Using Equation , the slopes of the rate-stress (Figure ) plots were used to calculate ΔV * , while the slopes and intercepts of the rate-temperature (Figure ) plots were used to calculate Ea and ln(A), respectively (see Table and Figure ). The ΔV * values are also normalized by the molecular volumes, Vmol. The Vmol values shown in Table were calculated using the 3V software with a probe size of 3 Å 3 . The ZDDP structures used were relaxed in ChemBio3D Ultra 14 Suite (PerkinElmer, Waltham, MA) using the MM2 forcefield. The ΔV * is slightly smaller for the primary oct-1-yl (0.09 nm 3 ) ZDDP than the secondary oct-2-yl (0.10 nm 3 ) ZDDP, implying a similar stress-dependency of their reactivity. The Ea is higher for the oct-2-yl (66 kJ mol -1 ) than the oct-1-yl (56 kJ mol -1 ) ZDDP, despite the lower thermal stability of secondary ZDDPs. These Ea values are consistent with previous measurements for commercial secondary (82 kJ mol -1 ), primary (56 kJ mol -1 ), and mixed primary/secondary (79 kJ mol -1 ) ZDDPs. On the other hand, A is around than two orders of magnitude larger for the oct-2-yl (2.5 × 10 5 nm s -1 ) than the oct-1-yl (5.9 × 10 3 nm s -1 ) ZDDP. Since the difference in Ea suggests faster rates for oct-1-yl than oct-2-yl, the experimentally observed larger rates for oct-2-yl must be primarily due to the differences in ΔV * and particularly A. This can be visualized in Figure , where the rates for the oct-2-yl ZDDP are shifted vertically (larger intercept) with respect to the oct-1yl ZDDP. These observations are also consistent with the differences noted by Zhang et al. , although the differences in A between the commercial secondary (2.0 × 10 7 nm s -1 ) and primary (1.1 × 10 3 nm s -1 ) were even larger than in the current study. The current experiments support the hypothesis that the mechanochemical decomposition of primary and secondary ZDDPs proceeds through different chemical mechanisms. While previous glassware experiments in the absence of steel surfaces have suggested a common mechanism for thermal decomposition of ZDDP, first principles calculations have shown that the C-O bonds are broken through different mechanisms for primary and secondary ZDDPs; alkyl group transfer (from -OR to -SR) for the former and alkene elimination for the latter. 68 The ΔV * /Vmol values in Table suggest that a 7-17 % deformation of the ZDDP molecules occurs when moving from the ground state to the transition state for the mechanochemical reaction. These values fall within the range estimated for other tribological processes involving molecular systems (2-27 %), which have been derived both from experiments and molecular simulations. The ΔV * values are similar to those obtained previously for commercial primary (0.13 nm 3 ) and secondary (0.15 nm 3 ) ZDDPs. There do not seem to be any systematic differences between the ΔV * values for primary and secondary ZDDPs. However, the primary C8