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Subtle differences in how pyridine is introduced can direct mechanochemical reactivity by enabling bond activation and favoring one reaction pathway over another. 

his study examined six phosphonium-based room-temperature ionic liquids (PRTILs) having trihexyltetradecyl- or tributyltetradecyl-phosphonium cations with saccharinate, salicylate, or benzoate anions, and obtained a feature parameter to correlate their cationic chain length, anionic ring size, and contact angle with tribological properties. PRTILs with trihexyltetradecyl-phosphonium cations had lower coefficient of friction (COF) and wear than PRTILs with tributyltetradecyl- phosphonium cations, a trend attributed to the additional methylene groups providing lower contact angle. For either cation, PRTILs with the saccharinate anion exhibited much lower COF and wear than single-ring anions, due to the formation of a low-shear-strength-tribofilm facilitated by the double-ring structure and sulfur of saccharinate. Overall, this study revealed PRTIL interfacial mechanisms that can be used to identify anion-cation combinations with optimal tribological performance. 

Abstract Mechanical stress can directly activate chemical reactions by reducing the reaction energy barrier. A possible mechanism of such mechanochemical activation is structural deformation of the reactant species. However, the effect of deformation on the reaction energetics is unclear, especially, for shear stress-driven reactions. Here, we investigated shear stress-driven oligomerization reactions of cyclohexene on silica using a combination of reactive molecular dynamics simulations and ball-on-flat tribometer experiments. Both simulations and experiments captured an exponential increase in reaction yield with shear stress. Elemental analysis of ball-on-flat reaction products revealed the presence of oxygen in the polymers, a trend corroborated by the simulations, highlighting the critical role of surface oxygen atoms in oligomerization reactions. Structural analysis of the reacting molecules in simulations indicated the reactants were deformed just before a reaction occurred. Quantitative evidence of shear-induced deformation was established by comparing bond lengths in cyclohexene molecules in equilibrium and prior to reactions. Nudged elastic band calculations showed that the deformation had a small effect on the transition state energy but notably increased the reactant state energy, ultimately leading to a reduction in the energy barrier. Finally, a quantitative relationship was developed between molecular deformation and energy barrier reduction by mechanical stress. 

Reactive molecular dynamics simulations differentiate thermal and mechanochemical pathways for oxygen release from polymers with implications for therapeutic applications. 

The molecules with higher tribochemical reactivity exhibited smaller activation volume, implying that less mechanical energy was required to initiate tribochemical reactions. 

Surface chemistry influences the mechanochemical decomposition of antiwear additives.