Charged species in the radiolysis of supercritical CO2

The pulse radiolysis technique has been employed in studying charge-transfe r reactions of anionic C2O4- and cationic C2O4+ species in supercritical ca rbon dioxide (scCO(2)) over a range of reduced densities rho(r) = 0.36-1.5 and at a reduced temperature of T-r = 1.03. The absorption spectrum measure d in the visible region with a maximum around 700 nm is assigned to the dim er cation C2O4+. The pressure dependence of charge-transfer reactions was e xamined using dimethylaniline (DMA), benzoquinone (BQ), and oxygen as charg e accepters. The reaction rates of DMA with cations, and BQ with anions are at or near the diffusion-controlled limit. The rates decrease an order of magnitude with increase of pressure. The reaction of C2O4+ With oxygen is m uch slower with an almost constant rate over the pressure range examined. T he measured race constants of electron-transfer reactions are analyzed in t erms of the diffusion constants of reactants in scCO(2), and the dependence of measured and theoretical values on the bulk density is discussed.