The Three-Phase Contact Potential Difference Modulates the Water Surface Charge

The surface charge of an open water surface is crucialfor solvationphenomena and interfacial processes in aqueous systems. However, themagnitude of the charge is controversial, and the physical mechanismof charging remains incompletely understood. Here we identify a previouslyoverlooked physical mechanism determining the surface charge of water.Using accurate charge measurements of water microdrops, we demonstratethat the water surface charge originates from the electrostatic effectsin the contact line vicinity of three phases, one of which is water.Our experiments, theory, and simulations provide evidence that a junctionof two aqueous interfaces (e.g., liquid-solid and liquid-air)develops a pH-dependent contact potential difference Delta phi due to the longitudinal charge redistribution between two contactinginterfaces. This universal static charging mechanism may have implicationsfor the origin of electrical potentials in biological, nanofluidic,and electrochemical systems and helps to predict and control the surfacecharge of water in various experimental environments.

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Aleksandra Radenovic, Andrei Kiselev, Vasily Artemov, Roman Doronin
2023
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https://infoscience.epfl.ch/record/303713?ln=en

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Liquid

A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a nearly constant volume independent of pressure. It is one of the four fundamental states of matter (the others being solid, gas, and plasma), and is the only state with a definite volume but no fixed shape. The density of a liquid is usually close to that of a solid, and much higher than that of a gas. Therefore, liquid and solid are both termed condensed matter.

Drop (liquid)

A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant drop. Drops may also be formed by the condensation of a vapor or by atomization of a larger mass of solid. Water vapor will condense into droplets depending on the temperature. The temperature at which droplets form is called the dew point. Liquid forms drops because it exhibits surface tension.

Surface tension

Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) to float on a water surface without becoming even partly submerged. At liquid–air interfaces, surface tension results from the greater attraction of liquid molecules to each other (due to cohesion) than to the molecules in the air (due to adhesion). There are two primary mechanisms in play.