New findings reveal that organosulfate groups, not carboxyls, control graphene oxide’s surface charge in water, challenging long-standing models and reshaping its chemical profile.
(Nanowerk Spotlight) In laboratories and cleanrooms around the world, graphene oxide is used as a starting point for designing ultrathin membranes, flexible electronics, and materials that can sift ions, store energy, or guide molecules through nanoscale channels. Its appeal lies not in perfection, but in complexity. Made by oxidizing graphite with strong acids and oxidants, graphene oxide becomes a tangled sheet of carbon atoms draped with oxygen-rich chemical groups. These modifications make it messy but useful, more chemically reactive, more water-compatible, and much easier to process than pristine graphene.
But this chemical disorder comes at a cost. Scientists do not fully understand what happens when graphene oxide touches water. The moment it enters a solution, it acquires charge, forms stable suspensions, and begins to behave like a highly acidic material. This behavior determines how it self-assembles, binds ions, and interfaces with biological or environmental systems. Yet the source of this charge, the precise molecular features responsible for it, remains a point of confusion.
For years, the explanation seemed simple. Graphene oxide was thought to behave like a weak acid, with carboxyl groups on its edges losing protons in water. These negatively charged sites, in turn, were believed to drive the electrostatic repulsion that keeps graphene oxide sheets suspended and separated. But this model has never fully aligned with experimental data. It fails to explain why GO dispersions can remain stable at very low pH, why they behave like strong acids in titration experiments, or why GO made with sulfuric acid exfoliates easily while sulfur-free variants do not.
There is a deeper chemical mystery hiding in plain sight. During the most widely used synthesis method, the Hummers method, graphite is treated with potassium permanganate in concentrated sulfuric acid. This introduces not just oxygen, but sulfur into the structure. For decades, researchers assumed that any residual sulfuric acid or sulfate was a contaminant that could be removed by washing. But what if sulfur was not just an impurity? What if it was structurally embedded in the form of organosulfate groups, sulfate units covalently bonded to carbon atoms, and what if those groups were central to how GO interacts with water?
This question lies at the heart of a study published in Small Structures (“Redefining Graphene Oxide: The Role of Organosulfate Groups in Charging Dynamics and Colloidal Stability”). Through a detailed experimental and theoretical investigation, the team shows that these organosulfate groups, rather than carboxylic acids, are the dominant source of charge in graphene oxide suspensions. Their findings challenge the standard textbook model and provide a molecular explanation for several unresolved anomalies in GO behavior, including spontaneous exfoliation and persistently high acidity.
How sulfur groups make graphene oxide sheets fall apart in water. A) Graphene oxide with more sulfur shows brighter signals, but sulfur is lost when the material is heated or washed for too long. B) The overall carbon–oxygen balance stays about the same, showing that only the sulfur groups are removed. C) A diagram shows that when enough sulfur groups are present, they give the sheets a strong negative charge, forcing them apart and allowing water to slip in between. D) Under polarized light, sheets soaked in acid begin to separate at a certain concentration, confirming that sulfur groups, not oxygen groups, are responsible for the exfoliation process. (Image: Reprinted from DOI:10.1002/sstr.202500035, CC BY) (click on image to enlarge)
To test their hypothesis, the researchers prepared highly oxidized graphene oxide using a refined version of the Hummers method. They then tracked the sulfur content, acidity, and surface charge of GO under different conditions. Elemental analysis showed sulfur content as high as 8.4 percent by weight in freshly prepared samples, indicating a substantial presence of organosulfate groups. When these samples were washed or dried, the sulfur content dropped significantly, confirming that the groups were covalently bonded but could hydrolyze in water and release sulfuric acid.
Potentiometric titration showed that even dilute GO dispersions, containing only 0.3 percent GO by weight, reached a pH of 3.05. The concentration of free protons scaled linearly with the amount of GO present. This behavior resembles that of a strong acid and cannot be explained by carboxyl or phenol groups, whose acidity is much weaker and does not produce such a consistent proton release across concentrations. Moreover, when GO was chemically treated to remove sulfur-containing groups, its surface charge and acidity dropped sharply, even though the amount of carboxylic acid groups remained nearly unchanged.
Osmotic pressure measurements provided additional evidence. The researchers placed GO dispersions in contact with pure water across a membrane and found that osmotic pressure increased linearly with GO concentration. This pattern indicates that the charged sites are permanently attached to the GO sheets and are not due to free ions or impurities. If the protons had come from residual acid or loosely bound ions, the pressure would not have increased in this way.
To model the results, the team used a diffuse layer theory that describes how ions arrange themselves near charged surfaces. They included the measured surface density of sulfate groups and used a single variable, the pKa of the organosulfate group, set at 1.2. The model closely matched the measured pH, osmotic pressure, and surface charge values. When they used a model based only on carboxyl groups, even with generous assumptions about density and acidity, it failed to reproduce the data.
Zeta potential measurements, which quantify the electric potential at the slipping plane of moving particles, reinforced these findings. The team compiled data from over thirty studies and found that GO maintains a negative zeta potential of approximately minus 25 millivolts even at pH 2. This suggests that surface charges remain even in acidic environments where carboxyl groups would typically be neutral. Their model predicted this behavior when sulfate groups were included but not when they were excluded. It also explained the lack of a point of zero charge, a feature that contradicts models based on only oxygen-containing groups.
The study also clarified why GO made with sulfuric acid exfoliates in water, while GO prepared using sulfur-free methods does not. The researchers calculated that spontaneous delamination requires a minimum surface charge density of about 26 millicoulombs per square meter. Fresh GO samples, with sulfur content near 8.4 percent, exceeded this threshold. As the material is washed or aged in water, the sulfate groups hydrolyze and the charge density falls below the level needed to maintain exfoliation. GO from Brodie’s method, which introduces no sulfur, remains below this threshold unless pH is raised to encourage deprotonation of weaker acidic groups.
Spectroscopic measurements confirmed the presence of sulfate groups in freshly prepared GO. Vibrational bands in the infrared spectrum matched known signatures of sulfur-oxygen bonds. These features diminished when GO was dried or subjected to warm water washing, indicating hydrolysis and loss of organosulfate content. When dried GO was redispersed in water, the supernatant contained a sharply increased concentration of dissolved sulfur, consistent with hydrolyzed sulfuric acid from formerly bonded organosulfate groups.
This work calls for a reevaluation of how GO is modeled and processed. In applications that depend on colloidal stability, ion transport, or surface reactivity, the presence of organosulfate groups is not a minor detail. It defines the charge landscape of the material and directly affects its behavior in solution. Synthetic protocols that involve sulfuric acid, as many do, inherently introduce these groups. Controlling their retention or removal becomes essential, depending on whether high surface charge or neutral interfaces are desired.
The study provides a clear and consistent framework for understanding the charging behavior of graphene oxide. Their data-driven model aligns with long-standing anomalies in GO chemistry and reveals that much of the material’s behavior in water depends not on traditional acidic groups but on covalently bonded sulfates with strong acidity and high surface density. For scientists and engineers working with GO, this insight offers a more precise basis for designing, tuning, and applying this widely used material.
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ORCID information
Seyed Hamed Aboutalebi (Institute for Research in Fundamental Sciences)
, 0000-0002-3711-332X corresponding author
Mohsen Moazzami Gudarzi (University of Manchester)
, 0000-0001-7134-6082 first author
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