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Functionalization of nanoparticles with specific ligands is helpful to control specific diagnostic and therapeutic responses such as protein adsorption, cell targeting, and circulation. Precision delivery critically depends on a fundamental understanding of the interplay between surface chemistry, ligand dynamics, and interaction with the biochemical environment. Due to limited atomic-scale insights into the structure and dynamics of nanoparticle-bound ligands from experiments, relationships of grafting density and ligand chemistry to observable properties such as hydrophilicity and protein interactions remain largely unknown. In this work, we uncover how self-assembled monolayers (SAMs) composed of multisegment ligands such as thioalkyl-PEG-(

A classical model in the framework of the INTERFACE force field has been developed for treating the LiCoO2 (LCO) (001)/water interface.

Hydroxyapatite (HAP) is the major mineral phase in bone and teeth. The interaction of individual amino acids and citrate ions with different crystallographic HAP surfaces has remained uncertain for decades, creating a knowledge gap to rationally design interactions with peptides, proteins, and drugs. In this contribution, we quantify the binding mechanisms and binding free energies of the 20 end-capped natural amino acids and citrate ions on the basal (0 0 1) and prismatic (0 1 0)/(0 2 0) planes of hydroxyapatite at pH values of 7 and 5 for the first time at the molecular scale. We utilized over 1500 steered molecular dynamics simulations with highly accurate potentials that reproduce surface and hydration energies of (hkl) hydroxyapatite surfaces at different pH values. Charged residues demonstrate a much higher affinity to HAP than charge-neutral species due to the formation of superficial ion pairs and ease of penetration into layers of water molecules on the mineral surface. Binding free energies range from 0 to −60 kJ/mol and were determined with ∼ 10% uncertainty. The highest affinity was found for citrate, followed by Asp(-) and Glu(-), and followed after a gap by Arg(+), Lys(+), as well as by His(+) at pH 5. The (hkl)-specific area density of calcium ions, the protonation state of phosphate ions, and subsurface directional order of the ions in HAP lead to surface-specific binding patterns. Amino acids without ionic side groups exhibit weak binding, between −3 and 0 kJ/mol, due to difficulties to penetrate the first layer of water molecules on the apatite surfaces. We explain recognition processes that remained elusive in experiments, in prior simulations, discuss agreement with available data, and reconcile conflicting interpretations. The findings can serve as useful input for the design of peptides, proteins, and drug molecules for the modification of bone and teeth-related materials, as well as control of apatite mineralization.

Predictive computational modeling of polymer materials is necessary for the efficient design of composite materials and the corresponding processing methods. Molecular dynamics (MD) modeling is especially important for establishing accurate processing-structure-property relationships for neat resins. For MD modeling of amorphous polymer materials, an accurate force field is fundamental to reliable prediction of properties. Reactive force fields, in which chemical bonds can be formed or broken, offer further capability in predicting the mechanical behavior of amorphous polymers subjected to relatively large deformations. To this end, the Reactive Interface Force Field (IFF-R) has been recently developed to provide an efficient means to predict the behavior of materials under these conditions. Although IFF-R has been proven to be accurate for some crystalline organic and inorganic systems, it has not yet been proven to be accurate for amorphous polymer systems. The objective of this study is to use IFF-R to predict the thermo-mechanical properties of three different epoxy systems and validate with experimental measurements. The results indicate that IFF-R predicts thermo-mechanical properties that agree closely with experiment. Therefore, IFF-R can be used to reliably establish mechanical properties of polymers on the molecular level for future design of new composite materials and processing methods.

Gas molecules and interfaces with liquids and solids play a critical role in living organisms, sorption, catalysis, and the environment. Monitoring adsorption and heterogeneous interfaces remains difficult in experiments, and earlier models for molecular simulations lead to errors over 100% in fundamental molecular properties. We introduce conceptually new force field parameters for molecular oxygen, nitrogen, and hydrogen that reduce deviations to

Cement and concrete are rapidly growing in demand and pose many unresolved chemistry questions at particle interfaces, during hydration reactions, regarding the role of electrolytes and organic additives. Solutions through developing greener, more sustainable formulations are needed to reduce the high carbon footprint that amounts to 11% of global CO2 emissions. Cement is a multiphase material composed of calcium silicates, aluminates, and other mineral phases, produced from natural and low-cost industrial sources, which undergoes complex hydration reactions. This perspective highlights current research challenges and opportunities for new chemistry insight, including intriguing colloid and interface science problems that involve mineral surfaces, electrolytes, polymers, and hydration reactions. Specifically, we discuss (1) characteristics of cement phases, supplementary cementitious materials, and other constituents, (2) hydration reactions and the characterization by imaging and NMR spectroscopy, (3) the structure of hydrated cement phases including calcium-silicate-hydrates at different scales, (4) quantitative simulation techniques from the atomic scale to microscale kinetic models, and (5) the function of organic additives. Focusing on new directions, we explain the benefits of integrating knowledge from inorganic chemistry, acid-base chemistry, polymer chemistry, reaction mechanisms, and theory to describe mesoscale cement properties and bulk properties upon manufacturing.

Boron nitride nanotubes (BNNTs) are a very promising reinforcement for future high-performance composites because of their excellent thermo-mechanical properties. To take full advantage of BNNTs in composite materials, it is necessary to have a comprehensive understanding of the wetting characteristics of various high-performance resins. Molecular dynamics (MD) simulation provides an accurate and efficient approach to establish the contact angle values of engineering polymers on reinforcement surfaces. In this research, MD simulations are used to predict the contact angle values of different epoxy systems on a BNNT surface. A reactive INTERFACE force field was parameterized and utilized in the simulations to accurately describe the interaction of the epoxy monomers with the BNNT surface. The effect of epoxy monomer type, hardener type, and temperature on the contact angle is established. The results show that contact angles decrease with increases in temperature for all the epoxy/hardener systems. The bisphenol-A based epoxy system demonstrates better wettability with the BNNT surface than the bisphenol-f based epoxy system. As wetting properties drive the resin infusion in the reinforcement materials, these results are important for the future manufacturing of high-quality BNNT/epoxy nanocomposites for high-performance applications.

We highlight the development of an integrated cyberinfrastructure for molecular dynamics simulations of nanomaterials and biomaterials from atoms to micrometers in high accuracy and with high level automation, suitable for use by experts and significantly lowering the barrier for use by non-experts. We integrated the INTERFACE force field (IFF) and the IFF surface model database for a wide range of inorganic materials into CHARMM-GUI and OpenKIM, resulting in a new CHARMM-GUI Nanomaterial Modeler as an easy-to-use, web-based interactive platform for the simulation of inorganic, organic, and biological hybrid materials ("bionanomaterials"). OpenKIM was expanded for new functionality to facilitate performance comparisons of bonded force fields such as IFF and CHARMM in addition to nonbonded (reactive) force fields. The chemistry coverage of IFF was more than doubled, including metals, oxides, minerals, 2D materials, gases, and full compatibility with solvents, drugs, proteins, DNA, lipids, polymers, and unlimited materials combinations. The project has to-date supported 5 graduate students (including minority), 3 postdocs, and multiple undergraduate students. We held a 2-day workshop with nationwide and international participation in 2021. Please see details and next steps in the poster., Award number NSF 1931587

Protein scaffolds direct the organization of amorphous precursors that transform into mineralized tissues, but the templating mechanism remains elusive. Motivated by models for the biomineralization of tooth enamel, wherein amyloid-like amelogenin nanoribbons guide the mineralization of apatite filaments, we investigated the impact of nanoribbon structure, sequence, and chemistry on amorphous calcium phosphate (ACP) nucleation. Using full-length human amelogenin and peptide analogs with an amyloid-like domain, films of β-sheet nanoribbons were self-assembled on graphite and characterized by in situ atomic force microscopy and molecular dynamics simulations. All sequences substantially reduce nucleation barriers for ACP by creating low-energy interfaces, while phosphoserines along the length of the nanoribbons dramatically enhance kinetic factors associated with ion binding. Furthermore, the distribution of negatively charged residues along the nanoribbons presents a potential match to the Ca–Ca distances of the multi-ion complexes that constitute ACP. These findings show that amyloid-like amelogenin nanoribbons provide potent scaffolds for ACP mineralization by presenting energetically and stereochemically favorable templates of calcium phosphate ion binding and suggest enhanced surface wetting toward calcium phosphates in general.

Heterogeneous catalysts play a key role in the chemical and energy industries1. To date, most industrial-scale heterogeneous catalytic reactions have relied on nanocatalysts2,3. However, despite significant progress from theoretical, experimental and computational studies4-18, identifying the active sites of alloy nanocatalysts remains a major challenge. This limitation is mainly due to an incomplete understanding of the three-dimensional (3D) atomic and chemical arrangement of different constituents and structural reconstructions driven by catalytic reactions19-22. Here, we use atomic electron tomography23 to determine the 3D local atomic structure, surface morphology and chemical composition of 11 Pt alloy nanocatalysts for the electrochemical oxygen reduction reaction (ORR). We reveal the facet, surface concaveness, structural and chemical order/disorder, coordination number, and bond length with unprecedented 3D atomic detail. The experimental 3D atomic coordinates are used by first-principles trained machine learning to identify the active sites of the nanocatalysts, which are corroborated by electrochemical measurements. A striking feature is the difference of the ORR activity of the surface Pt sites on the nanocatalysts by several orders of magnitude. Furthermore, by analyzing the structure-activity relationship, we formulate an equation named the local environment descriptor to balance the strain and ligand effects and gain quantitative insights into the ORR active sites of the Pt alloy nanocatalysts. The ability to determine the 3D atomic structure and chemical composition of realistic nanoparticles coupled with machine learning could transform our fundamental understanding of the catalytic active sites and provide a guidance for the rational design of optimal nanocatalysts.

The earlier integration of validated Lennard–Jones (LJ) potentials for 8 fcc metals into materials and biomolecular force fields has advanced multiple research fields, for example, metal–electrolyte interfaces, recognition of biomolecules, colloidal assembly of metal nanostructures, alloys, and catalysis. Here we introduce 12-6 and 9-6 LJ parameters for classical all-atom simulations of 10 further fcc metals (Ac, Ca (α), Ce (γ), Es (β), Fe (γ), Ir, Rh, Sr (α), Th (α), Yb (β)) and stainless steel. The parameters reproduce lattice constants, surface energies, water interfacial energies, and interactions with (bio)organic molecules in 0.1 to 5% agreement with experiment, as well as qualitative mechanical properties under standard conditions. Deviations are reduced up to a factor of one hundred in comparison to earlier Lennard–Jones parameters, embedded atom models, and density functional theory. We also explain a quantitative correlation between atomization energies from experiments and surface energies that supports parameter development. The models are computationally very efficient and applicable to an exponential space of alloys. Compatibility with a wide range of force fields such as the Interface force field (IFF), AMBER, CHARMM, COMPASS, CVFF, DREIDING, OPLS-AA, and PCFF enables reliable simulations of nanostructures up to millions of atoms and microsecond time scales. User-friendly model building and input generation are available in the CHARMM-GUI Nanomaterial Modeler. As a limitation, deviations in mechanical properties vary and are comparable to DFT methods. We discuss the incorporation of reactivity and features of the electronic structure to expand the range of applications and further increase the accuracy.

Human transport to Mars and deep-space explorations demand the development of new materials with extraordinarily high performance-to-mass ratios. Promising candidates to fulfill these requirements ...

Systematic control of grain boundary densities in various platinum (Pt) nanostructures was achieved by specific peptide-assisted assembly and coagulation of nanocrystals. A positive quadratic correlation was observed between the oxygen reduction reaction (ORR) specific activities of the Pt nanostructures and the grain boundary densities on their surfaces. Compared to commercial Pt/C, the grain-boundary-rich strain-free Pt ultrathin nanoplates demonstrated a 15.5 times higher specific activity and a 13.7 times higher mass activity. Simulation studies suggested that the specific activity of ORR was proportional to the resident number and the resident time of oxygen on the catalyst surface, both of which correlate positively with grain boundary density, leading to improved ORR activities.

Molybdenum disulfide (MoS2) is a layered material with outstanding electrical and optical properties. Numerous studies evaluate the performance in sensors, catalysts, batteries, and composites that can benefit from guidance by simulations in all-atom resolution. However, molecular simulations remain difficult due to lack of reliable models. We introduce an interpretable force field for MoS2 with record performance that reproduces structural, interfacial, and mechanical properties in 0.1% to 5% agreement with experiments. The model overcomes structural instability, deviations in interfacial and mechanical properties by several 100%, and empirical fitting protocols in earlier models. It is compatible with several force fields for molecular dynamics simulation, including the interface force field (IFF), CVFF, DREIDING, PCFF, COMPASS, CHARMM, AMBER, and OPLS-AA. The parameters capture polar covalent bonding, X-ray structure, cleavage energy, infrared spectra, bending stability, bulk modulus, Young's modulus, and contact angles with polar and nonpolar solvents. We utilized the models to uncover the binding mechanism of peptides to the MoS2 basal plane. The binding strength of several 7mer and 8mer peptides scales linearly with surface contact and replacement of surface-bound water molecules, and is tunable in a wide range from −86 to −6 kcal mol−1. The binding selectivity is multifactorial, including major contributions by van-der-Waals coordination and charge matching of certain side groups, orientation of hydrophilic side chains towards water, and conformation flexibility. We explain the relative attraction and role of the 20 amino acids using computational and experimental data. The force field can be used to screen and interpret the assembly of MoS2-based nanomaterials and electrolyte interfaces up to a billion atoms with high accuracy, including multiscale simulations from the quantum scale to the microscale., We introduce a cutting-edge force field for molybdenum disulfide and use it to uncover mechanisms of peptide recognition and design.

Ligand-protected metal nanoparticles are widely used in heterogeneous catalysis and biomass upgrading. Thiolate surfactants can greatly improve the overall yield; however, the dynamics of the react...

Oxidation and corrosion have a significant economic footprint. Mo-based alloys are a strong candidate for structural materials with oxidation resistance at high temperatures. However, understanding of the mechanisms remains limited as experimental techniques do not reach atomic-scale resolution. We examined the mechanism of oxidation of Mo3Si (A15 phase) in Mo-Si-B alloys, the emergence of a superficial silica scale, and explain available experimental data up to the large nanometer scale using chemically detailed reactive simulations. We introduce new simulation protocols for layer-by-layer oxidation and simple force fields for the reactants, intermediates, and products. Growth of thin superficial silica layers as a function of temperature and oxidation rate on the (001) surface involves the formation of silica clusters, rings, and chains with pore sizes of 0 to 2 nm. An increase in temperature from 800 to 1000°C slightly decreased the pore size and lead to less accumulation of Mo oxides at the interface, consistent with observations by electron tomography and energy dispersive X-ray spectroscopy (EDS). The elimination of gaseous MoOx is essential to form open channels and much larger pores up to 100 nm size as observed by 3D tomography, in-situ transmission electron microscopy (TEM) and scanning electron microscopy (SEM) as the oxide phase grows. According to the simulation, these large pores would otherwise be closed. The rate of oxidation, represented by successive oxidation of layers of variable thickness per unit time, influences the structure and cohesion of silica layers. High rates of oxidation can destabilize and break apart the silica layer, supported by a very wide pore size distribution in electron tomography. Limitations of the simulations in time scale currently restrict the analysis to few-layer oxidation. Within these bounds, the proposed simulation protocols can provide insight into the oxidation of (hkl) surfaces, grain boundaries, and various alloys compositions up to the 100 nm scale in atomic-level detail.

Obtaining insights into the adsorption and assembly of polyelectrolytes on chemically variable calcium silicate hydrate (C-S-H) surfaces at the atomic scale has been a longstanding challenge in the chemistry of sustainable building materials and mineral–polymer interactions. Specifically, polycarboxylate ethers (PCEs) based on acrylate and poly(ethylene glycol) acrylate co-monomers are widely used to engineer the fluidity and hydration of cement and play an important role in the search for building materials with a lower carbon footprint. We report the first systematic study of PCE interactions with C-S-H surfaces at the molecular level using simulations at single molecule coverage and comparisons to experimental data. The mechanism of adsorption of the ionic polymers is a two-step process with initial cation adsorption that reverses the mineral surface charge, followed by adsorption of the polymer backbone through ion pairing. Free energies of binding are tunable in a wide range of 0 to −5 kcal mol−1 acrylate monomer. Polymer attraction increases for higher calcium-to-silicate ratio of the mineral and higher pH value in solution, and varies significantly with PCE composition. Thereby, successive negatively charged carboxylate groups along the backbone induce conformation strain and local detachment from the surface. Polyethylene glycol (PEG) side chains in the copolymers avoid contact with the C-S-H surfaces. The results guide in the rational design of adsorption strength and conformations of the comb copolymers, and lay the groundwork to explore the vast phase space of C-S-H compositions, surface morphologies, electrolyte conditions, and PCE films of variable surface coverage. Chemically similar minerals and copolymers also find applications in other structural and biomimetic materials.

Ionic liquids (ILs) derive their useful properties from molecularly tunable compositions, but methods to diversify anion identities and probe ion speciation remain limited. Here, we demonstrate post-synthetic modification of perhalometallate anions to achieve ionic liquid-to-ionic liquid transformations. Rheological measurements of the metal-containing ILs indicate that minor alterations to anion coordination spheres induces considerable changes to IL viscosities. UV-vis spectra confirm the purities for most ILs, while revealing a surprising cation dependence of perchlorovanadate speciation and supramolecular structure. The intermolecular interactions studied here span a wide range from dispersion to covalent bonding, permitting their impact on IL viscosity to be decoupled and quantified. Together, synthetic strategies from coordination chemistry paired with conventional UV-vis spectroscopy provide a powerful tool for expanding IL compositions and investigating fundamental nanoscale behavior.

Molecular modeling and simulation are invaluable tools for nanoscience that predict mechanical, physicochemical, and thermodynamic properties of nanomaterials and provide molecular-level insight into underlying mechanisms. However, building nanomaterial-containing systems remains challenging due to the lack of reliable and integrated cyberinfrastructures. Here, we present Nanomaterial Modeler in CHARMM-GUI, a web-based cyberinfrastructure that provides an automated process to generate various nanomaterial models, associated topology, and configuration files to perform state-of-the-art molecular dynamics simulations using most simulation packages. The nanomaterial models are based on the interface force field (IFF), one of the most reliable FFs. The transferability of nanomaterial models among the simulation programs was assessed by single-point energy calculations, which yielded 0.01% relative absolute energy differences for various surface models and equilibrium nanoparticle shapes. Three widely-used Lennard-Jones (LJ) cut-off methods are employed to evaluate the compatibility of nanomaterial models with respect to conventional biomolecular FFs: simple truncation at r = 12 Å (12 cut-off), force-based switching over 10 to 12 Å (10–12 fsw), and LJ particle mesh Ewald with no cut-off (LJPME). The FF parameters with these LJ cut-off methods are extensively validated by reproducing structural, interfacial, and mechanical properties. We find that the computed density and surface energies are in good agreement with reported experimental results, although the simulation results increase in the following order: 10–12 fsw < 12 cut-off < LJPME; nanomaterials in which LJ interactions are a major component show relatively higher deviations (up to 4% in density and 8% in surface energy differences) compared to the experiment. Nanomaterial Modeler’s capability is also demonstrated by generating complex systems of nanomaterial-biomolecule and nanomaterial-polymer interfaces with a combination of existing CHARMM-GUI modules. We hope that Nanomaterial Modeler can be used to carry out innovative nanomaterial modeling and simulation to acquire insight into the structure, dynamics, and underlying mechanisms of complex nanomaterial-containing systems.

Nucleation plays a critical role in many physical and biological phenomena that range from crystallization, melting and evaporation to the formation of clouds and the initiation of neurodegenerative diseases1-3. However, nucleation is a challenging process to study experimentally, especially in its early stages, when several atoms or molecules start to form a new phase from a parent phase. A number of experimental and computational methods have been used to investigate nucleation processes4-17, but experimental determination of the three-dimensional atomic structure and the dynamics of early-stage nuclei has been unachievable. Here we use atomic electron tomography to study early-stage nucleation in four dimensions (that is, including time) at atomic resolution. Using FePt nanoparticles as a model system, we find that early-stage nuclei are irregularly shaped, each has a core of one to a few atoms with the maximum order parameter, and the order parameter gradient points from the core to the boundary of the nucleus. We capture the structure and dynamics of the same nuclei undergoing growth, fluctuation, dissolution, merging and/or division, which are regulated by the order parameter distribution and its gradient. These experimental observations are corroborated by molecular dynamics simulations of heterogeneous and homogeneous nucleation in liquid-solid phase transitions of Pt. Our experimental and molecular dynamics results indicate that a theory beyond classical nucleation theory1,2,18 is needed to describe early-stage nucleation at the atomic scale. We anticipate that the reported approach will open the door to the study of many fundamental problems in materials science, nanoscience, condensed matter physics and chemistry, such as phase transition, atomic diffusion, grain boundary dynamics, interface motion, defect dynamics and surface reconstruction with four-dimensional atomic resolution.

Zinc and stannous ions are commonly used in oral care to reduce tooth enamel degradation. However, mechanistic understanding of the role of the ions in the protection of enamel against acid insults...

Carbohydrate hydrogels are extensively used in pharmaceuticals and engineered biomaterials. Molecular conformations, assembly, and interactions of the carbohydrate strands with stabilizers such as clay minerals in aqueous solution are difficult to quantify in experiments and the hydrogel properties remain largely a result of trial-and-error studies. We analyzed the assembly of gellan gum in aqueous solution and interactions with dispersed clay minerals in all-atomic detail using molecular dynamics simulation, atomic force microscopy (AFM), and comparisons to earlier measurements. Gellan strands associate at low pH values of 2 and gradually disassemble to double strands with weak association of -0.4 kcal per mole carbohydrate ring as the pH values increases to 9. Ionization of the carbonic acid side groups in the backbone extends the chains and accelerates the conformational dynamics via rapidly changing intramolecular ion bridges. Gellan interactions with clay minerals depend on the strength of electric triple layers between clay, cations, and anionic polymer strands, as well as weaker hydrogen bonds along the edges, which are tunable as a function of the clay surface chemistry, local ionic strength, and pH values. Interaction energies range from -4 to +6 kcal per mol carbohydrate ring and were most favorable for electric triple layers with high charge mobility, which can be achieved for intermediate cation exchange capacity of the clay mineral and high pH values to increase ionization of the clay edges and of the polymer. The findings provide understanding and help control the dynamics and stabilization of carbohydrate hydrogels by clay minerals.

Polypeptide based biosilica composites show promise as next generation multi-functional nano-platforms for diagnostics and bio-catalytic applications. Following the identification of a strong silica binder (LDHSLHS) by phage display, we conduct structural analysis of the polypeptide at the interface with amorphous silica nanoparticles in an aqueous environment. Our approach relies on modelling infrared and Raman spectral responses using predictions of molecular dynamics simulations and quantum studies of the normal modes for several potential structures. By simultaneously fitting both infrared and Raman responses in the amide spectral region, we show that the main structural conformer has a beta-like central region and helix-twisted terminals. Classical simulations, as conducted previously (Chem. Mater., 2014, 26, 5725), predict that the association of the main structure with the interface is stimulated by electrostatic interactions though surface binding also requires spatially distributed sodium ions to compensate for negatively charged acidic silanol groups. Accordingly, diffusion of sodium ions would contribute to a stochastic character of the peptide association with the surface. Consistent with the described dynamics at the interface, the results obtained from isothermal titration calorimetry (ITC) confirm a significant enhancement of polypeptide binding to silica at higher concentrations of Na+. The results of this study suggest that the tertiary structure of a phage capsid protein plays a significant role in regulating the conformation of peptide LDHSLHS, increasing its binding to silica during the phage display process. The results presented here support design-led engineering of polypeptide-silica nanocomposites for bio-technological applications.

Chemical admixtures are an essential ingredient of modern concrete mix, but many of their molecular scale working mechanisms remain poorly described. In this respect, recent advances in computational methods, provide a unique opportunity to gain the needed molecular level insights into the mechanism of action of chemical admixtures in cementitious systems. Such studies are slowly increasing in number and this paper proposes a review on approaches that deal with molecular simulations of chemical admixtures. The key properties studied so far are mainly adsorption behaviors and conformations of monomers, oligomers and polymers (molecular weight ~ 10,000 g/mol). Our aim is to identify opportunities, challenges and give perspectives on the future of molecular modeling of chemical admixture-cement interactions., Cement and Concrete Research, 156, ISSN:0008-8846

Core-shell nanoparticles find applications in catalysts, sensors, and theranostics. The full internal 3D atomic structure, however, cannot be resolved by current imaging and diffraction techniques. We analyzed the atomic positions and stress-release mechanism in a cubic Au-Pd core-shell nanoparticle in approximately 1000 times higher resolution than current experimental techniques using large-scale molecular dynamics simulation to overcome these limitations. The core-shell nanocube of 73 nm size was modeled similarly to solution synthesis by random epitaxial deposition of a 4 nm thick shell of Pd atoms onto a Au core of 65 nm side length using reliable interatomic potentials. The internal structure reveals specific deformations and stress relaxation mechanisms that are caused by the +4.8% lattice mismatch of gold relative to palladium and differential confinement of extended particle facets, edges, and corners by one, two, or three Au-Pd interfaces, respectively. The three-dimensional lattice strain causes long-range, arc-like bending of atomic rows along the faces and edges of the particle, especially near the Au-Pd interface, a bulging deformation of the Pd shell, and stacking faults in the Pd shell at the corners of the particle. The strain pattern and mechanism of stress release were further characterized by profiles of the atomic layer spacing in the principal crystallographic directions. Accordingly, strain in the Pd shell is several times larger in the extended facets than near the edges and corners of the nanoparticle, which likely affects adsorption, optical, and electrochemical properties. The findings are consistent with available experimental data, including 3D reconstructions of the same cubic nanoparticle by coherent diffractive imaging (CDI) and may be verified by more powerful experimental techniques in the future. The stress release mechanisms are representative for cubic core-shell nanoparticles with fcc structure and can be explored for different shapes by the same methods.

The adsorption and decomposition of furfuryl alcohol on Pt(111) was studied using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). TPD studies showed that furfuryl alcohol underwent decarbonylation to furan, propylene, carbon monoxide, and hydrogen, as well as hydrodeoxygenation to produce methylfuran and water. Benzene was also produced as an unexpected ring decomposition product. Experiments conducted on deuterium-precovered Pt(111) were used to determine the extent of hydrogen exchange between the surface and organic intermediates during TPD. These experiments suggested that furan was likely evolved in part through dehydrogenated C4H2O intermediates. In contrast to earlier observation of furfuryl alcohol chemistry on Pd(111), deoxygenation of furfuryl alcohol only occurred via hydrogenation by surface hydrogens and not through an interadsorbate hydrogen exchange. The HREELS studies revealed that at low coverages, the furyl ring was adsorbed in a flat-lying configuration on the Pt(111) surface, while at higher coverages intermediates with more upright structures were observed. Furfuryl alcohol decomposed through a furfural intermediate, which underwent decarbonylation and decomposition at relatively low temperatures to produce propylene and benzene. These results elucidate key differences between Pt and Pd surfaces important for designing catalysts to have high selectivity towards a given process.

Interfacial melting of an internal solid hydrocarbon “skeletal” phase initiates vaporization of liquid fluorocarbon droplets., Liquid emulsion droplet evaporation is of importance for various sensing and imaging applications. The liquid-to-gas phase transformation is typically triggered thermally or acoustically by low–boiling point liquids, or by inclusion of solid structures that pin the vapor/liquid contact line to facilitate heterogeneous nucleation. However, these approaches lack precise tunability in vaporization behavior. Here, we describe a previously unused approach to control vaporization behavior through an endoskeleton that can melt and blend into the liquid core to either enhance or disrupt cohesive intermolecular forces. This effect is demonstrated using perfluoropentane (C5F12) droplets encapsulating a fluorocarbon (FC) or hydrocarbon (HC) endoskeleton. FC skeletons inhibit vaporization, whereas HC skeletons trigger vaporization near the rotator melting transition. Our findings highlight the importance of skeletal interfacial mixing for initiating droplet vaporization. Tuning molecular interactions between the endoskeleton and droplet phase is generalizable for achieving emulsion or other secondary phase transitions, in emulsions.

After more than 50 years of development, carbon fiber composites exhibit an order of magnitude higher specific strength as compared to structural metals such as steel. However, the strength of the current state-of-the-art carbon fiber composites remains less than 10% of their theoretical value. Recent studies show that the polymer-carbon nanotube (CNT) interphase, i.e., the region of carbon components in contact with multiple organic components in its vicinity, plays a major role. Engineering the polymer-CNT interphase at the molecular level is a promising pathway to improve the mechanical properties of nano-composite materials on the way to fully realize the potential of mechanical properties of carbon nanotubes. Examples for using pristine and flattened CNTs, combinations of polymers, and surface grafting, as well as analogies to biological systems to prepare strong polymer/CNT composites are reviewed. In support of these developments, molecular simulations have revealed the binding mechanisms of polymers to CNTs and relationships to mechanical properties such as modulus, tensile strength, and interfacial shear strength in the interphase. Recent computational models enable increasingly quantitative predictions, and examples that explain the influence of the type of polymers, polymer crystallinity, carbon nanotubes, and nanotube surface modification on the interphase properties are discussed. The developments in molecular engineering of interphases by experiment and simulations advance rational composite design.

Peptides in nanomaterials assembly allow powerful control over shape development, surface functionalization, and hierarchical 3D organization for many applications. Monitoring and imaging of the flexible biomolecules in experiment remains a major challenge and limits the design of tailor-made peptide nanomaterials. We highlight recent developments in understanding peptide-surface interactions on the basis of experimental data and simulation data for silica, apatites, carbon nanomaterials, and mica. Quantitative insights into molecular recognition mechanisms and structure–function relationships are described, including binding and unbinding, basic sequence design principles, and impacts on nanoparticle shape and crystal growth. In particular, findings from the combination of measurements and molecular dynamics simulations are surveyed that satisfy the growing demand for control down to the atomic level. Design principles remain specific for a given materials type due to large differences in surface chemistry and surface energies. Opportunities for future directions are described.

Polyacrylonitrile (PAN)/carbon nanotube (CNT) composites are used as precursors for ultrastrong and lightweight carbon fibers. However, insights into the structure at the nanoscale and the relationships to mechanical and thermal properties have remained difficult to obtain. In this study, molecular dynamics simulation with accurate potentials and available experimental data were used to describe the influence of different degrees of PAN preorientation and CNT diameter on the atomic-scale structure and properties of the composites. The inclusion of CNTs in the polymer matrix is favored for an intermediate degree of PAN orientation and small CNT diameter whereas high PAN crystallinity and larger CNT diameter disfavor CNT inclusion. The glass transition at the CNT/PAN interface involves the release of rotational degrees of freedom of the polymer backbone and increased mobility of the protruding nitrile side groups in contact with the carbon nanotubes. The glass-transition temperature of the composite increases in correlation with the amount of CNT/polymer interfacial area per unit volume, i.e., in the presence of CNTs, for higher CNT volume fraction, and inversely with CNT diameter. The increase in glass-transition temperature upon CNT addition is larger for PAN of lower crystallinity than for PAN of higher crystallinity. Interfacial shear strengths of the composites are higher for CNTs of smaller diameter and for PAN with preorientation, in correlation with more favorable CNT inclusion energies. The lowest interfacial shear strength was observed in amorphous PAN for the same CNT diameter. PAN with ∼75% crystallinity exhibited hexagonal patterns of nitrile groups near and far from the CNT interface which could influence carbonization into regular graphitic structures. The results illustrate the feasibility of near-quantitative insights into macroscale properties of polymer/CNT composites from simulations of nanometer-scale composite domains. Guidance is most effective when key assumptions in experiment and simulation are closely aligned, such as exfoliation versus bundling of CNTs, size, type, potential defects of CNTs, and precise measures for polymer crystallinity.

Debundling and dispersion of carbon nanotubes (CNTs) in polymer solutions play a major role in the preparation of carbon nanofibers due to early effects on interfacial ordering and mechanical properties. A roadblock toward ultrastrong fibers is the difficulty to achieve homogeneous dispersions of CNTs in polyacrylonitrile (PAN) and poly(methyl methacrylate) (PMMA) precursor solutions in solvents such as dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAc), and N,N-dimethylformamide (DMF). In this contribution, molecular dynamics simulations with accurate interatomic potentials for graphitic materials that include virtual π electrons are reported to analyze the interaction of pristine single wall CNTs with the solvents and polymer solutions at 25 °C. The results explain the barriers toward dispersion of SWCNTs and quantify CNT-solvent, polymer-solvent, as well as CNT-polymer interactions in atomic detail. Debundling of CNTs is overall endothermic and unfavorable with dispersion energies of +20 to +30 mJ...

The function of nanomaterials and biomaterials greatly depends on understanding nanoscale recognition mechanisms, crystal growth and surface reactions. The Interface Force Field (IFF) and surface m...

The impact of polyanions on the formation of lipid bilayers on top of polyelectrolyte multilayers (PEMs) with poly(allylamine hydrochloride) (PAH) as the top layer is studied for the deposition of vesicles of mixed lipid composition, 50:50 molar ratio of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and negatively charged 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS). PEMs are assembled with polystyrene sulfonate (PSS), poly(acrylic acid) (PAA), and alginic acid sodium salt (Alg) as polyanions. The assembly of the vesicles on the PEMs is followed by means of the quartz crystal microbalance with dissipation. Fluorescence recovery after photobleaching measurements are applied to evaluate bilayer formation. Whereas a bilayer is formed on top of PAH/PSS multilayers, the vesicles are adsorbed on top of PAH/Alg and PAH/PAA multilayers, remaining unruptured or only partially fused. The influence of the surface composition of the PEM and of the bulk properties of the film are analyzed. The phosphate ions present in phosphate-buffered saline (PBS) play a fundamental role in bilayer formation on top of PAH/PSS as they complex with PAH and render the surface potential close to zero. For PAH/PAA and PAH/Alg, PBS renders the surface negative. X-ray photoelectron spectroscopy shows that the dibasic phosphate ions from PBS complex preferentially with PAH in PAH/PAA and PAH/Alg multilayers, whereas monobasic phosphates complex with PAH in PAH/PSS. An explanation for the absence of bilayer formation on PAH/PAA and PAH/Alg is given on the basis of the different affinities of phosphate ions for PAH in combination with the different polyanions.

Colloidal metal nanocrystals find many applications in catalysis, energy conversion devices, and therapeutics. However, the nature of ligand interactions and implications on shape control have remained uncertain at the atomic scale. Large differences in peptide adsorption strength and facet specificity were found on flat palladium surfaces versus surfaces of nanoparticles of 2 to 3 nm size using accurate atomistic simulations with the Interface force field. Folding of longer peptides across many facets explains the formation of near-spherical particles with local surface disorder, in contrast to the possibility of nanostructures of higher symmetry with shorter ligands. The average particle size in TEM correlates inversely with the surface coverage with a given ligand and with the strength of ligand adsorption. The role of specific amino acids and sequence mutations on the nanoparticle size and facet composition is discussed, as well as the origin of local surface disorder that leads to large differences in catalytic reactivity.