Finite-Size Charged Species Diffusion and pH Change in Nanochannels

Area of Science:

• Nanofluidics
• Physical Chemistry
• Surface Science

Background:

• Molecular transport in nanofluidic systems is governed by nanoscale phenomena, particularly surface interactions.
• The electrical double layer (EDL) influences ion diffusivity, but prior models often treat ions as point charges and ignore pH variations.
• Existing research on nanoscale molecular transport has limitations due to simplified ion models and overlooked pH effects in confined spaces.

Purpose of the Study:

• To provide a comprehensive analysis of molecular diffusion across micro- to ultra-nanoscale dimensions.
• To investigate the impact of finite-size ions and self-consistently computed pH on molecular transport.
• To address the overlooked aspect of pH changes within nanochannels and their effect on ion behavior.

Main Methods:

• Development of an all-encompassing analysis framework for molecular diffusion.
• Incorporation of finite-size ion effects in transport calculations.
• Self-consistent computation of the pH within nanochannels.

Main Results:

• Observed significant changes (over 2 orders of magnitude) in H+ ion concentration within channels compared to bulk.
• Demonstrated that counterions can be both enriched and excluded depending on nanochannel size.
• Validated findings with experimental data, highlighting the crucial role of pH and finite ion size.

Conclusions:

• The study reveals that nanoscale pH variations profoundly affect molecular transport, contrary to previous assumptions.
• Finite ion size and EDL effects are critical for accurate modeling of nanofluidic transport.
• A deeper understanding of nanoscale transport properties is essential for advancing medical and industrial applications.