Radiolysis of Supercritical Water at 400°C: A Sensitivity Study of the Density Dependence of the Yield of Hydrated Electrons on the (eaq−+eaq−) Reaction Rate Constant

The temperature dependence of the rate constant (⁠$k$⁠) of the bimolecular reaction of two hydrated electrons (⁠$eaq−$⁠) measured in alkaline water exhibits an abrupt drop between 150°C and 200°C; above 250°C, it is too small to be measured reliably. Although this result is well established, the applicability of this sudden drop in $k(eaq−+eaq−)$⁠) above $∼150°C$ to neutral or slightly acidic solution, as recommended by some authors, still remains uncertain. In fact, the recent work suggested that in near-neutral water the abrupt change in $k$ above $∼150°C$ does not occur and that $k$ should increase, rather than decrease, at temperatures greater than 150°C with roughly the same Arrhenius dependence of the data below 150°C. In view of this uncertainty of $k$⁠, Monte Carlo simulations were used in this study to examine the sensitivity of the density dependence of the yield of $eaq−$ in the low–linear energy transfer (LET) radiolysis of supercritical water (⁠$H2O$⁠) at 400°C on variations in the temperature dependence of $k$⁠. Two different values of the $eaq−$ self-reaction rate constant at 400°C were used: one was based on the temperature dependence of $k$ above 150°C as measured in alkaline water (⁠$4.2×108M−1s−1$⁠), and the other was based on an Arrhenius extrapolation of the values below 150°C (⁠$2.5×1011M−1s−1$⁠). In both cases, the density dependences of our calculated $eaq−$ yields at $∼60ps$ and 1 ns were found to compare fairly well with the available picosecond pulse radiolysis experimental data (for $D2O$⁠) for the entire water density range studied (⁠$∼0.15–0.6g/cm3$⁠). Only a small effect of $k$ on the variation of $G(eaq−)$⁠) as a function of density at 60 ps and 1 ns could be observed. In conclusion, our present calculations did not allow us to unambiguously confirm (or deny) the applicability of the predicted sudden drop of $k(eaq−+eaq−)$ at $∼150°C$ in near-neutral water.