A recent study published in Advanced Materials Interfaces examines how water nanofilms affect heat transfer and adhesion at the interface between hematite (α-Fe2O3) and hydrocarbons.
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Background
When oxide surfaces like hematite are exposed to ambient humidity, they become fully hydroxylated. This leads to the formation of thin water layers—nanofilms—that vary in thickness depending on humidity levels. These nanofilms are key in determining how well hydrocarbon fluids wet or adhere to the surface, which is critical for applications such as lubrication and coating.
Water films also influence interfacial thermal resistance (ITR), a measure of how efficiently heat moves across the solid-liquid boundary. Previous studies have shown that surface hydroxylation and interfacial water layers can impact both adhesion and thermal transport. However, many simulations so far have focused on dry surfaces, which don’t capture the full complexity of hydrated interfaces.
The Current Study
This work uses classical molecular dynamics (MD) simulations to investigate hematite-hydrocarbon interfaces under ambient-like humidity. The authors modeled a fully hydroxylated hematite surface, consistent with conditions below 60 % relative humidity. They introduced water nanofilms ranging from 0.5 to 4 monolayers (ML) in thickness, based on experimental observations of water uptake.
To explore how solid-liquid interaction strength influences results, the study used three types of interatomic potential models: ClayFF (weak), INTERFACE-FF (intermediate), and a strong interaction model. These models span a realistic range of interaction strengths.
Simulations were run using LAMMPS. Each setup included energy minimization, equilibration under constant pressure and temperature (300 K, 1 atm), and data collection under constant volume and constant pressure ensembles. Two main quantities were computed: the work of adhesion, via thermodynamic integration and free energy methods, and the interfacial thermal resistance, using non-equilibrium MD (NEMD) with applied temperature gradients.
The researchers varied surface hydroxylation and water coverage to examine how each affects adhesion and heat transfer.
Results and Discussion
The simulations showed that water layers between hematite and hydrocarbons significantly affect both adhesion and heat transfer. In models without water, the calculated work of adhesion was higher than what has been measured in experiments. When water nanofilms were added—especially those thicker than one monolayer—the simulated adhesion values matched experimental data more closely.
Hydroxylation of the hematite surface improved the accuracy of the adhesion estimates, even without added water. However, including water films led to further improvement, particularly when the coverage was greater than one monolayer.
The simulations also examined thermal resistance at the interface. When water films thicker than one monolayer were present, the ITR remained consistent at about 10 m2K/W, regardless of which interatomic potential model was used.
This suggests that, once a certain amount of water is present, the details of the hematite surface have a smaller effect on heat transfer. Instead, the properties of the water layer and its contact with the hydrocarbon become the main factors.
The thermal slip length—used to describe how heat flows across the interface—was estimated to be around 1.4 nanometers. This is consistent with other studies of similar systems.
The presence of surface hydroxyl groups increased the hydrophilicity of the hematite surface. This made it easier for water to adsorb and remain stable, leading to thicker water layers. These water layers reduced direct interactions between the hydrocarbon and the solid surface.
As a result, both adhesion and thermal conductivity were affected. The findings are consistent with experimental evidence showing that adsorbed water can reduce adhesion and increase thermal resistance at solid-liquid interfaces.
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Conclusion
This study shows that including surface hydration and water nanofilms in simulations of hematite-hydrocarbon interfaces leads to more accurate predictions of adhesion and thermal resistance. Surface hydroxylation promotes water film formation, which affects both wettability and heat transfer at the nanoscale. The results reflect conditions common in real environments, where humidity and hydrated surfaces are typical.
These findings are relevant for technologies involving oxide-hydrocarbon contact, such as lubrication, thermal interfaces, and surface coatings. Future research could extend this approach to other oxides and explore the effects of higher humidity, temperature changes, and evolving surface chemistry.
Accounting for surface-adsorbed water is essential for realistic modeling of solid-liquid interfaces and for improving material design in practical applications.
Journal Reference
Carman F., et al. (2025). Water Nanofilms Mediate Adhesion and Heat Transfer at Hematite–Hydrocarbon Interfaces. Advanced Materials Interfaces, e2500267. DOI: 10.1002/admi.202500267, https://advanced.onlinelibrary.wiley.com/doi/10.1002/admi.202500267