Transport Mechanism of Acetamide in Deep Eutectic Solvents

TL;DR

This study investigates the molecular diffusion mechanisms of acetamide in deep eutectic solvents using neutron scattering and molecular dynamics, revealing restricted mobility due to hydrogen bonding and complex formation.

Contribution

It provides a detailed nanoscopic diffusion model of acetamide in DES, combining experimental neutron scattering data with MD simulations to explain viscosity and freezing point depression.

Findings

Long range diffusion of acetamide is three times slower in DES.

Hydrogen bonding leads to complex formation, restricting molecular mobility.

Complexation inhibits crystallization and depresses freezing point.

Abstract

Over the last couple of decades, deep eutectic solvents (DESs) have emerged as novel alternatives to ionic liquids that are extensively used in synthesis of innovative materials, metal processing, catalysis, etc. However, their usage is limited, primarily because of the large viscosity and poor conductivity. Therefore, an understanding of the molecular origin of these properties is essential to improve their industrial applicability. Here, we present the report of the nanoscopic diffusion mechanism of acetamide in a DES synthesized with lithium perchlorate as studied using neutron scattering and molecular dynamics (MD) simulation techniques. Although, the acetamide based DES (ADES) has remarkably lower freezing point compared to pure acetamide, the molecular mobility is found to be enormously restricted in the former. MD simulation indicates a diffusion model with two distinct processes, corresponding to, long range jump diffusion and localised diffusion within a restricted volume. This model is validated by analysis of neutron scattering data in both molten acetamide and ADES. The long range diffusion process of acetamide is slower by a factor of three in ADES in comparison with molten acetamide. MD simulation reveals that the long range diffusion in ADES is restricted mainly due to the formation of hydrogen bond mediated complexes between the ionic species of the salt and acetamide molecules. Hence, the origin of higher viscosity observed in ADES can be attributed to the complexation. The complex formation also explains the inhibition of the crystallisation process while cooling and thereby results in depression of the freezing point of ADES.