Molecular Insight into CO2 Improving Oil Mobility in Shale Inorganic Nanopores Containing Water Films

CO2 injection into shale reservoirs has been recognized as one of the most promising techniques for enhanced oil recovery and carbon capture, utilization, and storage. However, the omnipresent nanopores and the water films formed near the pore walls affect the understanding of mechanisms of CO2 regulating crude oil mobility in shale nanopores. In this work, we employ molecular dynamics simulations to study the occurrence and flow of CO2 and octane (nC8) mixtures in quartz nanopores containing water films, to illustrate the impact mechanisms of CO2 on nC8 mobility. The results indicate that nC8 exists between water films, and CO2 is mainly miscible with nC8 in the pore center, and a small portion of it accumulates at the interface between nC8 and the water film. CO2 significantly decreases the apparent viscosity of nC8 in both the bulk nC8 region and the nC8-water interface region, improving nC8 fluidity. As the percentage of CO2 in the CO2 and nC8 mixtures increases from 0 to 50%, the mean flow velocities of nC8 in the bulk phase region and the nC8-water interface region increase by 92.85 and 60.64%, respectively. Three major microscopic mechanisms of CO2 improving nC8 fluidity in quartz nanopores with water films are summarized: (i) CO2 reduces friction between nC8 and the water film by increasing the angle between nC8 molecules and the plane of the water film; (ii) CO2 widens the distance between nC8 molecules, causing the volume expansion of nC8 and its viscosity reduction; (iii) CO2 significantly increases the most probable and average velocities of nC8 molecules, thus improving their mobility. Our results enhance the comprehension of how CO2 facilitates oil flow in water-bearing shale reservoirs and the exploitation of unconventional oil resources.

Passive fractionating mechanism for oil spill using shear-wettability modulation

Oil spillage and organic solvent leakage have been a frequent occurrence in recent years, which pose a significant threat not only to the aquatic ecosystems but also result in substantial economic burdens. This has necessitated the search for materials capable of separating oil from water at enhanced efficiency with superior mechanical and thermal properties. In this study, we conduct a set of systematic molecular dynamics simulations to investigate the potential of two-dimensional graphene-like channels under extreme confinement to achieve efficient oil-water separation. Effective modulation of the wetting characteristics of graphene-like surfaces juxtaposed with unconventional flow behavior at the nanoscale unveils differential interaction of water and oil molecules towards the wall, thereby resulting in distinct separation zones for varying compositions of the oil-water mixture. Such separation zones have been observed to be highly correlated with mixture temperature, which provides effective separation pathways across diverse environmental conditions. Our study offers a paradigm shift in oil-water separation strategies, which not only provides deeper insights into the equilibrium and dynamic behavior of a two-phase mixture but also holds immense implications for the development of smart, wettability-based oil separation devices.

Unraveling Flow Effect on Capacitive Energy Extraction from Salinity Gradients

The harvesting of salinity gradient energy through a capacitive double-layer expansion (CDLE) technique is directly associated with ion adsorption and desorption in electrodes. Herein, we show that energy extraction can be modulated by regulating ion adsorption/desorption through water flow. The flow effects on the output energy, capacitance, and energy density under practical conditions are systematically investigated from a theoretical perspective, upon which the optimal operating condition is identified for energy extraction. We demonstrate that the net charge accumulation displays a negative correlation with the water flow velocity and so does the surface charge density, and this causes a nontrivial variation in the magnitude of output energy when water flows are introduced. When high water flows are introduced in both the charging and discharging processes, the energy extraction can be significantly reduced by 47.69-49.32%. However, when a high flow is solely exerted in the discharging process, the energy extraction can be enhanced by 12.94-14.49% even at low operation voltages. This study not only offers a comprehensive understanding of the microscopic mechanisms of surface-engineered energy extraction with water flows but also provides a novel direction for energy extraction enhancement.

Wettability-modulated behavior of polymers under varying degrees of nano-confinement

Extreme confinement in nanochannels results in unconventional equilibrium and flow behavior of polymers. The underlying flow physics dictating such paradigms remains far from being understood and more so if the confining substrate is composed of two-dimensional materials, such as graphene. In this study, we conducted systematic molecular dynamics simulations to explore the effect of wettability, confinement, and chain length on polymer flow through graphene-like nanochannels. Altering the wetting properties of these membranes that structurally represent graphene results in substantial changes in the behavior of polymers of disparate chain lengths. Longer hydrocarbon chains (n-dodecane) exhibit negligible wettability-dependent structuring in narrower nanochannels compared to shorter chains (n-hexane) culminating in higher average velocities and interfacial slippage of n-dodecane under less wettable conditions. We demonstrate that the wettability compensation comes from chain entanglement attributed to entropic factors. This study reveals a delicate balance between wettability-dependent enthalpy and chain-length-dependent entropy, resulting in a unique nanoscale flow paradigm, thus not only having far-reaching implications in the superior discernment of polymeric flow in sub-micrometer regimes but also potentially revolutionizing various applications in the oil industry, including innovative oil transport, oil extraction, ion transport polymers, and separation membranes.

Immobilizing Ionic Liquids onto Functionalized Surfaces for Sensing Volatile Organic Compounds

Herein, a highly sensitive volatile organic compound (VOC) gas sensor is demonstrated using immobilized ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate, onto surfaces functionalized by the quaternary ammonium group -N⁺R, -COOH, and -NH₂, i.e., N⁺-IL, COOH-IL, and NH₂-IL, respectively. These functional groups ensure highly tunable interactions between the IL and surfaces, efficiently modulating the electrical resistance of the immobilized IL upon exposure to acetone and toluene. The immobilized IL to both acetone and toluene displays significant electronic resistance changes at a concentration of 150 ppm, falling in the order NH₂-IL > N⁺-IL > COOH-IL for acetone while COOH-IL > NH₂-IL > N⁺-IL for toluene. A better gaseous sensing ability is achieved in COOH-IL for toluene than acetone, while this does not hold in the case of NH₂-IL and N⁺-IL surfaces because of the completely different ion structuring of the IL at these functionalized surfaces. The accelerated ion mobility in the IL that is immobilized onto functionalized surfaces is also responsible for the strong gaseous sensing response, which is demonstrated further by the atomic force microscopy-measured smaller friction coefficient. This is highly encouraging and suggests that ILs can be immobilized by a network formed by surface functionalization to easily and cheaply detect VOCs at ppm concentrations.

Thermodynamic Barrier for Nanoparticle Penetration into Nanotubes

It is promising yet challenging to develop efficient methods to separate nanoparticles (NPs) with nanochannel devices. Herein, in order to guide and develop the separation method, the thermodynamic mechanism of NP penetration into solvent-filled nanotubes is investigated by using classical density functional theory. The potential of mean force (PMF) is calculated to evaluate the thermodynamic energy barrier for NP penetration into nanotubes. The accuracy of the theory is validated by comparing it with parallel molecular dynamics simulation. By examining the effects of nanotube size, solvent density, and substrate wettability on the PMF, we find that a large tube, a low bulk solvent density, and a solvophilic substrate can boost the NP penetration into nanotubes. In addition, it is found that an hourglass-shaped entrance can effectively improve the NP penetration efficiency compared with a square-shaped entrance. Furthermore, the minimum separation density of NPs in solution is identified, below which the NP penetration into nanotubes requires an additional driving force. Our findings provide fundamental insights into the thermodynamic barrier for NP penetration into nanotubes, which may provide theoretical guidance for separating two components using microfluidics.

Apparent Liquid Permeability in Mixed-Wet Shale Permeable Media

Apparent liquid permeability (ALP) in ultra-confined permeable media is primarily governed by the pore confinement and fluid–rock interactions. A new ALP model is required to predict the interactive effect of the above two on the flow in mixed-wet, heterogeneous nanoporous media. This study derives an ALP model and integrates the compiled results from molecular dynamics (MD) simulations, scanning electron microscopy, atomic force microscopy, and mercury injection capillary pressure. The ALP model assumes viscous forces, capillary forces, and liquid slippage in tortuous, rough pore throats. Predictions of the slippage of water and octane are validated against MD data reported in the literature. In up-scaling the proposed liquid transport model to the representative-elementary-volume scale, we integrate the geological fractals of the shale rock samples including their pore size distribution, pore throat tortuosity, and pore-surface roughness. Sensitivity results for the ALP indicate that when the pore size is below 100 nm pore confinement allows oil to slip in both hydrophobic and hydrophilic pores, yet it also restricts the ALP due to the restricted intrinsic permeability. The ALP reduces to the well-established Carman–Kozeny equation for no-slip viscous flow in a bundle of capillaries, which reveals a distinguishable liquid flow behavior in shales versus conventional rocks. Compared to the Klinkenberg equation, the proposed ALP model reveals an important insight into the similarities and differences between liquid versus gas flow in shales.

Nanoconfinement Effect on Surface Tension: Perspectives from Molecular Potential Theory

Liquid-vapor surface tension (ST) in nanopores attracts great attention in many industries because of the prosperity of nanoscience and nanotechnology. Here, considering the important emerging new physical phenomena induced by nanoconfinement effects, including curvature-dependent and shift-critical temperature (T_c)-dependent effects, the anomalous variation of ST in nanopores is captured from the molecular potential perspective. Furthermore, a simple analytical model is proposed to determine the ST in nanopores by correlating these two effects with an easily accessible parameter, that is, normalized pore dimension, which is the ratio of the pore radius to Lennard-Jones size parameter. The model is validated to be reliable for determining the STs of different substances both in the bulk phase as well as nanopores through comparison with the experimental results and molecular simulations. Our results show that the reduction of ST induced by the nanoconfinement effects is visible when the pore diameter is within tens of nanometers, and the reduction is more sensitive as the pore size decreases. In detail, the curvature-dependent effect is remarkable in the pores with diameters ranging from a few nanometers to tens of nanometers. Moreover, a simply generalized formula is obtained to determine the curvature-dependent effect and the Tolman length for different substances. The shift-T_c-dependent effect is not only related to the pore dimension but also depends on the temperature. As the pore size decreases, the critical temperature of confined fluids diverges significantly from the bulk values. While at high temperatures, the range of pore size impacted by the shift-Tc-dependent effect is enlarged. Additionally, the nanoconfined STs of different substances are calculated and compared. Overall, the new model captures the underlying physics behind the variation of STs in nanopores and can determine the nanoconfined STs reasonably. Moreover, the simple formulation of the model is beneficial to the practical applications in many chemical engineering processes, such as chemical separation, nucleation phenomenon, and capillary condensation.

The Role of Interface Ions in the Control of Water Transport through a Carbon Nanotube

Controlling the water transport toward a given direction is still challenging, particularly due to thermal fluctuations of water motion at the nanoscale. While most of the previous works focus on the symmetric hydrophobic membrane systems, the role of the membrane in affecting the water transport remains largely unexplored. In this work, by using extensive molecular dynamics simulations, we find an interesting electropumping phenomenon, that is, the flowing counterions on an asymmetric hydrophobic-hydrophilic membrane can significantly drive the single-file water transport through a carbon nanotube, suggesting a nanometer water pump in a highly controllable fashion. The ion-water coupling motion in electric fields on the charged surface provides an indirect driving force for this pumping phenomenon. The water dynamics and thermal dynamics demonstrate a unique behavior with the change in electric fields, surface charge density, and even charge species. Particularly, due to the ion flux bifurcation for the positive and negative surfaces, the water dynamics such as the water flow, flux, and translocation time also exhibit similar asymmetry. Surprisingly, the positive surface charge induces an abnormal three-peak dipole distribution for the confined water and subsequent high flipping frequency. This can be attributed to the competition between the surface charge and interface water orientation on it. Our results indicate a new strategy to pump water through a nanochannel, making use of the counterion flowing on an asymmetric charged membrane, which are promising for future studies.

Dynamic Adsorption of Ions into Like-Charged Nanospace: A Dynamic Density Functional Theory Study

The adsorption processes of ions into charged nanospace are associated with many practical applications. Whereas a large number of microporous materials have been prepared toward efficient adsorption of ions from solutions, theoretical models that allow for capturing the characteristics of ion dynamic adsorption into like-charged nanopores are still few. The difficulty originates from the overlapping of electric potentials inside the pores. Herein, a theoretical model is proposed by incorporating dynamic density functional theory with modified Poisson equation for investigating the dynamic adsorption of ions into like-charged nanoslits. This model is rationalized by comparing the theoretical predictions with corresponding simulation results. Afterward, by analyzing the adsorption dynamics, we show that the overlapping effect is associated with the pore size, ion bulk concentration, and surface charge density, and it plays a dominant role in the coupling between the total adsorption amount of ions and total adsorption time. Specifically, with weak overlapping effect, the total adsorption amount is intuitively proportional to the total adsorption time; however, when the overlapping effect is strong, the total adsorption amount may be inversely proportional to the total adsorption time, indicating that both high adsorption amount and short adsorption time can be achieved simultaneously. This work provides a meaningful insight toward the rational design and optimization of microporous materials for efficient ion adsorption.

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.

Anomalous Capillary Rise under Nanoconfinement: A View of Molecular Kinetic Theory

Understanding the capillary filling behaviors in nanopores is crucial for many science and engineering problems. Compared with the classical Bell-Cameron-Lucas-Washburn (BCLW) theory, anomalous coefficient is always observed because of the increasing role of surfaces. Here, a molecular kinetics approach is adopted to explain the mechanism of anomalous behaviors at the molecular level; a unified model taking account of the confined liquid properties (viscosity and density) and slip boundary condition is proposed to demonstrate the macroscopic consequences, and the model results are successfully validated against the published literature. The results show that (1) the effective viscosity induced by the interaction from the pore wall, as a function of wettability and the pore dimension (nanoslit height or nanotube diameter), may remarkably slow down the capillary filling process more than theoretically predicted. (2) The true slip, where water molecules directly slide on the walls, strongly depends on the wettability and will increase as the contact angle increases. In the hydrophilic nanopores, though, the magnitude may be comparable with the pore dimensions and promote the capillary filling compared with the classical BCLW model. (3) Compared with the other model, the proposed model can successfully predict the capillary filling for both faster or slower capillary filling process; meanwhile, it can capture the underlying physics behind these behaviors at the molecular level based on the effective viscosity and slippage. (4) The surface effects have different influence on the capillary filling in nanoslits and nanotubes, and the relative magnitude will change with the variation of wettability as well as the pore dimension.