Sonoluminescence quenching in aqueous solutions containing weak organic acids and bases and its relevance to sonochemistry

Multibubble sonoluminescence from water was found to be quenched by the addition of low levels of unsubstituted or 2-chloro-substituted aliphatic carboxylic acids and the aromatic solutes, phenol and aniline. In all cases, the sonoluminescence intensity could be restored to the level obtained in pure water by adjusting the solution pH to a point where the solutes were completely in their ionized form. A comparison of the sonochemical decomposition of phenol at pH levels above and below its pK(a) revealed that only when the solute was in its neutral form was there significant decomposition. It is shown that both the sonochemical decomposition of phenol and the sonoluminescence quenching by all solutes studied can be directly correlated with the amount of solute adsorbed at the bubble/solution interface. The relationship between sonochemical activity and sonoluminescence quenching by water-soluble organic solutes is discussed.