Molecular Dynamics Simulation of Salt Diffusion in Polyelectrolyte Assemblies.

Area of Science:

• Materials Science
• Physical Chemistry
• Computational Chemistry

Background:

• Polyelectrolyte (PE) assemblies are crucial in various applications.
• Salt ion diffusion in PEs is often modeled by ion hopping.
• Experimental verification of ion diffusion pathways at the nanoscale remains challenging.

Purpose of the Study:

• To investigate the molecular mechanisms of salt diffusion in polyelectrolyte assemblies.
• To explore the interplay between ion movement, water dynamics, and polymer chain motion.
• To provide a more comprehensive understanding of salt transport in hydrated PE systems.

Main Methods:

• All-atom molecular dynamics simulations were employed.
• Simulations focused on salt diffusion within poly(sodium-4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDAC) assemblies.
• Analysis of ion trajectories and water pocket dynamics.

Main Results:

• Observed ion diffusion beyond simple hopping, including a jump process linked to PE relaxation and water pocket dynamics.
• Anomalous subdiffusion of ions and water was identified due to trapping in water pockets.
• Jump events, though less frequent than hopping, significantly enhance salt diffusion with increasing temperature.

Conclusions:

• Salt diffusion in hydrated PDAC/PSS is a hybrid process combining ion hopping and jump motion.
• This study offers a novel molecular explanation for the coupling of salt and polymer chain motion.
• The findings elucidate the nonlinear increase in salt diffusion near the glass-transition temperature.