Macro- versus microscopic view on the electrokinetics of a water-membrane interface

Area of Science:

• Physical Chemistry
• Interface Science
• Computational Biophysics

Background:

• Electrophoresis is a key technique for probing interfacial electrostatic properties.
• Macroscopic theories, like the Helmholtz-Smoluchowski model, are widely applied but rely on assumptions about sharp interfaces and length scales.

Purpose of the Study:

• To investigate the validity of macroscopic electrophoresis theory for a 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) lipid bilayer in a high salt concentration (500 mM NaCl).
• To explore deviations from classical electrokinetic models using molecular dynamics simulations.

Main Methods:

• Conducting molecular dynamics (MD) simulations of a POPC bilayer in an aqueous NaCl solution.
• Analyzing electrostatic properties, ion adsorption, and fluid dynamics at the lipid-water interface.

Main Results:

• POPC bilayers exhibit positive electrophoretic mobility, attributed to sodium ion adsorption onto lipid headgroups.
• The assumption of a sharp interface relative to the Debye screening length is violated.
• Observed deviations include ill-defined surfaces, inapplicability of continuum hydrodynamics, increased effective viscosity, and zeta potential dominated by dipole potential.

Conclusions:

• Classical macroscopic electrophoresis theories may not accurately describe electrokinetic phenomena at lipid interfaces under certain conditions.
• The findings necessitate re-evaluation of electrokinetic data interpretation, particularly in complex interfacial systems.
• Results have broad implications for understanding interfacial electrostatics in biological and material science contexts.