Anomalous ζ potential in foam films

Area of Science:

• Surface science
• Physical chemistry
• Microfluidics

Background:

• Electrokinetic effects are crucial for controlling micro- and nanofluidic flows.
• Understanding electrokinetic phenomena at liquid-solid interfaces is established, but liquid-air interfaces remain under-explored.
• Ionic surfactants are key to modifying interfacial properties.

Purpose of the Study:

• To investigate electrokinetic transport at the molecular level in liquid films covered with ionic surfactants.
• To elucidate the behavior of zeta potential (ζ potential) with varying surfactant coverage at liquid-air interfaces.
• To rationalize the observed dependence of zeta potential on surfactant coverage.

Main Methods:

• Molecular-level simulations of electrokinetic transport.
• Analysis of interfacial hydrodynamics.
• Incorporation of an ion-binding mechanism.

Main Results:

• Zeta potential (ζ potential) shows an unexpected dependence on surfactant coverage.
• ζ potential increases as surfactant coverage decreases from saturation.
• ζ potential approaches a finite, non-zero value at low surfactant coverage.

Conclusions:

• Interfacial hydrodynamics and ion-binding mechanisms are critical for understanding electrokinetic transport at liquid-air interfaces.
• The findings challenge existing models for zeta potential measurements at free interfaces.
• Results have implications for electrokinetic transport in liquid foams and other systems with free interfaces.