Aromatic hydrocarbons suppress diffusion in carbon nanotubes during gasification

Mar 19, 2026

Aromatic molecules adsorb onto carbon nanotube walls via pi-pi interactions, slowing supercritical water flow and reducing gasification efficiency, a new simulation study finds.

(Nanowerk News) A molecular dynamics study shows that aromatic hydrocarbons form strong π–π interactions with carbon nanotube walls, severely reducing the diffusion of both organic solutes and supercritical water inside nanoscale pores. The research, published in Sustainable Carbon Materials (“Molecular dynamics study on the transport and structural behaviors of supercritical water–organic mixtures under nanoscale confinement”) by Hui Jin’s team at Xi’an Jiaotong University, provides molecular-level guidance for improving supercritical water gasification and related energy conversion technologies.

Key Findings

Supercritical water forms when water is heated and pressurized beyond its critical point of 647.1 K and 22.1 MPa. Under these extreme conditions, its physical properties shift. Density, viscosity, and dielectric constant all drop sharply, while the hydrogen-bond network between water molecules weakens or breaks apart. The resulting fluid behaves less like ordinary water and more like an organic solvent, capable of dissolving a broad spectrum of organic compounds and gases. These properties make supercritical water an attractive medium for converting biomass, waste plastics, and fossil feedstocks into fuels and useful chemicals through processes such as supercritical water gasification, waste plastic recycling, and in situ resource extraction. In practice, however, supercritical water and organic reaction intermediates coexist within nanoporous structures, where tight confinement alters how molecules move and react. Understanding how different molecular structures affect transport under these confined conditions is essential for optimizing gasification performance. The researchers validated their computational model by calculating the self-diffusion coefficients of bulk water, benzene, and methane and comparing the results against experimental data. The simulated values deviated by less than 7% from measured values, confirming that the model reliably reproduces how both water and organic molecules move. With this validated framework, the team systematically examined how organic solute type, pore size, solute concentration, and temperature influence mass transport inside carbon nanotubes under typical supercritical water gasification conditions. Wider nanotubes enhanced diffusion for both solutes and the surrounding supercritical water, reflecting weaker wall restrictions in larger pores. Aromatics, however, consistently diffused far more slowly than alkanes, and this suppression grew more pronounced as the number of aromatic rings increased. Anthracene, a three-ring aromatic, cut solute diffusion by more than 80% and reduced supercritical water diffusion by up to 50% compared to methane systems. Energy analysis revealed that interactions between the carbon nanotube wall and the solute molecules dominate the system’s energy balance, accounting for 60–80% of total interaction energy. In aromatic systems, these interactions are an order of magnitude stronger than in alkane systems, driven by π–π interactions between the conjugated ring structures of the aromatics and the carbon surface. Radial density profiles confirmed this picture, showing dense adsorption layers of aromatic molecules at approximately 3.3–3.5 angstroms from the nanotube wall, matching the characteristic spacing of π–π stacking and explaining why aromatics become effectively trapped near the surface. Increasing the solute concentration from 1% to 30% further slowed diffusion. Higher concentrations disrupted the supercritical water hydrogen-bond network and shifted the system’s energy balance toward solute-dominated interactions. This effect was especially pronounced for multi-ring aromatics, which formed clustered structures near the tube wall that further impeded molecular movement. Temperature offered a partial countermeasure. Raising the system temperature from 673 to 973 K boosted diffusivity by weakening the adsorption forces that pin aromatic molecules to the nanotube surface. At higher temperatures, more aromatic molecules detached from the wall, with near-wall aromatic fractions dropping by 16–22%. The added thermal energy disrupted π–π stacking interactions enough to free some molecules and restore a degree of flow. The results show that molecular structure, pore diameter, concentration, and temperature all interact to regulate mass transport in nanoconfined supercritical water systems. Aromatic hydrocarbons adsorb strongly onto pore walls and cluster into π–π stacked assemblies that suppress diffusion, potentially reducing gasification efficiency or promoting coking, the buildup of solid carbon deposits. By mapping these adsorption and clustering mechanisms at the molecular level, the study provides a theoretical basis for tuning supercritical water gasification through temperature management and pore design to improve mass transfer and reaction performance.