Interrelations between charging, structure and electrokinetics of nanometric polyelectrolyte films

Area of Science:

• Materials Science
• Physical Chemistry
• Surface Science

Background:

• Polyelectrolyte thin films exhibit complex electrokinetic behavior due to dissociable groups.
• Surface conductivity analysis is crucial for understanding ion transport in these films.
• Diffuse soft interface theory provides a framework for analyzing swelling and electrokinetic phenomena.

Purpose of the Study:

• To analyze streaming current, surface conductivity, and swelling data for poly(acrylic acid) (PAA) and poly(ethylene imine) (PEI) thin films.
• To differentiate electroosmotic and migration contributions to surface conductivity in highly charged films.
• To investigate the relationship between film swelling, pH, and interfacial properties.

Main Methods:

• Analysis of streaming current, surface conductivity, and swelling data.
• Application of diffuse soft interface theory.
• Ellipsometry for determining film thickness and swelling.

Main Results:

• PAA film swelling with increasing pH enhances interfacial diffuseness, leading to non-monotonous streaming current dependence on pH.
• PEI film swelling upon protonation (lowering pH) shows a decay length nearly independent of pH.
• Heterogeneous swelling significantly impacts electrokinetic measurements.

Conclusions:

• Electrokinetic and surface conductivity measurements are vital for characterizing coupled electrohydrodynamic and structural properties of swollen polyelectrolyte films.
• The diffuse soft interface model effectively describes the observed phenomena.
• Careful analysis is needed to interpret surface conductivity in films with high densities of dissociable groups.