Sonochemistry of volatile and nonvolatile solutes in aqueous solutions: A spin trapping study

Abstract

The very high temperatures and pressures induced by acoustic cavitation in collapsing gas bubbles in aqueous solutions exposed to ultrasound lead to the thermal dissociation of water vapor into H atoms and OH radicals. Their formation has been confirmed by spin trapping for continuous wave and pulsed ultrasound. Sonochemical reactions occur in the gas-phase (pyrolysis reactions), in the gas-liquid interface, and in the bulk of the solution (radiation-chemistry reactions). The high-temperature gradients in the interfacial regions lead to pyrolysis products from nonvolatile solutes present at sufficiently high concentrations. The sonochemically generated radicals from carboxylic acids, amino acids, dipeptides, sugars, pyrimidine bases, nucleosides, and nucleotides were identified by spin trapping with a nonvolatile nitroso spin trap. At low concentrations of nonvolatile solutes, the spin trapped radicals produced by sonolysis are due to H atom and OH radical reactions. At higher concentrations of these nonvolatile solutes, sonolysis leads to the formation of additional reactions due to pyrolysis processes (typically methyl radicals). A preferred localization of nonvolatile surfactants at the gas-liquid interface was demonstrated. The volatile solutes methanol, ethanol, acetone, and acetonitrile were studied over the complete range of solvent composition. By the use of rare gases with different thermal conductivities, the contributions of individual reaction steps with widely different energies of activation can be evaluated.