Sonolysis of aqueous 4-nitrophenol at low and high pH

The sonolysis of 4-nitrophenol in argon-saturated aqueous solution has been studied at 321 kHz. In order to evaluate separately the effect of OH radic als that are formed in the cavitational bubble and part of which react in t he aqueous phase with this substrate, radiolytic studies in N2O-saturated s olutions were carried out for comparison. A detailed product study of the s onolysis of 4-nitrophenol solutions shows that at pH 10, where 4-nitropheno l is deprotonated (pK(a) = 7.1), its sonolytic degradation is fully account ed for by OH-radical-induced reactions in the aqueous phase. At this pH, th e sonolytic yield of H2O2 resulting from OH radical recombination in the so lution, measured as a function of the 4-nitrophenol concentration, is reduc ed in line with the scavenging capacity of the 4-nitrophenolate. In contras t, at pH 4 the formation of H2O2 is already fully suppressed when the solut ion is 7 x 10(-4) mol dm(-3) in 4-nitrophenol, and oxidative-pyrolytic degr adation predominates, as exemplified by the large yields of CO and CO2 whic h are accompanied by a large H-2 yield. The basis of this difference in beh avior is a hydrophobic enrichment of 4-nitrophenol (which is undissociated at pH 4) at the interface of the cavitational bubble by a factor of about 8 0. The pH dependence of the yields of the pyrolytic products reflects the h ydrolytic equilibrium concentration of 4-nitrophenol. The paper also demons trates that the complexity of this sonochemical system precludes its use as a gauge to determine the temperature in the interior of the cavitational b ubble. (C) 2000 Elsevier Science B.V. All rights reserved.