Our group is one of relatively few groups studying continuous reactions in supercritical water, scH2O. As water is heated towards its critical point (Tc 374oC, Pc 218 atm.), it undergoes a transformation considerably more dramatic than that of most other substances. It changes from the familiar polar liquid to an almost non-polar fluid. The change occurs over a relatively wide temperature range; even at 200oC, the density drops to 0.8 g/ml and, at Tc, the fluid becomes miscible both with organics and with gases. Diffusivity increases and the acidity is enhanced more than would be expected purely on the basis of higher temperatures.
Over the past decade, a major research effort has been focused on the total oxidation of toxic organics in scH2O, The process is highly effective but there can be serious problems of corrosion associated with large scale waste destruction, so serious indeed that many chemists have been discouraged from even contemplating possible uses of scH2O as a medium for chemical reactions.
Recently, however, there have been a number of reports which show that high temperature and scH2O can be used constructively for reaction chemistry. It is clear that increasing the temperature renders water increasingly acidic and favours ionic processes over radical or purely thermal pathways. The challenge lies in finding out how to exploit these effects safely on a larger scale over a wide range of chemistry. The field has been extensively reviewed by P. E. Savage “Organic chemical reactions in supercritical water” CHEMICAL REVIEWS 99 (2): 603-621 1999