Phase Behavior in Confined Fluid Systems

Phase behaviour in confined fluid systems diverges markedly from bulk‐phase phenomena owing to the influence of pore geometry, surface interactions and nanoscale effects. When fluids occupy pores with dimensions approaching the molecular scale, capillary pressures arise from curvature of liquid–vapour interfaces and surface tension, leading to depression or elevation of vapour pressures relative to bulk conditions. Confinement alters critical properties, shifting critical temperature and pressure and modifying the shape and position of the phase envelope. Fluid–solid interactions, including adsorption layers and wettability at pore walls, further influence density distributions and component selectivity in multicomponent mixtures. These effects collectively govern phase transitions, miscibility thresholds, interfacial tension and transport characteristics in applications as diverse as shale gas extraction, carbon capture and storage, hydrogen storage, nano‐separation technologies and catalytic reactors. A fundamental understanding of these phenomena underpins predictive models that extend classical equations of state through coupling with capillary and adsorption terms, offering both qualitative insight and quantitative tools for design and optimisation in energy and environmental technologies.

Research from Nature Portfolio

Recent studies have refined classical thermodynamic models by integrating confinement‐induced shifts into established equations of state. One investigation extended the Kelvin equation and the Peng–Robinson equation of state to capture vapour‐liquid equilibria of hydrocarbons in capillary media, demonstrating that real rock samples exhibit vapour pressures on average 15 % lower than bulk predictions. This work emphasised the roles of pore size, surface tension and wettability in depressing equilibrium pressures. Another contribution developed an analytical equation of state for supercritical methane storage in nanoporous materials, showing that a single fitted parameter linking molecule–wall interaction strength to intermolecular interactions accurately reproduces storage isotherms over a broad pressure range. The resulting model enables direct correlation of macroscopic storage capacity with microscopic parameters such as pore geometry, wall chemistry and roughness.

Phase Behavior in Confined Fluid Systems publication trend

The graph below shows the total number of articles in phase behavior in confined fluid systems across all publications each year (not limited to Nature Index journals).

Technical terms

Capillary pressure: Pressure difference across a curved liquid–vapour interface arising from surface tension and pore curvature.

Equation of state: Mathematical relation linking pressure, temperature and volume (or density) of a fluid to predict phase equilibria.

Phase envelope: Boundary in pressure–temperature space defining regions of single‐phase and multiphase stability.

Adsorption: Accumulation of fluid molecules at a solid surface, forming layers that alter local density and phase transitions.

Critical temperature shift: Change in the temperature at which liquid and vapour phases become indistinguishable, due to confinement or surface interactions.

References

1. Methane storage in nanoporous material at supercritical temperature over a wide range of pressures. Scientific Reports (2016).
2. Combined Experimental, Theoretical, and Molecular Simulation Approach for the Description of the Fluid-Phase Behavior of Hydrocarbon Mixtures within Shale Rocks. Energy & Fuels (2018).
3. Revisiting Kelvin equation and Peng–Robinson equation of state for accurate modeling of hydrocarbon phase behavior in nano capillaries. Scientific Reports (2021).
4. Phase Behavior and Composition Distribution of Multiphase Hydrocarbon Binary Mixtures in Heterogeneous Nanopores: A Molecular Dynamics Simulation Study. Nanomaterials (2021).
5. Nanopore Confinement Effect on the Phase Behavior of CO2/Hydrocarbons in Tight Oil Reservoirs considering Capillary Pressure, Fluid‐Wall Interaction, and Molecule Adsorption. Geofluids (2021).
6. The Influence of Wettability Effect and Adsorption Thickness on Nanoconfined Methane Phase Behavior: Vapor-Liquid Co-Existence Curves and Phase Diagrams. Processes (2024).