Balancing Group I Monatomic Ion-Polar Compound Interactions for Condensed Phase Simulation in the Polarizable Drude Force Field.

Molecular dynamics (MD) simulations are a commonly used method for investigating molecular behavior at the atomic level. Achieving reliable MD simulation results necessitates the use of an accurate force field. In the present work, we present a protocol to enhance the quality of group 1 monatomic ions (specifically Li+, Na+, K+, Rb+, and Cs+) with respect to their interactions with common polar model compounds in biomolecules in condensed phases in the context of the Drude polarizable force field. Instead of adjusting preexisting individual parameters for ions, model compounds, and water, we employ atom-pair specific Lennard-Jones (LJ) (known as NBFIX in CHARMM) and through-space Thole dipole screening (NBTHOLE) terms to fine-tune the balance of ion-model compound, ion-water, and model compound-water interactions. This involved establishing a protocol for the optimization of NBFIX and NBTHOLE parameters targeting the difference between molecular mechanical (MM) and quantum mechanical (QM) potential energy scans (PES). It is shown that targeting PES involving complexes that include multiple model compounds and/or ions as trimers and tetramers yields parameters that produce condensed phase properties in agreement with experimental data. Validation of this protocol involved the reproduction of experimental thermodynamic benchmarks, including solvation free energies of ions in methanol and N-methylacetamide, osmotic pressures, ionic conductivities, and diffusion coefficients within the condensed phase. These results show the importance of including more complex ion-model compound complexes beyond dimers in the QM target data to account for many-body effects during parameter fitting. The presented parameters represent a significant refinement of the Drude polarizable force field, which will lead to improved accuracy for modeling ion-biomolecular interactions.

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.

Blending to Make Nonhealable Polymers Healable: Nanophase Separation Observed by CP/MAS 13 C NMR Analysis.

Can commodity polymers are made to be healable just by blending with self-healable polymers? Here we report the first study on the fundamental aspect of this practically challenging issue. Poly(ether thiourea) (PTUEG3 ; Tg =27 °C) reported in 2018 is extraordinary in that it is mechanically robust but can self-heal even at 12 °C. In contrast, poly(octamethylene thiourea) (PTUC8 ; Tg =50 °C), an analogue of PTUEG3 , cannot heal below 92 °C. We found that their polymer blend self-healed in a temperature range above 32 °C even when its PTUEG3 content was only 20 mol %. Unlike PTUEG3 alone, this polymer blend, upon exposure to high humidity, barely plasticized, keeping its excellent mechanical properties due to the non-hygroscopic nature of the PTUC8 component. CP/MAS 13 C NMR analysis revealed that the polymer blend was nanophase-separated, which possibly accounts for why such a small amount of PTUEG3 provided the polymer blend with humidity-tolerant self-healable properties.

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.

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.

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.

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.

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.

A hybrid theoretical method for predicting electrokinetic energy conversion in nanochannels.

The traditional methods to predict electrokinetic energy conversion (EKEC) in nanochannels are mostly based on the Navier-Stokes (NS) equation for ionic flow and the Poisson-Boltzmann (PB) equation for charge distributions, which is questionable for ion transport through highly charged nanochannels. In this work, the classical density functional theory (cDFT) is used together with molecular dynamics (MD) simulation and the Navier-Stokes (NS) equation to predict the electrical current and the thermodynamic efficiency of electrokinetic energy conversion in nanochannels. By introducing numerical results for the slip length calculated from MD simulation, a significant increase of the electrokinetic current is predicted in comparison to that obtained from the traditional electrokinetic equations with the non-slip boundary condition, leading to the theoretical predictions of the thermodynamic efficiency for electrokinetic energy conversion in nanochannels in good agreement with recent experiments. The hybrid method predicts that maximum electrokinetic efficiency can be achieved by tuning the channel height and solution conditions including electrolyte concentrations, ion valences, and surface energies. The theoretical results provide new insights into pressure-driven electrical energy generation processes and helpful guidelines for engineering design and optimization of electrokinetic energy conversion.

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.

Mechanically robust, readily repairable polymers via tailored noncovalent cross-linking.

Expanding the range of healable materials is an important challenge for sustainable societies. Noncrystalline, high-molecular-weight polymers generally form mechanically robust materials, which, however, are difficult to repair once they are fractured. This is because their polymer chains are heavily entangled and diffuse too sluggishly to unite fractured surfaces within reasonable time scales. Here we report that low-molecular-weight polymers, when cross-linked by dense hydrogen bonds, yield mechanically robust yet readily repairable materials, despite their extremely slow diffusion dynamics. A key was to use thiourea, which anomalously forms a zigzag hydrogen-bonded array that does not induce unfavorable crystallization. Another key was to incorporate a structural element for activating the exchange of hydrogen-bonded pairs, which enables the fractured portions to rejoin readily upon compression.