Sonochemistry: Historical developments and modern aspects

Four types of sonochemical reactions are known; 1, the acceleration of conventional reactions; 2, redox processes in aqueous solution; 3, the degradation of polymers; and 4, the decomposition of and reactions in organic solvents. The electrical discharge theory, developed in the late thirties, was gradually substituted by hot spot theories after a bank of knowledge had been accumulated about the behaviour of cavitation bubbles in sonic fields. Sonochemical reaction may take place in hot gas bubbles, and these gas reactions may be understood in terms of what is known from combustion chemistry. Other reactions occur in the cooler interfacial region between the gas bubble and the liquid, and these reactions may be discussed in the light of radiation chemistry of solutions. Solute molecules may be decomposed by free radical attack (indirect action) and by direct thermal action. An important feature in the kinetics of these reactions is the accumulation of solute molecules at the interface, a process depending on their hydrophobicity. Finally, the chemical effects of pulsed ultrasound are described and discussed with respect to the use of ultrasonic pulses in medical diagnosis.