Advanced #54

Water adsorption can cause negative electrical resistance

On certain surfaces, water adsorption leads to an unusual decrease in resistance.

Scientific Explanation

When water is adsorbed on the surface of certain materials — particularly metal oxides such as aluminum oxide or zinc oxide — the electrical resistance of the material can decrease rather than increase or remain unchanged as expected. This “negative resistance effect” from water adsorption is a surprising finding.

The mechanism is based on the formation of a proton-conducting layer. Water molecules adsorbed on the oxide surface partially dissociate into protons and hydroxide groups. The protons can then move along the adsorbed water layer via the Grotthuss mechanism (anomaly 48). When humidity is sufficient, a continuous water film forms that acts as a proton conductor, lowering the overall electrical resistance of the system.

Crucially, this effect cannot be attributed simply to the conductivity of liquid water — pure water has very low conductivity. Rather, the interaction between water and the oxide surface catalyzes dissociation and enables proton conductivity that far exceeds that of pure water. Surface chemistry plays a decisive role here.

Negative Resistance from Water Adsorption Schematic showing how water molecules adsorbed on certain surfaces create a proton-conducting layer that causes the electrical resistance to decrease (negative resistance effect) rather than increase as humidity rises. Water Adsorption — Negative Resistance Solid surface (e.g., oxide) Adsorbed water film Proton conduction along film Expected (normal) Humidity → Observed (anomalous) Humidity → R R H₂O molecules H⁺ hopping Resistance drops as water layer conducts protons
Negative resistance change from water adsorption: protons conduct along the water film on the surface.

Everyday Relevance

This effect is the basis of many humidity sensors. Ceramic moisture sensors use the change in electrical resistance upon water adsorption to measure relative humidity. They are found in weather stations, air conditioning systems, industrial processes, and smartphones. Understanding the mechanism allows the development of sensors with high sensitivity and fast response times. In semiconductor technology, uncontrolled water adsorption is also an important concern, as it can alter the electrical properties of devices in unwanted ways.