FREE-RADICAL PRODUCTION BY SONOLYSIS OF AQUEOUS MIXTURES OF N,N-DIMETHYLFORMAMIDE - AN EPR SPIN-TRAPPING STUDY

The 50-kHz sonochemistry of argon-saturated aqueous mixtures of N,N-dimethylformamide (DMF) was investigated by spin trapping with EPR detection, using 3,5-dibromo-4-nitrosobenzene sulfonate (DBNBS) as the spin trap. Methyl radicals, produced by pyrolysis in the hot cavitation regions and (CH2N)-C-.(CH3)CHO radicals, formed by hydrogen abstraction from DMF, were spin trapped over a wide range of DMF concentrations (0.004-12.9 mol/L (100% DMF)). The thermal decomposition of the spin trap did not contribute to the observed radical yield. Sonochemical yields of both types of radical adducts increased with increasing DMF concentration in spite of the decreased production of H-. and (OH)-O-. radicals from the thermolysis of water. The increase of the radical yield is not due to the increased availability of the monomeric form of the spin trap at higher DMF concentrations but can be explained by an increased amount of DMF thermolysis in the hot cavitation region. This behavior is in contrast to that of aqueous mixtures of volatile solvents (solvents with vapor pressures higher than water), which show a maximum followed by a decrease of radical yields at higher mole fractions of the volatile component, due to the decrease of the effective gamma = C-p/C-v in the imploding cavitation bubbles.