The explicit characterization of counterion dynamics around a flexible polyelectrolyte

TL;DR

This study uses coarse-grained simulations to analyze counterion dynamics around a flexible polyelectrolyte, revealing sub-diffusive behavior, coupling effects, and how ion valency and salt concentration influence ion mobility and binding times.

Contribution

It provides a detailed characterization of counterion dynamics, including diffusion and binding behavior, around a flexible polyelectrolyte, highlighting effects of ion valency and salt concentration.

Findings

Counterions exhibit sub-diffusive behavior relative to the chain's COM.

Effective diffusivity is lowest near the chain and increases with distance.

Higher valency ions have drastically reduced diffusivity and longer binding times.

Abstract

The article presents a comprehensive study of counterion dynamics around a generic linear polyelectrolyte (PE) chain with the help of coarse-grained computer simulations. The ion-chain coupling is discussed in the form of binding time, mean-square-displacement (MSD) relative to the chain, local ion transport coefficient, and spatio-temporal correlations in the effective charge. We have shown that a counterion exhibits sub-diffusive behavior $\langle \delta R^2 \rangle \sim t^\delta$, $\delta\approx0.9$ w.r.t. chain's centre of mass (COM). The MSD of ions perpendicularly outwards from the chain segment exhibits a smaller sub-diffusive exponent compared to the one relative to the chain's COM. Further, we confirm that the effective diffusion-coefficient of counterions is strongly coupled with the chain. The effective diffusivity of ion is the lowest in chain's close proximity, extending up to length-scale of the radius of gyration Rg. Beyond Rg at larger distances, they attain diffusivity of free ion with a smooth cross-over from the adsorbed regime to the free ion regime. We have shown that the effective diffusivity drastically decreases for the higher valent ions, while the crossover length scale remains the same. Conversely, with increasing salt concentration the coupling-length scale reduces, while the diffusivity remains unaltered. The effective diffusivity of adsorbed-ion reveals an exponential reduction with electrostatic interaction strength. We further corroborate this from the binding time of ions on the chain, which also grows exponentially with the coupling strength of the ion-polymer duo. Moreover, the binding time of ions exhibits a weak dependence with salt concentration for the monovalent salt, while for higher valent salts the binding time decreases dramatically with concentration.