Pulse radiolysis of supercritical water. 1. Reactions between hydrophobic and anionic species

Reaction rates of solvated electrons with oxygen and with sulfur hexafluoride were measured in hydrothermal and supercritical water using transient absorption spectroscopy and electron pulse radiolysis. Under alkaline conditions, the reaction of hydrogen atoms with hydroxide ions to generate solvated electrons was also observed in the presence of the SF6 scavenger. At temperatures below 300 degreesC, the rate constants for scavenging by O-2 or SF6 follow Arrhenius behavior but become increasingly dependent on water density (pressure) at higher temperatures. Above 100 degreesC, the rate constant for the H reaction with OH- falls well below the numbers extrapolated from the Arrhenius behavior in the one atmosphere liquid. At a fixed temperature above the water critical temperature (380 degreesC, T/T-c = 1.01), rate constants for all three reactions reach a distinct minimum near 0.45 g/cm(3). We propose an explanation for this behavior in terms of the potential of mean force separating an ion (OH- or (e-)(aq)) from a hydrophobic species (H, O-2, or SF6) in the compressible fluid. The data also reveal an increasing initial yield of atomic hydrogen relative to solvated electrons as water density decreases. The initial yield of H appears to surpass that of solvated electrons when the water density is below 0.6 g/cm(3) at 380 degreesC.