Effects of Water on Mica–Ionic Liquid Interfaces

A growing body of work shows that water can affect the structure and properties of the ionic liquids near solid surfaces, which has rich ramifications in applications of ionic liquids such as lubrication and energy storage. Using molecular dynamics simulations, we investigate how water affects the three-dimensional structure of ionic liquids [BMIM]­[Tf₂N] near mica surfaces with two different charge densities. We show that water can alter not only the layering of ions near the mica surface but also their lateral and orientation ordering and the aggregation of cations’ hydrophobic tails. Water often, but not always, weakens the structuring of interfacial ionic liquids. The multifaceted impact of water on the interfacial structure of ionic liquids can be traced back to the fact that water is both a dielectric solvent and a molecular liquid. Based on the additional observations that the adsorption of water at mica–ionic liquid interfaces is enhanced by ionic liquids and surface charge, we suggest that the structure of ionic liquids near solid surfaces is governed by the three-way coupling between the self-organization of ions, the adsorption of interfacial water, and the electrification of the solid surfaces

Recovery of Multicomponent Shale Gas from Single Nanopores

The adsorption of multicomponent gas mixtures in shale formations and their recovery are of great interest to the shale gas industry. Here we report molecular dynamics simulations of the adsorption of methane/ethane mixtures in 2 and 4 nm-wide nanopores and their recovery from these nanopores. Surface adsorption contributes significantly to the storage of methane and ethane inside the pores, and ethane is enriched inside the nanopores in equilibrium with bulk methane–ethane mixtures. The enrichment of ethane is enhanced as the pore is narrowed but is weakened as the pressure increases due to entropic effects. These effects are captured by the ideal adsorbed solution (IAS) theory, but the theory overestimates the adsorption of both gases. Upon opening the mouth of the nanopores to gas baths with lower pressure, both gases enter the bath. The production rates of both gases show only weak deviation from the square root scaling law before the gas diffusion front reaches the dead end of the pores. The ratio of the production rate of ethane and methane is close to their initial mole ratio inside the nanopore despite the fact that the mobility of pure ethane is smaller than that of pure methane inside the pores. Scale analysis and calculation of the Onsager coefficients for the transport of binary mixtures of methane and ethane inside the nanopores suggest that the strong coupling between methane and ethane transport is responsible for the effective recovery of ethane from the nanopores

Electrical Double Layers near Charged Nanorods in Mixture Electrolytes

The electrical double layers (EDLs) in the mixtures of room-temperature ionic liquids and water play an important role in many applications but have only begun to receive widespread attention recently. Here, we report the molecular dynamics simulations of EDLs near rigid polyanion nanorods immersed in electrolytes containing ionic liquids [C₂mim]­[TfO], water, and Na⁺ ions. When the water content in bulk electrolyte is high, the EDLs near the rods are similar to those in aqueous electrolytes except that TfO^– ions accumulate notably near the cation layer adsorbed on the rods. When the water content in the bulk electrolyte becomes very low, even though water is greatly enriched in the interfacial region, the microenvironment in the region offers weak dielectric screening and the EDL exhibits features common to those in neat ionic liquids, e.g., charge overscreening. Na⁺ ions are readily adsorbed on the sulfonate groups of the polyanion rod in the range of water contents explored here but can experience an energy barrier when moving from the bulk electrolyte to the rod surface if the water content is very low. Introducing Na⁺ ions into the system displaces C₂mim⁺ ions from the rod’s surface. However, the number of displaced C₂mim⁺ ions is far less than the Na⁺ newly adsorbed on the rod, and thus, charge overscreening is enhanced. We highlight the important role of ion–ion correlations in determining the EDL characteristics and their response to the variation of mixture electrolytes’ water/Na⁺ ion contents revealed here and discuss their technical implications

Water in Ionic Liquids at Electrified Interfaces: The Anatomy of Electrosorption

Complete removal of water from room-temperature ionic liquids is nearly impossible. For the electrochemical applications of ionic liquids, how water is distributed in the electrical double layers when the bulk liquids are not perfectly dry can potentially determine whether key advantages of ionic liquids, such as a wide electrochemical window, can be harnessed in practical systems. In this paper, we study the adsorption of water on electrode surfaces in contact with humid, imidazolium-based ionic liquids using molecular dynamics simulations. The results revealed that water molecules tend to accumulate within sub-nanometer distance from charged electrodes. At low amount of water in the bulk, the distributions of ions and of electrostatic potential in the double layer are affected weakly by the presence of water, but the spatial distribution of water molecules is strongly dependent on both. The preferential positions of water molecules in double layers are determined by the balance of several factors: the tendency to follow the positions of the maximal absolute value of the electrical field, the association with their ionic surroundings, and the propensity to settle at positions where more free space is available. The balance between these factors changes with charging the electrode, but the adsorption of water generally increases with voltage. The ion specificity of water electrosorption is manifested in the stronger presence of water near positive electrodes (where anions are the counterions) than near negative electrodes (where cations are counterions). These predictions await experimental verification

Enhanced Recovery of Oil Mixtures from Calcite Nanopores Facilitated by CO₂ Injection

Slow production, preferential recovery of light hydrocarbons, and low recovery factors are common challenges in oil production from unconventional reservoirs dominated by nanopores. Gas injection-based techniques such as CO2 Huff-n-Puff have shown promise in addressing these challenges. However, a limited understanding of the recovery of oil mixtures on the nanopore scale hinders their effective optimization. Here, we use molecular dynamics simulations to study the recovery of an oil mixture (C10 + C19) from a single 4 nm-wide calcite dead-end pore, both with and without CO2 injection. Without CO2 injection, oil recovery is much faster than expected from oil vaporization and features an undesired selectivity, i.e., the preferential recovery of lighter C10. With CO2 injection, oil recovery is accelerated and its selectivity toward C10 is greatly mitigated. These recovery behaviors are understood by analyzing the spatiotemporal evolution of C10, C19, and CO2 distributions in the calcite pore. In particular, we show that interfacial phenomena (e.g., the strong adsorption of oil and CO2 on pore walls, their competition, and their modulation of transport behavior) and bulk phenomena (e.g., solubilization of oil by CO2 in the middle portion of the pore) play crucial roles in determining the oil recovery rate and selectivity

Self-Diffusiophoresis of Janus Catalytic Micromotors in Confined Geometries

The self-diffusiophoresis of Janus catalytic micromotors (JCMs) in confined environment is studied using direct numerical simulations. The simulations revealed that, on average, the translocation of a JCM through a short pore is moderately slowed down by the confinement. This slowdown is far weaker compared to the transport of particles through similar pores driven by forces induced by external means or passive diffusiophoresis. Pairing of two JCMs facilitates the translocation of the one JCM entering the pore first but slows down the second JCM. Depending on its initial orientation, a JCM near the entrance of a pore can exhibit different rotational motion, which determines whether it can enter the pore. Once a JCM enters a narrow pore, it can execute a self-alignment process after which it becomes fully aligned with the pore axis and moves to the center line of the pore. Analysis of these results showed that, in addition to hydrodynamic effect, the translation and rotation of JCM is also affected by the “chemical effects”, i.e., the modification of the chemical species concentration around a JCM by confining walls and neighboring JCMs. These chemical effects are unique to the self-diffusiophoresis of JCMs and should be considered in design and operations of JCMs in confined environment

Marangoni Flow Induced Collective Motion of Catalytic Micromotors

A new collective motion of non-bubble-propelled spherical Janus catalytic micromotors has been observed. When the local concentration of micromotors is high, bubbles start to form between the motors. As the bubble grows, micromotors move collectively toward the center of the bubble regardless of the orientations of their catalyst surface, eventually become aggregated, and captured around the perimeter of the bubble. It is suggested that this collective motion of the micromotors, too fast for the diffusiophoresis, can be caused by the entrainment of micromotors by the evaporation-induced Marangoni flow near the bubble. Numerical simulations confirmed that the direction and strength of such Marangoni flow are consistent with the fast, collective motion of micromotors observed experimentally

Self-Diffusiophoresis of Janus Catalytic Micromotors in Confined Geometries

The self-diffusiophoresis of Janus catalytic micromotors (JCMs) in confined environment is studied using direct numerical simulations. The simulations revealed that, on average, the translocation of a JCM through a short pore is moderately slowed down by the confinement. This slowdown is far weaker compared to the transport of particles through similar pores driven by forces induced by external means or passive diffusiophoresis. Pairing of two JCMs facilitates the translocation of the one JCM entering the pore first but slows down the second JCM. Depending on its initial orientation, a JCM near the entrance of a pore can exhibit different rotational motion, which determines whether it can enter the pore. Once a JCM enters a narrow pore, it can execute a self-alignment process after which it becomes fully aligned with the pore axis and moves to the center line of the pore. Analysis of these results showed that, in addition to hydrodynamic effect, the translation and rotation of JCM is also affected by the “chemical effects”, i.e., the modification of the chemical species concentration around a JCM by confining walls and neighboring JCMs. These chemical effects are unique to the self-diffusiophoresis of JCMs and should be considered in design and operations of JCMs in confined environment

Electro-Induced Dewetting and Concomitant Ionic Current Avalanche in Nanopores

Electrically driven ionic transport of room-temperature ionic liquids (RTILs) through nanopores is studied using atomistic simulations. The results show that in nanopores wetted by RTILs a gradual <i>dewetting</i> transition occurs upon increasing the applied voltage, which is accompanied by a sharp <i>increase</i> in ionic current. These phenomena originate from the solvent-free nature of RTILs and are in stark contrast with the transport of conventional electrolytes through nanopores. Amplification is possible by controlling the properties of the nanopore and RTILs, and we show that it is especially pronounced in charged nanopores. The results highlight the unique physics of nonequilibrium transport of RTILs in confined geometries and point to potential experimental approaches for manipulating ionic transport in nanopores, which can benefit diverse techniques including nanofluidic circuitry and nanopore analytics

Multicomponent Gas Storage in Organic Cage Molecules

Porous liquids are a promising new class of materials featuring nanoscale cavity units dispersed in liquids that are suitable for applications such as gas storage and separation. In this work, we use molecular dynamics simulations to examine the multicomponent gas storage in a porous liquid consisting of crown-ether-substituted cage molecules dissolved in a 15-crown-5 solvent. We compute the storage of three prototypical small molecules including CO₂, CH₄, and N₂ and their binary mixtures in individual cage molecules. For porous liquids in equilibrium with a binary 1:1 gas mixture bath with partial gas pressure of 27.5 bar, a cage molecule shows a selectivity of 4.3 and 13.1 for the CO₂/CH₄ and CO₂/N₂ pairs, respectively. We provide a molecular perspective of how gas molecules are stored in the cage molecule and how the storage of one type of gas molecule is affected by other types of gas molecules. Our results clarify the molecular mechanisms behind the selectivity of such cage molecules toward different gases