Molecular-scale Structure Research Articles

Achieving Significantly Boosted Dielectric Energy Density of Polymer Film via Introducing a Bumpy Gold/Polymethylsilsesquioxane Granular Blocking Layer.

Polymer dielectrics are the key materials for pulsed energy storage systems, but their low energy densities greatly restrict the applications in integrated electronic devices. Herein, a unique bumpy granular interlayer consisting of gold nanoparticles (Au NPs) and polymethyksesquioxane (PMSQ) microspheres is introduced into a poly(vinylidene fluoride) (PVDF) film, forming trilayered PVDF-Au/PMSQ-PVDF films. Interestingly, the Au/PMSQ interlayer arouses a dielectric enhancement of 47% and an ultrahigh breakdown strength of 704MVm-1, which reaches 153% of pure PVDF. It is revealed that the greatly enhanced breakdown strength originated from the Coulomb-blockade effect of Au NPs and the excellent insulating properties of PMSQ microspheres with a special molecular-scale organic-inorganic hybrid structure. Benefiting from the concurrently enhanced dielectric and breakdown performances, an outstanding energy density of 22.42Jcm-3 with an efficiency of 67.1%, which reaches 249% of that of the pure PVDF, is achieved. It is further confirmed that this design strategy is also applicable to linear dielectric polymer polyethyleneimine. The composites exhibit an energy density of 8.91Jcm-3 with a high efficiency of ≈95%. This work offers a novel and efficient strategy for concurrently enhancing the dielectric and breakdown performances of polymers toward pulsed power applications.

Long timescale molecular dynamics simulations of carboxylic acid-modified anatase TiO[formula omitted](101)-water interfaces using ab-initio deep neural network potentials

Carboxylic acid-modified anatase TiO2-water interfaces are widely relevant, yet understanding of their molecular scale structure is limited. To help improve this understanding, we here construct a deep neural network potential (DP) that accurately represents the potential energy surface of the formic (FA) and acetic acid (AA)-covered anatase TiO2(101) (A101) interfaces with water predicted by Density Functional Theory (DFT) with the SCAN exchange–correlation functional. Long time-scale (ns) Molecular Dynamics simulations employing such DP provide insight into the hydration structure at the interface, showing how the water density profile and radial distribution functions depend on the coverage and adsorption configurations of the acids. The developed model sets the stage for estimating the adsorption energetics of these small carboxylic acids on the A101 surface in an aqueous environment.

Spatial organization of the ions at the free surface of imidazolium-based ionic liquids

HypothesisExperimental information on the molecular scale structure of ionic liquid interfaces is controversial, giving rise to two competing scenarios, namely the double layer-like and “chessboard”-like structures. This issue can be resolved by computer simulation methods, at least for the underlying molecular model. Systematically changing the anion type can elucidate the relative roles of electrostatic interactions, hydrophobic (or, strictly speaking, apolar) effects and steric restrictions on the interfacial properties. SimulationsMolecular dynamics simulation is combined with intrinsic analysis methods both at the molecular and atomic levels, supplemented by Voronoi analysis of self-association. FindingsWe see no evidence for the existence of a double-layer-type arrangement of the ions, or for their self-association at the surface of the liquid. Instead, our results show that cation chains associate into apolar domains that protrude into the vapour phase, while charged groups form domains that are embedded in this apolar environment at the surface. However, the apolar chains largely obscure the cation groups, to which they are bound, while the smaller and more mobile anions can more easily access the free surface, leading to a somewhat counterintuitive net excess of negative charge at the interface. Importantly, this excess charge could only be identified by applying intrinsic analysis.

Molecular geochemistry of radium: A key to understanding cation adsorption reaction on clay minerals

Adsorption reactions of various cations on clay minerals have different effects on their environmental behaviors depending on the molecular-scale adsorption structure. Some cations form outer-sphere complexes via hydration, while others create inner-sphere complexes through dehydration. This preference dictates their environmental impact. However, the factors controlling these complex formations remain unclear. Furthermore, research on the adsorption preferences of radium (Ra) is lacking. Thus, this study conducted the first EXAFS study of Ra2+ adsorbed on clay minerals and showed that Ra2+ forms inner-sphere complexes on vermiculite, which can be surprising because Ra2+ is a divalent cation and prefers to be hydrated. In order to investigate the factors controlling the complex formations, this study conducted systematic EXAFS measurements and DFT calculations for alkali and alkaline earth metal cations. The results showed the importance of the size-matching effect between the adsorbed cation and the cavity of the tetrahedral sheets and that the complex formation can be estimated by the combination of the ionic radius and hydration enthalpy of the adsorbed cation. Furthermore, this study also analyzed environmental core samples. Their results showed the fixation of Ra2+ by clay minerals and the controlling factors can effectively predict cation environmental behavior.

The role of surface thermodynamics and kinetics in the removal of PFOA from aqueous solutions

Perfluorooctanoic acid (PFOA) has been extensively used as surfactant in industrial applications. Human exposure to PFOA through contaminated water has been linked to serious adverse health effects. In this work, the removal of PFOA from water in all-silica zeolites, which are hydrophobic materials with diverse pore geometries and exceptional hydrothermal stability, is studied. Molecular scale structure, dynamics, kinetics, and free energy landscapes associated with PFOA adsorption are characterized. Interfacial adsorption constitutes the rate limiting step and the adsorption of PFOA is orientation competitive. The PFOA orientation where the hydrophobic perfluorinated methyl group is adsorbed first on the zeolite surface is thermodynamically favored; whereas the adsorption kinetics is faster when the hydrophilic carboxyl group is adsorbed first. Furthermore, the adsorption of PFOA in deprotonated state in hydrophobic pores is thermodynamically prohibitive. Based on computed permeabilities in the pores and kinetic rates associated with the adsorption of PFOA from water, three zeolites, MTW, VET and GON, are estimated to exhibit several orders of magnitude better PFOA removal performance compared to the benchmark material, zeolite Beta (BEA).

Comparison between the intermolecular interactions in the liquid and solid forms of propellant 1,1,1,2 – Tetrafluoroethane

A multitude of metered dose inhaler (MDI) formulation properties, such as permanent particle growth or separation, can affect the device’s drug delivering performance. 1,1,1,2 – Tetrafluoroethane (HFA-134a) is a liquid propellant commonly employed in MDIs. Here molecular modelling approaches are reported to evaluate the effect weak hydrogen bonding and steric repulsions have on its solid and liquid state molecular-scale structure. Molecular dynamics (MD) simulations of pure liquid HFA-134a were completed for a range of temperatures (203 – 323 K) and intermolecular interactions were analysed using the molecular trajectory files generated. Empirical force fields were used to calculate the strength of interactions seen in the low-temperature crystal structure, and these were compared to the liquid.The solid-state intermolecular structure arranges to maximise attractive interactions between permanent dipoles which enables the formation of ‘weak hydrogen bonds’, these were found to guide the transition from body centred cubic to its low temperature, monoclinic phase. Reducing HFA-134a’s liquid phase temperature increases its resemblance to the believed body centred cubic structure, with improved similarity between intermolecular radial distribution functions and vector angles. Weak hydrogen bonds in the liquid occurred more frequently at lower temperatures but the minimisation of steric repulsions is identified as the main cause of structural ordering, this shows liquid propellant is relatively non-polar. Understanding the structure of pure propellant is useful when evaluating the variation in wetting between different formulation materials and has the potential to be a useful resource for digital drug product design.

Molecular Scale Structure and Kinetics of Layer-by-Layer Peptide Self-Organization at Atomically Flat Solid Surfaces.

Understanding the mechanisms of self-organization of short peptides into two- and three-dimensional architectures are of great interest in the formation of crystalline biomolecular systems and their practical applications. Since the assembly is a dynamic process, the study of structural development is challenging at the submolecular dimensions continuously across an adequate time scale in the natural biological environment, in addition to the complexities stemming from the labile molecular structures of short peptides. Self-organization of solid binding peptides on surfaces offers prospects to overcome these challenges. Here we use a graphite binding dodecapeptide, GrBP5, and record its self-organization process of the first two layers on highly oriented pyrolytic graphite surface in an aqueous solution by using frequency modulation atomic force microscopy in situ. The observations suggest that the first layer forms homogeneously, generating self-organized crystals with a lattice structure in contact with the underlying graphite. The second layer formation is mostly heterogeneous, triggered by the crystalline defects on the first layer, growing row-by-row establishing nominally diverse biomolecular self-organized structures with transient crystalline phases. The assembly is highly dependent on the peptide concentration, with the nucleation being high in high molecular concentrations, e.g., >100 μM, while the domain size is small, with an increase in the growth rate that gradually slows down. Self-assembled peptide crystals are composed of either singlets or doublets establishing P1 and P2 oblique lattices, respectively, each commensurate with the underlying graphite lattice with chiral crystal relations. This work provides insights into the surface behavior of short peptides on solids and offers quantitative guidance toward elucidating molecular mechanisms of self-assembly helping in the scientific understanding and construction of coherent bio/nano hybrid interfaces.

Characterization and Analysis of Molecular-Scale Pore Structure of Coal with Different Metamorphic Degrees

To explore the micropore structure of coal, this paper proposes a method to reconstruct coal pores at the molecular scale, characterizing and analyzing the micropore structure of coal with different metamorphic degrees. Based on the construction of coal molecules, the molecular-scale pore models of coal were reconstructed by the Monte Carlo method. The pore models were visualized and quantitatively characterized via the principles of the maximum sphere algorithm. The molecular-scale pore structure parameters and morphological characteristics of coal with different metamorphic degrees were analyzed. Combined with CO2 adsorption experiments, the rationality of coal molecular-scale pore reconstruction and characterization was verified. The results are as follows: the pore size of molecular-scale pores is distributed over 0.2–0.7 nm, and the throat diameter is mainly distributed over 0.1–0.5 nm. With the increase of metamorphic degree, the specific surface area and pore volume first decreased and then increased, and the number of pores showed a rapid–slow–rapid rise trend. In addition, the pore shape tends to be simpler and then more complex, the pore connectivity decreases and then increases, while the coal development becomes progressively more uniform. During the evolution of the molecular structure, the aliphatic structure and oxygen-containing functional groups in the coal are shed and the intermolecular pores gradually decrease. As the aromatic structure increases, the internal molecular pores gradually increase. The research provides a new method for exploring the molecular-scale pore structure of coal and has practical significance for understanding the microscopic characteristics of coal with different metamorphic degrees.

Molecular-scale structural changes of silicate melts under tension revealed by time-resolved X-ray diffraction

The non-Newtonian behavior of silicate melts influences magma flow dynamics in a volcanic conduit. Shear thinning causes a reduction in viscosity and brittle failure induces magma fragmentation, resulting in explosive eruptions that release magma fragments into the surface with volcanic gases. The conditions under which they occur have been investigated by macroscopic rheological experiments; however, the understanding of the molecular-scale origin of shear thinning and brittle failure of silicate melts remains poor, partially because of the experimental difficulty of in situ investigations of molecular-scale melt structures under deformation. In this study, we performed tension experiments for silicate melts at temperatures of 760–840 °C and investigated the melt structure forming intermediate-range ordering using time-resolved X-ray diffraction. Our experiments demonstrate that the contribution of anisotropic elastic deformation to molecular-scale strain increases with increasing stress. The mechanical data for stress relaxation indicate that the relaxation timescale decreases with an increase in stress, implying viscosity reduction with an increase in stress. Based on this, we infer that when the stress is large, the molecular-scale structure elastically dilates in the direction of the deformation under tension, causing a decrease in melt viscosity, and the melt may cause macroscopic failure through local anisotropic dilation and the resultant cavitation. This process seems to depend on experimental geometry, such as tension, compression, and torsion; hence, the relationship between experimental geometry, molecular-scale structure, and non-Newtonian behavior must be further investigated in future studies.

The role of water at electrified metal-water interfaces unravelled from first principles

Electrified metal-water interfaces are a key feature in many devices and processes, finding applications in a variety of fields, including corrosion, photo/electrocatalysis, and energy. For over 200 years after Faraday established his ground-breaking law, electrochemistry has been subject to intense research, due to its broad applications and interdisciplinary nature. However, it remains an open challenge to provide a comprehensive description of molecular scale structure and processes at electrified interfaces in electrochemical conditions. In this review, we highlight the most recent advancements in the application of ab initio molecular dynamics (AIMD) to study metal-water interfaces, noting how this work aligns with experimental and other theoretical models. We show how AIMD simulations have unveiled new perspectives on the crucial impact of water on determining surface potentials, the chemistry of surface adsorbates, and capacitive response under electrochemical conditions. We highlight how this work aligns with experimental and other simplified theoretical models.

Molecular Resolution Imaging of Adsorption of Dodecane on a Lignite Surface by Frequency Modulation AFM

The consumption of dodecane, a critical flotation collector, in the flotation of low-rank coal is particularly high; however, the adsorption mechanism of dodecane on coal still remains unclear. To shed light on this issue, the molecular-scale adsorption structure of dodecane on a lignite surface was investigated in situ by frequency modulation atomic force microscopy (FM-AFM) for the first time. The adsorption structure of dodecane on highly oriented pyrolytic graphite (HOPG) and mica has been detected in advance. Meanwhile, molecular dynamics (MD) simulations are utilized to elucidate and illustrate the adsorption structure on HOPG, mica, and lignite. Experimental results from FM-AFM show that a layered structure of dodecane molecules is formed on HOPG, while there is no stable adsorption structure on mica. Meanwhile, a few local monolayer structures of dodecane molecules are observed on the surface of lignite, and their thickness is approximately 1 nm, which is less than the molecular chain length of dodecane. It is speculated that the dodecane molecules are adsorbed on lignite selectively, with the capacity to form a stable adsorption structure. As indicated by MD simulations, the interaction energy between dodecane and the mentioned surfaces is in the order HOPG > lignite > mica. And the MD simulation results show a desirable match with direct observation by FM-AFM. In accordance with the characterization of the lignite sample surface, large amounts of polar oxygen-containing functional groups may hinder dodecane adsorption behavior. Through in situ molecular resolution imaging of the interface structure between dodecane and a raw lignite surface, we provide a new perspective for exploring the interaction mechanism between nonpolar collectors and a lignite surface.

Molecular-scale elucidating of lignocellulose biomass char steam gasification for ultimately converting to syngas

Lignocellulosic biomass char gasification plays an irreplaceable role in alleviating environmental problems caused by greenhouse effects. Elucidating the molecular-scale gasification mechanism is significant in ultimately converting the lignocellulosic biomass char to clean syngas and reducing pollutant emissions. In this study, the molecular-scale structure of sawdust char (SC) is carefully established using 13C NMR, FTIR, and XPS. The results compare well with the results of previous work. The mechanism of its steam gasification is investigated by molecular dynamics simulation. Results show that the main products of the gasification process after complete reaction are H2 and CO, and CO is always released before H2. The gasification equivalent ratio decreases, H2 increases significantly and is more from steam. The maximum carbon number of single molecule in the product is re-increased, and the increase of H2O molecules hinders the decomposition of light tar into small molecules of gas. The reaction mechanism is clearly defined: the aromatic carbon rings substituted by oxygen-containing functional groups are firstly gasified, followed by bridge bond breakage of the macromolecular char into several multiple aromatic rings. Each of the rings is gasified separately and ring-opening to form C5-C13 (light tar). Finally, C5-C13 completes the conversion to generate a large amount of H2, accompanied by CH4 reforming and dehydrogenating small molecule compounds. The migration paths of N elements are pyrrole (N)→CN→CON→C1-3H1-3N→HCN. This work elucidates the steam gasification process of lignocellulosic biomass char at a molecular scale, which provides a theoretical basis for the clean utilization of biomass and CO2 reduction.

Optically Induced Anisotropy in Time-Resolved Scattering: Imaging Molecular-Scale Structure and Dynamics in Disordered Media with Experiment and Theory.

Time-resolved scattering experiments enable imaging of materials at the molecular scale with femtosecond time resolution. However, in disordered media they provide access to just one radial dimension thus limiting the study of orientational structure and dynamics. Here we introduce a rigorous and practical theoretical framework for predicting and interpreting experiments combining optically induced anisotropy and time-resolved scattering. Using impulsive nuclear Raman and ultrafast x-ray scattering experiments of chloroform and simulations, we demonstrate that this framework can accurately predict and elucidate both the spatial and temporal features of these experiments.

Structured Water Molecules on Membrane Proteins Resolved by Atomic Force Microscopy.

Water structuring on the outer surface of protein molecules called the hydration shell is essential as well as the internal water structures for higher-order structuring of protein molecules and their biological activities in vivo. We now show the molecular-scale hydration structure measurements of native purple membrane patches composed of proton pump proteins by a noninvasive three-dimensional force mapping technique based on frequency modulation atomic force microscopy. We successfully resolved the ordered water molecules localized near the proton uptake channels on the cytoplasmic side of the individual bacteriorhodopsin proteins in the purple membrane. We demonstrate that the three-dimensional force mapping can be widely applicable for molecular-scale investigations of the solid-liquid interfaces of various soft nanomaterials.

Water-Induced Restructuring of the Surface of a Deep Eutectic Solvent.

We study the molecular-scale structure of the surface of Reline, a DES made from urea and choline chloride, using heterodyne-detected vibrational sum frequency generation (HD-VSFG). Reline absorbs water when exposed to the ambient atmosphere, and following structure-specific changes at the Reline/air interface is crucial and difficult. For Reline (dry, 0 wt %, w/w, water) we observe vibrational signatures of both urea and choline ions at the surface. Upon increase of the water content, there is a gradual depletion of urea from the surface, an enhanced alignment, and an enrichment of the surface with choline cations, indicating surface speciation of ChCl. Above 40% w/w water content, choline cations abruptly deplete from the surface, as evidenced by the decrease of the vibrational signal of the −CH2– groups of choline and the rapid rise of a water signal. Above 60% w/w water content, the surface spectrum of aqueous Reline becomes indistinguishable from that of neat water.

Molecular Control of Interfacial Fibronectin Structure on Graphene Oxide Steers Cell Fate.

The use of graphene-based materials (GBMs) for tissue-engineering applications has been growing exponentially because of the seemingly endless multifunctional and tunable physicochemical properties of graphene that can be exploited to influence cellular behavior. Despite many demonstrations wherein cell physiology has been modulated on different GBMs, a clear mechanism connecting the different physicochemical properties of GBMs to cell fate has remained elusive. In this work, we demonstrate how different GBMs can be used to bias cell fate in a multiscale study-starting from serum protein (fibronectin) adsorption and its molecular scale morphology, structure, and bioactivity and ending with stem cell response. Using heat to chemically reduce graphene oxide without changing physical properties, we show that graphene chemistry controls surface-adsorbed molecular conformation and morphology, epitope presentation, and stem cell attachment. Moreover, this subtle change in the protein structure was found to drive increased bone differentiation of stem cells, suggesting that the physicochemical properties of graphene biases cell fate by directly influencing the adsorbed protein structure and subsequent biochemical activity.

Three-Dimensional Molecular Mapping of Ionic Liquids at Electrified Interfaces.

Electric double layers (EDLs), occurring ubiquitously at solid-liquid interfaces, are critical for electrochemical energy conversion and storage processes such as capacitive charging and redox reactions. However, to date the molecular-scale structure of EDLs remains elusive. Here we report an advanced technique, electrochemical three-dimensional atomic force microscopy (EC-3D-AFM), and use it to directly image the molecular-scale EDL structure of an ionic liquid under different electrode potentials. We observe not only multiple discrete ionic layers in the EDL on a graphite electrode but also a quasi-periodic molecular density distribution within each layer. Furthermore, we find pronounced 3D reconfiguration of the EDL at different voltages, especially in the first layer. Combining the experimental results with molecular dynamics simulations, we find potential-dependent molecular redistribution and reorientation in the innermost EDL layer, both of which are critical to EDL capacitive charging. We expect this mechanistic understanding to have profound impacts on the rational design of electrode-electrolyte interfaces for energy conversion and storage.

In-situ/operando X-ray absorption spectroscopic investigation of the electrode/electrolyte interface on the molecular scale

A method for performing X-ray absorption spectroscopy (XAS) in the soft X-ray region was developed to investigate the surface-sensitive electron signal of electrode/electrolyte interfaces in common multivalent based organic solvents under in situ/operando conditions. Our approach enables us to probe the molecular-scale structure of electrode interfaces by measuring total electron yield and is suitable for redox systems exhibiting low intrinsic electrochemical current. In-situ F K-edge XAS measurements in a 0.5 M Magnesium bis(trifluoromethanesulfonimide)/2-Methyltetrahydrofuran (Mg(TFSI)2/2-MeTHF) electrolyte were carried out to determine the evolution of interfacial species during the electrochemical charging/discharging process. Time-dependent density-functional theory (TD-DFT) simulation indicate that the F K-edge evolution is the result of interfacial chemical environment change driven by electrochemical potential. In addition, we performed the “operando XAS” which runs the CV and collects spectra simultaneously. By using this method, the non-equilibrium state of condensed matter interfaces and interfacial dynamical transient process can be revealed.

Cross-Plane and In-Plane Heat Conductions in Layer-by-Layer Membrane: Molecular Dynamics Study.

A material with anisotropic heat conduction characteristics, which is determined by molecular scale structure, provides a way of controlling heat flow in nanoscale spaces. As such, here, we consider layer-by-layer (LbL) membranes, which are an electrostatic assembly of polyelectrolyte multilayers and are expected to have different heat conduction characteristics between cross-plane and in-plane directions. We constructed models of a poly(acrylic acid)/polyethylenimine (PAA/PEI) LbL membrane sandwiched by charged solid walls and investigated their anisotropic heat conduction using molecular dynamics simulations. In the cross-plane direction, the thermal boundary resistance between the solid wall and the LbL membrane and that between the constituent PAA and PEI layers decrease with increasing degree of ionization (solid surface charge density and the number of electric charges per PAA/PEI molecule). When the degree of ionization is low, the cross-plane thermal conductivity of a constituent layer is higher than that of the bulk state. As the degree of ionization increases, however, the cross-plane thermal conductivity of PAA, a linear polymer, decreases because of the increase in the number of in-plane oriented polymer chains. In the in-plane direction, we investigated the heat conduction of each layer and found the enhancement of effective in-plane thermal conductivity again due to the in-plane oriented chain alignment. The heat conduction in the LbL membrane is three-dimensionally enhanced compared to those in the bulk states of the constituent polymers because of the electrostatic interactions in the cross-plane direction and the molecular alignment in the in-plane direction.

Negatively Charged Lipids Exhibit Negligible Effects on the Water Repellency of Montmorillonite Films.

Amphiphilic molecules can alter the wettability of soil minerals. To determine how the headgroup chemistry of amphiphiles determines these effects, we investigate a system of the clay montmorillonite with long-chain phospholipids. We use phosphatidylglycerol (PG) phospholipids to contrast with our previous work using phosphatidylethanolamine (PE) lipids. Zwitterionic PE lipids can sorb to the negatively charged montmorillonite surface, whereas negatively charged PG lipids cannot. Employing a suite of techniques from molecular dynamics, atomic force microscopy, fluorescence microscopy, and contact angle measurements, we define sample characteristics from molecular-scale structure to the macroscopic wettability. We find that PG lipids do not significantly alter montmorillonite wetting characteristics, such as the contact angle, flow viscosity, and the characteristic time scale for droplet imbibition. On comparing PE and PG lipid/clay films, we find that, among the phospholipids compared, they must have three characteristics to change clay/lipid film wettability: they must bind to the mineral surface, be solid at room temperature, and have a relatively continuous distribution throughout the film.