Resist the surface field: the H-bond network decides if water aligns at metal electrodes

TL;DR

This paper presents a new physical model explaining water molecule orientation at metal electrodes, highlighting the competing effects of surface templating and electric surface fields, supported by both theoretical and experimental evidence.

Contribution

It introduces a model that balances hydrogen-bond network effects and surface field forces to explain water alignment at electrodes, reconciling traditional and recent experimental findings.

Findings

Water alignment is opposed by surface templating effects.

Surface hydrophilicity influences water orientation.

The model explains discrepancies in water alignment observations.

Abstract

At an electrode, water molecules align to the surface field upon voltage application. This initiates important electrochemical reactions, e.g., hydrogen and oxygen evolution reactions. Recently developed non-linear optical techniques challenge the traditional picture by quantifying a lack of water alignment at metal electrodes. We here provide theoretical and experimental evidences for the existence of a driving force that opposes water alignment to the surface field. Such driving force originates from the ordering templated by the metal surface on the physisorbed water layer, and scales with surface hydrophilicity. We hence propose a physical model for water alignment at electrodes based on a balance of H-bond network and surface field driving forces, which reconciles the traditional picture with the new experimental observations.