Experiments with methyl methacrylate prove that polymerization can be initiated in a pure vinyl monomer with intense ultrasound. Variation of the polymerization rate with time, volume, and intensity are explained by a simple reaction mechanism. The number average molecular weight of the polymer produced is about 400000 g mol-I; rate constants for the polymerization mechanism obtained from the molecular weight data agree well with literature values. Presence of the radical scavenger DPPH inhibits the polymerization. The temperature deperldence of the DPPH reaction is explained well over a wide (-17 to 40 "C) temperature range by relations based on reversible or irreversible adiabatic collapse of the cavitation bubble.Furthermore it is in agreement with sonochemical experiments where composition rather than temperature was changed to vary the vapor pressure. Initiation rates calculated from polymerization rates and DPPH consumption rates are of the same order of magnitude but show an unexplainable difference in temperature dependence. A colored compound appears if the cavitation is carried out at lower temperatures.
The formation of nitrite and nitrate ions in water under irradiation with 900 kHz ultrasound was studied as a function of time, temperature and gas (oxygen/nitrogen) composition. The rate decreases as temperature increases, and is below the detection limit when there is no O2 gas present. The absolute rate of formation of NOx- ions obtained (about 30 x 10(-9) mol min-1 W-1) agrees well with previous similar studies. The differences in the NO2-/NO3- ratio found between various studies can be satisfactorily explained though a mechanism where HNO2 and HNO3 are formed in the gas phase of the imploding cavity, and then dissolve in the water and dissociate to ions. The NO2- species is initially substantially favoured, as considerably more NO is formed than NO2. Thermodynamic calculations indicate that at the 'hot spot' temperature of about 5000 K believed to be present, there would very large amounts of NO and OH radicals present, and at such high temperatures, thermodynamics would be a good approximation of the situation, since the rates of reactions would be very rapid. The reaction needs O2 in order to proceed to a significant degree; no NOx- was detected in the absence of oxygen gas.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.