How Voltage Tunes Glycine’s Acid–Base Chemistry at a Gold/Water Interface

Voltage can change how an acid or base behaves at an electrode because the molecular environment next to the metal is not the same as bulk water. In a 2024 study, glycine’s two acid-dissociation constants shifted at a gold/water interface, and changing the applied potential added an electric-field control over proton transfer. At fixed bulk pH, the interface therefore can favor different protonation states than a beaker measurement would predict.

The result is a measured example—not a claim that every electrochemical catalyst behaves identically. The work used glycine as a model molecule, gold as the electrode surface, electrochemical surface-enhanced Raman spectroscopy (SERS), and constant-potential molecular-dynamics simulations.

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What the gold/water experiment showed

Steffen Murke, Wanlin Chen, Simone Pezzotti, and Martina Havenith measured glycine at an electrified gold/water interface while varying the applied voltage. Their paper, Tuning Acid–Base Chemistry at an Electrified Gold/Water Interface, appeared online in the Journal of the American Chemical Society on April 10, 2024, and in volume 146, issue 18 (May 8, 2024), pages 12423–12430. The article is available through the American Chemical Society by DOI 10.1021/jacs.3c13633.

Glycine is a useful test molecule because it has two ionizable groups. Its carboxyl group (COOH) can lose a proton, while its ammonium group (NH3+) can also lose a proton to become neutral amine (NH2). In water away from a surface, those equilibria are commonly summarized by bulk pH and pKa values. The study demonstrates why those bulk numbers cannot simply be transferred to the first few molecular layers beside an electrified metal.

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Bulk and interfacial pKa values are different

The central comparison is glycine’s measured acid–base behavior in bulk solution versus at the gold/water interface. The pKa is the pH at which the relevant protonated and deprotonated forms are present in equal amounts for that equilibrium. A higher pKa for the carboxyl group means it is harder to deprotonate; a lower pKa for the ammonium group means it is easier to remove its proton.

Both interface values move toward neutral pH. Using the two pKas as boundaries, the broad bulk zwitterion-dominant interval spans roughly 2.4 to 9.8, or 7.4 pH units. At the interface, the corresponding interval is about 3.6 to 8.8, or 5.2 pH units. The difference is not a simple change in proton concentration: the two equilibria move in opposite numerical directions because the local environment changes the relative stability of the molecular states.

Why local solvation shifts glycine’s equilibria

Charged groups lose some of their normal hydration

In bulk water, charged chemical groups are stabilized by surrounding solvent molecules. Water’s polarity helps screen charge and lowers the free-energy cost of maintaining ionic forms. A molecule immediately beside a metal surface experiences a different arrangement of solvent, surface atoms, and electric field. The study’s interpretation is that the region near gold has a hydrophobic-like solvation character compared with fully hydrated bulk water.

That local environment partially desolvates glycine’s charged groups. The zwitterion—carrying both NH3+ and COO−—then loses some of the hydration stabilization it receives in solution. Destabilizing the zwitterion shifts the carboxyl equilibrium toward the protonated COOH form and the ammonium equilibrium toward the deprotonated NH2 form. Those tendencies correspond to the measured carboxyl pKa increase from 2.4 to 3.6 and ammonium pKa decrease from 9.8 to 8.8.

Why the two pKas move toward one another

It is tempting to treat an interface as a tiny volume with a different pH. That picture would predict a common offset in protonation behavior, not the opposing shifts observed here. The two glycine groups respond to how each protonation state interacts with local water, the metal, and the rest of the molecule. A change in solvation free energy can therefore raise one apparent pKa while lowering another.

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The reported values are specific to the measured system: glycine at gold in contact with water. They should not be used as universal replacements for glycine’s bulk pKas in ordinary solution calculations.

How applied voltage adds a second control

The metal electrode and the nearby water form an interfacial electric environment. When the applied potential changes, the electric field at the surface and through the first layers of water changes as well. Proton transfer changes the charge distribution of glycine, so that field can alter the relative stability of protonated and deprotonated forms.

The study reports that voltage-dependent fields can further drive protonation or deprotonation. The direction is not a universal “more positive means more protonated” rule that can be applied without specifying the electrode, reference convention, solvent structure, and molecular orientation. In a particular setup, the field couples to the local dipoles and charges and can favor one state or the other.

Simone Pezzotti, the lead study author, summarized the implication in an interview with Chemistry World: “If we fix the pH and we just play with the voltage, we can induce chemistry.” The important qualifier is “can”: the statement describes a mechanistic control principle for the interfacial system, not a demonstrated production process or a guaranteed response for every reaction.

How the researchers separated the effects

Electrochemical SERS observes the interface

Surface-enhanced Raman spectroscopy uses the electromagnetic enhancement near a rough or nanostructured metal surface to make molecular vibrations easier to detect. Under electrochemical conditions, SERS can follow spectral changes as the electrode potential and solution pH are varied. For this study, the spectra provided signatures of glycine protonation states at the gold/water boundary, enabling a spectroscopic titration rather than relying only on bulk solution measurements.

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The authors describe their approach in the paper’s abstract: “Here, we measure the pKa values of an amino acid, glycine, at a gold/water interface under a varying applied voltage by means of spectroscopic titration.” This is a direct measurement of a model molecule’s interfacial acid–base equilibria, not an inference from catalytic output.

Constant-potential molecular dynamics supplies molecular detail

Constant-potential molecular-dynamics simulations model the metal electrode while allowing its charge distribution to respond to the imposed potential. In combination with the SERS observations, the simulations were used to examine solvent organization, partial desolvation, and electric fields near the surface. This pairing supports the proposed mechanism: local solvation helps set the baseline interfacial pKas, while changing potential modifies the field contribution.

Neither technique alone would answer the full question. Spectra reveal potential-dependent molecular signatures but do not directly show every solvent configuration. Simulations provide atomistic interpretation but depend on the chosen molecular model and force-field description. Agreement between the two lines of evidence strengthens the explanation for this gold/water/glycine system.

What “fine-tuning” means in practice

For an interfacial reaction, there are at least two independent levers:

• Surface-controlled solvation: changing the metal or its surface chemistry can alter how strongly nearby water stabilizes charged groups.
• Potential-controlled electric fields: changing the applied voltage can alter the local field and shift proton-transfer equilibria while the bulk pH remains fixed.

These controls act on different physical factors. A surface modification changes the molecular environment in a relatively persistent way; voltage is an operational variable that can be adjusted during an experiment. In principle, a catalyst designer could use both, but the useful voltage range would still be bounded by competing processes such as water electrolysis, surface reconstruction, adsorption, and loss of selectivity. The glycine study did not map those engineering limits.

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Why bulk pH alone can mislead at an electrode

Bulk pH is measured or calculated from the solution away from the electrode. The interfacial region can differ because solvent molecules are oriented by the metal, ions are redistributed, and the electric potential changes over a short distance. A molecule adsorbed on, or held close to, the surface samples that local environment rather than the average solution.

Consequently, a glycine species expected from a bulk Henderson–Hasselbalch calculation may not be the dominant species at the surface. The practical workflow is to treat bulk pH as one boundary condition, then account for interfacial solvation and potential when interpreting spectra or reaction rates.

What the study does—and does not—demonstrate

Directly established

• Glycine’s interfacial pKas at gold/water were measured with electrochemical SERS and interpreted with constant-potential molecular dynamics.
• The carboxyl pKa was 3.6 at the interface versus 2.4 in bulk; the ammonium pKa was 8.8 versus 9.8 in bulk.
• The authors connect the low-voltage shifts to partial desolvation in a hydrophobic-like local environment and identify voltage-dependent electric fields as an additional influence.

Still prospective

The authors write that surface and voltage control offer “promising, but still unexplored, paths to regulate reactivity.” That wording matters. The paper proposes a framework that could be tested with other molecules, metals, and electrochemical reactions; it does not report a higher catalytic turnover, improved selectivity, industrial hydrogen production, or a better battery electrode.

Ian McCrum of Clarkson University, an external electrochemistry researcher quoted by Chemistry World, described the broader value this way: “I think that [the research] is going to move us in the direction of understanding how solvent and electric field work together to affect reactions occurring at surfaces.” The statement is about understanding and future reaction design, not a completed application.

How to reproduce the idea responsibly

A laboratory attempting a related experiment needs an electrochemical cell with a gold working electrode, controlled solution pH, a potentiostat, and a Raman system capable of collecting surface-enhanced spectra. The measurement must distinguish signals from molecules at the interface from signals in the bulk solution. Potential should be reported with its reference electrode and sign convention, because “increasing voltage” is otherwise ambiguous.

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Glycine reagent is the appropriate chemical description for replication. Dietary glycine supplements are not interchangeable with a characterized laboratory reagent: grade, impurities, water content, and suitability for spectroscopy can differ. The study does not validate any retail listing or supplement product.

For interpretation, record the pH, ionic composition, electrode preparation, potential range, equilibration time, laser conditions, and spectral assignments. A change in Raman intensity alone is not automatically a pKa shift; the protonation-state model and calibration procedure must support that conclusion.

Where this leaves electrochemical reaction design

The key lesson is a change in scale. Acid–base chemistry at an electrified interface is governed by the molecular neighborhood immediately next to the electrode, not only by the composition of the reservoir solution. In the measured glycine system, a gold surface narrowed the pH interval associated with the zwitterion, and applied potential provided another way to perturb proton transfer.

That insight gives researchers a concrete design question for future work: can a chosen surface create the desired solvation environment, and can a controlled potential stabilize the transition state or protonation state needed for a specific reaction? Answering it will require measurements beyond glycine and beyond equilibrium pKas, including reaction rates, selectivity, durability, and behavior on different metals. Those tests remain open.

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acid-base chemistryElectrochemistryglycinegold electrodesmolecular dynamicssurface-enhanced Raman spectroscopy