivity as a function of water thickness.
• Analyze the role of hydrogen-bonding organization in confined water on polarization and proton exchange.

Experimental results

Research questions

• RQ1 How does confinement thickness affect the in-plane dielectric constant of water compared to bulk values?
• RQ2 How does proton conductivity evolve with decreasing water thickness under nanoscale confinement?
• RQ3 What microscopic mechanisms (e.g., hydrogen-bonding disorder) drive observed changes in polarization and ionic transport?
• RQ4 Is there a thickness threshold where properties shift from bulk-like to giant, ferroelectric-like behavior?

Key findings

• For confinement exceeding a few nanometers, in-plane dielectric constant is close to bulk water value and proton conductivity is enhanced.
• As thickness decreases, conductivity and dielectric response gradually increase, with dramatic changes when water is only a few molecular layers thick.
• In the few-molecule-thick regime, the in-plane dielectric constant reaches ~1000 and conductivity peaks at a few S/m.
• The enhancement is attributed to strongly disordered hydrogen bonding that facilitates dipole polarization and faster proton exchange.
• These findings illuminate electrical properties of nanoconfined water at aqueous interfaces and in nanoscale pores.