The kinetics of a Knoevenagel condensation between vanillin and barbituric acid has recently provided insight into the differences between mechanical and traditional solution-based synthetic methods. The solution-based reaction follows first-order reaction kinetics, while mechanochemical reaction kinetics follow a sigmoid pattern, with a rapid acceleration of reactivity following a slow induction period. Previous works theorize the source of the reaction acceleration to be either particle fracture evolution or changes to the reactant's rheological properties. Here, we examined the reaction kinetics in stainless steel, Teflon, zirconia, and aluminum reaction vessels using different milling frequencies to determine the role of reagent mechanics under varying mechanical environments. Reaction vessels with interchangeable midsections and end-caps of different materials were used to discern the role of jar/ball material surface energy and localized shear vs normal loading forces on the reaction kinetics. The kinetics remained sigmoidal regardless of milling jar/ball materials, milling frequency, and observed rheological changes. Based on a kinetic energy model, the reaction is consistent with a force-accelerated autocatalytic process. High mixing in a low-force environment (Teflon) resulted in conversion similar to 8x higher than high-force environments (stainless steel), which also showed more ready formation of solid volumes of product that were not free-flowing, reducing yields, suggesting that optimizing interfacial adhesion, impact force, and the relative amounts of shear vs normal forces yields significant rate improvements. Previously reported "cohesive states" likely arise from conditions of shear flow in reaction systems, making the methods introduced here broadly applicable to a host of mechanochemically accelerated chemical systems.

Ionic liquids (ILs) are promising electrotunable lubricants due to their unique molten salt properties. Here, we elucidate how surface curvature modulates the electrotunability of friction at single-asperity contacts lubricated by 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Single-layer graphene (SLG) with nanoscale curvatures was prepared by coating graphene onto SiO2 nanoparticles (NPs) of varying diameters (10, 20, and 30 nm), supported on a flat, oxidized Si wafer. Friction tests performed by atomic force microscopy (AFM) showed that SLG on 10 nm NPs significantly enhanced the electrotunability of friction compared to SLG on larger NPs and flat graphene. Molecular dynamic simulations revealed changes in ionic rearrangement with varying surface potential. Correlating frictional response with interfacial ionic structure reveals a two-fold mechanism: when the curvature approaches the nanoscale comparable to ion size, reduced steric interactions arise, improving friction electrotunability. These findings demonstrate how nanoscale surface geometry at electrified interfaces can be exploited to control friction, advancing IL-based lubrication in micro- and nanoelectromechanical systems.

The kinetics of a Knoevenagel condensation between vanillin and barbituric acid has recently provided insight into the differences between mechanical and traditional solution-based synthetic methods. The solution-based reaction follows first-order reaction kinetics, while mechanochemical reaction kinetics follow a sigmoid pattern, with a rapid acceleration of reactivity following a slow induction period. Previous works theorize the source of the reaction acceleration to be either particle fracture evolution or changes to the reactant's rheological properties. Here, we examined the reaction kinetics in stainless steel, Teflon, zirconia, and aluminum reaction vessels using different milling frequencies to determine the role of reagent mechanics under varying mechanical environments. Reaction vessels with interchangeable midsections and end-caps of different materials were used to discern the role of jar/ball material surface energy and localized shear vs normal loading forces on the reaction kinetics. The kinetics remained sigmoidal regardless of milling jar/ball materials, milling frequency, and observed rheological changes. Based on a kinetic energy model, the reaction is consistent with a force-accelerated autocatalytic process. High mixing in a low-force environment (Teflon) resulted in conversion ∼8x higher than high-force environments (stainless steel), which also showed more ready formation of solid volumes of product that were not free-flowing, reducing yields, suggesting that optimizing interfacial adhesion, impact force, and the relative amounts of shear vs normal forces yields significant rate improvements. Previously reported "cohesive states" likely arise from conditions of shear flow in reaction systems, making the methods introduced here broadly applicable to a host of mechanochemically accelerated chemical systems.

With the rapid advancement of nano-electromechanical systems (NEMS), friction at the nanoscale has become a critical factor influencing the stability, precision, and energy efficiency of moving components. Unlike macroscopic friction, which is primarily governed by mechanical interlocking, nanoscale friction is strongly influenced by molecular interactions and surface properties. Therefore, developing lubricants with low and controllable friction at the nanoscale is of paramount importance for improving the performance and longevity of NEMS devices. Ionic liquids (ILs) have recently garnered significant attention as advanced lubricants due to their unique physicochemical properties, such as negligible vapor pressure, high thermal stability, and excellent lubrication performance. One of the key advantages of ILs is their ability to undergo electrotunable friction, a property arising from the ability of their cations and anions to reorganize in response to applied electric fields. This allows for dynamic control over friction by modulating the adsorption configuration of ions on solid surfaces. While extensive studies have been conducted on the tribological behavior of ILs on atomically smooth surfaces like graphene, their performance on surfaces with nanoscale curvature remains largely unexplored. Understanding this effect is crucial, as real-world applications often involve rough and curved interfaces rather than ideal flat surfaces. The frictional properties of ILs are highly dependent on the location of the slippage plane, where ionic motion occurs. According to the electrical double-layer model, both surface potential and curvature can influence the position of this plane. The surface potential alters electrostatic interactions between the solid substrate and adsorbed ions, while nanoscale curvature introduces steric hindrance effects that impact ion packing density and structural ordering. As curvature increases, the available space for ion adsorption changes, potentially leading to variations in lubrication behavior. To address this knowledge gap, this study investigates the frictional electrotunability of ILs on graphene surfaces with controlled nanoscale curvatures. Single layer graphene will be transferred onto nanoparticles of different diameters (10 nm, 20 nm, and 30 nm) to systematically vary the curvature. The nanofriction tests will be performed using atomic force microscopy (AFM) under single-asperity contact mode, utilizing a sharp tip (radius ~9 nm) to study the friction under varying normal loads. The IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) will be employed as the lubricant due to its strong π-π interactions with graphene, which significantly influence adsorption behavior and subsequent friction. By applying external electrical potentials to the system, we will evaluate how surface curvature modifies the electrotunable frictional response of the IL. The force-distance curve measurements using AFM will provide insight into the ionic layering and structural organization of ILs on curved graphene surfaces. The correlation between surface curvature, ionic structuring, and electrotunable friction will be analyzed to elucidate the fundamental mechanisms governing this behavior. This work aims to provide a deeper understanding of IL lubrication on non-ideal (curved or rough) surfaces, offering new strategies for designing adaptive nanoscale lubrication systems. The findings will contribute to the development of next-generation lubricants for nanodevices where controlled friction at the nanoscale is crucial.

Using mechanical force to induce chemical reactions with two-dimensional (2D) materials provides an approach for both understanding mechanochemical processes on the molecular level, and a potential method for using mechanical strain as a means of directing the functionalization of 2D materials. To investigate this, we have designed a modular experimental platform which allows for in situ monitoring of reactions on strained graphene via Raman spectroscopy as a function of time. Both the strain present in graphene and the corresponding chemical changes it undergoes in the presence of a reagent can be followed concomitantly. As a case study, we have experimentally monitored and theoretically modeled the reactivity of a suspended single-layer graphene membrane under strain with water, where the graphene is strained via an applied backing pressure. While exposure of the unstrained membrane to water does not drive a chemical reaction, distortion of the membrane causes a rise in the ID/IG peak ratio, indicating an initial lattice conversion from crystalline to nanocrystalline due to reaction with water. With continued reaction, a decrease in the ID/IG peak ratio is then seen, indicative of a nanocrystalline to amorphous lattice transition. Using density functional theory (DFT) calculations, the reaction of water on graphene has been determined to be nucleated by epoxide defects, with the reaction barrier decreasing by nearly 5x for the strained vs. unstrained graphene. While demonstrated here for graphene, this approach also provides the opportunity to examine a host of force-driven chemical reactions with 2D materials.

Direct mechanocatalysis has arisen as a promising tool to achieve synthetic transformations by utilizing reaction vessels and/or media made of a material capable of acting as a source of catalysis. One common metal utilized for this purpose is copper, the surface of which is not a static environment but rather undergoes many transformations (particularly upon exposure to water and oxygen). Here we have utilized in-house developed equipment for work under controlled atmosphere milling, including a composite milling jar design that enables investigating the behaviour of the copper surface in either impact- or shear-dominated milling regimes. We exploit the unique sensitivity to Cu(ii) species of the mechanocatalytic coupling of isocyanate and sulfonamide to form the sulfonylurea diabetic drug tolbutamide to establish methods to control the copper surface composition and reveal the factors that influence copper transformation into different states during milling. We reveal the active catalyst formed through direct mechanocatalysis to be a hydroxylated copper species and demonstrate that the reaction proceeds via surface wear and subsequent catalyst formation. Any initial surface oxide is observed to be insignificant to the overall process. The use of the composite reaction jar further revealed that wear dominates from the end regions of the vessel, undergoing primarily impact forces. Based on these finding and density functional theory (DFT) calculations, we present a reaction mechanism which explains the different yields of in situ generated Cu(OH)2 and a traditional CuCl2 pre-catalyst. These results highlight the importance of systematic investigations of surface characteristics for understanding and controlling direct mechanocatalysis and demonstrate methods to realize these goals.

Co-Editors-in-Chief James Batteas and Tomislav Friščić introduce RSC Mechanochemistry.

Ionic liquids (ILs) are a promising class of lubricants that allow dynamic friction control at electrified interfaces. In the real world, surfaces inevitably exhibit some degree of roughness, which can influence lubrication. In this work, we deposited single-layer graphene onto 20 nm silica nanoparticle films to investigate the effect of surface curvature and electrostatic potential on both the lubricious behavior and interfacial layering structure of 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide on graphene. Normal force and friction force measurements were conducted by atomic force microscopy using a sharp silicon tip. Our results reveal that the friction coefficient at the lubricated tip-graphene contacts significantly depends on surface curvature. Two friction coefficients are measured on graphene peaks and valleys with a higher coefficient measured at lower loads (pressures), whereas only one friction coefficient is measured on smooth graphene. Moreover, the electrotunability of the friction coefficient at low loads is observed to be significantly enhanced in peaks and valleys compared with smooth graphene. This is associated with the promoted overscreening of surface charge on convex interfaces and the steric hindrance at concave interfaces, which leads to more layers of ions (electrostatically) bound to the surface, i.e., thicker boundary films (electrical double layers). This work opens new avenues to control IL lubrication on the nanoscale by combining topographic features and an electric field.

In the current issue of Chem, Gugin et al. are showcasing the power of energy-dispersive X-ray diffraction to track kinetics and chemical phenomena in situ during twin screw extrusion. This technique holds great promise for industrial scale-up of green chemical syntheses.

We provide a systematic investigation of the role of atmospheric oxygen and choice of milling assembly (i.e., the milling jar and ball materials) on a prototypical medicinal mechanochemistry reaction: the...

Attaining controllable molecular motion at the nanoscale can be beneficial for multiple reasons, spanning from optoelectronics to catalysis. Here we study the movement of a two-legged molecular walker by modeling the migration of a phenyl aziridine ring on curved graphene. We find that directional ring migration can be attained on graphene in the cases of both 1D (wrinkled/rippled) and 2D (bubble-shaped) curvature. Using a descriptor approach based on graphene's frontier orbital orientation, we can understand the changes in binding energy of the ring as it translates across different sites with variable curvature and the kinetic barriers associated with ring migration. Additionally, we show that the extent of covalent bonding between graphene and the molecule at different sites directly controls the binding energy gradient, propelling molecular migration. Importantly, one can envision such walkers as carriers of charge and disruptors of local bonding. This study enables a new way to tune the electronic structure of two-dimensional materials for a range of applications.

Conventional mechanochemical synthetic tools, such as ball mills, offer no methodology to quantitatively link macroscale reaction parameters, such as shaking frequency or milling ball radius, to fundamental drivers of reactivity, namely the force vectors applied to the reactive molecules. As a result, although mechanochemistry has proven to be a valuable method to make a wide variety of products, the results are seldom reproduceable between reactors, difficult to rationally optimize, and hard to ascribe to a specific reaction pathway. Here we have developed a controlled force reactor, which is a mechanochemical ball mill reactor with integrated force measurement and control during each impact. We relate two macroscale reactor parameters-impact force and impact time-to thermodynamic and kinetic transition state theories of mechanochemistry utilizing continuum contact mechanics principles. We demonstrate force controlled particle fracture of NaCl to characterize particle size evolution during reactions, and force controlled reaction between anhydrous copper(II) chloride and (1, 10) phenanthroline. During the fracture of NaCl, we monitor the evolution of particle size as a function of impact force and find that particles quickly reach a particle size of ∼100 μm largely independent of impact force, and reach steady state 10-100× faster than reaction kinetics of typical mechanochemical reactions. We monitor the copper(II) chloride reactivity by measuring color change during reaction. Applying our transition state theory developed here to the reaction curves of copper(II) chloride and (1, 10) phenanthroline at multiple impact forces results in an activation energy barrier of 0.61 ± 0.07 eV, distinctly higher than barriers for hydrated metal salts and organic ligands and distinctly lower than the direct cleavage of the CuCl bond, indicating that the reaction may be mediated by the higher affinity of Fe in the stainless steel vessel to Cl. We further show that the results in the controlled force reactor match rudimentary estimations of impact force within a commercial ball mill reactor Retsch MM400. These results demonstrate the ability to quantitatively link macroscale reactor parameters to reaction properties, motivating further work to make mechanochemical synthesis quantitative, predictable, and fundamentally insightful.

The performance of electrochemical devices using ionic liquids (ILs) as electrolytes can be impaired by water uptake. This work investigates the influence of water on the behavior of hydrophilic and hydrophobic ILs─with ethylsulfate and tris(perfluoroalkyl)trifluorophosphate or bis(trifluoromethyl sulfonyl)imide (TFSI) anions, respectively─on electrified graphene, a promising electrode material. The results show that water uptake slightly reduces the IL electrochemical stability and significantly influences graphene's potential of zero charge, which is justified by the extent of anion depletion from the surface. Experiments confirm the dominant contribution of graphene's quantum capacitance (CQ) to the total interfacial capacitance (Cint) near the PZC, as expected from theory. Combining theory and experiments reveals that the hydrophilic IL efficiently screens surface charge and exhibits the largest double layer capacitance (CIL ∼ 80 μF cm-2), so that CQ governs the charge stored. The hydrophobic ILs are less efficient in charge screening and thus exhibit a smaller capacitance (CIL ∼ 6-9 μF cm-2), which governs Cint already at small potentials. An increase in the total interfacial capacitance is observed at positive voltages for humid TFSI-ILs relative to dry ones, consistent with the presence of a satellite peak. Short-range surface forces reveal the change of the interfacial layering with potential and water uptake owing to reorientation of counterions, counterion binding, co-ion repulsion, and water enrichment. These results are consistent with the charge being mainly stored in a ∼2 nm-thick double layer, which implies that ILs behave as highly concentrated electrolytes. This knowledge will advance the design of IL-graphene-based electrochemical devices.

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Robert W. Carpick opened a discussion of the paper by Lars Borchardt: For your contact angle measurements, did you measure the surface of the Pd ball, or a flat Pd reference sample? Lars Borchardt replied: We measured on both geometries – the Pd ball and a flat steel substrate th

RSC Advances is proud to present the 2021 Emerging Investigators series. Guest Edited by Professor James Batteas (Texas A&M University), this series showcases some of the very best work from chemists in the early stages of their independent careers.

There is significant interest in using single- and few-from the ability to tune its optoelectronic properties. One method investigated to tune the mechanical and electronic properties of MoS2 is through aryl radical addition reactions with compounds such as 4-nitrobenzenediazonium tetrafluoroborate (4-NBD). Here we investigated this for single-layer MoS2 (SLM) and multilayer MoS2 (MLM) on Au(111) substrates. The optical, chemical, and tribological properties of SLM and MLM on Au(111) were investigated before and after functionalization with an aqueous solution of 4-NBD. Atomic force microscopy (AFM) was used to characterize the pristine and functionalized MoS2 on Au(111) and illustrated the formation of a film on the MoS2 surfaces from the diazonium compound. This film density was greater on SLM than on MLM, likely due to the increased electronic coupling between SLM and the Au(111) substrate. Interestingly, we find that the films formed appear to be weakly bound on the surface of SLM and MLM and were easily worn away through AFM when contact forces exceeded 5 and 3 nN, respectively, suggesting limited covalent binding to the surfaces. While some of the diazonium reacts with the surfaces, the remaining diazonium film on the surface of SLM and MLM is stabilized through electron transfer from the underlying Au(111) substrate yielding a self-terminating dendritic growth on the surface with a film thickness of about 1.5 nm, regardless of the thickness of the MoS2 or the initial concentration of the diazonium solution.

In mechanochemistry, the application of controlled forces is key to altering reaction rates and pathways to direct product yields and selectivity. However, a fundamental knowledge gap exists between what is occurring on the atomic scale in mechanically driven reactions and the resulting macroscale outcomes. Two-dimensional (2D) materials, such as graphene, proffer a model system to study the impact of mechanical forces, such as strain, on chemical reactivity, as force distributions may be applied across a well-organized atomic-scale structure comprising a single layer of C atoms. Here, using Raman micro-spectroscopy and first-principles calculations, we have investigated the reaction of graphene, under varying degrees of strain, with 4-nitrobenzenediazonium tetrafluoroborate (4-NBD). We find that only with increased out-of-plane distortion (shifting the C atoms of graphene from sp2 toward sp3 electronic states) would the reactivity be increased, with larger out-of-plane distortions yielding greater reactivity. Density functional theory (DFT) calculations reveal that increasing the curvature of graphene decreases the activation barrier of 4-NBD functionalization and enhances the thermodynamic favorability of the reaction. Furthermore, we find that curvature affects the orientation of the graphene 2pz orbitals, and we then relate the thermodynamic feasibility of 4-NBD functionalization with the orbital orientation. These studies point to how the precise application of forces can be used to direct the functionalization of graphene for C-C bond forming reactions, which has significant implications for controlling its corresponding electronic structure in a well-defined fashion.

Graphene has unique mechanical, electronic, and optical properties that make it of interest for an array of applications. These properties can be modulated by controlling the architecture of graphene and its interactions with surfaces. Self-assembled monolayers (SAMs) can tailor graphene-surface interactions; however, spatially controlling these interactions remains a challenge. Here, we blend colloidal lithography with varying SAM chemistries to create patterned architectures that modify the properties of graphene based on its chemical interactions with the substrate and to study how these interactions are spatially arrayed. The patterned systems and their resulting structural, nanomechanical, and optical properties have been characterized using atomic force microscopy, Raman and infrared spectroscopies, scattering-type scanning near-field optical microscopy, and X-ray photoelectron spectroscopy.