Elucidating contact electrification mechanism of water

TL;DR

This study investigates the charge mechanisms of water surfaces, revealing pH-dependent charge polarity and proposing a contact potential difference model based on interface ion adsorption, with implications for biological and electrochemical systems.

Contribution

It introduces a new understanding of water surface electrification through experiments, theory, and simulations focusing on interface ion adsorption and contact potential differences.

Findings

Water surface charge polarity depends on pH and capillary type.

A pH-dependent contact potential difference of up to 52 mV is observed.

The charge transfer mechanism explains the origin of electrical potentials in water surfaces.

Abstract

The open water surface is known to be charged. Yet, the magnitude of the charge and the physical mechanism of the charging remain unclear, causing heated debates across the scientific community. Here we directly measure the charge Q of microdrops ejected from hydrophilic and hydrophobic capillaries and show that the water surface can take both positive or negative charge values depending on pH and the capillary type. Our experiments, theory, and simulations provide evidence that a junction of two aqueous interfaces with a different ion adsorption energy (e.g., liquid-solid and liquid-air interfaces) develops a pH-dependent contact potential difference {\Delta}{\phi} up to 52 mV. The longitudinal charge transfer between the interfaces stimulated by {\Delta}{\phi} determines the charge of the open water surface. The suggested static electrification mechanism provides far-reaching insights into the origin of electrical potentials in biological and electrochemical energy systems.