Machine Learning-Based Interfacial Tension Equations for (H2 + CO2)-Water/Brine Systems over a Wide Range of Temperature and Pressure.

Large-scale underground hydrogen storage (UHS) plays a vital role in energy transition. H2-brine interfacial tension (IFT) is a crucial parameter in structural trapping in underground geological locations and gas-water two-phase flow in subsurface porous media. On the other hand, cushion gas, such as CO2, is often co-injected with H2 to retain reservoir pressure. Therefore, it is imperative to accurately predict the (H2 + CO2)-water/brine IFT under UHS conditions. While there have been a number of experimental measurements on H2-water/brine and (H2 + CO2)-water/brine IFT, an accurate and efficient (H2 + CO2)-water/brine IFT model under UHS conditions is still lacking. In this work, we use molecular dynamics (MD) simulations to generate an extensive (H2 + CO2)-water/brine IFT databank (840 data points) over a wide range of temperature (from 298 to 373 K), pressure (from 50 to 400 bar), gas composition, and brine salinity (up to 3.15 mol/kg) for typical UHS conditions, which is used to develop an accurate and efficient machine learning (ML)-based IFT equation. Our ML-based IFT equation is validated by comparing to available experimental data and other IFT equations for various systems (H2-brine/water, CO2-brine/water, and (H2 + CO2)-brine/water), rendering generally good performance (with R2 = 0.902 against 601 experimental data points). The developed ML-based IFT equation can be readily applied and implemented in reservoir simulations and other UHS applications.

Dependence of Methane Transport on Pore Informatics in the Amorphous Nanoporous Kerogen Matrix.

Fluid transport in kerogen is mainly diffusion-driven, while its dependence on pore informatics is still poorly understood. It is challenging for experiments to identify the effect of pore informatics (such as pore connectivity and tortuosity) on fluid transport therein. Therefore, in this work, we use molecular dynamics simulations to study methane transport behaviors in amorphous kerogen matrices with broad pore properties. The pore properties including porosity, pore connectivity, pore size, and diffusive tortuosity are characterized. Next, self-diffusion coefficients in the connected pores (DeffS) and in the total pores without distinguishing its connectivity (DtotS) are calculated in all the kerogen matrices based on the free volume theory. We find that both DeffS and DtotS exponentially decreases with methane loading with two controlled parameters: fitting constant αeff and DeffS(0) (DeffS at infinitely small loading) for DeffS and fitting constant αtot and DtotS(0) (DtotS at infinitely small loading) for DtotS. However, in the kerogen models with relatively low pore connectivity, αeff and αtot as well as DeffS(0) and DtotS(0) can be quite different, inducing the different estimations of DeffS and DtotS. Since methane in the unconnected pores does not contribute to the actual transport, it is important to recognize connected pores when evaluating the fluid transport in kerogen. On the other hand, DeffS(0) strongly depends on the effective limiting pore size (rlim_eff) of the dominant flow path and effective diffusive tortuosity (τeff), in which DeffS(0) linearly increases with (rlim_eff/τeff)2. We also find that αeff is a multivariable function of ϕeff, τeff, and rlim_eff, but their generalized relation requires more data to obtain. This work provides important insights into fluid transport in kerogen based on the kerogen pore informatics, which are important to shale gas development.

Effects of Interfacial Molecular Structures on Pressure-Driven Brine Flow in Silica Mesopores.

In this work, we conduct molecular dynamics simulations to investigate pressure-driven brine flow in silica mesopores under typical reservoir conditions (323 K and 20 MPa). While surface counterions accumulate strongly in the vicinity of fully deprotonated silica surfaces, water forms multilayer structures due to hydrogen bonding, counterion hydration, and excluded-volume effect. Brine flow behaviors exhibit adsorption, transition, and bulk-like regions in fully deprotonated silica mesopores, while the transition region is negligible in fully protonated ones. In the adsorption region, strong surface hydrogen bonding and a high degree of counterion hydration collectively hinder water mobility. Even without surface hydrogen bonding, persistent ion hydration impedes water flow, leading to the transition region in fully deprotonated silica mesopores and higher viscosity of brine (with 10 wt % NaCl) in the bulk region. This work elucidates the collective effects of surface chemistry and interfacial water structures on brine flow behaviors in silica mesopores from molecular perspectives.

Two-Dimensional Living Supramolecular Polymerization: Improvement in Edge Roughness of Supramolecular Nanosheets by Using a Dummy Monomer.

Supramolecular polymers are formed through nucleation (i. e., initiation) and polymerization processes, and kinetic control over the nucleation process has recently led to the realization of living supramolecular polymerization. Changing the viewpoint, herein we focus on controlling the polymerization process, which we expect to pave the way to further developments in controlled supramolecular polymerization. In our previous study, two-dimensional living supramolecular polymerization was used to produce supramolecular nanosheets with a controlled area; however, these had rough edges. In this study, the growth of the nanosheets was controlled by using a 'dummy' monomer to produce supramolecular nanosheets with smoothed edges.

Pore-Scale Study on Shale Oil-CO2-Water Miscibility, Competitive Adsorption, and Multiphase Flow Behaviors.

Due to the fracturing fluid imbibition and primary water, oil-water two-phase fluids generally exist in shale nanoporous media. The effects of water phase on shale oil recovery and geological carbon sequestration via CO2 huff-n-puff is non-negligible. Meanwhile, oil-CO2 miscibility after CO2 huff-n-puff also has an important effect on oil-water two-phase flow behaviors. In this work, by considering the oil-CO2 competitive adsorption behaviors and the effects of oil-CO2 miscibility on water wettability, an improved multicomponent and multiphase lattice Boltzmann method is proposed to study the effects of water phase on CO2 huff-n-puff. Additionally, the effects of oil-CO2 miscibility on oil-water flow behaviors and relative permeability are also discussed. The results show that due to Jamin's effect of water droplets in oil-wetting pores and the capillary resistance of bridge-like water phase in water-wetting pores, CO2 can hardly diffuse into the oil phase, causing a large amount of remaining oil. As water saturation increases, Jamin's effect and the capillary resistance become more pronounced, and the CO2 storage mass gradually decreases. Then, based on the results from molecular dynamics simulations, the influences of oil-CO2 miscibility on oil-water relative permeability in calcite nanoporous media are studied, and as the oil mass percentage in the oil-CO2 miscible system decreases, the oil/water relative permeability decreases/increases. The improved lattice Boltzmann model can be readily extended to quantitatively calculate geological CO2 storage mass considering water saturation and calculate the accurate oil-water relative permeability based on the real 3D digital core.

The multiple roles of phenols in the degradation of aniline contaminants by sulfate radicals: A combined study of DFT calculations and experiments.

Recent research revealed inhibition or enhancement of dissolved organic matter (DOM) to the degradation of trace organic contaminants (TrOC) in natural and engineered water systems. Phenols containing acetyl, carboxyl, formyl, hydroxy, and methoxy groups were selected as the model DOM to quantitatively study their roles in the degradation of simple anilines, sulfonamide antibiotics, phenylurea pesticides by sulfate radicals (SO4•-). Experimental results found that p-methoxyphenol inhibited aniline and sulfamethoxazole degradation by thermally activated peroxydisulfate (TAP), while p-acetylphenol slightly promoted aniline degradation. Quantum chemical calculations were applied to study the microscopic mechanism and kinetics of phenols affecting the degradation of aniline pollutants (AN) in three ways: competitively reacting with SO4•-, repairing aniline cationic radicals (AN•+) and phenylaminyl radicals (AN(-H)•), and generating phenoxy radicals to degrade anilines. Generally, the degradation of sulfonamides and phenylureas prefer to be inhibited by hydroxy- and methoxy-phenols with low oxidation potential (Eox), due to their diffusion-limiting reaction with SO4•- and rapid back-reduction AN•+ with the calculated rate constants of (0.02 - 6.38) × 109 M-1 s-1. Phenols repairing AN(-H)• through H abstraction reaction is speculated to possibly dominate the joint degradation of phenols and anilines by TAP, which has a poor correlation with Eox. This study provides mechanistic insight into the chemical behavior of complex and heterogeneous DOM in complex aqueous environments.

Effect of molecular structure and ionization state on aggregation of carboxymethyl chitosan: A molecular dynamics study.

The aggregation behavior of carboxymethyl chitosan (CMCS) plays an important role in its extensive applications. Here, we perform molecular dynamics simulations to investigate aggregation behaviors of CMCS in water and the effects of degrees of deacetylation (DD) and substitution (DS) and ionization states (equivalently different pH conditions). CMCS prefers to aggregate in neutral condition, self-assembling into multimeric forms with interlaced stacking of molecular chains, while forming dimer or trimer through twisted stacking and parallel stacking in acidic and alkaline conditions, respectively. With the increase of DD and DS, the aggregation becomes weaker when in neutral and alkaline conditions, while gets stronger in acidic environment. The presence of intramolecular hydrogen bonds and exo-anomeric effect causes twisted, coplanar and extended conformations of individual chain in acidic, alkaline and neutral conditions, respectively, contributing to their distinct inter-chain stacking structures. Subsequently, the specific intermolecular hydrogen bonding, hydrophobic and electrostatic interactions stabilize the aggregation structures.

Molecular structure-tuned stability and switchability of CO2-responsive oil-in-water emulsions.

HYPOTHESIS: Pseudo-Gemini surfactants (PGS) possessing switchable and recyclable features have drawn increasing attention on generating high-performance CO2-responsive emulsions for wide range and versatile applications. However, there is a lack of fundamental understanding on how the molecular structure of PGS affects the stability and switchability of emulsions. We hypothesize that the length and type of the spacer in PGS play a decisive role in controlling interfacial and switching properties. EXPERIMENTS: Two series of PGS with different spacers were prepared through electrostatic association between amines and oleic acid. The interfacial activity and CO2-responsive properties of corresponding emulsions were systematically investigated by well-designed experiments and molecular dynamics simulations. FINDINGS: Increasing the spacer length to allow the bent configuration leads to more tight arrangement of oleic molecules, consequently improving the interfacial activity. In addition, the introduction of amino group into the spacer dramatically promotes CO2 response of resulting PGS due to ehanced migration of the spacer from the interface to the aqueous phase after CO2 addition. These results are inspiring in designing controllable CO2-responsive emulsions for a wide range of industrial applications (e.g., enhanced oil recovery and oil-contaminated soil remediation).

Breakthrough pressure of oil displacement by water through the ultra-narrow kerogen pore throat from the Young-Laplace equation and molecular dynamic simulations.

As one common unconventional reservoir, shale plays a pivotal role to compensate the depletion of conventional oil resources. There are numerous nanoscale pores and ultra-narrow pore throats (sub 2 nm) in shale media. To displace oil through ultra-narrow pore throats by water, one needs to overcome excessively-high capillary pressure. Understanding the water-oil two-phase displacement process through pore throats is critical to numerical simulation on tight/shale oil exploitation and ultimate oil recovery estimation. In this work, we use molecular dynamics simulations to investigate oil (represented by n-octane) displacement by water through a ~2 nm kerogen (represented by Type II-C kerogen) pore throat. Besides, the applicability of the Young-Laplace equation to the ultra-narrow kerogen pore throat has been assessed. We find that although the Type II-C kerogen is generally oil-wet, water has an excellent displacement efficiency without the oil film on the substrate, thanks to the hydrogen bonding formed between water and heteroatoms (such as O, N, and S) on the kerogen surface. Unlike previous studies, the capillary pressure obtained from the widely used Young-Laplace equation shows a good agreement with the breakthrough pressure obtained from MD simulations for the ∼2 nm kerogen pore throat. Our work indicates that explicitly considering intermolecular interactions as well as atomistic and molecular level characteristics is imperative to study the two-phase displacement process through ultra-narrow pore throats.

Molecular Dynamics Studies on Effective Surface-Active Additives: Toward Hard Water-Resistant Chemical Flooding for Enhanced Oil Recovery.

Divalent ions, which are omnipresent in brine, may be detrimental to surfactant functionalities during chemical flooding in the enhanced oil recovery (EOR) process. Surfactant blending is one potential solution to overcome such an adverse effect. Herein, we report a molecular dynamics (MD) study to investigate the molecular arrangement and possible applications of surfactant blending in hard water-resistant chemical flooding for oil recovery. We chose commonly used anionic surfactants, sodium dodecyl sulfate (SDS), as primary surfactants. The non-ionic (propanol) and cationic [cetrimonium bromide (CTAB)] surfactants with a wide range of concentrations are introduced to the primary system. We demonstrate that CTAB can disaggregate the cation bridging when their concentration is above a certain threshold. This threshold value is related to the surfactant and cosurfactant surface charge in the interface region. The cation bridging density is maintained at a low level when the sum of surfactants and cosurfactant interface charges is neutral or positive. On the other hand, propanol barely disaggregates the cation bridging. When propanol concentration is above a certain value, it even facilitates the cation bridging formation. Both propanol and CTAB can further decrease the oil-brine interfacial tension (IFT) while having different efficacies (IFT decrement rate is different as their interface concentration increases). More rapid IFT decrement is observed when cation bridging is disaggregated (i.e., in systems with high CTAB concentrations). Increasing propanol concentration barely affects hydrogen bond (H-bond) formation between SDS and H2O because of a low propanol distribution around SDS. On the other hand, the first increasing and then decreasing trend in H-bond density between SDS and H2O is observed as CTAB concentration increases. Our work should provide important insights into designing chemical formulas in chemical flooding applications.

Ethanol Blending to Improve Reverse Micelle Dispersity in Supercritical CO2: A Molecular Dynamics Study.

Despite a great promise in the enhanced oil recovery in tight formations, CO2 flooding with surfactants is hindered due to the low surfactant solubility in supercritical CO2 (scCO2). Alcohol blending can increase the sodium bis(2-ethylhexyl) sulfosuccinate (AOT) solubility in scCO2. While this finding offers a promising solution to CO2 flooding in tight oil reservoirs, to the best of our knowledge, their working mechanism still remains elusive. Herein, we report a molecular dynamics simulation study to explore the working mechanism of alcohols in reverse micelle (RM) dispersity ("solubility") increment. The spontaneous aggregation process in two systems (System A consisting of AOT and scCO2; System B consisting of AOT, scCO2, and 10 wt % ethanol) are conducted under a typical tight oil reservoir condition (333 K and 200 bar). After 600 ns runs, the AOT molecules aggregate together and form rod-like RMs in System A, while form several small sphere-like RMs in System B. We observe that the aggregation process in System A occurs when two clusters approach each other end-to-end. More CO2 molecules are around the Na+ ion at the end of the clusters, which can be readily replaced by AOT molecules. On the other hand, the ethanol molecules can better solvate and surround Na+ ions, preventing the further aggregation of AOT clusters in System B. The potential of mean force calculations also reveal that while two small clusters formed by four AOT molecules attract each other in System A, they repel each other in System B. Our work should provide important insights into the design of scCO2-soluble surfactant formulas.

Ion Valency and Concentration Effect on the Structural and Thermodynamic Properties of Brine-Decane Interfaces with Anionic Surfactant (SDS).

Salt ion valency and concentration vary in actual oil reservoirs, which play an important role in the functionalities of surfactant formula during chemical flooding processes to enhance oil recovery. Herein, we report a molecular dynamics (MD) study to investigate the ion valency and concentration effect on the structural and thermodynamic properties of brine-decane interfaces with anionic surfactant (SDS), under typical reservoir conditions (353 K and 200 bar). We use two different cations (Na+ and Ca2+) and a wide range of ion concentrations (up to 3.96 M) to simulate reservoir conditions. We find that ion valency has a significant effect on the molecular configurations, which further influences the thermodynamic properties. Ca2+ ions can have a strong adsorption at the interface due to the strong electrostatic interactions between Ca2+ ions and SDS, which also results in the Cl- ion enrichment at the interface. Furthermore, Ca2+ ions can form pentagon-like SDS-Ca2+ complexes through SDS-Ca2+-SDS cation bridging, which renders a nonuniform distribution of SDS at the interface. On the other hand, the cation bridging density monotonically increases as ion concentration increases for the systems without Ca2+ ions, while first increases, then decreases for the systems with Ca2+ ions. This is because the accumulation of Cl- ions at the interface at high salt concentrations can melt SDS-Ca2+ complexes. This work should provide new insights into the structural and thermodynamic properties of brine-oil interfaces with an anionic surfactant, which can facilitate the optimization of chemical flooding processes.

Electrolyte Structure of Lithium Polysulfides with Anti-Reductive Solvent Shells for Practical Lithium-Sulfur Batteries.

The lithium-sulfur (Li-S) battery is regarded as a promising secondary battery. However, constant parasitic reactions between the Li anode and soluble polysulfide (PS) intermediates significantly deteriorate the working Li anode. The rational design to inhibit the parasitic reactions is plagued by the inability to understand and regulate the electrolyte structure of PSs. Herein, the electrolyte structure of PSs with anti-reductive solvent shells was unveiled by molecular dynamics simulations and nuclear magnetic resonance. The reduction resistance of the solvent shell is proven to be a key reason for the decreased reactivity of PSs towards Li. With isopropyl ether (DIPE) as a cosolvent, DIPE molecules tend to distribute in the outer solvent shell due to poor solvating power. Furthermore, DIPE is more stable than conventional ether solvents against Li metal. The reactivity of PSs is suppressed by encapsulating PSs into anti-reductive solvent shells. Consequently, the cycling performance of working Li-S batteries was significantly improved and a pouch cell of 300 Wh kg-1 was demonstrated. The fundamental understanding in this work provides an unprecedented ground to understand the electrolyte structure of PSs and the rational electrolyte design in Li-S batteries.

Structural properties of deprotonated naphthenic acids immersed in water in pristine and hydroxylated carbon nanopores from molecular perspectives.

We use molecular dynamic simulations to study the structural properties of deprotonated cyclohexanoic acid (DCHA) and heptanoic acid (DHA) immersed in water in pristine and hydroxylated carbon nanopores (PACNs and HACNs) in relation to NA removal by activated carbons (ACs). In PACNs, both NAs can aggregate on the pore surface by depleting water molecules, while water molecules accumulate in the area where there is no NA aggregation. The hydrophobic tails of NAs are generally in the interface water region (IWR), while the hydrophilic head groups prefer to be hydrated by water and form pairing with Na+ ions outside the IWR. The linear carbon tails of DHA tend to be parallel to the pore surface, while a slightly inclined configuration of the carbon ring in DCHA is observed. In HACNs, water forms a predominant interface layer by forming hydrogen bonding with -OH groups, while the hydrophobic tails of NAs are driven away from the surface. Both NAs have a more perpendicular orientation close to the pore surface with their hydrophilic head groups forming hydrogen bonding with -OH groups. The strong water film greatly reduces hydrophobic interactions as well as decreases the available adsorption sites for NAs.

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.

Role of Alcohol as a Cosurfactant at the Brine-Oil Interface under a Typical Reservoir Condition.

A cosurfactant is a chemical used in combination with a surfactant to enrich the properties of the primary surfactant formulation. Understanding the roles of a cosurfactant is of great importance in designing a chemical solution with desired features. Herein, we report a molecular dynamics simulation study to explore the roles of alcohol (propanol) as a cosurfactant at a brine-oil interface in chemical flooding under a typical reservoir condition (353 K and 200 bar). We demonstrate that propanol, as a cosurfactant, can be transported through oil and brine phases; such a dislocation of propanol in the system is a dynamic process. The interfacial tension between brine and oil decreases as propanol concentration in the system increases. This is because propanol can form hydrogen bonds with water molecules while it decreases the density of hydrogen bonds formed between the surfactant and water. The introduction of propanol does not always increase the local fluidity of surfactants at the interfaces. A local maximum fluidity was observed when the surfactants are more perpendicular to the interfaces. Our work should provide important insights into the design of the surfactant formulas for chemical flooding during enhanced oil recovery.

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.

Effects of Moisture Contents on Shale Gas Recovery and CO2 Sequestration.

Enhanced recovery of shale gas with CO2 injection has attracted extensive attention as it combines the advantages of improved efficiency of shale gas recovery and reduced greenhouse gas emissions via CO2 geological sequestration. On the other hand, the microscopic mechanism of enhanced shale gas recovery with CO2 injection and the influence of the subsurface water confined in the shale nanopores remain ambiguous. Here, we use grand canonical Monte Carlo (GCMC) simulations to investigate the effect of moisture on the shale gas recovery and CO2 sequestration by calculating the adsorption of CH4 and CO2 in dry and moist kerogen slit pores. Simulation results indicate that water accumulates in the form of clusters in the middle of the kerogen slit pore. Formation of water clusters in kerogen slit pores reduces pore filling by methane molecules, resulting in a decrease in the methane sorption capacity. For the sorption of CH4/CO2 binary mixtures in kerogen slit pores, the CH4 sorption capacity decreases as the moisture content increases, whereas the effect of moisture on CO2 sorption capacity is related to its mole fraction in the CH4/CO2 binary mixture. Furthermore, we propose a reference route for shale gas recovery and find that the pressure drawdown and CO2 injection exhibit different mechanisms for gas recovery. Pressure drawdown mainly extracts the CH4 molecules distributed in the middle of kerogen slit pores, while CO2 injection recovers CH4 molecules from the adsorption layer. When the water content increases, the recovery ratio of the pressure drawdown declines, while that of CO2 injection increases, especially in the first stage of CO2 injection. The CO2 sequestration efficiency is higher under higher water content. These findings provide the theoretical foundation for optimization of the shale gas recovery process, as well as effective CO2 sequestration in depleted gas reservoirs.

Natural gas vaporization in a nanoscale throat connected model of shale: multi-scale, multi-component and multi-phase.

Production of hydrocarbons from shale is a complex process that necessitates the extraction of multi-component hydrocarbons trapped in multi-scale nanopores. While advances in nanofluidics have allowed researchers to probe thermodynamics and transport in single, discrete nanochannels, these studies present a highly simplified version of shale reservoirs with homogeneous pore structures and/or single-component fluid compositions. In this study, we develop and apply a 30 000-pore nanomodel that captures the inherent heterogeneity in reservoir pore sizes (100 nm pores gated by 5 nm-pores) to study vaporization of a representative natural gas hydrocarbon mixture. The nanomodel matches major North American formations in the volumetric and number contributions of the pore sizes, porosity (10.5%), and ultra-low permeability (44 nD). Combined experimental and analytical results show 3000× slower vaporization owing to the nanoscale throat bottlenecks. At low temperatures, mixture effects reduce rates further with stochastic vaporization of light components in large pores dominating. Collectively this approach captures the coupled complexity of multicomponent, multiphase fluids in multiscale geometries that is inherent to this resource.

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.