How molecules move in extreme water environments depends on their shape

(Press-News.org) Water behaves in remarkable ways when heated and pressurized beyond its critical point. Under these extreme conditions, known as supercritical water, it no longer acts like an ordinary liquid. Instead, it takes on properties similar to organic solvents, dissolving hydrocarbons efficiently and transporting molecules rapidly. These unique features make supercritical water a promising green medium for energy conversion technologies such as biomass gasification, plastic recycling, and in situ fuel extraction.

Yet many of these reactions occur inside extremely small spaces, including nanopores in rocks, polymers, or catalytic materials. Until now, scientists have had limited insight into how supercritical water and organic molecules move together under such nanoscale confinement.

In a new study published in Science of Carbon Materials, researchers from Xi’an Jiaotong University used advanced molecular dynamics simulations to uncover how the molecular structure of organic compounds controls their transport behavior when mixed with supercritical water inside carbon nanotubes.

The team focused on two major classes of hydrocarbons that commonly appear in energy systems: alkanes, which are straight chain molecules such as methane and propane, and aromatics, which contain ring structures such as benzene, naphthalene, and anthracene. Carbon nanotubes were used as model nanochannels because their size and surface properties resemble many natural and engineered porous materials.

The simulations revealed a striking contrast between the two types of molecules. Alkanes were able to move relatively freely through the confined space, maintaining high diffusion rates alongside supercritical water. Aromatic molecules, however, significantly slowed down not only their own motion but also the movement of surrounding water molecules.

“Our results show that molecular shape really matters under nanoscale confinement,” said corresponding author Hui Jin. “Aromatic compounds interact strongly with carbon surfaces and with each other, which leads to aggregation near the nanotube wall and restricts overall mass transport.”

At the molecular level, the effect is driven by strong pi interactions between the flat aromatic rings and the carbon nanotube surface. These interactions cause aromatic molecules to stick to the wall and form clusters, creating a dense adsorption layer that limits mobility. The more rings an aromatic molecule has, the stronger this effect becomes.

The study also found that temperature plays a key regulatory role. As temperature increases, the attractive interactions between aromatic molecules and the nanotube wall weaken. This allows the molecules to detach from the surface and diffuse more readily through the channel.

“Raising the temperature can partially overcome the transport limitations imposed by aromatic compounds,” Jin explained. “This provides useful guidance for optimizing operating conditions in supercritical water gasification and related processes.”

By systematically analyzing the effects of pore size, temperature, solute concentration, and molecular structure, the researchers were able to link microscopic interactions with macroscopic transport behavior. The findings help explain why some organic intermediates linger in confined reactors while others are removed efficiently.

Beyond energy applications, the results are relevant to any system where fluids move through nanoscale spaces under extreme conditions, including catalysis, carbon based membranes, and subsurface geochemical processes.

“This work provides a molecular level picture of how structure and confinement work together to control transport,” said Jin. “Such insights are essential for designing more efficient nanoscale reactors and cleaner energy technologies.”

The study demonstrates how computer simulations can reveal hidden mechanisms that are difficult or impossible to observe experimentally, offering a powerful tool for guiding the development of next generation supercritical water systems.

 

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Journal reference: Rong X, Wu H, Zhang B, Zhang J, Zhang T, et al. 2026. Molecular dynamics study on the transport and structural behaviors of supercritical water–organic mixtures under nanoscale confinement. Sustainable Carbon Materials 2: e002 doi: 10.48130/scm-0025-0015   

https://www.maxapress.com/article/doi/10.48130/scm-0025-0015  

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Sustainable Carbon Materials (e-ISSN 3070-3557) is a multidisciplinary platform for communicating advances in fundamental and applied research on carbon-based materials. It is dedicated to serving as an innovative, efficient and professional platform for researchers in the field of carbon materials around the world to deliver findings from this rapidly expanding field of science. It is a peer-reviewed, open-access journal that publishes review, original research, invited review, rapid report, perspective, commentary and correspondence papers.

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