pH drives electron density fluctuations that enhance electric field-induced liquid flow

Area of Science:

• Physical Chemistry
• Nanotechnology
• Surface Science

Background:

• Classical continuum theory describes liquid flow near charged interfaces using uniformly distributed point charges.
• Electrophoretic mobility of hydrophobic nanodroplets in water increases significantly with pH.
• Current theories attribute this mobility change to increased surface charge.

Purpose of the Study:

• To investigate the mechanism behind the increased electrophoretic mobility of hydrophobic nanodroplets with varying pH.
• To reconcile experimental observations with classical continuum theory.
• To elucidate the role of surface charge and molecular structure in nanodroplet dynamics.

Main Methods:

• All-optical measurements of nanodroplet surface charge and molecular structure.
• Electronic structure calculations.
• Comparison of experimental data with classical continuum theory predictions.

Main Results:

• Surface charge and molecular structure of nanodroplets remain consistent across neutral and mildly basic pH.
• Nanodroplet propulsion is driven by surface negative charge from water charge transfer and electric field-induced anisotropic polarization gradients.
• Replacing chloride with hydroxide doubles charge conductivity via the Grotthuss mechanism and nanodroplet mobility.

Conclusions:

• The observed increase in nanodroplet mobility with pH is not solely due to surface charge but involves complex charge transfer and polarization effects.
• Classical continuum theory is insufficient to explain these phenomena.
• The findings have broad implications for biological, chemical, and nanotechnology processes involving charged interfaces and pH-dependent hydrodynamics.