Journal Study Uncovers Entropy’s Role in Field-Driven Water Ionization

Researchers led by Dr. Yair Litman at the Max Planck Institute for Polymer Research, working in collaboration with Professor Angelos Michaelides at the Yusuf Hamied Department of Chemistry, University of Cambridge, have reported new insights into how water behaves under strong electric fields. Their study, published in the Journal of the American Chemical Society, shows that electric fields common in electrochemical devices fundamentally change the driving forces behind water autodissociation.

Litman, Y., & Michaelides, A. (2025). Entropy Governs the Structure and Reactivity of Water Dissociation Under Electric Fields. Journal of the American Chemical Society, 147(49), 44885–44894. https://doi.org/10.1021/jacs.5c12397

Water autodissociation refers to the spontaneous splitting of a water molecule into a proton and a hydroxide ion. Under everyday conditions, this process occurs only rarely. In neutral water at room temperature, the balance between intact molecules and ions is tightly constrained. Traditional thermodynamic descriptions explain this by noting that water splitting is both energetically unfavorable and entropically disfavored in bulk liquid conditions.

Dr. Yair Litman at University of Cambridge stated,

“These results point to a new paradigm. To understand and improve water-splitting devices, we need to consider not just energy, but entropy—and how electric fields reshape the molecular landscape of water.”

Energy and entropy are the two principal factors governing chemical reactions. A reaction proceeds spontaneously when it lowers the overall energy of a system or increases disorder, which is quantified as entropy. For water under standard conditions, neither factor strongly supports dissociation. The reaction is uphill in energy and does not provide a significant entropic benefit, which keeps ion concentrations low.

However, inside electrochemical systems such as electrolyzers, batteries and fuel cells, water molecules are exposed to intense electric fields near charged interfaces. These environments differ substantially from a simple glass of water. Despite the importance of these conditions for hydrogen production and related energy technologies, the microscopic mechanisms governing water behavior under strong fields have remained less well understood.

Using advanced molecular dynamics simulations, Litman and Michaelides examined how water responds to electric fields comparable to those found at electrode surfaces. Their computational models revealed that electric fields reorganize water molecules into more ordered arrangements. Dipoles align along the field direction, forming structured networks rather than the relatively dynamic hydrogen-bonded structure seen in bulk liquid water.

This increased order has important thermodynamic consequences. In a highly ordered state, the system’s entropy is reduced. When water molecules dissociate into ions under these conditions, the resulting disruption of the ordered network introduces greater disorder. In other words, dissociation becomes entropically favorable.

The researchers found that under sufficiently strong electric fields, the entropic contribution can outweigh the energetic cost of splitting water. This represents a reversal of the situation in bulk water, where entropy typically resists dissociation. Under bias, entropy instead drives the reaction forward.

The findings challenge the assumption that water splitting under electrochemical conditions is governed primarily by energetic considerations. In many models of electrolysis and interfacial chemistry, reaction rates are treated largely in terms of energy barriers. The new work indicates that entropy, reshaped by the electric field, plays a central role in determining reactivity.

The simulations also suggest measurable consequences. Under strong fields, the effective acidity of water can increase substantially. The calculated pH in confined regions near electrodes may drop from neutral values to levels comparable with acidic solutions. Such shifts have implications for catalyst stability, corrosion and reaction selectivity in electrochemical systems.

The study builds on a growing body of theoretical and experimental work examining interfacial water structure. Previous surface-sensitive spectroscopic measurements have shown that water near charged surfaces differs from bulk liquid in orientation and hydrogen bonding patterns. The present research extends that understanding by linking structural changes directly to thermodynamic driving forces.

From an engineering perspective, these insights are relevant to hydrogen production through electrolysis, a process expected to play a role in low-carbon energy systems. The efficiency of water splitting depends not only on electrode materials but also on how water molecules reorganize and react at interfaces. A better grasp of entropic effects under strong fields may inform the design of catalysts and electrode architectures.

The findings may also apply beyond classical electrolysis. Many electrochemical and so-called on-water reactions involve strong local electric fields that can alter solvent structure. Incorporating entropic contributions into computational models could improve predictions of reaction pathways and rates in these systems.

The researchers emphasize that their conclusions arise from detailed atomistic simulations rather than simplified thermodynamic assumptions. By explicitly modeling water molecules under electric bias, they were able to capture collective effects that would be difficult to infer from bulk measurements alone.

While further experimental validation at electrode interfaces will be important, the work suggests that water’s behavior under electric fields is more complex than previously thought. Instead of acting solely as a passive medium, water can adopt field-induced structures that reshape its own chemistry.

For scientists and engineers working on electrochemical technologies, the message is that energy landscapes are only part of the picture. Entropy, modulated by electric fields, can alter reaction tendencies in ways not captured by conventional models.

As efforts to scale hydrogen production and develop next-generation electrochemical systems continue, integrating these thermodynamic insights may lead to more accurate simulations and more efficient device designs. In systems where water is both solvent and reactant, understanding how electric fields reorganize molecular structure could prove essential for optimizing performance.