Supervisors: Ph.D Siim Salmar, prof. Jaak Järv
Opponent: Prof. Giancarlo Gravotto, PhD, University of Turin, Italy
"Kinetic effects of ultrasound in aqueous acetonitrile solutions"
In the present work kinetics of the pH-independent hydrolysis of 4-methoxyphenyl dichloroacetate was investigated with and without ultrasonic irradiation in acetonitrile-water binary mixtures with molar ratios of acetonitrile from 0.008 to 35 wt % and the kinetic sonication effects (kson/knon) were calculated. All experiments were carried out with an on-line spectrophotometric system designed for this study.
Our results revealed that addition of acetonitrile to the reaction medium substantially decreased the rate of hydrolysis of the ester, while rate constants determined under ultrasound showed little susceptibility to changes in solvent composition. Evidently, the size of kinetic sonication effects are related to the size of solvent effects.
In parallel, MD simulations of the structure of the water-ethanol and water-acetonitrile solutions were performed with ethyl acetate as the model ester to cast light on solvent effects in aqueous-organic solutions. Ethyl acetate was preferentially solvated by co-solvent in all of these cases, however, the different solvation patterns in acetonitrile and ethanol aqueous solutions were found.
MD simulations of ethyl acetate solution in aqueous ethanol medium were performed at 280 K and 330 K. With increasing temperature, a significant proportion of alcohol molecules were replaced by water molecules in the first solvation shell. This result confirmed the principle of thermo-solvatochromism, which predicts depletion of the solvation shell of the organic solvent with rising temperature. The addition of ethanol to the aqueous solution of ethyl acetate decreased the rate of hydrolysis of the ester. It was found that the decrease of the reaction rate was almost linearly related to the increase in the ethanol content in the first solvation shell of the ester.
The abundance of acetonitrile relative to water in the solvation shell grew fast with the increase in the co-solvent content in the bulk solution. In parallel to this, the formation of a second acetonitrile-rich solvation shell was observed. This mode of ester solvation caused dramatic drop in the reaction rate with the increase in the acetonitrile content in the reaction mixture, as enhanced co-solvent content in the solvation shell decreased water activity in the microenvironment and lowered reaction rate.
The principal conclusion from the MD simulation study was that ultrasound enhanced the kinetic energy of molecules and through this effect shifted solvation equilibrium and affected reaction rate. At the same time it appeared that ultrasound did not completely break down the solvent shells or clusters in the solution, as was believed previously. In contrast to this, application of high energy was not required to affect reaction rates.