The kinetics of pH-independent hydrolysis of 4-methoxyphenyl dichloroacetate were investigated under ultrasonic irradiation with an application of 10% of the maximum power of the equipment and without sonication in acetonitrile-water binary mixtures with a content of acetonitrile ranging from 0.008 to 35 wt.%. Similar kinetic investigations were performed at intensities corresponding to 10%, 20%, 30%, 40%, and 50% of the input energy in solvent mixtures containing 10 wt.% and 25 wt.% acetonitrile. In parallel, the responses of KI and terephthalic acid dosimeters at applied irradiation levels were registered under the same experimental conditions. Significant kinetic sonication effects were found at sound intensities presumably not inducing cavitation in the solution. This result provides an experimental evidence of kinetic effects of ultrasound in the absence of cavitation. A disturbing impact of cavitation on the ultrasonic acceleration of the reaction was found. The implications of these findings were discussed.

Molecular dynamics (MD) simulation of the structure of ethyl acetate solutions in two water-ethanol mixtures was performed at 280 and 330K. The MD simulations revealed that ethyl acetate was preferentially solvated by ethanol, water being mainly located in the next solvation layer. With increasing temperature ethanol was gradually replaced by water in the first solvation shell. These findings explain the decrease in the rate of ester hydrolysis with increasing molar ratio of ethanol in the solution as the reaction rate was linearly dependent on the relative ethanol content in the first solvation shell of the ester. Predominance of ethanol results in decreased polarity and water activity in the shell and accordingly in a decreased reaction rate. Based on the results of the MD simulations, the principal conclusion of this work is that ultrasound enhances the kinetic energy (the effective temperature) of species in the solution and, in this way, evokes shifts in the solvation equilibria thus affecting the reaction rate. It appears that ultrasound does not completely break down the solvent shells or clusters in the solution as previously believed. Phenomena of thermo-solvatochromism and reaction rate levelling by ultrasound in binary solvents are described.

The kinetics of the pH-independent hydrolysis of 4-methoxyphenyl dichloroacetate were investigated with and without ultrasonic irradiation in acetonitrile-water binary mixtures containing 0.008 to 35 wt.% of acetonitrile and the kinetic sonication effects (kson/knon) were calculated. Molecular dynamics (MD) simulations of the structure of the solutions were performed with ethyl acetate as the model ester. The ester is preferentially solvated by acetonitrile. The excess of acetonitrile over water in the solvation shell grows fast with an increase in the co-solvent content in the bulk solution. In parallel, the formation of a second solvation shell rich in acetonitrile takes place. Significant kinetic sonication effects for the hydrolysis were explained with facile destruction of the diffuse second solvation shell followed by a rearrangement of the remaining solvent layer under sonication. The rate levelling effect of ultrasound was discussed. In an aqueous-organic binary solvent, independent of the solvent composition, the ultrasonic irradiation evokes changes in the reaction medium which result in an almost identical solvation state of the reagent thus leading to the reaction rate levelling.

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A unique ultrasonic online spectrophotometric system for kinetic measurements of homogeneous polar reactions under ultrasound was developed. The setup consists of an ultrasonic bath, an HPLC UV/Vis detector, an HPLC pump, a refrigerated heating circulator bath, and a data processing unit. An analogue-digital converter was constructed and a data processing program AD_com was written for registration of detector signal. The pH-independent hydrolysis of 4-nitrophenyl chloroacetate as a model reaction and also sonolytic degradation of 4-nitrophenol were used for the examination of the online spectrophotometric system. Preliminary kinetic studies proved that the system can be successfully applied for ultrasonic kinetic investigations of polar homogeneous reactions. This online system allows a sensitive and reproducible monitoring of various homogeneous processes, which can be detected spectrophotometrically under ultrasonic irradiation.