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.
Oil–brine
interfaces play an important role in oil recovery and oil–brine
separation, in which the effects of salinity on interfacial tension
(IFT) have been much of debate in the past in experiments and modeling
studies owing to complex oil compositions. In this work, we use molecular
dynamics (MD) simulations to study the oil–brine interfacial
properties by designing seven systems containing different oil compositions
(decane with/without polar compounds) and the salinity in brine of
up to ∼14 wt %. We carefully investigate the salinity and polar
component effects by analyzing IFTs, density profiles, orientation
parameters, hydrogen bond densities, and charge density profiles.
The results indicate that O-bearing compounds (phenol and decanoic
acid) can significantly reduce the oil–brine IFT and exhibit
the highest Gibbs surface excess relative to water, while the others,
including N-bearing compounds (pyridine and quinoline) and S-bearing
compounds (thiophene and benzothiophene), only slightly decrease the
oil–brine IFTs and show a relatively small Gibbs surface excess.
Increasing salinity can slightly increase the oil–brine IFT
except in the system containing phenol, which shows a decrease. Phenol
and decanoic acid incline to be perpendicular to the interface and
generate numerous hydrogen bonds with water in the interfacial region,
while others prefer to be parallel to the interface with much fewer
hydrogen bonds with water. On the other hand, salinity has an insignificant
effect on the orientation of polar molecules and hydrogen bond density
in the interfacial region. The charges at the interfaces on the brine
and oil sides are negative and positive, respectively, and the polar
compounds disturb the arrangement of water molecules in the interfacial
regions, while the addition of salt ions result in the higher peak
values of charges in terms of water and system. Our study should provide
new insights into the oil–brine interfacial issues and clarify
some unsettled disputes.
The lithium-sulfur (Li-S) battery is regarded as ap romising secondary battery.H owever,c onstant 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.H erein, the electrolyte structure of PSs with anti-reductive solvent shells was unveiled by molecular dynamics simulations and nuclear magnetic resonance.T he reduction resistance of the solvent shell is proven to be ak ey reason for the decreased reactivity of PSs towards Li. With isopropyl ether (DIPE) as acosolvent, DIPE molecules tend to distribute in the outer solvent shell due to poor solvating power. Furthermore,D IPE is more stable than conventional ether solvents against Li metal. The reactivity of PSs is suppressed by encapsulating PSs into anti-reductive solvent shells.C onsequently,the cycling performance of working Li-S batteries was significantly improved and ap ouchc ell of 300 Wh kg À1 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.
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 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.
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