Sonolytic Hydrolysis of p-Nitrophenyl Acetate: The Role of Supercritical Water

Ultrasonic irradiation is shown to accelerate the rate of hydrolysis of p-nitrophenyl acetate (PNPA) in aqueous solution by 2 orders of magnitude over the pH range of 3-8. In the presence of ultrasound, the observed 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, and k_(obs) = 4.6 x 10^(-4) s^(-1) with He as the cavitating gas. The apparent activation parameters for sonolytic catalysis are ΔH^≠ (sonified) = 211 Kj/mol, ΔS^≠(sonified) = -47/Jl(mol K), and ΔG^≠ (sonified) = 248 kJ/mol. Under ambient conditions and in the absence of ultrasound, k_(obs) is a strong function of pH where k_(obs) = k_(H_2O)[H_2O] + k_(OH)[OH-] with k_(H_2O) = 6.0 x 10^(-7) s^(-1) and k_(OH)- = 11.8 M^(-1) s^(-1) at 25 °C. The corresponding activation parameters are ΔH^≠ = 71.5 kJ/mol, ΔS^≠ = -107 J/(mol K), and ΔG^≠ = 155 kJ/mol. During cavitational bubble collapse, high temperatures and pressures exceeding the critical values 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 supercritical water (SCW) appears to be an important factor in the acceleration of chemical reactions in the presence of ultrasound. The apparent activation entropy, ΔS^≠, is decreased substantially during the sonolytic catalysis of PNPA hydrolysis, while ΔG^≠ and ΔH^≠ are increased. The decrease ΔS^≠ is attributed to differential solvation effects due to the existence of supercritical water (e.g., lower ρ and є) while the increases in ΔG^≠ and ΔH^≠ are attributed to changes in the heat capacity of the water due to the formation of a transient supercritical state. A dynamic heat-transfer model for the formation, lifetime, and spatial extent of transient supercritical water at cavitating bubble interfaces is presented.