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

Abstract

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 150آ°C and 200آ°C; above 250آ°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 ∼150آ°C to neutral or slightly acidic solution, as recommended by some authors, still remains uncertain. In fact, the recent work suggested that in nearneutral water the abrupt change in k above ∼150آ°C does not occur and that k should increase, rather than decrease, at temperatures greater than 150آ°C with roughly the same Arrhenius dependence of the data below 150آ°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 400آ°C on variations in the temperature dependence of k. Two different values of the eaq− selfreaction rate constant at 400آ°C were used: one was based on the temperature dependence of k above 150آ°C as measured in alkaline water (4.2أ—108  M−1 s−1), and the other was based on an Arrhenius extrapolation of the values below 150آ°C (2.5أ—1011  M−1 s−1). In both cases, the density dependences of our calculated eaq− yields at ∼60  ps and 1آ ns were found to compare fairly well with the available picosecond pulse radiolysis experimental data (for D2O) for the entire water density range studied (∼0.15–0.6  g/cm3). Only a small effect of k on the variation of G(eaq−)) as a function of density at 60آ ps and 1آ 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 ∼150آ°C in nearneutral water.