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

Hua, Inez and Hoechemer, Ralf H. and Hoffmann, Michael R. (1995) Sonolytic Hydrolysis of p-Nitrophenyl Acetate: The Role of Supercritical Water. Journal of Physical Chemistry, 99 (8). pp. 2335-2342. ISSN 0022-3654. doi:10.1021/j100008a015.

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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.

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Hoffmann, Michael R.0000-0001-6495-1946
Additional Information:© 1995 American Chemical Society. Received: July 12, 1994; In Final Form: September 30, 1994. The authors are grateful to EPRI (contract No. RP8003-36) and ARPA (Grant No. NAV 5HFMN N0101492J1901) for generous financial support. We appreciate the support and encouragement of Drs. Ira Skumick and Myron Jones and the helpful discussions provided by Professor Kenneth Suslick.
Funding AgencyGrant Number
Electric Power Research Institute (EPRI)RP8003-36
Office of Naval Research (ONR)NAV 5HFMN N0101492J1901
Advanced Research Projects Agency (ARPA)UNSPECIFIED
Issue or Number:8
Record Number:CaltechAUTHORS:20150810-083412223
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Official Citation:Sonolytic Hydrolysis of p-Nitrophenyl Acetate: The Role of Supercritical Water Inez Hua, Ralf H. Hoechemer, and Michael R. Hoffmann The Journal of Physical Chemistry 1995 99 (8), 2335-2342 DOI: 10.1021/j100008a015
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:59335
Deposited By: Ruth Sustaita
Deposited On:10 Aug 2015 21:37
Last Modified:10 Nov 2021 22:18

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