A study of the subdiffusion of small molecules in charged polyelectrolyte multilayers

Area of Science:

• Materials Science
• Chemical Physics
• Polymer Science

Background:

• Polyelectrolyte multilayers (PEMs) are versatile materials with applications in coatings, drug delivery, and separation technologies.
• Understanding diffusion within PEMs is crucial for optimizing their performance and predicting their behavior in various environments.
• Sodium dithionite is a reducing agent often used in chemical processes and biological studies.

Purpose of the Study:

• To theoretically describe and experimentally investigate the diffusion of sodium dithionite in polyelectrolyte multilayers.
• To determine if the diffusion process follows classical Fickean laws or exhibits anomalous behavior.
• To elucidate the underlying mechanisms responsible for the observed diffusion characteristics.

Main Methods:

• Development of a theoretical model for slow diffusion in PEMs.
• Experimental measurement of sodium dithionite diffusion using fluorescence quenching of NBD-labeled polyallylamine hydrochloride (PAH).
• Assembly of PAH/PSS PEMs on silica particles and analysis using flow cytometry.

Main Results:

• Demonstrated that sodium dithionite diffusion in PAH/PSS PEMs is a case of subdiffusion, not following classical Fickean diffusion.
• Observed slow diffusion kinetics, indicating a non-Markovian diffusion process.
• Explained the slow diffusion by fractional time derivative diffusion equations, attributing it to the trapping of negatively charged dithionite in positively charged PAH layers.

Conclusions:

• The diffusion of sodium dithionite in polyelectrolyte multilayers is characterized by subdiffusion, a deviation from Fickean behavior.
• The observed anomalous diffusion can be accurately modeled using fractional time derivative equations.
• The trapping of dithionite ions within the positively charged PAH layers is the primary cause of the slow diffusion observed in these PEMs.