In the present work, methyl viologen (1,1′-dimethyl-4,4′-bipyridinium dichloride) is used as a scavenger to estimate the radiolytic yields of water decomposition products from room temperature to 400 °C by pulse radiolysis method. {G(e aq -) + G(OH) + G(H)} has been studied using a 0.5 mM MV 2+ solution in the presence of 10 mM NaCOOH up to 200 °C and in the presence of 0.2 M ethanol up to 400 °C. The results show that the {G(e aq -) + G(OH) + G(H)} increases with temperature up to 350 °C at 25 MPa, while it depends also on pressure in supercritical conditions. The G(e aq -) was estimated using MV 2+ solutions in the presence of 0.2 M tert-butyl alcohol. The results agree well with the reported data up to around 300 °C at 25 MPa; however, in supercritical conditions a very significant density effect was observed. At a given temperature, G(e aq -) and {G(e aq -) + G(OH) + G(H)} decrease with increasing density while at a fixed density they decrease with increasing temperature.

The structures and diffusion behaviors of a series of ionic liquids [C(n)mim][PF(6)] (n = 1, 4, 8 and 12) on a graphite surface have been investigated by means of molecular dynamics simulation. It was found that three or four ordering layers of ionic liquids were formed near the graphite surface, and this layering structure was stable over the temperature range investigated. At the liquid/vacuum interface, the ionic liquid with a butyl chain had a monolayer ordering surface, while [C(8)mim][PF(6)] and [C(12)mim][PF(6)] exhibited a bilayer ordering with a polar domain sandwiched between two orientational nonpolar domains. More impressively, the simulated results showed that for the ionic liquids with alkyl chains longer than C(4), the adjacent alkyl chains in the whole film tended to be parallel to each other, with the imidazolium rings packed closely together. This indicated that the ionic liquids have a better regulated short-range structure than was previously expected. It was also found that both in the bottom layer and in the bulk region, the diffusion of the alkyl chains was much faster than that of the polar groups. However, as the alkyl chain length increased, the charge delocalization in the cation and the enhanced van der Waals interaction between the nonpolar groups contributed by reducing this difference in the diffusivity of major groups.

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The carbonate radical was investigated by pulse radiolysis in both NaHCO 3 and Na 2 CO 3 solutions over 15-400 °C. There was almost no change in the absorption spectrum of the carbonate radical within the temperature range studied. However, the absorbance of carbonate radical increased with temperature for NaHCO 3 solution at any concentration considered, whereas the absorbance for Na 2 CO 3 solution increased with temperature at high concentrations and decreased at low concentrations e 2 × 10 -3 m (mol/kg). The second-order rate constant for the decay of the carbonate radical exhibited a complicated temperature dependence: it decreased with temperature up to about 120 °C, and then it slowly increased with temperature up to 300 °C followed by a rapid increase with temperature at 300-400 °C. The decay was also dependent on the concentration of NaHCO 3 or Na 2 CO 3 . If considering the carbonate radical as CO 3 •-, the experimentally measured yield was always smaller than the simulated one, and the discrepancy was enlarged at lower carbonate/bicarbonate concentrations. We propose that the carbonate radical is present in the form of a dimer anion, either (CO 3 ) 2 •3or H(CO 3 ) 2 •2-. The equilibrium constants for the formation of (CO 3 ) 2 •3and H(CO 3 ) 2 •2were obtained for various temperatures. The measured radical yield can correlate well with the simulated result using the dimer model. We further estimated pK a of H(CO 3 ) 2 •2at temperatures up to 250 °C. The pK a at 25 °C was 9.30(0.15, in good agreement with the literature value of 9.5-9.6 obtained by measuring the pH dependence of the reactivity.

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