SONOLYTIC HYDROLYSIS OF P-NITROPHENYL ACETATE - THE ROLE OF SUPERCRITICAL WATER

Ultrasonic irradiation is shown to accelerate the rate of hydrolysis o f p-nitrophenyl acetate (PNPA) in aqueous solution by 2 orders of magn itude over the pH range of 3-8. In the presence of ultrasound, the obs erved first-order rate constant for the hydrolysis of PNPA is found to be independent of pH and ionic strength with k(obs) = 7.5 x 10(-4) s( -1) with Kr as the cavitating gas, k(obs) = 4.6 x 10(-4) s(-1) with Ar as the cavitating gas, and k(obs) = 1.2 x 10(-4) s(-1) with He as the cavitating gas. The apparent activation parameters for sonolytic cata lysis are Delta H-double dagger(sonified) = 211 kJ/mol, Delta S-double dagger(sonified) = -47 J/(mol K), and Delta G(double dagger)(sonified ) = 248 kJ/mol. Under ambient conditions and in the absence of ultraso und, k(obs) is a strong function of pH where k(obs) = k(H2O)[H2O] + k( OH-)[OH-] with k(H2O) = 6.0 x 10(-7) s(-1) and k(OH-) = 11. 8 M(-1) s( -1) at 25 degrees C. The corresponding activation parameters are Delta H-double dagger = 71.5 kJ/mol, Delta S-double dagger = -107 J/(mol K) , and Delta G(double dagger) = 155 kJ/mol. During cavitational bubble collapse, high temperatures and pressures exceeding the critical value s of water (T > T-c = 647 K and P > P-c = 221 bar) occur in the vapor phase of the cavitating bubbles and at the interfaces between the hot vapors and the cooler bulk aqueous phase. The formation of transient s upercritical water (SCW) appears to be an important factor in the acce leration of chemical reactions in the presence of ultrasound. The appa rent activation entropy, Delta S-double dagger, is decreased substanti ally during the sonolytic catalysis of PNPA hydrolysis, while Delta G( double dagger) and Delta H-double dagger are increased. The decrease D elta S-double dagger is attributed to differential solvation effects d ue to the existence of supercritical water (e.g., lower rho and epsilo n) while the increases in Delta G(double dagger) and Delta H-double da gger are attributed to changes in the heat capacity of the water due t o the formation of a transient supercritical state. A dynamic heat-tra nsfer model for the formation, lifetime, and spatial extent of transie nt supercritical water at cavitating bubble interfaces is presented.