Effect of confinement in nano-porous materials on the solubility of a supercritical gas

Hydrogen Solubility in Confined Water

Confined water has demonstrated distinct structural and dynamic properties compared to bulk water. Although many studies have explored the water structure within simple geometries using materials such as carbon and silica, studies on gas solubility in confined water and the underlying physics of water structure-solubility remain limited. Recent research has illuminated the concept of "oversolubility", wherein gases display increased solubility within liquids confined in small pores compared to their bulk form. This study focuses on zeolites, naturally abundant materials with versatile applications, to study the hydrogen solubility within confined water through careful experimentation. Our findings underscore the relationship between the pore dimension and gas solubility enhancement within confined water. Hydrogen solubility is closely associated with the rearrangement of water molecules within the porous framework of the zeolite. Our research shows that a 2 nm pore size results in the greatest increase in hydrogen solubility in the water trapped inside the zeolite framework. The double donor-double acceptor (DDAA) bonds play a critical role in hydrogen solubility. Our research provides fundamental insight into the role of the molecular bonding type on hydrogen solubility in water, paving the way for potential applications in hydrogen storage and utilization.

Molecular Dynamics Study on CO2 Storage in Water-Filled Kerogen Nanopores in Shale Reservoirs: Effects of Kerogen Maturity and Pore Size

CO₂ sequestration in shale reservoirs is an economically viable option to alleviate carbon emission. Kerogen, a major component in the organic matter in shale, is associated with a large number of nanopores, which might be filled with water. However, the CO₂ storage mechanism and capacity in water-filled kerogen nanopores are poorly understood. Therefore, in this work, we use molecular dynamics simulation to study the effects of kerogen maturity and pore size on CO₂ storage mechanism and capacity in water-filled kerogen nanopores. Type II kerogen with different degrees of maturity (II-A, II-B, II-C, and II-D) is chosen, and three pore sizes (1, 2, and 4 nm) are designed. The results show that CO₂ storage mechanisms are different in the 1 nm pore and the larger ones. In 1 nm kerogen pores, water is completely displaced by CO₂ due to the strong interactions between kerogen and CO₂ as well as among CO₂. CO₂ storage capacity in 1 nm pores can be up to 1.5 times its bulk phase in a given volume. On the other hand, in 2 and 4 nm pores, while CO₂ is dissolved in the middle of the pore (away from the kerogen surface), in the vicinity of the kerogen surface, CO₂ can form nano-sized clusters. These CO₂ clusters would enhance the overall CO₂ storage capacity in the nanopores, while the enhancement becomes less significant as pore size increases. Kerogen maturity has minor influences on CO₂ storage capacity. Type II-A (immature) kerogen has the lowest storage capacity because of its high heteroatom surface density, which can form hydrogen bonds with water and reduce the available CO₂ storage space. The other three kerogens are comparable in terms of CO₂ storage capacity. This work should shed some light on CO₂ storage evaluation in shale reservoirs.

Enhancing Gas Solubility in Nanopores: A Combined Study Using Classical Density Functional Theory and Machine Learning

Geometrical confinement has a large impact on gas solubilities in nanoscale pores. This phenomenon is closely associated with heterogeneous catalysis, shale gas extraction, phase separation, etc. Whereas several experimental and theoretical studies have been conducted that provide meaningful insights into the over-solubility and under-solubility of different gases in confined solvents, the microscopic mechanism for regulating the gas solubility remains unclear. Here, we report a hybrid theoretical study for unraveling the regulation mechanism by combining classical density functional theory (CDFT) with machine learning (ML). Specifically, CDFT is employed to predict the solubility of argon in various solvents confined in nanopores of different types and pore widths, and these case studies then supply a valid training set to ML for further investigation. Finally, the dominant parameters that affect the gas solubility are identified, and a criterion is obtained to determine whether a confined gas-solvent system is enhance-beneficial or reduce-beneficial. Our findings provide theoretical guidance for predicting and regulating gas solubilities in nanopores. In addition, the hybrid method proposed in this work sets up a feasible platform for investigating complex interfacial systems with multiple controlling parameters.

Effect of Extra Gas Amount on Liquid Outflow from Hydrophobic Nanochannels: Enhanced Liquid-Gas Interaction and Bubble Nucleation

Understanding liquid motion in nanoenvironment is of fundamental importance in nanofluidics-based systems. While the liquid outflow from hydrophobic nanochannels can significantly affect system performance, its underlying mechanism remains unclear so far. Here, we present an experimental study of the gas-phase effect on liquid outflow behavior from hydrophobic nanochannels in a liquid nanofoam (LN) system. Four LN samples, consisting of same liquid-solid composition but different amounts of the gas phase, are characterized by cyclic quasi-static compression tests. A remarkable difference in the LN system reusability has been observed, indicating that the liquid outflow behavior is highly sensitive to the amount of the gas phase. As the gas amount increases, the degree of liquid outflow from hydrophobic nanochannels is considerably promoted. This promotive effect is because of the suppression of gas outflow and acceleration of bubble nucleation in the nanochannels. These fundamental findings open a new perspective on liquid outflow behavior and can facilitate the design of reusable nanofluidics-based energy absorbers.

Wetting Transition of Ionic Substrate by Modulating Surface Charge Distribution

Surface wettability regulation plays a crucial role in antifouling and related applications. For regulating surface wettability, one of the effective approaches is to modulate the surface charge distribution. Herein, we report a theoretical study for unraveling the mechanistic relation between surface charge distribution and ionic substrate wettability. Specifically, acetonitrile liquids at ambient condition in contact with various ionic substrates are considered. At different surface charge distributions, the interfacial thermodynamic properties are investigated by means of molecular density functional theory. We find that the variation of the spatial interval among the discrete charges strongly alters the substrate-acetonitrile interaction and leads to an oscillation in the interfacial tension, indicating that the substrate can be tuned from a solvophobic one to a solvophilic one. This trend can be further enhanced by increasing the charge quantity. The underlying mechanisms are extensively discussed and expatiated. Our work provides theoretical guidance to engineer and regulate surface wettability.

Comment on “Pressure enhancement in carbon nanopores: a major confinement effect” by Y. Long, J. C. Palmer, B. Coasne, M. Śliwinska-Bartkowiak and K. E. Gubbins, Phys. Chem. Chem. Phys., 2011, 13, 17163

A standard thermodynamic interpretation unambiguously explains the observed properties of fluids confined in pores, while a “pressure enhancement” effect emerges only from calculations in which particular choices are selected from an arbitrary set.

Effect of Electrolytes on Gas Oversolubility and Liquid Outflow from Hydrophobic Nanochannels

We have experimentally studied the effect of electrolytes on gas oversolubility and liquid outflow from hydrophobic nanochannels. By immersing nanoporous material with the same porous structure and surface properties into four different aqueous electrolyte solutions with the same surface tension, the excessive solid-liquid interfacial tension of the resulted liquid nanofoam (LN) systems has been set as a constant. Upon unloading, partial liquid outflow has been observed and quantified. As the four LN systems show different degrees of recoverability, it suggests that the degree of liquid outflow is highly sensitive to the ion species. In addition, different from bulk phase scenario, the anions have a more profound effect than cations on gas oversolubility. Lower bulk gas solubility and larger gas oversolubility factor lead to higher degree of liquid outflow and recoverability of the LN systems. This fundamental understanding on the mechanism of liquid outflow enables the development of nanofluidics-based system into reusable energy absorbers.

Confinement Effect on Molecular Conformation of Alkanes in Water-Filled Cavitands: A Combined Quantum/Classical Density Functional Theory Study

The depletion force exerted on an alkane molecule from surrounding solvent may greatly alter its conformation. Such a behavior is closely related to the selective molecular recognition, molecular sensors, self-assembly, and so on. Herein, we report a multiscale theoretical study on the conformational change of a single alkane molecule confined in water-filled cavitands, in which the quantum and classical density functional theories (DFTs) are combined to determine the grand potential of alkane-water system. Specifically, the intrinsic free energy of the alkane molecule is tackled by quantum DFT, while the solvent effect arising from the solvent density inhomogeneity in confined space is addressed by classical DFT. By varying the alkane chain length, pore size, and wettability of inner pore surface, we find that pore confinement and hydrophilic inner surface facilitate the alkane conformational change from extended state to helical state, which becomes more significant as the alkane chain length increases. Our findings, which are in line with previous experimental observations, provide not only the microscopic mechanism but also theoretical guidance for elaborately manipulating molecular conformation at the nanoscale.