A statistical thermodynamic approach to sonochemical reactions

The calculation of the equilibrium constants K of the sonolysis reactions of CO2 into CO and O atom, the recombination of O atoms into O2 and the formation of H2O starting with H and O atoms, has been studied by means of statistical thermodynamic. The constants have been calculated at 300 kHz versus the pressure and the temperature according to the extreme conditions expected in a cavitation bubble, e.g. in the range from ambient temperature to 15 200 K and from ambient pressure to 300 bar. The decomposition of CO2 appears to be thermodynamically favored at 15 200 K and 1 bar with a constant K1 = 1.52 × 106, whereas the formation of O2 is not expected to occur (K2 = 1.8 × 10−8 maximum value at 15 200 K and 300 bar) in comparison to the formation of water (K3 = 3.4 × 1047 at 298 K and 300 bar). The most thermodynamic favorable location of each reactions is then proposed, the surrounding shell region for the thermic decomposition of CO2 and the wall of the cavitation bubble for the formation of water.Starting from a work of Henglein on the sonolysis of CO2 in water at 300 kHz, the experimental amount of CO formed (7.2 × 1020 molecules L−1) is compared to the theoretical CO amount (1.4 × 1027 molecules L−1) which can be produced by the sonolysis of the same starting amount CO2. With the help of the literature data, the number of cavitation bubble has been evaluated to 6.2 × 1015 bubbles L−1 at 300 kHz, in 15 min. This means that about 1 bubble on 1 900 000 is efficient for undergoing the sonolysis of CO2.