Activation Volume in Shear-Driven Chemical Reactions

Abstract

Interfacial shear-driven or shear-assisted chemical reactions play an important role in many engineering processes, including reactions between lubricant additives and the surfaces of mechanical components and fabrication of surface topographic features. Mechanistic studies of shear-driven chemical reactions often employ a mechanically assisted thermal-activation model from which a so-called activation volume can be defined. Activation volume is important because it quantifies the efficiency of interfacial shear to drive the reaction. Recent advancements in methods have enabled calculation of activation volume from both nano-and macro-scale experiments as well as simulations. However, the calculated volumes differ by orders of magnitude, even for the same reactant species, and the physical interpretations vary correspondingly. Here, we review how activation volume has been measured and interpreted for shear-driven reactions in the literature with the goal of guiding future efforts to understand and use this important parameter for engineering design through tribochemistry.

Substituent effects on the mechanochemical response of zinc dialkyldithiophosphate

“…Moreover, at 80 °C, the primary C 8 ZDDPs containing branched (2-ethylhexyl, 0.13 nm 3 ) and particularly the bulky cycloaliphatic (2-cyclohexylethyl, 0.20 nm 3 ) substituents have larger Δ V * values than those with linear (oct-1-yl, 0.09 nm 3 ) alkyl groups. The exact physicochemical meaning of Δ V * remains unclear for tribochemical reactions; 79,80 however, a larger activation volume (or activation length 24,25 ) implies that, at a given stress, there is a greater distortion of the molecule from the ground state to the transition state along the reaction path. 74 In other words, compared to the oct-1-yl groups, the longer dodec-1-yl, branched 2-ethylhexyl and particularly bulky 2-cyclohexylethyl groups act as metaphorical crowbars, allowing the shear (or tensile 13 ) stress to be transmitted across a greater distance as the initial C–O (or C–S) bond-breaking reaction proceeds.…”

Developing Reaction Chemistry Models from Reactive Molecular Dynamics: TATB

“…The separate branches for the first reaction stage, B and E , do follow quite different reaction pathways and we do think their separation arises from how pressure affects the reaction processes [68–70]. The lower pressure branch E is dominated by the water elimination reaction, which is net dissociative in nature.…”

“…This does contain larger molecular weight species than the final products (i. e., 6-10 atoms per molecule vs. 3-4 atoms per product species), so pressure-dependent equilibrium would be harder to establish. The separate branches for the first reaction stage, B and E, do follow quite different reaction pathways and we do think their separation arises from how pressure affects the reaction processes [68][69][70]. The lower pressure branch E is dominated by the water elimination reaction, which is net dissociative in nature.…”

Substituent Control of Tribofilm Growth under Mechanochemical Conditions

“…ZDDPs containing branched (2-ethylhexyl, 0.13 nm 3 ) and particularly the bulky cycloaliphatic (2cyclohexylethyl, 0.20 nm 3 ) substituents have larger ΔV * values than those with linear (oct-1-yl, 0.09 nm 3 ) alkyl groups. The exact physicochemical meaning of ΔV * remains unclear for tribochemical reactions; 74,75 however, a larger activation volume (or activation length 21,22 ) implies that, at a given stress, there is a greater distortion of the molecule from the ground state to the transition state along the reaction path. 69 In other words, compared to the oct-1-yl groups, the 2-ethylhexyl and particularly 2-cyclohexylethyl groups act as metaphorical crowbars, allowing the shear (or tensile 13 ) stress to be transmitted across a greater distance as the C−O bondbreaking reaction proceeds.…”