Kinetic sonication effects in light of molecular dynamics simulation of the reaction medium

Abstract

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.

Reaction kinetics and solubility in water-organic binary solutions are governed by similar solvation equilibria

“…The MD calculations were carried out for naphthalene and ethyl acetate solutions in water–acetonitrile mixtures because the largest solvent effects were found experimentally in these media. Ethyl acetate was used, as in our previous work, as a model ester because its chemical structure is less complicated than that of 4‐MPDA. The presence of phenyl ring brings with it multiple substrate conformations, thereby complicating the interpretation of the MD simulations, as reported by Rispens et al …”

“…The authors were able to prove the possibility of calculating the trends in rate retardation effects versus cosolute concentration. We are not aware of further MD study of ester hydrolyses except of our very limited simulation work indicating the linear dependence for the rate of ethyl acetate hydrolysis in aqueous ethanol on the relative ethanol content in the first solvation shell of the ester.…”

Ultrasound-assisted theophylline polymorphic transformation: Selective polymorph nucleation, molecular mechanism and kinetics analysis

“…Prasad et al [27] summarized the nucleation mechanism of different polymorphs with the application of ultrasound, and indicated that the ultrasound had the potential to alter the intermolecular interaction of solute. During the studies of ester hydrolysis [28] , [29] , the ultrasonic energy was identified to shift the balance of solvation state by improving the kinetic energy of species, and finally influencing the reaction rate. However, a thorough understanding of the effect of solute–solvent interaction affected by ultrasound on the nucleation process is still unclear, which needs attention.…”