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; however, a larger activation volume (or activation length 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. 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 ) stress to be transmitted across a greater distance as the C-O bondbreaking reaction proceeds. This implies that the lever-arm effect, previously identified in polymer mechanochemistry under applied force, can also affect mechanochemical reactivity of comparatively small organometallic compounds such as ZDDP under applied stress. From a practical perspective, ZDDPs with branched and cycloaliphatic substituents could be particularly useful for wear protection in low-temperature, high-stress applications, such as gear oils. 76 Figure shows that, for the various ZDDPs studied here, there is a linear relationship between Ea and ln(A), which is indicative of the kinetic compensation effect. ZDDPs that have a large Ea also show higher ln(A) (e.g. dodec-1-yl), which suggests both a high energy barrier and a high frequency of successful collisions. From transition state theory, this can also be interpreted as the tribofilm growth reactions having a larger enthalpic barrier, but also a smaller entropic penalty. ZDDPs with primary linear alkyl groups sit on the linear fit in Figure (within statistical uncertainty), while those with branched (2-ethylhexyl) and cycloaliphatic (2-ethylcylohexyl) primary alkyl groups sit above it. Thus, ZDDPs with branched and cycloaliphatic groups have a larger Ea for a given ln(A) compared to ZDDPs with linear alkyl groups, which can be attributed to their higher thermal stability due to increased steric bulk. Conversely, most of the ZDDPs with secondary alkyl groups (oct-2-yl and but-2-yl) fall below the linear fit in Figure , implying a smaller Ea for a given ln(A), which can be attributed to their lower thermal stability, due to the increased stability of the secondary carbocation. In this study, we have demonstrated that the Bell model 13 can be used to explain the large differences in mechanochemical reactivity of the various ZDDPs studied (Figure ). Linear secondary alkyl ZDDPs are inherently less stable than primary ones with the same chain length under mechanochemical conditions, which is mostly due to a higher pre-exponential factor (Table ). Since they form SAMs with a higher packing density, ZDDPs with longer chain lengths require higher temperatures to form tribofilms, which manifests as a higher activation energy. We observe an increase in the activation volume moving from linear, to branched, and to cycloaliphatic substituents, which suggests a molecular-scale analogue of the lever-arm effect that has been observed in polymer mechanochemistry. These kinetic parameters can be used directly to inform macroscale tribology models that consider tribofilm growth inside rubbing contacts. We also expect that, coupled with information regarding their friction and wear performance under mixed/boundary conditions, the improved understanding of substituents effects on tribofilm growth identified here under mechanochemical conditions to be useful in the rational design of antiwear additives with improved performance for specific applications such as heavy-duty diesel engines and gear oils. More generally, the methods proposed here can be used to identify molecular structure-performance relationships for lubricant additives under well-controlled conditions, as recently achieved for viscosity modifier additives and synthetic base oils.