Dispersion truncation affects the phase behavior of bulk and confined fluids: Coexistence, adsorption, and criticality

Mild-Temperature Supercritical Water Confined in Hydrophobic Metal-Organic Frameworks

Fluids under extreme confinement show characteristics significantly different from those of their bulk counterpart. This work focuses on water confined within the complex cavities of highly hydrophobic metal-organic frameworks (MOFs) at high pressures. A combination of high-pressure intrusion-extrusion experiments with molecular dynamic simulations and synchrotron data reveals that supercritical transition for MOF-confined water takes place at a much lower temperature than in bulk water, ∼250 K below the reference values. This large shifting of the critical temperature (Tc) is attributed to the very large density of confined water vapor in the peculiar geometry and chemistry of the cavities of Cu2tebpz (tebpz = 3,3',5,5'-tetraethyl-4,4'-bipyrazolate) hydrophobic MOF. This is the first time the shift of Tc is investigated for water confined within highly hydrophobic nanoporous materials, which explains why such a large reduction of the critical temperature was never reported before, neither experimentally nor computationally.

Wetting Behavior of Kerogen Surfaces: Insights from Molecular Dynamics

In this study, the wettability of a kerogen surface, a key component of shale reservoirs, is investigated by using molecular dynamics simulations. Specifically, we examined the impact of droplet size and morphology as well as surface roughness on the water contact angles. The findings highlighted that the contact angle dependency on the droplet size intensifies with increased rigidity of the surface. Conversely, as the surface becomes more flexible and rougher, it gains hydrophilicity. The higher hydrophilicity stems from the ability of water molecules to penetrate the kerogen corrugations and form more hydrogen bonds with heteroatoms, particularly oxygen. Notably, the contact angle of kerogen hovers between 65 and 75°, thereby crossing the transition from an underoil hydrophilic to an underoil hydrophobic state. Consequently, minor alterations in the kerogen nanostructure can dramatically alter the wetting preference between water and oil. This insight is of paramount significance for refining strategies in managing fluid interactions in shale reservoirs such as geological carbon storage or oil extraction.

Interfacial properties of binary mixtures of Lennard-Jones chains in planar interfaces by molecular dynamics simulation

Binary mixtures of fully flexible linear tangent chains composed of bonded Lennard-Jones interaction sites (monomers) were studied using the molecular dynamics simulation in the NVT ensemble. Their interfacial properties were investigated in planar interfaces by direct simulation of an explicit liquid film in equilibrium with its vapor. A method for the calculation of long-range interactions in inhomogeneous fluids was implemented to take into account the potential truncation effects. Surface tension and the pressure tensor were calculated via the classical Irving-Kirkwood method; vapor pressure, orthobaric densities, density profiles, and Gibbs relative adsorption of the volatile component with respect to the heavy component were also obtained. The properties were studied as a function of the temperature, molar concentration of the heavy component, and the asymmetry of the mixture. According to the results of this work, the temperature loses influence on the surface tension, vapor pressure, and Gibbs relative adsorption curves as the molecular length of the heavy component increases. This suggests that the universal behavior observed in pure fluids of Lennard-Jones chains also holds for binary mixtures. The contribution of the long-range interactions turned out to account for about 60%, 20%, and 10% of the surface tension, vapor pressure, and orthobaric density final values, respectively. This contribution was even larger at high temperatures and for large molecules. Strong enrichment of the volatile component at the interface was observed in the asymmetric mixtures. One of these mixtures even showed a barotropic effect at elevated pressures and a class III phase behavior.

A Poromechanical Model for Sorption Hysteresis in Nanoporous Polymers

Sorption hysteresis in nanoporous polymer is an intriguing phenomenon that involves coupling between sorption and deformation. Based on the mechanism revealed at the microscopic level by use of molecular simulation, a poromechanical model is developed capturing all relevant physics and yielding a quantitative description. In this model, the coupling between sorption and deformation is described by a poromechanics framework. More in detail, an upscaling process from the molecular mechanism is implemented to model the hysteresis through the state change of each element upon deformation. We provide two solutions of the model: a numerical one based on the finite element method and an analytical one based on uniform strain assumption. The results from both solutions agree well with the molecular simulation and experimental results, therefore capturing and describing adequately sorption hysteresis. The developed model illustrates that water forms different structural distributions upon adsorption and desorption. A parametric study shows that sorption hysteresis is influenced by material properties. We find that a softer material with stronger adsorbent-adsorbate interaction tends to exhibit more profound sorption hysteresis. The developed model, which relies on the concepts of sorption-deformation coupling and multiscale modeling from atomistic simulations to domain dependent theory, paves the way for a new direction of modeling sorption hysteresis.

Probing the concept of line tension down to the nanoscale

A novel mechanical approach is developed to explore by means of atom-scale simulation the concept of line tension at a solid-liquid-vapor contact line as well as its dependence on temperature, confinement, and solid/fluid interactions. More precisely, by estimating the stresses exerted along and normal to a straight contact line formed within a partially wet pore, the line tension can be estimated while avoiding the pitfalls inherent to the geometrical scaling methodology based on hemispherical drops. The line tension for Lennard-Jones fluids is found to follow a generic behavior with temperature and chemical potential effects that are all included in a simple contact angle parameterization. Former discrepancies between theoretical modeling and molecular simulation are resolved, and the line tension concept is shown to be robust down to molecular confinements. The same qualitative behavior is observed for water, but the line tension at the wetting transition diverges or converges toward a finite value depending on the range of solid/fluid interactions at play.