Diffusion of Small Solute Particles in Viscous Liquids: Cage Diffusion, a Result of Decoupling of Solute–Solvent Dynamics, Leads to Amplification of Solute Diffusion

Abstract

We study the diffusion of small solute particles through solvent by keeping the solute-solvent interaction repulsive and varying the solvent properties. The study involves computer simulations, development of a new model to describe diffusion of small solutes in a solvent, and also mode coupling theory (MCT) calculations. In a viscous solvent, a small solute diffuses via coupling to the solvent hydrodynamic modes and also through the transient cages formed by the solvent. The model developed can estimate the independent contributions from these two different channels of diffusion. Although the solute diffusion in all the systems shows an amplification, the degree of it increases with solvent viscosity. The model correctly predicts that when the solvent viscosity is high, the solute primarily diffuses by exploiting the solvent cages. In such a scenario the MCT diffusion performed for a static solvent provides a correct estimation of the cage diffusion.

Fickian yet non-Gaussian behaviour: A dominant role of the intermittent dynamics

“…In order to understand how slowing down of solvent dynamics, which leads to the decoupling, effects the solute motion, we study a set of systems where we vary the solvent mass. As observed earlier 11,25 with the increase in solvent mass the decoupling between the solute and the solvent dynamics increases as seen from the D 2 /D 1 values in Table 1. In these studies the solutesolvent interaction is kept moderately attractive (ǫ 12 =6).…”

“…In this study we work with a solute-solvent system where the solute size is chosen to be small so that the solute can explore the inter-solvent cage 11 . In earlier studies 2,3 it has been shown that for such small solute particles the non-Gaussian behaviour of the van Hove correlation function is due to decoupling in solute-solvent dynamics.…”

“…In our present system there is no domain formation and the intermittency is the effect of attractive solute-solvent interaction. While undergoing random walk, the solute explores the transient solvent cage 4,11 . As the solute is small when it passes through the neck of the cage it feels uneven attraction from the different solvent particles and spends longer time near a particular solvent, which can be envisaged as a sticky point and leads to the intermittency in the solute dynamics.…”

“…Diffusion of smaller particles in a crowded medium is ubiquitous in nature and has huge industrial and academic relevance [1][2][3][4][5][6][7][8][9][10][11][12][13] . The diffusion of sodium chloride through granular soil bed is important in understanding the ground water contamination from solid waste landfills 14 .…”

Molecular simulations of cellular processes

“…Experimentally unexpected, this result was attributed to nonspecific interactions with bystander macromolecules. The importance of the electrostatic contribution to small molecule diffusion in crowded environments has also been emphasized recently (Acharya et al 2015;Kekenes-Huskey et al 2016). The impressive size of the simulated MG cytoplasm allowed a statistically accurate calculation of macromolecule transport properties.…”