Decoupling between Solvent Viscosity and Diffusion of a Small Solute Induced by Self-Motion

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• The Stokes-Einstein relation is a cornerstone for understanding solute diffusion in liquids.
• Experimental studies have shown discrepancies between the Stokes-Einstein relation and observed diffusion coefficients for small solutes.
• The molecular-level mechanisms driving these deviations remain incompletely understood.

Purpose of the Study:

• To investigate the self-diffusion of monatomic solutes in liquid 1-octanol and n-tetradecane using molecular dynamics simulations.
• To elucidate the relationship between solute diffusion, solvent dynamics, and the validity of the Stokes-Einstein relation at the molecular level.
• To identify the key factors contributing to the faster-than-expected diffusion of small solutes.

Main Methods:

• Molecular dynamics (MD) simulations were employed to model the system.
• The diffusion coefficient of a monatomic solute (e.g., argon) was calculated.
• Memory functions and autocorrelation functions (shear stress, force) were analyzed to probe solvent dynamics and solute-solvent interactions.

Main Results:

• The simulated diffusion coefficient for small solutes significantly exceeded predictions from the Stokes-Einstein relation, consistent with experimental findings.
• A faster relaxation of the solute's memory function was observed compared to the solvent's shear stress autocorrelation function.
• When the solute was spatially fixed, the diffusion coefficient calculated from the force-force autocorrelation function aligned with the Stokes-Einstein behavior, with force relaxation mirroring shear stress relaxation.

Conclusions:

• The fast diffusion of small solutes is attributed to a decoupling between solvent structural relaxation and solute diffusion.
• The self-motion of the solute plays a crucial role in this decoupling.
• The findings highlight the limitations of hydrodynamic theories for small solutes and emphasize the importance of molecular-level dynamics.