Role of molecular structure in defining the dynamical landscape of deep eutectic solvents

The molecular dynamics of deep eutectic solvents (DESs) are highly complex, characterized by pronounced spatial and temporal heterogeneity. Understanding these dynamics is crucial for tailoring transport properties such as diffusion, viscosity, and ionic conductivity. Molecular diffusion in DESs stems from transient caging and translation jumps, necessitating an understanding of how molecular structure regulates these processes. This study explores the influence of alkyl chain length on the dynamical behavior of alkylamide-lithium perchlorate based DESs using quasielastic neutron scattering (QENS) and molecular dynamics simulations. QENS results show that, despite its shorter chain length and lighter mass, acetamide exhibited the lowest mobility among the alkylamides, including propanamide (PRM) and butyramide (BUT). Detailed analysis of distinct degrees of freedom including the long-range jump diffusion of the alkylamide center of mass and localized diffusion, a clear trend emerges. The jump dynamics typically slowed with increasing chain length, essentially due to their differences in molecular size, mass, and also enhanced complexation in longer alkyl chain molecules. However, localized dynamics, dictated by the interplay between molecular flexibility and caging effects, exhibit an unusual trend, with PRM emerging as the fastest due to its optimal balance of molecular flexibility and reduced caging effects. In contrast, although BUT exhibited greater flexibility due its longer chain, its localized dynamics were slower, owing to stronger caging effects. Our findings highlight the complex interplay between alkyl chain length and the dynamical properties of DESs, demonstrating the relevance of molecular structure in governing the dynamics and transport properties of these systems.