Effects of Water Content on the Transport and Thermodynamic Properties of Phosphonium Ionic Liquids.

We present a numerical investigation of the influence of water content on the dynamic properties of a family of phosphonium-based room-temperature ionic liquids. The study presents a compelling correlation between structural changes in water-ionic liquid solutions and thermodynamic and transport properties across diverse systems. The results for phosphonium ionic liquids are compared with 1-butyl-3-methylimidazolium hexaphosphate ([bmim]PF6) as a reference. Through this approach, phosphonium cation structure-related characteristics can be identified and placed within the broader context of ionic liquids. These insights are underpinned by observed changes in interaction energy, boiling point, diffusion rate, and viscosity, highlighting the crucial role of water molecules in weakening the strength of interactions between ions within the ionic liquid. The investigation also explains temperature-dependent trends in phosphonium cations, showing that alkyl group length and molecular symmetry are important tuning parameters for the strength of Coulomb interactions. These results contribute to a refined understanding of phosphonium ionic liquid behavior in the presence of water, offering valuable insights for optimizing their use in diverse fields.

Reactive Simulations of Silica Functionalization with Aromatic Hydrocarbons.

Reactive molecular dynamics simulations are used to model the covalent functionalization of amorphous silica with aromatic hydrocarbons. Simulations show that the surface density of silanol-terminated phenyl, naphthyl, and anthracenyl molecules is lower than the maximum value calculated based on molecule geometry, and the simulation densities decrease faster with the number of aromatic rings than the geometric densities. The trends are analyzed in terms of the surface-silanol bonding configurations, tilt angles, local conformational ordering, and aggregation of surface-bound molecules under steady-state conditions. Results show that the surface density is affected by both the size and symmetry of the aromatic hydrocarbons. The correlations among bonding, orientation, and surface density identified here may guide the selection or design of molecules for functionalized surfaces.

Temperature-Dependent Density and Viscosity Prediction for Hydrocarbons: Machine Learning and Molecular Dynamics Simulations.

Machine learning-based predictive models allow rapid and reliable prediction of material properties and facilitate innovative materials design. Base oils used in the formulation of lubricant products are complex hydrocarbons of varying sizes and structure. This study developed Gaussian process regression-based models to accurately predict the temperature-dependent density and dynamic viscosity of 305 complex hydrocarbons. In our approach, strongly correlated/collinear predictors were trimmed, important predictors were selected by least absolute shrinkage and selection operator (LASSO) regularization and prior domain knowledge, hyperparameters were systematically optimized by Bayesian optimization, and the models were interpreted. The approach provided versatile and quantitative structure-property relationship (QSPR) models with relatively simple predictors for determining the dynamic viscosity and density of complex hydrocarbons at any temperature. In addition, we developed molecular dynamics simulation-based descriptors and evaluated the feasibility and versatility of dynamic descriptors from simulations for predicting the material properties. It was found that the models developed using a comparably smaller pool of dynamic descriptors performed similarly in predicting density and viscosity to models based on many more static descriptors. The best models were shown to predict density and dynamic viscosity with coefficient of determination (R2) values of 99.6% and 97.7%, respectively, for all data sets, including a test data set of 45 molecules. Finally, partial dependency plots (PDPs), individual conditional expectation (ICE) plots, local interpretable model-agnostic explanation (LIME) values, and trimmed model R2 values were used to identify the most important static and dynamic predictors of the density and viscosity.

Processing bulk natural wood into a high-performance structural material.

Synthetic structural materials with exceptional mechanical performance suffer from either large weight and adverse environmental impact (for example, steels and alloys) or complex manufacturing processes and thus high cost (for example, polymer-based and biomimetic composites). Natural wood is a low-cost and abundant material and has been used for millennia as a structural material for building and furniture construction. However, the mechanical performance of natural wood (its strength and toughness) is unsatisfactory for many advanced engineering structures and applications. Pre-treatment with steam, heat, ammonia or cold rolling followed by densification has led to the enhanced mechanical performance of natural wood. However, the existing methods result in incomplete densification and lack dimensional stability, particularly in response to humid environments, and wood treated in these ways can expand and weaken. Here we report a simple and effective strategy to transform bulk natural wood directly into a high-performance structural material with a more than tenfold increase in strength, toughness and ballistic resistance and with greater dimensional stability. Our two-step process involves the partial removal of lignin and hemicellulose from the natural wood via a boiling process in an aqueous mixture of NaOH and Na2SO3 followed by hot-pressing, leading to the total collapse of cell walls and the complete densification of the natural wood with highly aligned cellulose nanofibres. This strategy is shown to be universally effective for various species of wood. Our processed wood has a specific strength higher than that of most structural metals and alloys, making it a low-cost, high-performance, lightweight alternative.

Understanding and Preventing Lubrication Failure at the Carbon Atomic Steps.

Two-dimensional (2D) lamellar materials are normally capable of rendering super-low friction, wear protection, and adhesion reduction in nanoscale due to their ultralow shear strength between two basal plane surfaces. However, high friction at step edges prevents the 2D materials from achieving super-low friction in macroscale applications and eventually leads to failure of lubrication performance. Here, taking graphene as an example, the authors report that not all step edges are detrimental. The armchair (AC) step edges are found to have only a minor topographic effect on friction, while the zigzag (ZZ) edges cause friction two orders of magnitude larger than the basal plane. The AC step edge is less reactive and thus more durable. However, the ZZ structure prevails when step edges are produced mechanically, for example, through mechanical exfoliation or grinding of graphite. The authors found a way to make the high-friction ZZ edge superlubricious by reconstructing the (6,6) hexagon structure to the (5,7) azulene-like structure through thermal annealing in an inert gas environment. This will facilitate the realization of graphene-based superlubricity over a wide range of industrial applications in which avoiding the involvement of step edges is difficult.

Shear-activated chemisorption and association of cyclic organic molecules.

Mechanochemical activation has created new opportunities for applications such as solvent-free chemical synthesis, polymer processing, and lubrication. However, mechanistic understanding of these processes is still limited because the mechanochemical response of a system is a complex function of many variables, including the direction of applied stress and the chemical features of the reactants in non-equilibrium conditions. Here, we studied shear-activated reactions of simple cyclic organic molecules to isolate the effect of chemical structure on reaction yield and pathway. Reactive molecular dynamics simulations were used to model methylcyclopentane, cyclohexane, and cyclohexene subject to pressure and shear stress between silica surfaces. Cyclohexene was found to be more susceptible to mechanochemical activation of oxidative chemisorption and subsequent oligomerization reactions than either methylcyclopentane or cyclohexane. The oligomerization trend was consistent with shear-driven polymerization yield measured in ball-on-flat sliding experiments. Analysis of the simulations showed the distribution of carbon atom sites at which oxidative chemisorption occurred and identified the double bond in cyclohexene as being the origin of its shear susceptibility. Lastly, the most common reaction pathways for association were identified, providing insight into how the chemical structures of the precursor molecules determined their response to mechanochemical activation.

Effects of surface chemistry on the mechanochemical decomposition of tricresyl phosphate.

The growth of protective tribofilms from lubricant antiwear additives on rubbing surfaces is initiated by mechanochemically promoted dissociation reactions. These processes are not well understood at the molecular scale for many important additives, such as tricresyl phosphate (TCP). One aspect that needs further clarification is the extent to which the surface properties affect the mechanochemical decomposition. Here, we use nonequilibrium molecular dynamics (NEMD) simulations with a reactive force field (ReaxFF) to study the decomposition of TCP molecules confined and pressurised between sliding ferrous surfaces at a range of temperatures. We compare the decomposition of TCP on native iron, iron carbide, and iron oxide surfaces. We show that the decomposition rate of TCP molecules on all the surfaces increases exponentially with temperature and shear stress, implying that this is a stress-augmented thermally activated (SATA) process. The presence of base oil molecules in the NEMD simulations decreases the shear stress, which in turn reduces the rate constant for TCP decomposition. The decomposition is much faster on iron surfaces than iron carbide, and particularly iron oxide. The activation energy, activation volume, and pre-exponential factor from the Bell model are similar on iron and iron carbide surfaces, but significantly differ for iron oxide surfaces. These findings provide new insights into the mechanochemical decomposition of TCP and have important implications for the design of novel lubricant additives for use in high-temperature and high-pressure environments.

Origin of Pressure-Dependent Adhesion in Nanoscale Contacts.

The adhesion between nanoscale components has been shown to increase with applied load, contradicting well-established mechanics models. Here, we use in situ transmission electron microscopy and atomistic simulations to reveal the underlying mechanism for this increase as a change in the mode of separation. Analyzing 135 nanoscale adhesion tests on technologically relevant materials of anatase TiO2, silicon, and diamond, we demonstrate a transition from fracture-controlled to strength-controlled separation. When fracture models are incorrectly applied, they yield a 7-fold increase in apparent work of adhesion; however, we show that the true work of adhesion is unchanged with loading. Instead, the nanoscale adhesion is governed by the product of adhesive strength and contact area; the pressure dependence of adhesion arises because contact area increases with applied load. By revealing the mechanism of separation for loaded nanoscale contacts, these findings provide guidance for tailoring adhesion in applications from nanoprobe-based manufacturing to nanoparticle catalysts.

Simulations of Friction Anisotropy on Self-Assembled Monolayers in Water.

Molecular dynamics simulations were performed to study nanoscale friction on hydrophilic and hydrophobic self-assembled monolayers (SAMs) immersed in water. Sliding was simulated in two different directions to capture anisotropy due to the direction of motion relative to the inherent tilted orientation of the molecules. It was shown that friction depends on both hydrophobicity and sliding direction, with the highest friction observed for sliding on hydrophobic SAM in the direction against the initial orientation of the molecules. The origins of the friction trends were analyzed by differentiating the tip-SAM and tip-water force contributions to friction. The tip-water force was higher on the hydrophilic SAM, and this was shown to be due to the presence of a dense layer of water adjacent to the surface and hydrogen bonding. In contrast, the tip-SAM force was higher on the hydrophobic SAM due to a water depletion layer, which enabled the tip to be closer to the SAM terminal group. The higher-friction cases all exhibited greater penetration of the tip below the surface of the SAM, accommodated by further tilting and reorientation of the SAM molecules.

Simulation of Subnanometer Contrast in Dynamic Atomic Force Microscopy of Hydrophilic Alkanethiol Self-Assembled Monolayers in Water.

Atomic resolution imaging of surfaces in liquid environments using atomic force microscopy (AFM) is challenging in terms of both reproducibility and measurement interpretation. To understand the origins of these challenges, we used molecular dynamics simulations of AFM on hydrophilic self-assembled monolayers (SAMs) in water. The force on the model AFM tip was calculated as a function of lateral and vertical position relative to the SAM surface. The contributions of the water and SAMs to the overall force were analyzed, and the former was correlated to the water density distribution. Then, dynamic AFM was modeled by oscillating the tip at a driving amplitude. It was found that the contrast between amplitudes at different lateral positions on the surface was dependent on the vertical position of the tip. Lastly, amplitude maps were produced for two vertical positions at constant height, and the ability to capture atomic resolution was related to the force on the tip. These results offer an explanation for the observed instability in atomic scale imaging using AFM and more generally provide insight into the contrast mechanisms of surface images obtained in liquid environments.

Origin of Nanoscale Friction Contrast between Supported Graphene, MoS2, and a Graphene/MoS2 Heterostructure.

Ultralow friction can be achieved with 2D materials, particularly graphene and MoS2. The nanotribological properties of these different 2D materials have been measured in previous atomic force microscope (AFM) experiments sequentially, precluding immediate and direct comparison of their frictional behavior. Here, friction is characterized at the nanoscale using AFM experiments with the same tip sliding over graphene, MoS2, and a graphene/MoS2 heterostructure in a single measurement, repeated hundreds of times, and also measured with a slowly varying normal force. The same material systems are simulated using molecular dynamics (MD) and analyzed using density functional theory (DFT) calculations. In both experiments and MD simulations, graphene consistently exhibits lower friction than the MoS2 monolayer and the heterostructure. In some cases, friction on the heterostructure is lower than that on the MoS2 monolayer. Quasi-static MD simulations and DFT calculations show that the origin of the friction contrast is the difference in energy barriers for a tip sliding across each of the three surfaces.

Understanding contact between platinum nanocontacts at low loads: The effect of reversible plasticity.

Metal nanocontacts play a critical role in atomic force microscopy, functional nanostructures, metallic nanoparticles, and nanoscale electromechanical devices. In all cases, knowledge of the area of contact, and its variation with load, is critical for the quantitative prediction of behavior. Often, the contact area is predicted using continuum mechanics models which relate contact size to geometry, material properties, and load. Here we show for platinum nanoprobes that the contact size deviates significantly from these continuum predictions, even at low applied loads and in the absence of irreversible shape change. We use in situ transmission electron microscopy (TEM) with matched molecular dynamics (MD) simulations to investigate the load-dependent size of the contact. Direct measurements of contact radius from MD and TEM exceed the predictions of continuum mechanics by 24%-164%, depending on the model applied. The physical mechanism for this deviation is found to be dislocation activity in the near-surface material, which is fully reversed upon unloading. These findings demonstrate that contact mechanics models are insufficient for predicting contact area in real-world platinum nanostructures, even at ultra-low applied loads.

Quantitative measurement of contact area and electron transport across platinum nanocontacts for scanning probe microscopy and electrical nanodevices.

Conductive modes of atomic force microscopy are widely used to characterize the electronic properties of materials, and in such measurements, contact size is typically determined from current flow. Conversely, in nanodevice applications, the current flow is predicted from the estimated contact size. In both cases, it is very common to relate the contact size and current flow using well-established ballistic electron transport theory. Here we performed 19 electromechanical tests of platinum nanocontacts with in situ transmission electron microscopy to measure contact size and conductance. We also used molecular dynamics simulations of matched nanocontacts to investigate the nature of contact on the atomic scale. Together, these tests show that the ballistic transport equations under-predict the contact size by more than an order of magnitude. The measurements suggest that the low conductance of the contact cannot be explained by the scattering of electrons at defects nor by patchy contact due to surface roughness; instead, the lower-than-expected contact conductance is attributed to approximately a monolayer of insulating surface species on the platinum. Surprisingly, the low conductance persists throughout loading and even after significant sliding of the contact in vacuum. We apply tunneling theory and extract best-fit barrier parameters that describe the properties of this surface layer. The implications of this investigation are that electron transport in device-relevant platinum nanocontacts can be significantly limited by the presence and persistence of surface species, resulting in current flow that is better described by tunneling theory than ballistic electron transport, even for cleaned pure-platinum surfaces and even after loading and sliding in vacuum.

Mechanochemical Association Reaction of Interfacial Molecules Driven by Shear.

Shear-driven chemical reaction mechanisms are poorly understood because the relevant reactions are often hidden between two solid surfaces moving in relative motion. Here, this phenomenon is explored by characterizing shear-induced polymerization reactions that occur during vapor phase lubrication of α-pinene between sliding hydroxylated and dehydroxylated silica surfaces, complemented by reactive molecular dynamics simulations. The results suggest that oxidative chemisorption of the α-pinene molecules at reactive surface sites, which transfers oxygen atoms from the surface to the adsorbate molecule, is the critical activation step. Such activation takes place more readily on the dehydroxylated surface. During this activation, the most strained part of the α-pinene molecules undergoes a partial distortion from its equilibrium geometry, which appears to be related to the critical activation volume for mechanical activation. Once α-pinene molecules are activated, association reactions occur between the newly attached oxygen and one of the carbon atoms in another molecule, forming ether bonds. These findings have general implications for mechanochemistry because they reveal that shear-driven reactions may occur through reaction pathways very different from their thermally induced counterparts and specifically the critical role of molecular distortion in such reactions.

Combined Experimental and Simulation Study of Amplitude Modulation Atomic Force Microscopy Measurements of Self-Assembled Monolayers in Water.

Atomic force microscopy (AFM) can be used to measure surface properties at the nanoscale. However, interpretation of measurements from amplitude modulation AFM (AM-AFM) in liquid is not straightforward due to the interactions between the AFM tip, the surface being imaged, and the water. In this work, amplitude-distance measurements and molecular dynamics simulations of AM-AFM were employed to study the effect of surface chemistry on the amplitude of tip oscillation in water. The sample surfaces consisted of self-assembled monolayers where the hydrophilicity or hydrophobicity was determined by the terminal group of the alkanethiols. Analysis showed that surface chemical composition influences the hydration structure near the interface which affects the forces experienced by the tip and in turn changes the amplitude profile. This observation could aid our understanding of AM-AFM measurements of interfacial phenomena on various surfaces in water.

Reactive Molecular Dynamics Simulations of Thermal Film Growth from Di- tert-butyl Disulfide on an Fe(100) surface.

Iron sulfide films are present in many applications, including lubricated interfaces where protective films are formed through the reactions of lubricant additive molecules with steel surfaces during operation. Such films are critical to the efficiency and useful lifetime of moving components. However, the mechanisms by which films form are still poorly understood because the reactions occur between two surfaces and so cannot be directly probed experimentally. To address this, we explore the thermal contribution to film formation of di- tert-butyl disulfide-an important extreme pressure additive-on an Fe(100) surface using reactive molecular dynamics simulations, where the reactive potential parameters are validated by comparison to ab initio calculations. The reaction pathway leading to the formation of iron sulfide surfaces is characterized using the reactive simulations. Then, the film formation process is mimicked by simulations where di- tert-butyl disulfide molecules are cyclically added to the surface and subjected to temperatures comparable to those expected due to frictional heating. The use of a reactive empirical potential is a novel approach to modeling the iterative nature of thermal film growth with realistic lubricant additive molecules.

Mechanical characterization of diesel soot nanoparticles: in situ compression in a transmission electron microscope and simulations.

Incomplete fuel burning inside an internal combustion engine results in the creation of soot in the form of nanoparticles. Some of these soot nanoparticles (SNP) become adsorbed into the lubricating oil film present on the cylinder walls, which adversely affects the tribological performance of the lubricant. In order to better understand the mechanisms underlying the wear caused by SNPs, it is important to understand the behavior of SNPs and to characterize potential changes in their mechanical properties (e.g. hardness) caused by (or during) mechanical stress. In this study, the behavior of individual SNPs originating from diesel engines was studied under compression. The experiments were performed in a transmission electron microscope using a nanoindentation device. The nanoparticles exhibited elasto-plastic behavior in response to consecutive compression cycles. From the experimental data, the Young's modulus and hardness of the SNPs were calculated. The Young's modulus and hardness of the nanoparticles increased with the number of compression cycles. Using an electron energy loss spectroscopy technique, it was shown that the sp2/sp3 ratio within the compressed nanoparticle decreases, which is suggested to be the cause of the increase in elasticity and hardness. In order to corroborate the experimental findings, molecular dynamics simulations of a model SNP were performed. The SNP model was constructed using carbon and hydrogen atoms with morphology and composition comparable to those observed in the experiment. The model SNP was subjected to repeated compressions between two virtual rigid walls. During the simulation, the nanoparticle exhibited elasto-plastic behavior like that in the experiments. The results of the simulations confirm that the increase in the elastic modulus and hardness is associated with a decrease in the sp2/sp3 ratio.

Tip convolution on HOPG surfaces measured in AM-AFM and interpreted using a combined experimental and simulation approach.

Amplitude modulated atomic force microscopy (AM-AFM) was used to examine the influence of the size of the AFM tip apex on the measured surface topography of single highly oriented pyrolytic graphite (HOPG) atomic steps. Experimental measurements were complemented by molecular dynamics simulations of AM-AFM and the results from both were evaluated by comparison of the measured or simulated width of the topography at the step to that predicted using simple rigid-body geometry. The results showed that the step width, which is a reflection of the resolution of the measurement, increased with tip size, as expected, but also that the difference between the measured/simulated step width and the geometric calculation was tip size dependent. The simulations suggested that this may be due to the deformation of the bodies and the effect of that deformation on the interaction force and oscillation amplitude. Overall, this study showed that the resolution of AM-AFM measurements of atomic steps can be correlated to tip size and that this relationship is affected by the deformation of the system.

Dynamics of atomic stick-slip friction examined with atomic force microscopy and atomistic simulations at overlapping speeds.

Atomic force microscopy (AFM) and atomistic simulations of atomic friction with silicon oxide tips sliding on Au(111) are conducted at overlapping speeds. Experimental data unambiguously reveal a stick-slip friction plateau above a critical scanning speed, in agreement with the thermally activated Prandtl-Tomlinson (PTT) model. However, friction in experiments is larger than in simulations. PTT energetic parameters for the two are comparable, with minor differences attributable to the contact area's influence on the barrier to slip. Recognizing that the attempt frequency may be determined by thermal vibrations of the larger AFM tip mass or instrument noise fully resolves the discrepancy. Thus, atomic stick-slip is well described by the PTT model if sources of slip-assisting energy are accounted for.