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-di methylformamide (DMF) was investigated by spin trapping with EPR detec tion, using 3,5-dibromo-4-nitrosobenzene sulfonate (DBNBS) as the spin trap. Methyl radicals, produced by pyrolysis in the hot cavitation re gions 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 tra p did not contribute to the observed radical yield. Sonochemical yield s of both types of radical adducts increased with increasing DMF conce ntration in spite of the decreased production of H-. and (OH)-O-. radi cals 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 incr eased amount of DMF thermolysis in the hot cavitation region. This beh avior is in contrast to that of aqueous mixtures of volatile solvents (solvents with vapor pressures higher than water), which show a maximu m 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.