Molecular Diffusion of Ions in Nanoscale Confinement

Area of Science:

• Physical Chemistry
• Surface Science
• Nanofluidics

Background:

• Ion diffusion in confined geometries is critical for various applications.
• The electrical potential of confining surfaces can significantly impact ion transport.
• Understanding these effects is essential for optimizing processes in micro- and nanofluidic devices.

Purpose of the Study:

• To investigate the influence of an inert electrolyte on the diffusion of fluorescein anions in nanoscale aqueous films.
• To elucidate the role of surface electrical potentials and electrostatic double layers in confined ion diffusion.
• To compare experimental findings with a finite difference method (FDM) diffusion model.

Main Methods:

• Measurement of fluorescein diffusion in a nanoscale aqueous film confined between angled glass walls.
• Controlled introduction of an inert electrolyte (NaCl) to study its effect on diffusion rates.
• Development and application of a finite difference method (FDM) model to simulate concentration profiles.

Main Results:

• Diffusion of fluorescein was approximately 10 times slower without an inert electrolyte compared to its presence.
• Unscreened surface potentials significantly hindered fluorescein ion diffusion in thin films.
• The presence of even low electrolyte concentrations effectively screened surface potentials, facilitating ion diffusion.
• Debye length had a minor effect on ion diffusion into the film when surface potential was initially screened.

Conclusions:

• Prewetting confined spaces with pure water is not advisable for applications requiring rapid ion diffusion.
• Inert electrolytes play a crucial role in mitigating the hindering effects of surface potentials on ion diffusion in confined aqueous films.
• The findings provide valuable insights for designing and optimizing nanofluidic systems and processes.